Remnant echoes of the past Andreas Viberg Archaeological geophysical prospection in Sweden

Remnant echoes of the past Andreas Viberg  Archaeological geophysical prospection in Sweden
Remnant echoes of the past
Archaeological geophysical prospection in Sweden
Andreas Viberg
Doctoral Thesis in Archaeological Science 2012
Stockholm University
Abstract
Viberg, A. 2012. Remnant echoes of the past. Archaeological geophysical prospection in
Sweden. Theses and Papers in Scientific Archaeology 13.
The aim of this thesis has been to investigate the benefits, pitfalls and possibilities of using
geophysical methods in archaeological projects. This is exemplified by surveys carried out at
archaeological sites in different geographical and chronological contexts. The thesis also aims
at investigating the cause for the under-use of the methods in Swedish archaeology by looking
at previously conducted surveys. The methods used during these surveys have been Groundpenetrating radar (GPR), magnetometer, slingram and a kappameter. The surveys in the
mountain tundra region of Lapland show that magnetic susceptibility surveys is a valuable aid
in discovering heaps of fire-cracked stones and when combined with magnetometry, also
hearths. GPR and magnetometer surveys within the Migration Period ringfort Sandbyborg
provided the spatial layout of the fort and indicated, along with results from recent
excavations and metal detections, many similarities with the ringfort Eketorp II. The nonmagnetic character of the sedimentary bedrock on Öland and Gotland is suitable for
magnetometer surveys and the method is also highly appropriate for the detection of the
remains of high-temperature crafts. GPR surveys at St. Mary’s Dominican convent in Sigtuna
produced the spatial layout of the central cloister area. The investigations also show that the
geology, pedology, land use and the character of commonly occurring prehistoric remains in
Sweden, in certain circumstances and in certain areas, have restricted the possibility of
successfully carrying out geophysical surveys. Care must therefore be taken to choose the
right instrument for the survey and to tailor the sampling density of each geophysical survey,
according to the character and size of the expected archaeological remains, in order to
maximize their information return. To increase the use of geophysical methods in Sweden the
educational opportunities, both for surveyors and professional archaeologists, need to
improve.
Keywords: Archaeological prospection, geophysical survey, Sweden, Ground-penetrating
radar, magnetometry, Slingram, magnetic susceptibility, Neolithic, Migration period, Viking
Age, Middle Ages, Öland, Gotland, Sigtuna, Lapland.
© Andreas Viberg, Stockholm 2012
ISSN 1400-7835
ISBN 978-91-7447-549-4
Printed in Sweden by Universitetsservice US-AB, Stockholm 2012
Distributor: Archaeological Research Laboratory, Stockholm University
Cover image: The text reads: “Commemoratio omnium fratrum familiarium et
benefactorum defunctorum ordinis nostri et alibi aliorum plurimorum sanctorum
martyrum et confessorum atque sanctarum virginum”. The text is written on a
wooden writing desk found at St. Mary’s Dominican Convent in Sigtuna. The gothic
window frame is published with permission from The Garden Factory and the use of
the font, Northumbria, by permission of David Kerkhoff. Cover image photos:
Kerstin Lidén & Siska Williams, Layout by the author.
To Helena and Elsa
the lights of my life
Acknowledgements
It is almost four years and eight months since I started off as a doctoral
student at the Archaeological Research Laboratory (ARL) and family,
friends and colleagues have been involved in the process and as such deserve
my deepest gratitude.
First I would like to thank my wife Helena and my daughter Elsa who
have put up with me during the intense process of writing the thesis. I am
sorry for the late nights, the weekends and vacations when I had to work to
get it finished. Thank you for encouraging me to do my best, even when it
was tough. This thesis is dedicated to you!
Thanks to my supervisor Kerstin Lidén who has been a solid support
during these past years. Thank you for keeping me calm and motivated
during times when the process of writing a thesis felt somewhat
overwhelming. Thank for your inspiration and contribution to the many
interesting projects we have been involved in together. We started off in the
mountain tundra region in Lapland and finally ended up on Öland. It has
been a fantastic time.
Next in line are my geophysical supervisors. Thanks to Immo Trinks for
introducing me to the world of archaeological geophysics and for your
supervision during my first year as a PhD-student. I am especially grateful to
Bo Olofsson, Rinita Dalan and Bruce Bevan who came to my aid when I
needed it the most, and who have been very patient and generous with their
time. This thesis would not have become a reality without your help! I would
also like to thank Kjell Persson for many interesting discussions on
archaeological geophysics and for stepping up and helping me with
instrument and computer issues whenever needed. I would also like to thank
you for your supervision during my time as an undergraduate student. It all
started with a GPR survey on a cold day in Fresta, Upplands Väsby, and
look where I ended up.
To my co-authors in the different articles presented in this thesis. It has
been inspiring to work with you and to share your passion for the different
projects and for archaeology. A special thanks to Anders Wikström who was
involved in my application to the ARL and who has been providing me with
suggestions to many suitable targets for geophysical prospections in Sigtuna.
It has been a privilege working with you.
I would especially like to thank Elin Fornander who has been like my
PhD mentor over the years. You were there when I arrived at the ARL and it
was inspiring to see you finish your thesis last year. Thank you for
everything and don’t forget to “frysa könsögonblicket” as often as you can,
just like our lecturer in university pedagogy so successfully did on many
occasions.
When I started as a PhD-student I got to share room with Björn
Hjulström. I really appreciate that you took the time to answer all my rookie
questions even though you were busy writing your own thesis. Remember
that it is never too late to pick up the tuba again. Thanks to Anna
Linderholm, who among other things helped me collect soil samples at
Stensborg. That day was hard work for us all but ended well at the golf
course restaurant. Heja Djurgården!
I have also tormented several other fellow PhD-students at the ARL to
help me during my geophysical measurements. Thank you Ny Björn
Gustafsson (and Erika of course) who joined in on several surveys in
miserable conditions. I especially remember the gale force winds on Öland,
the GPR survey in 0.4m of snow at Fornsigtuna and that remarkably wet day
on Gotland. Thank God for pear flavoured whipped cream! Thanks also to
Joakim Schultzén for helping me during the magnetometer surveys in
Hedeby and on Öland. A special thanks for taking me to Ö&B to gear up
when I accidentally turned my non-magnetic rain trousers into a miniskirt
while getting into your car. Thanks to my other fellow PhD-students at the
ARL: Anita Malmius, Christos Economou, Hans Ahlgren, Rachel Howcroft
and Sylvia Sandelin Löfgren. Thanks also to Christina Karlsson for
providing me with caffeine patches and for sharing my passion for coffee.
Snoozers are losers!
I have also had the privilege of spending a lot of time with past and
present researchers and staff at the ARL. Thanks to Anders Götherström,
Birgit Arrhenius, Charlotte Hedenstierna-Jonson, Christina Schierman,
Gunilla Eriksson, Gustav Trotzig, Ioannis Panagopoulos, Laila Kitzler
Åhfeldt, Lena Holmqvist, Liselotte Bergström, Maria Wojnar-Johansson,
Margaretha Klockhoff, Mikael Lundin, Sven Isaksson and Yvonne Fors for
your support that contributed to making my time as a PhD-student so
interesting and rewarding. I would also like to extend a big thank you to
Ludvig Papmehl-Dufay who was very generous with his time and help
during my surveys of Sandbyborg on Öland. How about surveying another
ringfort?
Many others have been involved in my different projects or been helping
me in one way or the other and I would like to thank Alain Tabbagh, Alois
Hinterleitner, Anders Angerbjörn, Anders Hedman, Andreas Forsgren, Anna
Kjellström, Bernth Johansson, Chris Gaffney, Christer Dahlström, Dario
Cianciarulo, Dattatray Parasnis, Dean Goodman, Elin Engström, Göran Alm,
Göran Burenhult, Henrik Alvarmo, Ivonne Dutra Leivas, Jaana Gustafsson,
Jan Storå, Joakim Kjellberg, Lars Larsson, Linda Qviström, Neil Linford,
Robin Blomdin, Svante Johansson, Tessa Evans, Timo Ibsen, Tom and
Charlotte Slettengren and Ulf Näsman. Thanks also to the participants of
Arctic Sweden, the Swedish Polar Research Secretariat, everyone
contributing to the material presented in the review article and to every
researcher and staff at the Department of Archaeology and Classical Studies
at Stockholm University. I would also like to thank everyone at the
department who provided me with valuable comments that greatly improved
the text. I hope you had “FUN” reading it.
The language in this thesis has been revised by Malcolm Hicks and I am
extremely grateful for your solid work and for the incredible speed with
which you corrected my texts.
This thesis would not have been possible to finish without the financial
support of Albert & Maria Bergström Stiftelse, Brigit and Gad Rausings
Stiftelse för Humanistisk Forskning, Helge Ax:son Johnsons Stiftelse and
Svenska Fornminnesföreningen.
My sincere gratitude also extends to everyone in the International Society
for Archaeological Prospection and especially: Adrian Butler, Alette
Kattenberg, Andrea Zeeb-Lanz, Arne Anderson Stamnes, Carmen Cuenca
Garcia, Christophe Benech, Ervan G Garrison, George Maloof, Guillaume
Hulin, Ian Moffat, James Bonsall, Jason Jeandron, Jarrod Burkes, Joep
Orbons, Johannes Frenzel, Jörg Fassbinder, Kayt Armstrong, Kelsey Lowe,
Kevin Barton, Norbert Buthmann, Piotr Szczepanik, Piotr Wroniecki, Roger
Ainslie, Steve de Vore, Tomasz Herbich, Walter Sevenants and Wesa
Perttola, who offered valuable insights into the use of archaeological
prospection methods in other countries. A special thanks to Lewis Somers
for interesting discussions on the early days of archaeological geophysics in
England.
To my dear friends Antje Wendt and Sven Kalmring. Thank you for your
valuable comments on my thesis, for being there and encouraging me during
the final stages of the writing process and for putting up with my feeble
attempts of learning German.
Thanks to Mats, Maria, Anna-Carin, Nils, Ingalill, Per and Monica for
your support throughout this journey. A special thanks to my mother and
father, Barbro and Wincent, who has cheered for me during my years at the
University and who has been helping out in various ways so that I could
spend the required time carrying out surveys and writing the thesis. Dignity,
always dignity!
All that remains to be said is:
Ahoi Landratten! See you next year in Tuscany!
List of papers
I.
Viberg, A., Trinks, I. & Lidén, K. 2011. A Review of the
Use of Geophysical Archaeological Prospection in Sweden.
Archaeological Prospection 18(1): 43-56.
II.
Viberg, A., Berntsson, A. & Lidén, K. Archaeological
Prospection of a High Altitude Neolithic Site in the Arctic
Mountain Tundra Region of Northern Sweden. Manuscript
submitted to Journal of Archaeological Science.
III.
Viberg, A., Victor, H., Fischer, S., Lidén, K. & Andrén, A.
A Room with a View. Archaeological Geophysical
Prospection and Excavations at Sandby ringfort, Öland,
Sweden. Manuscript.
IV.
Gustafsson, N-B. & Viberg, A. 2012. Tracing Hightemperature Crafts. Magnetometry on the island of Gotland,
Sweden. Archaeological Prospection, 19(3): 201-208.
V.
Viberg, A. & A. Wikström, 2011. St. Mary's Dominican
Convent in Sigtuna Revisited. Geophysical and archaeological investigations. Fornvännen 106: 322-33.
Contents
1.
Introduction, aim and structure of thesis................................................11
2.
Geophysical prospection methods .........................................................16
2.1. Magnetic methods ................................................................................ 16
2.1.1.
The Earth’s magnetic field............................................................ 17
2.1.2.
Magnetism, basic concepts ........................................................... 19
2.1.3.
Magnetic susceptibility and archaeological prospection ................. 20
2.1.4.
Thermoremanent magnetism and archaeological prospection ........ 22
2.1.5.
Passive instruments for magnetic prospection ............................... 22
2.2. Electromagnetic methods...................................................................... 26
2.2.1.
Ground penetrating radar, an introduction ..................................... 26
2.2.2.
GPR and soil properties ................................................................ 29
2.2.3.
GPR and topography .................................................................... 34
2.2.4.
GPR manufacturers and suitable archaeological targets ................. 34
2.2.5.
The metal detector ........................................................................ 36
2.2.6.
Magnetic susceptibility and the Bartington MS2D......................... 36
2.2.7.
Conductivity and susceptibility measurements using the Geonics
EM38 Slingram instrument ........................................................... 38
2.3. Geoelectric methods and other geophysical prospection methods .......... 40
2.4. Marine geophysical methods and geochemical prospection methods...... 42
2.5. The benefits of using an integrated approach ......................................... 43
3.
Development and current use of archaeological geophysical
prospection methods ............................................................................. 45
3.1. A brief history of archaeological geophysics ......................................... 45
3.1.1.
Geoelectric methods ..................................................................... 45
3.1.2.
Magnetometry .............................................................................. 46
3.1.3.
Electromagnetic methods.............................................................. 47
3.2. Use of archaeological geophysical prospection methods in other
countries .............................................................................................. 48
3.2.1.
Austria ......................................................................................... 48
3.2.2.
Denmark ...................................................................................... 48
3.2.3.
England........................................................................................ 49
3.2.4.
Finland......................................................................................... 49
3.2.5.
Germany ...................................................................................... 50
3.2.6.
Iceland ......................................................................................... 50
3.2.7.
The Netherlands and Flanders....................................................... 50
3.2.8.
Norway ........................................................................................ 51
3.2.9.
Poland.......................................................................................... 51
3.2.10. Scotland ....................................................................................... 51
3.2.11. Turkey ......................................................................................... 52
3.2.12. North and South America ............................................................. 52
3.2.13. Australia ...................................................................................... 52
3.3. Archaeological geophysics in Sweden................................................... 53
3.4. Concluding remarks ............................................................................. 60
4.
4.1.
4.2.
5.
Data collection, interpretation and the character of Swedish
archaeological remains ......................................................................... 69
5.1. Sampling density, data collection and georeferencing survey grids ........ 69
5.2. Interpretation........................................................................................ 76
5.3. Swedish archaeological remains ........................................................... 77
5.3.1.
Graves ......................................................................................... 77
5.3.2.
Prehistoric settlement remains ...................................................... 79
5.3.3.
Fortified sites ............................................................................... 80
5.3.4.
Fired structures............................................................................. 81
5.3.5.
Medieval buildings ....................................................................... 81
5.3.6.
Gardens........................................................................................ 82
5.3.7.
Detection of archaeological remains in present-day urban
environments................................................................................ 82
5.3.8.
Ferrous objects ............................................................................. 82
5.3.9.
Other non-archaeological features................................................. 82
6.
6.1.
6.2.
7.
Geological and pedological conditions in Sweden and their
implication for archaeological geophysics ............................................. 61
Geology, pedology and land use ........................................................... 61
Implications for archaeological geophysics ........................................... 67
Discussion ............................................................................................83
The Swedish situation and beyond ........................................................ 89
Brief guidelines for archaeologists ........................................................ 90
Conclusions and future prospects for archaeological geophysics in
Sweden ................................................................................................ 93
References............................................................................................................95
1. Introduction, aim and structure of thesis
For a very long time Sweden was in the forefront of the development and
use of geophysical prospection. One of the earliest documented applications
dates from the 1640s, when a simple declination compass was used to locate
iron ore in Närke (Carlborg 1963:13). Since then, Swedish researchers have
been involved in developing new methods and instruments, such as
magnetometers (Thalén 1879; Tiberg 1884), electrical resistivity instruments
(e.g. Petersson 1907; Bergström 1913; Lundberg, 1919, 1922), electromagnetic instrumentation (Sundberg et al. 1923) and the electromagnetic
instrument Slingram (Werner 1937). Metal detectors were already being
used for mine detection during the Second World War and groundpenetrating radar (GPR) investigations and theoretical studies were carried
out during the 1970s (Nilsson, 1973, 1976, 1978; Ulriksen 1982).
Geophysical surveys have gradually been adopted by researchers in many
other countries with an interest in locating buried archaeological remains,
providing important information on subsurface structures in a wide variety
of archaeological environments from 1938 onwards (see, for example,
Aitken 1958; Atkinson 1953; Bevan 2000). Thus the subdiscipline of
“archaeological geophysics”, defined as “the examination of the Earth’s
physical properties using non-invasive ground survey techniques to reveal
buried archaeological features, sites and landscapes” (Gaffney & Gater
2003:12), was born.
Even though geophysical prospection has been successfully used in
Sweden for a long period of time and the country is the home of three
geophysical equipment manufacturers, ABEM, Malå Geoscience and
Radarteam Sweden AB, this success has failed to be translated into
archaeology. The methods were not used in any Swedish archaeological
projects until the mid-1970s (Fridh 1982) with the exception of some early
metal detector investigations in 1959 and the early 1960s (Hagberg 1961;
Nylén 1972). Despite the fact that geophysical methods are non-destructive,
can pinpoint specific areas of interest within larger sites (and thus permit
targeted excavations) and can provide complementary information about
archaeological structures outside the areas directly affected by excavations,
they have only been used sporadically in archaeological projects. This can
partly be explained by the fact that many archaeologists who have used
geophysical prospection have been sceptical about its benefits and reliability,
as there has in many cases been a lack of correlation between the
geophysical and archaeological results (see, for example, Carelli 2003;
11
Hylén 2007; Eklund 2008; Steineke 2008; Bennström & Helgesson 2010;
Lindberg 2010).
Archaeological geophysics has now become an integrated part of
archaeological projects in many other countries, and its limited use in
Sweden is interesting. I have therefore, in parallel with my geophysical
surveys, been engaged in examining the possible reasons for this underdevelopment.
The importance of correcting this archaeological scepticism emerges very
clearly from the revised European Convention on the Protection of the
Archaeological Heritage, a convention ratified by Sweden1 (Trotzig, 1993),
which requires each signatory ‘to ensure that archaeological excavations
and prospecting are undertaken in a scientific manner’ and that ‘nondestructive methods of investigation are applied wherever possible’ (Article
3ib).
The overall aim of this thesis is to investigate the benefits of using
geophysical methods in Swedish archaeology. By surveying sites situated in
different geographical contexts, from the mountain tundra region in Lapland
(Paper II) to the arable lands of Gotland (Paper IV) and Öland (Paper III),
and at sites spanning a time scale from the Neolithic (Paper II) to the Middle
Ages (Paper V) (Fig. 1), I intend to gain a better understanding of the pitfalls
and possibilities involved in using archaeological geophysics in different
geological contexts.
The surveys have, with one exception, been carried out in the eastern part
of Sweden, and this is of course a limitation, as the natural environment and
archaeology of the rest of Sweden may be different. But even though the
surveys have an easterly focus, it is my belief that many of the conclusions
presented in this thesis are also valid for geophysical surveys in other parts
of the country and other areas with similar geological, pedological and
archaeological preconditions. The purposes and specific research questions
connected with these different projects are presented in papers II-V.
It became evident in the process of carrying out these investigations that
geophysical methods had only been used sporadically within Swedish
archaeology. With the purpose of understanding the reason for this limited
use, a review of prior geophysical surveys at archaeological sites in Sweden
was carried out (Paper I), and an attempt was made through correspondence
with several Swedish field archaeologists involved in these earlier surveys,
to investigate on a limited scale their attitude towards archaeological
geophysics and their level of satisfaction with the resulting data and
interpretations.
1
http://conventions.coe.int/Treaty/EN/Treaties/Html/143.htm
12
Figure 1. Sites prospected by the author since 2006. 1: Tierp Church, Uppland, 2:
Alunda Church, Uppland, 3: Bälinge Church, Uppland, 4: Sigtuna, Uppland
(several sites), 5: Fornsigtuna, Uppland (several surveys), 6: RAÄ 108 in the
parish of Fresta, Uppland, 7: Frescati , Uppland (several sites), 8: Karsvik,
Uppland (in collaboration with UV-Teknik, Swedish 7ational Heritage Board), 9:
Riddarholmen Church, Stockholm, 10: Stensborg in the parish of Grödinge,
Södermanland, 11: Korsnäs, in the parish of Grödinge, Södermanland, 12: Tjuls, in
the parish of Eskelhem, Gotland, 13: Stånga annex, in the parish of Stånga,
Gotland, 14: Odvalds in the parish of Linde, Gotland, 15: 7ygårds, in the parish of
Eke, Gotland, 16: Sandbyborg, in the parish of Södra Sandby, Öland, 17 Hedeby
Hochburg, Germany (not shown on the map), 18: RAÄ 1372, in the parish of
Sorsele, Lapland. Coordinates in SWEREF99 TM.
13
A review of the literature further enabled a study of data collection
strategies, sampling densities and the choice of instruments and methods, as
these are factors which might have affected the outcomes of the surveys. In
addition, the interpretations of the geophysical data were compared
whenever possible with the results of subsequent excavations in order to
evaluate the success of the surveys. This review was important, as I strongly
believe that it is essential to evaluate thoroughly both successful and failed
geophysical surveys if better decisions and strategies are to be arrived at in
the future.
Related research questions include, but are not restricted to:
• What methods are especially suitable for use in Sweden?
• In what way do Sweden’s geological environments and climatic
conditions affect the success of geophysical prospection?
• What are the sizes and physical properties of the archaeological
structures and remains commonly found in Sweden, and what
methods are best suited for their detection?
• How dense do the measurements have to be in order to identify the
archaeological features and interpret them correctly?
• How does the situation for archaeological geophysics in Sweden relate
to that in other countries?
Several overviews of archaeological geophysics have been written
previously (e.g. Scollar et al. 1990; Clark 1996; Conyers & Goodman 1997;
Bevan 1998; Gaffney & Gater 2003; Witten 2006; Aspinall et al. 2008;
Campana & Piro 2009) and it is not my intention to provide a complete
review of every aspect of the subject. As an archaeologist, my main interest
is in the interpretation and implementation of geophysical data in
archaeological projects, and not necessarily in the technical development of
the methods. As only one doctoral thesis (Fischer 1980), one licentiate thesis
(Persson 2005a) and a short text describing archaeological geophysics and
some of its benefits and disadvantages (Liljenstolpe 1998) have been written
on this topic in Sweden, I felt that the time was right to take this theme up
once again.
The thesis begins with a description of the theory behind the geophysical
methods most commonly used in archaeology, followed by a short
description of the development of archaeological prospection with an extra
focus on developments in Sweden. This chapter is followed by a discussion
on the impact of geological, pedological and geomorphological conditions,
14
which are vital factors for the planning and execution of any geophysical
survey. Another important factor that affects the success of geophysical
prospection is the character of the archaeological remains and structures. As
geophysical instruments detect only contrasts between the archaeological
remains and the surrounding soil matrix, one must understand the physical
characteristics and properties of archaeological features if a correct choice of
method and sampling density is to be made. This part of the thesis also
investigates existing guidelines for data collection and suggests suitable
strategies for the Swedish environment. Questions regarding sizes of survey
grids and the interpretation of geophysical maps are discussed. The thesis
ends with a discussion of future prospect for archaeological geophysics in
Sweden.
15
2. Geophysical prospection methods
Archaeological geophysics is defined as “the examination of the Earth’s
physical properties using non-invasive ground survey techniques to reveal
buried archaeological features, sites and landscapes” (Gaffney & Gater
2003:12), and the same authors have provided a concise list of methods for
doing this (Gaffney and Gater 2003:26), labelling them as either active or
passive. Active methods transmit an electromagnetic field, an electric
current, or a seismic wave into the ground and measure how the soil and
other materials alter its transmission, while passive methods detect voltages
or fields that are already at the Earth’s surface and determine how this
measurement changes within the surveyed area. GPR, Slingram, magnetic
susceptibility and electrical resistance or resistivity are examples of active
instruments or methods, whereas magnetometry is the most commonly used
passive method (see below).
The list of publications describing geophysical prospection is extensive
(e.g. Milsom 1989; Reynolds 1997; Mussett & Khan 2000; Kearey et al.
2002), and several books focusing on geophysical methods for archaeology
are available (e.g. Aitken 1974; Scollar et al. 1990; Clark 1996; Conyers &
Goodman 1997; Bevan 1998; Neubauer 2001; Gaffney & Gater 2003;
Witten 2006; Aspinall et al. 2008; Campana & Piro 2009). These provide
archaeologists with information about basic techniques and about the more
advanced theoretical and methodological aspects of geophysical prospection.
The summary presented here should be regarded as a basic introduction to
the more common concepts of archaeological geophysics, and the reader is
advised to seek more detailed information in the above publications. The
methods used in the projects described in this thesis include magnetometry
(Paper III and IV), magnetic susceptibility (Paper II) and GPR (Paper III and
V).
2.1. Magnetic methods
Ever since the first magnetic surveys at archaeological sites were carried
out by Aitken and Hall in 1958 the magnetic method of prospection has been
increasingly widely used in archaeological projects all over the world. The
fluxgate magnetometer (see below) has been referred to as the “work horse”
of archaeological geophysics, for example (see Clark 1996:69; David et al.
2008) and is currently one of the most popular and most frequently used
instruments for geophysical prospection at archaeological sites.
16
As with all geophysical methods, one is looking for a contrast between
the buried archaeological feature and the surrounding soil. In magnetic
prospection this contrast can either be related to enhancements in magnetic
susceptibility (the degree to which a substance can be magnetised when
subjected to a magnetic field), or to magnetic remanence. Here
thermoremanent alterations are of particular interest to archaeologists
because, being caused by pronounced heating and cooling of the
archaeological structure, they are related to burning and represent the earth’s
magnetic field at the time when this burning took place.
A basic introduction to magnetism in general and soil magnetism in
particular is presented below, together with a brief introduction to magnetic
susceptibility and thermoremanent magnetism and its implications for
archaeological geophysics.
2.1.1. The Earth’s magnetic field
The Earth’s magnetic field is created by movement of a liquid iron alloy
in the planet’s outer core (Marshak 2008:50ff); this field is essential for life
as it protects us from harmful particles emitted by the sun (Reynolds
1997:131). The Earth’s magnetic field resembles that of an ordinary bar
magnet, with a north and a south pole and magnetic flux lines passing
through the middle of the Earth and then travelling outside it from the north
magnetic pole to the south magnetic pole (see Fig. 2; Kearey & Brooks
2002:156). The strength of this magnetic field, expressed in nanotesla (nT),
differs depending on where you are on Earth’s surface and varies from
c.30,000 nT at the magnetic equator to 60,000 nT at the north and south
magnetic poles (Reynolds 1997:134).
Figure 2. The Earth’s magnetic field and the location of its magnetic and
geographical north poles.
17
As indicated in Fig. 2, the location of magnetic north differs from that of
true or geographical north. This angular difference is called the magnetic
declination and is a value that varies over time as the magnetic poles are
constantly moving at a slow pace (Marshak 2008:78f). The declination map
for Sweden is updated regularly by the Swedish Geological Survey (SGU)
and the map for 2012 is presented in Fig. 3, showing values varying between
c. 2-14 degrees. Previous locations of the Earth’s magnetic north pole are
recorded by directions frozen in magnetic features such as in situ igneous
rocks and fired archaeological structures such as kilns. As such, this
direction can be a means for roughly dating archaeological structures fired at
temperatures above the Curie point (Aitken & Weaver 1962; Sternberg
2008). The magnetic inclination, on the other hand, can be measured with a
compass that is allowed to tilt freely on a horizontal axis and can be defined
as the angle of dip that the compass takes at different latitudes on Earth,
being parallel to the ground surface (0°) at the equator and pointing straight
down (90°) at the north and south poles. The different locations of the
magnetic north pole during historic and prehistoric times and the current
magnetic declinations and inclinations have important implications for
interpreting the results of magnetic prospection. This will be examined in
greater detail below.
Figure 3. Magnetic declination values for 7orway, Sweden and Finland © Swedish
Geological Survey (SGU).
18
2.1.2. Magnetism, basic concepts
All materials show some reaction to a magnetic field, though this reaction
may be weak or strong. This magnetic field can arise from electron spin and
orbits at an atomic level (Thompson & Oldfield 1986:3). Some materials are
permanently magnetised and will be magnetic even if they are not subjected
to a magnetic field, while others are only magnetic in the presence of such a
field (Reynolds 1997:122). Remanent magnetism is the name given to
permanently magnetised materials, while induced magnetism describes
materials that are only magnetic in the presence of a magnetic field. The
magnetic property of any material is, among other things, dependent on the
number of electrons present in each subshell of the atom and in which of the
two possible directions the orbiting electrons are aligned when subjected to
an applied magnetic field (Aspinall et al 2008:11ff). Different materials can
thus be classified according to the direction of spin and the overall magnetic
moment (positive or negative) created by this motion.
These materials, which in some cases are chemical elements, can be
categorised as being diamagnetic, paramagnetic, ferromagnetic,
antiferromagnetic and ferrimagnetic. If one were to expose these materials or
chemical elements to a magnetic field, diamagnetic materials, such as quartz,
with complete electron subshells, will produce an opposing weak negative
magnetisation and paramagnetic materials, such as biotite, will align with the
applied field and produce a weak positive magnetisation, as their atomic
subshells are incomplete. Both diamagnetic and paramagnetic materials will
only display this characteristic behaviour in the presence of a magnetic field.
By comparison, ferromagnetic materials such as iron and nickel are able
to retain their magnetic properties without the presence of a magnetic field,
as they have unpaired electrons in their atomic subshells and magnetic
dipoles aligned in one direction, which produces a very strong
magnetisation. Antiferromagnetic materials, on the other hand, have dipoles
of equal strength that are aligned in opposite directions, resulting in a zero or
almost zero net magnetisation as the magnetic moments cancel each other
out, as is the case with the imperfect antiferromagnetic mineral haematite.
Magnetite is a ferrimagnetic material and has magnetic dipoles in opposite
directions but of unequal strength, resulting in a positive net magnetisation
(Fig. 4 & Table 1). For a more detailed description of this classification, see
Thompson and Oldfield (1986:3ff), Reynolds (1997:122ff) or Kearey and
Brooks (2002:157f). Many materials, such as a piece of rock or a soil
sample, will contain mixtures of different minerals and magnetic
components, and the total magnetic susceptibility of such a material will
consist of the magnetic susceptibilities of all these categories of magnetic
behaviour added together (Dearing 1999:6). Magnetic susceptibility values
for certain common rock types and minerals are presented in Reynolds
(1997:126ff).
19
Figure 4. Orientation of the magnetic
antiferromagnetic and ferrimagnetic materials.
moments
for
ferromagnetic,
As ferromagnetic materials rarely occur in nature, the most influential
components are the ferrimagnetic iron oxides such as magnetite and
maghemite. These ferrites are very common in many rock types and are
responsible for the very high magnetic susceptibilities of basic igneous rocks
such as basalt (Thompson & Oldfield 1986:13ff). Magnetite, and primarily
maghemite, are also very important for the detection of archaeological
features.
Table 1. Examples of diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic
and ferrimagnetic materials.
Diamagnetic Paramagnetic Ferromagnetic
Quartz
Feldspar
Calcite
Water
Olivine
Garnet
Biotite
Pyroxene
Iron
Cobalt
Nickel
Antiferromagnetic/
Imperfect
Antiferromagnetic
Haematite
Chromium
Ferrimagnetic
Magnetite
Maghemite
2.1.3. Magnetic susceptibility and archaeological prospection
Magnetic susceptibility is the degree to which a substance can be
magnetised when subjected to a magnetic field (Thompson & Oldfield
1986:25). Le Borgne has shown that the magnetic susceptibility of the soil
layers within the top 20-30cm of the surface is greater than that of the
underlying subsoil, and has suggested that this enhancement could be caused
by soil development processes (the fermentation effect) and burning (Le
Borgne 1955, 1960). These effects have been studied by Weston (2002). The
burning effect has also been studied both in the laboratory (e.g. Tite &
Mullins 1971; Mullins 1974; Graham & Scollar 1976) and during
experimental firing in different parts of the world (e.g. Bellomo 1993;
Marshall 1998; Linford & Canti 2001; Weston 2002). One important finding
has been that enhancement is achieved when the antiferromagnetic mineral
haematite is transformed to the highly magnetic mineral magnetite during
firing in a reducing environment. When the fire is put out and oxygen is
20
reintroduced, the magnetite continues to transform into the ferrimagnetic
mineral maghemite. This mineral does not have as high a magnetic
susceptibility as magnetite, but it is still much higher than that of haematite,
thus creating a clear contrast between the fired structure and the surrounding
soils. A prerequisite for this transformation is the presence of organic matter.
A relatively modest temperature of c. 150-200°C during firing is sufficient
for creating a slight enhancement of magnetic susceptibility (Thompson &
Oldfield 1986:75; Linford & Canti 2001:224; Aspinall et al. 2008:24).
Forest fires or slash-and-burn agriculture would also cause such a
susceptibility enhancement (Clark 1996:115) providing sufficient
temperatures and burning times were achieved. Ploughing will distribute the
magnetic soils of fires and burned features throughout the topsoil and cause
an increase in magnetic susceptibility. This mixing can also be caused by
natural factors such as bioturbation.
Enhancement of the magnetic susceptibility of the topsoil has also been
shown to be caused by other mechanisms such as micron-sized soil bacteria,
which contain a small amount of magnetite and found in accumulations in
the unburned remains of prehistoric wooden structures in southern Germany
and Austria (Fassbinder et al. 1990; Fassbinder & Stanjek 1993; Neubauer
2001). These bacteria give rise to weak magnetic anomalies that are only
detectable by very sensitive magnetometers. Given the abundance of glacial
till soils in Sweden (see chapter 4) it is unlikely that background levels in
such soils would be low enough for such unburned features caused by the
presence of magnetic bacteria to be detected, despite the use of very
sensitive instruments. This mechanism, enabling the detection of unburned
wood, has unfortunately been confused by one Swedish surveyor, resulting
in the erroneous claim that certain bugs (beetles) supposedly containing
magnetite die in situ after consuming decaying unburned wood, thus
providing a magnetic contrast that is readily detectable using a
magnetometer2 (Kockum 2006). Such claims are not supported by current
research, however. A concise list of the mechanisms currently known to
cause enhancement of magnetic susceptibility is presented by Aspinall et al.
(2008:25).
Even though magnetic susceptibility measurements are commonly carried
out using an active instrument such as the EM38 or MS2D, archaeological
features that cut through the topsoil into the subsoil and are then
subsequently refilled with magnetically-enhanced topsoil may also be
detected with a magnetometer (Clark 1996:65; Gaffney & Gater 2003:38).
Depending on the surrounding soils, this would cause either a positive or a
negative anomaly to be detected. Such prehistoric features could be pits,
postholes or ditches.
2
http://www.ostran.se/NYHETER/Oeland/sensationella_fynd_vid_fornborg
21
2.1.4. Thermoremanent magnetism and archaeological
prospection
Another important aspect of magnetism is the magnetic remanence of
different materials, and thermoremanent magnetisation is of particular
importance for archaeological prospecting. Thermoremanent magnetisation
(TRM) occurs when a magnetic material is heated above its Curie point and
subsequently cooled below its blocking temperature. Above the Curie point,
which is particular to given magnetic materials, the heated material is
essentially paramagnetic, carrying only a slightly positive magnetic
susceptibility. When the material cools, however, it regains its magnetic
characteristics and the direction of the magnetisation aligns with the
geomagnetic field at the time of cooling.
In prehistory, for example, this heating could be achieved by firing in
hearths and kilns to temperatures exceeding the Curie temperature of the
magnetic mineral present in the soil. (The Curie temperature of magnetite,
for example, is 578°C and that of hematite 675°C). Upon cooling, a typical
bipolar magnetic anomaly generated by thermoremanent features is created,
and this is detectable by a magnetometer. Bipolar anomalies can also be
caused by igneous rocks, iron artifacts or lightning (see Bevan 1995a, 2009;
Jones and Maki 2005; Maki 2005), and caution should therefore be exercised
when interpreting magnetic data.
It is worth noting that ploughed out hearths lose their thermoremanent
component as the hearth material slowly mixes with the surrounding soils.
This would lead to a general increase in the magnetic susceptibility of the
soil (Weston 2002:208). Note that hearths will in most cases have both
increased magnetic susceptibility values and a thermoremanence (Weston
2002:208).
2.1.5. Passive instruments for magnetic prospection
Magnetometers are sensitive geophysical instruments that are able to
detect very small fluctuations in the Earth’s magnetic field. The unit of
magnetic measurement is the tesla (T), but prehistoric remains usually only
produce fluctuations in the Earth’s magnetic field of a few nanoteslas (1nT=
10-9T) or even picoteslas (1pT= 10-12T). The Earth’s magnetic field in the
Stockholm area in Sweden has an approximate strength of 50 000 nT and
instruments are able to detect small deviations from this value caused by
archaeological features. The detectability of prehistoric features is also
dependant on factors such as sampling density and the spatial variability in
the magnetic properties of the soil and rock in the area surveyed.
A single-sensor magnetometer not only measures spatial patterns caused
by remains buried in the soil but also the constantly fluctuating magnetic
field of the Earth (i.e. the total field). These changes, caused by solar wind or
22
solar magnetic storms (see Aspinall et al. 2008:31f) can effectively be
removed from the data by using two sensors instead of one and subtracting
the two values from each other. The two sensors can be mounted in different
ways in relation to each other and the most common configuration for
archaeological prospection being that in which one sensor is placed
vertically on top of the other at a fixed distance. Both sensors are equally
affected by the Earth’s magnetic field, but the lower sensor is more strongly
affected by any remains buried in the soil. The Earth’s magnetic field can
then be filtered out simply by subtracting the values recorded by the upper
sensor from those of the lower sensor. The remainder is primarily a product
of the buried structures or features at the site but is also affected by the
natural geological and pedological environment. These buried structures,
objects or features could be either prehistoric or modern, or due to natural
soil variation. A geophysical interpretation based on the appearance of the
anomaly in the data must therefore be carried out before any archaeological
interpretation is attempted.
Fluctuations in the Earth’s magnetic field can also be removed by highpass filtering the collected data. Several authors have convincingly presented
data favouring the use of a pair of total-field sensors in a horizontal
configuration instead of arranging them vertically. This horizontal
configuration essentially halves the time needed for carrying out the survey
(e.g. Tabbagh 2003; Becker 2009).
Gradiometers can use different distances between the sensors, but for
archaeological purposes it is common to use a separation of 0.5–1m (Fig. 5).
The sensor separation governs the depth sensitivity, as a larger distance
between the sensors results in an increased sensitivity to deep features.
Archaeological remains can normally be detected with a gradiometer when
buried in the uppermost two metres of soil (Clark 1996:78f), but the nature
of the buried remains and the sensitivity of the magnetometer are also factors
that need to be considered (Fig. 5 & Table II), and highly magnetic materials
can be detected even when buried at greater depths (David et al. 2008:16).
The depth sensitivity also increases if a total-field magnetometer is used
instead of a gradiometer. Since the majority of archaeological remains in
Sweden are weakly magnetic, they would most likely not be detectable
below a depth of two metres. The possibility of detecting deeply buried
features is also dependent on the magnetic properties of the soil in which the
feature is located and on those of the underlying bedrock.
A short distance between the magnetometer’s sensors has the benefit of
being less sensitive to deeper structures or those that are more distant, such
as passing cars, and should therefore be more likely to focus on the
archaeological remains in the upper parts of the soil. Magnetometers are
only sensitive to ferromagnetic and ferrimagnetic features, however, so that
copper, gold and silver objects are not detected with the instrument.
23
Figure 5. Common gradiometers used for archaeological prospection (left)
Foerster DLG 4.032, (middle) Geoscan FM 256, (right) Bartington Grad 601-2.
Photos: 7y Björn Gustafsson & Siska Williams.
Different kinds of magnetometers such as proton magnetometers, electron
spin-resonance magnetometers, alkali vapour magnetometers, SQUID
(Superconducting Quantum Interference Device) magnetometers and
fluxgate magnetometers are available, all of which use different constructions and methods for measuring magnetic fields. A brief description of
these instruments is presented below with special focus on the fluxgate
gradiometer, as this was the instrument used for the investigations reported
here.
Table 2. The sensitivity of certain magnetometers. After Aspinall et al. (2008).
Proton
0.1-0.5 nT
Electron
spin
resonance
0.05 nT
AlkaliVapour
SQUID
Fluxgate
0.01 nT
0.00001 nT
0.1 nT
Proton magnetometers, as used in the early days of archaeological
geophysics (see, for example, Aitken 1958; Clark 1996), consist of a bottle
containing a hydrogen-rich fluid with a coil of wire around it through which
an electric current is passed. When the current is sent through the coil, the
protons in the fluid align with the direction of the applied field, and when the
current is turned off, they start to precess about the direction of the Earth’s
magnetic field at the site. This precession causes a small electric current to
be generated with a frequency of alternation proportional to the strength of
the magnetic field. The measurement process is quite slow, and only very
limited use is made of proton magnetometers in current archaeological
24
prospection. Some of these issues have been corrected in the electron spin
resonance magnetometer (Overhauser) in which a second liquid is
introduced which aids in the alignment of the protons and reduces the time
needed to complete a measurement. Even so, their use in archaeological
geophysics is limited.
Alkali vapour magnetometers use the vapour of various alkaline metals,
most commonly caesium. When a ray of polarised light is sent through the
alkali vapour, the translucency of the vapour is directly proportional to the
strength of the magnetic field at the point of measurement.
The SQUID magnetometer is by far the most
sensitive instrument available for archaeological
prospection but its use is restricted by the fact that
the sensors must be kept at a very low temperature
in order to work, making it expensive to use for
field work. The sensitivity of the measurements is
very high, however, and this allows weakly
magnetic prehistoric features to be measured in
areas where there are few other anomalies. More
detailed descriptions of the magnetometers are
presented by Aspinall et al. (2008:41ff).
The fluxgate gradiometer is the most frequently
used magnetometer in geophysical prospection at
archaeological sites. A fluxgate magnetometer was
first constructed for archaeological purposes by
Alldred (1964) and subsequently developed by
Philpot (1973). The fluxgate sensor consists of a
core made up of two pieces of metal having very
high magnetic permeability (Fig. 6). A copper
wire is wound around them and the core is driven
in and out of saturation by an electric current
passing through the coil. When the core is out of
saturation it is affected by the ambient magnetic
field, which generates an electric current in a
secondary detector coil proportional to its own
magnitude (Clark 1996:69).
Three of the most frequently used gradiometers
Figure 6. Layout and
(Foerster Ferex 4.032 DLG, Geoscan FM 256 and functioning of a fluxgate
Bartington Grad601) are depicted in Fig. 5. They gradiometer. After Clark
have vertical distances between the fluxgate (1996).
sensors of 0.65m, 0.5m and 1m, respectively,
giving them different strengths and weaknesses
(see the discussion above).
25
2.2. Electromagnetic methods
Several electromagnetic instruments are available for archaeological
prospecting, including metal detectors, ground-penetrating radar (GPR), the
Slingram and various kappameters. These are all active methods. This
section provides an introduction to the physical principles of operation of
some of these electromagnetic instruments, emphasising the methods used
for the surveys reported in this thesis.
2.2.1. Ground penetrating radar, an introduction
RADAR, an acronym for Radio Detection And Ranging, has been used
since the Second World War for the detection of airplanes (Conyers
2004a:16), and ground-penetrating radar, based on the same principles, has
been known from the 1920s onwards (Stern 1929). The history, theory and
application of GPR in general and GPR for archaeological use in particular
have been presented by Conyers & Goodman (1997).
Figure 7. An archaeological geophysical survey using an X3M system and a
500MHz antenna manufactured by Malå Geoscience. Photo: Joakim Schultzén.
GPR (Sw. georadar or markradar) is an electromagnetic method which is
similar to the echo localisation method, or sonar (Sound Navigation And
Ranging) used on boats (Fig. 7), but instead of emitting an acoustic or sound
wave into the water, the GPR device emits an electromagnetic pulse into the
ground from a transmitting antenna. The time in nanoseconds needed for a
pulse to travel from the antenna into the soil and reflect back to a receiving
antenna from a reflective structure of some kind is measured. This is called a
two-way travel time (Conyers 2004a:38). Such reflective structures could be
stones, bedrock, the ground-water table, archaeological features or soil
boundaries. The clearest reflections are produced when the radar pulse is
26
reflected at the interface between two materials having very different
electromagnetic properties (Conyers & Goodman 1997:27).
The reflected pulses recorded by the receiving
antenna result in what is known as a radar trace (Fig.
8). This takes the form of an irregular, decaying
sinusoid. The large amplitude reflections in the upper
part of the trace are caused by the air/ground
interface directly below the antenna and by the
transmitted signal going directly to the receiver. At
every soil layer boundary or object in the soil, some
of the radar energy is reflected back towards the
surface and the remainder of the wave travels deeper
into the ground and continues to diminish as more
and more energy is reflected back to the surface. As
Figure 8. Example of a consequence, reflections from objects or soil
a radar trace. The boundaries situated deep in the soil will be much less
large
amplitudes pronounced than those from objects or soil
noted in the upper boundaries with equal properties that are located at
parts of the pictures
represent the radar shallower levels. As the radar is moved along a
wave’s
interaction predetermined transect on the ground surface a set of
with the air/ ground numerous radar traces is collected, and by placing
interface at ground these next to each other a radar profile, or radargram,
level directly below is generated (Fig. 9). The maximum positive
the antenna.
amplitudes are depicted in black and the minimum
negative amplitudes in white.
Figure 9. Example of a radar profile or radargram created by placing a number of
radar traces next to each other.
A radargram is a two-dimensional vertical section containing stratigraphic
information about the soil below the surveyed transect. As the radar wave is
not transmitted in a narrow beam straight down into the soil but spread
roughly in the shape of a cone, radar reflections from small objects such as
stones will have the shape of a hyperbola (see Conyers 2004a:57; Witten
2006:229; Ernenwein & Hargrave 2009:34). This transmission cone,
commonly referred to as the radar footprint (Leckebusch 2003:215; Conyers
2004a:61ff), effectively means that the GPR not only reveals objects situated
27
directly below the antenna but also objects situated in front, behind and on
either side of it. The antenna begins to detect a point object when this comes
into contact with the front-most part of the radiation footprint, but the radar
echo is plotted as if it were directly below the antenna. This means that the
object is first recorded as being deeper than it really is, as the distance
between the antenna and the front part of the radiation footprint is greater
than the depth of the actual object. As the antenna passes over the object, the
distance, called the range, will gradually decrease, and when the antenna is
directly above the object the correct depth will be recorded, provided the
radar wave velocity is known (see section 2.2.2.). As the antenna continues
along the survey transect the apparent depth will increase until the rearmost
part of the footprint completely loses contact with the object. A hyperbolic
reflection in a radargram is depicted in the centre of Fig. 10 (left).
Figure 10. A reflection hyperbola created as a result of the shape of the transmitted
radar wave interacting with a buried object in the soil.
If many radar profiles are surveyed next to each other it is possible to
merge these into a three-dimensional cube or rectangular cuboid (Fig. 11).
This is mainly done to increase the interpretability of the original twodimensional data. The interpretability can be increased further by
subdividing the cube or rectangular cuboid into horizontal images, called
depth or time slices (see Goodman & Nishimura 1993; Goodman et al. 1995;
Fig. 11). By making an animated movie of these time slices one can gain a
better understanding of how structures change with depth (cf. Neubauer et
al. 2002).
Figure 11. GPR data: from densely surveyed radargrams to the creation of time or
depth slices.
28
The depth penetration of a GPR is partly governed by the soil properties,
but also by the antenna frequency, which is related to the wavelength of the
transmitted electromagnetic pulse. A high-frequency antenna transmitting a
short wavelength radio pulse will be able to resolve smaller features, but as
the frequency is high the depth of penetration will be low and deeply buried
features will not be detected. Low-frequency antennas have long
wavelengths and will penetrate deep into the ground but will only be able to
resolve larger features. For archaeological purposes this trade-off between
the depth of penetration and the resolution of smaller features has resulted in
a preference for antennas having frequencies in the central range of 400-500
MHz. This will provide a depth penetration of c. two metres, depending on
the properties of the soil (David et al. 2008:31). When the targets are thought
to be situated at shallow depths, higher-frequency antennas can be deployed.
Depending on the prerequisites and purpose of the archaeological project,
antenna frequencies can range between 250 and 1000 MHz.
2.2.2. GPR and soil properties
In addition to being reduced by reflections, the electromagnetic pulse will
be attenuated in conductive soil. Highly conductive soils and soils with a
high magnetic permeability will weaken the radar pulse, which in turn will
affect the penetration depth of the instrument. The soil properties are
consequently of vital importance to the success of any GPR survey.
One clear benefit of using GPR is the possibility for estimating the depths
of targets found in the data. In order to do this, it is important to have some
knowledge about the soil in which the radar wave is travelling. To obtain
this information, the Relative Dielectric Permittivity (RDP) εr can be
measured. RDP, defined as “a measure of the ability of a material to store a
charge from an applied electromagnetic field and then transmit that energy”
(Conyers 2004a:45), is related to the velocity with which the radar wave
travels through different soils or materials, and is generally negatively
correlated with velocity (see Table 3). The characteristics of soils and other
materials are of course highly diverse and the values in the table should only
be regarded as rough guidelines.
If the velocity with which the radar wave travels through the soil is
known, depth estimates can be produced. This can be done by one of several
methods (Conyers & Lucius 1996), of which the most reliable and exact one
involves hammering a reinforcement bar into the soil profile at different
depths and correlating the actual depth of the bar with the GPR results, thus
obtaining a body of RDP and velocity data for different depths. Note that a
freshly excavated soil profile must be used if correct values are to be
obtained. Another option is to use the depth of a known feature to calculate
the velocity.
29
Table 3. Relative dielectric permittivity (εr), velocity and conductivity of common
soil types and materials. Data modified from Reynolds (1997:104 and references
cited therein) and Cassidy (2009a:46).
Material
εr
Velocity (m/ns)
Conductivity (mS/m)
1
0.3
0
Air
81
0.033
0.1-10
Fresh water
81
0.033
4000
Seawater
2.5-8
0.078-0.157
10-100
Seawater Ice
1-8
0.106-0.3
0.1-10
Permafrost
10
0.095
0.01-1
Costal sand (dry)
3-6
0.12-0.17
0.0001-1
Sand (dry)
25-30
0.055-0.06
0.1-10
Sand (wet)
2-20
1-100
Clay (dry)
15-40
0.086-0.11
100-1000
Clay (wet)
3
0.173
0.1-100
Clayey soil (dry)
10-15
100-1000
Clayey soil (wet)
12
0.086
Marchland
15
0.077
Agricultural land
13
0.083
Pastoral land
16
0.075
5
Average soil
3-5
0.134-0.173
Asphalt
6-30
0.055-0.112
1-100
Concrete
If a soil profile is not available, one can perform either a common
midpoint test (CMP) (Ulriksen 1982:13; Jol & Bristow 2003:15f) or
hyperbola fitting, where a computer-generated hyperbola is remodelled to fit
the echo arc in the GPR data (see Leckebusch 2000:194f; Cassidy
2009b:159; Goodman et al. 2009:485). The hyperbolas get broader as the
velocity increases and narrower if it decreases (Ulriksen 1982:75f). For a
CMP test one needs two separate GPR antennas, and as most GPR systems
have the transmitting and receiving antenna housed in a closed case this is
not usually possible. It is advisable to make velocity measurements at
different places within the survey area as soil properties can vary within one
site. Furthermore, new velocity measurements should be carried out if the
measurements span several days, as the soil moisture content can vary
greatly with time (Conyers & Lucius 1996:38).
Attenuation of a radar wave is affected by the electrical conductivity (σ)
and magnetic permeability (µ) of the soils through which it passes, and also
slightly by the RDP. Electrical conductivity, defined as “the ability of a
material to pass free electric charges under the influence of an applied field”
(Cassidy 2009a:54), is partly governed by the water content of the soil, but
also by the salinity of the medium. As a consequence, highly conductive wet
clay soils will cause a high attenuation of the radar wave, reducing the depth
of penetration significantly (Leckebusch 2003:214f; Conyers 2004a:50).
30
Successful measurements have nevertheless been carried out on clay soils,
which do not per se preclude the use of GPR (Weaver 2006). Certain dry
soils, which have generally been accepted as suitable for GPR measurements, can also have a high conductivity and cause marked attenuation of the
radar wave, and thus other factors in addition to soil moisture need to be
taken into account when estimating the likely success of GPR (Conyers
2004a:50).
One option could be to perform an EM38 survey in order to measure the
electrical conductivity of the soil before the GPR survey, thus estimating the
likelihood of success (see Clay 2001:1f; Conyers 2004a:53; Viberg
2007:31). This is exemplified in Fig. 12, where an area of high conductivity
as measured with the EM38 (see below), is directly correlated with an area
of reduced depth penetration in a radargram.
Figure 12. Correlation between an area of high soil conductivity as identified by
the EM38 and an area of reduced depth penetration in GPR data.
The effects of rainfall on GPR data have been studied by Bevan
(1984:203f), who showed that the same profile before and after rainfall had
quite different characteristics. Many radar echoes seemed to derive from the
water as it percolated down through the upper unsaturated part of the soil,
and this effect was visible in the GPR data for three to four days after the
rainfall, depending on the site (Bevan 1984:204). Wet soils alone are not
always a problem, however, and in some areas certain kinds of structures can
actually be enhanced by rain, as the soil moisture contrast between the
archaeological structure and the surrounding soil could be increased (see
Bevan 1984:203f; Van Dam et al. 2002; Conyers 2004b).
This has also been confirmed during repeated seasonal measurements
performed on an Iron Age house terrace (no. 6) situated on a well-drained
glacial till soil at the Fornsigtuna, or Signhildsberg, site in Central Sweden
(Figs. 1 & 13). The archaeology at this site had been revealed by means of
an earlier small trench in the terrace and cultural material had been shown to
be situated in the upper c. 0.5 m, below which the bedrock became visible
(Damell 1986; Damell 1991:48ff).
31
Figure 13. Map of the Fornsigtuna site, showing the location of house terrace no 6.
The scale bar was missing from the original map, but the size of house terrace no. 6
is roughly 37.5m x 10m. After Damell (1991), with “no.6” added by the author.
GPR measurements were carried out in 2009-2010 using an X3M system
manufactured by Malå Geoscience connected to a 500MHz antenna. Data
were collected every 0.03m (inline sampling distance) in profiles located
0.25m apart (crossline sampling distance). A follow-up survey in August
2010 used a closer crossline sampling distance of 0.1m between the
transects.
Measurements performed in November 2009 produced slightly clearer
reflections from both shallow and deeply buried features than those made in
August 2009 and August 2010 (see Fig. 14), while the results collected in
April were somewhat clearer than those of the August measurements but not
as clear as the November measurements. The depth of penetration of the
GPR seemed to be similar for all the surveys but the features situated in the
upper part of the profile were more clearly defined in the spring and autumn
surveys. The bedrock, which is very shallow in the middle part of the radar
profile and deeper towards the ends, is clearly visible in all the profiles.
As the results were very similar, it must be concluded that future surveys
of house terraces in well-drained glacial till environments could be carried
32
out in any season, although the slightly improved results obtained in
November and April perhaps speak in favour of the spring or autumn.
Winter measurements in January were also attempted, but due to several
decimetres of snow covering the house terrace the survey wheel attached to
the GPR equipment had severe problems and did not record the correct
length of the radar profiles. This could perhaps have been solved by using
another data acquisition method, such as the hip chain, but this was not
available at the time of the surveys. Another problem with surveying in deep
snow is that the antenna tilts as a result of the compacted snow on the
previously surveyed transects. This problem could most likely be solved by
simply putting the radar antenna onto a broader sledge, which spreads its
weight more evenly. The results of the survey, presented as a time slice, are
given in section 5.1.
Figure 14. GPR profiles collected at the seasonal measurements of house terrace
no. 6 at Fornsigtuna in August 2009(top), 7ovember 2009 (second from top), April
2010 (second from bottom) and August 2010 (bottom).
The other factor affecting GPR measurements is magnetic permeability,
defined as “a measure of the ability of a medium to become magnetized when
33
an electromagnetic field is imposed upon it” (Conyers 2004a:53). This factor
has only been seen to affect GPR data in rare cases (e.g. Conyers 2004a:53f
and references cited therein), as most soils are weakly magnetic (Van Dam
& Schlager 2000:437; Cassidy 2009a:55f).
2.2.3. GPR and topography
The topography of the survey area is of vital importance when carrying
out GPR surveys (see Conyers & Goodman 1997; Conyers 2004a; Goodman
et al. 2006; Goodman et al. 2007; Leckebusch & Rychener 2007). Any tilt or
roll of the GPR antenna caused by the terrain will make the radar look in a
different direction. The GPR profiles will nevertheless still assume that all
the collected data apply to a point directly below the antenna, and as a result,
erroneous maps of the subsurface will be created. This can be remedied by
topographic tilt corrections applied to the radar profiles, which can be
carried out using GPR data management software such as GPR SLICE3.
Another important thing to remember when carrying out GPR surveys is that
smaller features such as piles of grass at the surface can cause the antenna to
tilt and lose its ground coupling, thereby introducing anomalies into the
radar profiles (Bevan 1984:202; Conyers 2004a:70f). Larger features of this
sort can be mapped with a RTK-GPS or total station to enable the correct
interpretation of such anomalies.
2.2.4. GPR manufacturers and suitable archaeological targets
In the early surveys carried out in Sweden and abroad the GPR equipment
was housed in a car while the antenna was pulled over the ground and the
results were printed continuously on a wide strip of paper (see. Fig. 15).
Then, as personal computers became available during the 1980s, the data
came to be stored on the computer’s hard disc, which enabled subsequent
filtering and data processing (Leckebusch 2003:214; Conyers 2004a:5).
Examples of common GPR systems used for archaeological prospection
are presented in Fig. 16. Sweden is the home of several geophysical
equipment manufacturers, such as Malå Geoscience and Radarteam
Sweden. 4 The results collected using different manufacturers’ GPR systems
have been evaluated in terms of quality by Goodman (2006) and Seren et al.
(2007). Before buying a GPR system, it is advisable to test different brands,
and also several antennas from the same manufacturer, as these can be
different and produce data of differing quality.
3
4
http://www.gpr-survey.com/
http://www.radarteam.se/
34
Figure 15. Early GPR measurements performed at the Stone Age cemetery of
Ajvide on Gotland in 1983. The GPR antenna in the top left picture is connected to
the data unit housed in the car. The radargrams are continuously recorded on a
roll of paper by the printer in the car. Photos: Göran Burenhult.
Figure 16. Common GPR systems and carts used for archaeological prospection by
Malå Geoscience (left), Sensors and Software (middle), Geophysical Survey
Systems Inc. (GSSI) (right). Photos: Joakim Schultzén and Siska Williams.
35
GPRs have most commonly, and most successfully, been used to detect
buried remains of buildings (e.g. Leckebusch 2000; Nishimura & Goodman
2000; Leucci 2002; Linford 2002; Neubauer et al. 2002; Trinks 2005;
Persson 2005a; Berard 2008; Novo 2010; Udphuay et al. 2010; Ercul et al.
2011; Löcker et al. 2011; Paper III, IV, V), although weakly reflective
archaeological structures such as graves (e.g. Goodman 1993; Conyers
2006a; Forte & Pipan 2008; Fiedler et al. 2009) and postholes (e.g. Trinks et
al. 2010b) have also been detected. Lists of suitable and unsuitable targets
for GPR surveys are provided by David et al. (2008) and Ernenwein &
Hargrave (2009).
2.2.5. The metal detector
The most popular instrument for use in Swedish archaeology is the metal
detector, an electromagnetic instrument using a transmitter coil to generate
an alternating magnetic field. When held over the soil, the field induces
currents in any buried conductive material, and these give rise to secondary
magnetic fields detectable by a receiving coil (Gaffney & Gater 2003:46f:
Linford 2006:2231). The majority of these metal detectors are VLF (Very
Low Frequency) instruments and essentially function as instruments of the
Slingram type (Scollar et al. 1990:570).
Metal detectors frequently use discrimination as a means of identifying
different metals. This procedure is primarily used to remove the signal
produced by iron in order to focus on precious metals such as gold, silver or
bronze. The discrimination procedure is, among other things, related to
frequency shifts and phase shifts in the transmitted signal (Scollar et al.
1990:570). The phase shift measured for different metals is compared with
values obtained during field measurements and is used as a basis for
discrimination. Iron, for example, is highly magnetic and will be in-phase, as
opposed to the excellent conductor, bronze, which is out of phase by 180°
(see Scollar et al. 1990:570). Care must be taken, however, as the shape of
the object and its orientation and size will affect the readings (Scollar et al.
1990:570; Connor & Scott 1998:80).
Other types of metal detector, such as the Pulse Induction Meter (PIM),
have been used in archaeological research but are not as common as the VLF
instrument described above. The basic operation of the PIM is described by
Gaffney and Gater (2003:47).
2.2.6. Magnetic susceptibility and the Bartington MS2D
Since magnetometers are affected by both the remanence and the
susceptibility of magnetic features, they are unable to determine the
magnetic susceptibility of materials directly or easily. Such measurements
36
can, however, be provided by means of active instruments such as the EM38
or MS2D (Dalan 2008:3).
There are several instruments available for measuring the magnetic
susceptibility of soils, as listed and discussed by Dalan (2008:3f) and David
et al. (2008:36). Two of the more common instruments are the MS2D
system of Bartington Instruments and the Slingram device EM38 by Geonics
Ltd.
The Bartington instrument (see Fig. 17) consists of an induction coil with
a diameter of 18.5cm that measures the volume magnetic susceptibility (κ)
of the top decimetre of the soil (Dearing 1999:28f). The results are expressed
in dimensionless SI units (x 10-5) and are calibrated to read 0.5 κ in rough
soils and 0.75 κ on smooth surfaces (see the operation manual for the MS2
magnetic susceptibility system, Bartington Instruments). The data therefore
needs to be corrected for roughness and then multiplied by 2 during
measurements made in rough terrain to achieve a correct value for the
volume magnetic susceptibility. The instrument has been described in detail
by several authors, e.g. Clark (1996:102ff), Dearing (1999), Gaffney &
Gater (2003:44ff) and Dalan (2008:5).
As enhanced magnetic susceptibility has been shown to be associated
with anthropogenic activity, areas with enhanced magnetic susceptibility
values in soils as compared with the prevalent background values at the site,
can be used as an indicator of prehistoric human activity (Gaffney & Gater
2003:44; David et al. 2008:36). Down-hole magnetic susceptibility
measurements have been shown to produce detailed stratigraphic profiles,
thus aiding the interpretation of archaeological sites (e.g. Dalan 2006,
2008:13ff). The method can not only be applied as a prospection tool but
also during excavations (e.g. Dalan 2008:7ff). Furthermore, as evidenced by
the brief theoretical discussion above, fired anthropogenic structures such as
hearths or kilns would probably have elevated magnetic susceptibility
values, making them readily detectable using any of the available
kappameters (Dalan 2008:3).
Figure 17. Surveying in the mountain tundra region of northern Sweden using the
MS2D by Bartington Instruments. Photos: Kerstin Lidén and Andreas Viberg.
37
The magnetic susceptibility of fire-affected limestone and sandstone has
been shown to be higher than that of unfired specimens of the same rock
type found at the same site, which would imply that burning or heating of
rocks increases their magnetic susceptibility (Gose 2000:417f).
2.2.7. Conductivity and susceptibility measurements using the
Geonics EM38 Slingram instrument
Slingram instruments, e.g. the SCM (Howell 1966), the SH3 (Parchas &
Tabbagh 1978) or the EM38 by Geonics, have been used for detecting
archaeological remains since the mid-1960s and have attracted the interest of
many researchers (e.g. Tite & Mullins 1970; Bevan 1983; Tabbagh 1984,
1985, 1986a, 1986b, 1994; Frohlich & Lancaster 1986; Tabbagh et al. 1988;
Benech & Marmet 1999; Simpson et al. 2009).
Slingram instruments generate a primary magnetic field in a transmitting
coil and measures the secondary magnetic fields created by buried features
or soil materials in the ground in a receiving coil (Fig. 18). When the
instrument is held up in the air the primary field will travel unaffected from
the transmitter to the receiving coil, but if the instrument is held near the
ground the transmitted electromagnetic field will induce eddy currents in
buried conductive features or objects. These currents will in turn generate
secondary magnetic fields which can be detected by the receiving coil. The
response in the receiving antenna will thus tell the operator something about
the presence and properties of buried conductive features or objects (Kearey
et al. 2002; Witten 2006:151ff).
Figure 18. Principles behind electromagnetic survey instruments and their
detection of buried objects. After Kearey et al. (2002:209).
The depth penetration is governed by the distance between the coils, so that
a larger distance will enable a greater depth penetration. However, as
increasing depth also increases the measured volume of soil, it will lower the
resolution and the detectability of smaller features within the soil volume.
The distance between the coils is 1m for the EM38.
38
One survey reported in this thesis (Paper II) was carried out using the EM38
from Geonics (Fig. 19). This is a two-in-one Slingram instrument measuring
both the in-phase and quadrature components of the electromagnetic field.
The in-phase component is essentially a
measure of the magnetic susceptibility of a
material and the quadrature component is a
measure of its electrical conductivity. The
instrument can be operated either in vertical
mode, with a vertical orientation of the
dipoles (see Fig. 19), or in horizontal mode
(lying on its side), with a horizontal
orientation of the dipoles. Different depth
sensitivities are achieved depending on the
orientation. The greatest sensitivity in the
vertical mode is at a depth of c. 0.3-0.4m
Figure 19. Surveying in the
when the instrument is measuring electrical
mountain tundra region of
northern Sweden using the conductivity, and the greatest sensitivity in
EM38
manufactured
by the horizontal mode is directly below the
Geonics Ltd. Photo: Kerstin instrument, gradually diminishing with
Lidén.
increasing depth (Fig. 20). The device can
nevertheless measure down to c. 1.5m in the vertical mode and 0.75m in the
horizontal mode. When it is measuring magnetic susceptibility its depth
sensitivity is somewhat less, being greatest at a depth of c. 0.2m in the
vertical mode below the instrument and c. 0.25m in the horizontal mode
(Fig. 20). The greatest depth penetration when measuring the magnetic
susceptibility in the vertical mode is c. 0.5m (Geonics 1992:15).
Figure 20. Sensitivity of the EM38 in conductivity mode (left) and magnetic
susceptibility mode (right). After Bevan (2004).
Early versions of the EM38 are also prone to drifting, as temperatures at the
site change during the day (Bevan 2004:15), and it may be important to take
39
regular measurements with the instrument lifted into the air in order to check
for the magnitude of the drift at particular times of day, at least when
measuring susceptibility. Bevan (2004:15) made measurements in this
manner before starting each survey transect in order to be able to correct for
drift. The correction can be made in several ways (see Bevan 2004:15; dos
Santos et al. 2011), but failure to do so may mask important anomalies
hidden in the data. An additional peculiarity of the EM38 when measuring
magnetic susceptibility and operating in the vertical mode is that the data can
include negative values. According to the response curve for the instrument,
any magnetic layer located at c. 0.6m or below will cause the instrument to
show negative values (Dalan 2008:4).
The unit for measurements of electrical conductivity is the
millisiemens/m (mS/m), and in-phase measurements are expressed in parts
per thousand (ppt) of the primary magnetic field. Electrical conductivity is
the inverse of electrical resistivity, and consequently the two methods react
to similar kinds of archaeological remains (Gaffney & Gater 2003:43),
especially when comparing EM38 conductivity measurements carried out in
the horizontal mode with earth resistance data (see Linford 2006:223 and
references cited therein). As the EM38 does not require the inserting of
probes into the ground, however, the instrument can be used in areas where
ground conditions make electrical resistance or resistivity measurements
impractical or impossible. This is exemplified by the electrical resistance
surveys carried out by Fischer (1980) on Cyprus, where the electrodes had to
be hammered some 0.4m into the soil at each measurement station to achieve
sufficient contact (Fischer 1980:19). In such an environment an EM38
survey would have been less time consuming and more practical.
Archaeological features detectable by the EM38 include ditches, pits,
stone constructions, brick structures, metals, etc. Several good introductions
to surveying using Slingram instruments are available for interested
archaeologists (see Bevan 1998; Gaffney & Gater 2003; David et al. 2008),
and more technical descriptions of the method are available in McNeill
(1980a & b, 1999), Reynolds (1997) and Kearey et al. (2002).
2.3. Geoelectric methods and other geophysical
prospection methods
There are several other geophysical prospection methods that have been
used both frequently and infrequently for archaeological purposes. One of
the more popular among these is electrical resistance or resistivity, but as
this method has not been used in any of the surveys reported here, it will
only be described briefly. Other less commonly used methods such as selfpotential (SP) (see Bevan 1990; Drahor 2004), induced polarisation (IP) (see
Schleifer 2002; Meyer et al. 2007), gravimetry (see Padin et al. 2012), VLF,
40
apart from the VLF metal detectors described above (see Persson 2005a;
Drahor et al. 2009) and seismic reflection and refraction (see Carson 1962;
Goulty & Hudson 1994; Andrén & Lindeberg 1997; Hildebrand et al. 2002;
Metwaly 2005; Hildebrand et al. 2007; Forte & Pipan 2008) will not be
described here at all.
Earth resistance measurements are probably the most frequently used
method of archaeological prospection apart from magnetometry, and as the
historical review above has shown, electrical methods have been used within
archaeological research since 1938 (Bevan 2000). Since this is such a
popular method, it has been described extensively in the archaeogeophysical
literature (see Scollar et al. 1990; Clark 1996; Gaffney & Gater 2003; Witten
2006; Campana & Piro 2009). The instruments basically measure the
resistance of soils to an electrical current. As mentioned above, resistance is
the inverse of conductance, and the archaeological features detectable by
resistance measurements are similar to those detectable by conductivity
instruments. Detectable features include ditches, graves and masonry
constructions.
Electrical resistance surveying involves inserting metal electrodes into the
soil and passing an alternating electric current between them. Two additional
electrodes measure the electrical potential difference at every measurement
point. The strength of the driving current (I), in amperes, is known and the
electrical potential difference (V), in volts, is measured, whereupon the
electrical resistance (R) can be calculated simply by dividing the voltage by
the current, and the resulting resistance, in ohms, can be read from the
instrument’s display. Resistivity in ohm/m may be calculated by multiplying
the resistance by an array constant that is dependent on the geometry and
spacing of the electrode configuration.
The electrodes can be arranged in different ways. If they are arranged in a
straight line with the two current electrodes at the ends and the two potential
electrodes between them, all spaced at equal distances, the configuration is
called the Wenner array. Many alternative arrays exist, but the most popular
for archaeological prospection is probably the Twin Probe array, which is
described by Clark (1996:44) as a Wenner array divided in two. This was
first used for archaeological prospection by Aspinall & Lynam (1970).
The distance between the probes in electrical resistance prospection is
roughly equal to the depth of greatest sensitivity (see Scollar et al.
1990:321ff: Gaffney & Gater 2003:32). An electrode distance of one metre,
for example, is excellent for detecting features at a depth of one metre. The
Twin Probe array with 0.5m electrode separation can measure to an
approximate depth of 0.75m, however (Gaffney & Gater 2003:32). By
extending the distance between the electrodes the depth of investigation can
be increased, and this technique is used when carrying out vertical electrical
soundings (VES) (Reynolds 1997:441). For a VES, the spacing between the
electrodes is stepped up, thereby measuring to greater depths, while in a
41
related technique called resistivity tomography, or CVES, a large number of
electrodes can be inserted into the soil along a profile and a switching box
determines which ones are activated. The measured values, representing
different soil volumes and penetration depths, are presented in a
“pseudosection”, which is a vertical representation of the soil resistance
beneath the row of electrodes. For archaeological applications of this
technique, see Aspinall and Crummet (1997).
The most commonly used electrical resistance instrument for
archaeological prospection is the RM15 by Geoscan Research, as depicted,
configured in the Twin probe array, in Fig. 21. This model was recently
replaced by the RM85 (Walker 2012).
Figure 21. The Geoscan RM15. Photos: Siska Williams and Andreas Viberg.
2.4. Marine geophysical methods and geochemical
prospection methods
Several marine geophysical methods, such as side scan sonar, multibeam
sonar, magnetometry, etc. (see Lurton 2002; Lawrence et al. 2003) are used
in archaeology, and their appropriateness for the visualisation and
investigation of submerged Swedish archaeological remains have been
demonstrated repeatedly (see Hjulhammar 2008, 2010). The methods are
also recommended in the guidelines on marine archaeology published by the
Swedish National Heritage Board as a way of saving both money and
time5.As this thesis explicitly deals with land-based archaeological
geophysical survey methods, marine techniques will not be discussed any
further, even though they are similar in their underlying physical principles
to land-based methods.
5
http://www.raa.se/publicerat/9789172095250.pdf
42
The situation is similar for geochemical prospection methods (see Bethell
& Máté 1989; Crowther 1997; Heron 2001), e.g. soil phosphate analysis,
which has been very popular in Sweden since the 1930s (see Arrhenius
1935). Even though soil phosphate analysis was used in one of the projects
described in this thesis (Paper II), the main focus is on geophysical
prospection methods.
2.5. The benefits of using an integrated approach
Since many of the geophysical methods used for archaeological
prospection measure different physical parameters, an integrated approach
using two or more methods within the same area can potentially provide a
more complete understanding of the archaeological remains. This
complementary nature of geophysical methods has been mentioned
frequently in the literature (see Weymouth 1986; Clay 2001; David 2001;
Neubauer 2001; Persson 2005a; Kvamme 2006a; Kvamme & Ahler 2007;
David et al. 2008; Watters 2009). The National Park Service’s annual
workshop on archaeological geophysics in the U.S.A. has on many occasions
provided the possibility of surveying a site by multiple methods, and
excellent European examples also exist where multiple methods have been
used for the study of Roman towns (Neubauer & Eder-Hinterleitner 1997;
Buteux et al. 2000; Neubauer et al. 2002). The benefit of using more than
one geophysical method is also exemplified by the survey of Sandbyborg
(Paper III), where the GPR and magnetometer provided complementary data,
enabling a more complete and reliable geophysical and archaeological
interpretation to be achieved. GPR provided information regarding the
spatial layout of the houses within the fort and the magnetic survey provided
information regarding the presence of possible hearths and pits within the
houses and the fort as a whole. The magnetometer surveys on Gotland
(Paper IV) also benefited from information from earlier metal detection
surveys, enabling the pinpointing of interesting buried remains of hightemperature crafts. The combined use of magnetometry and magnetic
susceptibility has also been suggested for mountain tundra sites in northern
Sweden (Paper II), as this would enable the differentiation of in situ fired
structures from magnetic stones.
Geophysical methods have frequently been part of wider archaeological
prospection schemes, including their combination with remote sensing and
aerial photography, satellite imagery, laser scanning, LIDAR surveys,
geochemical prospection etc. (see Persson 2005a; Powlesland 2006, 2009;
Kvamme & Ahler 2007; Kvamme 2008; Gaffney et al. 2012). These other
methods have an enormous potential of their own, but they become even
more valuable as integrated parts of a wider archaeological approach to
prospection.
43
The results of such multi-method surveys are commonly presented side
by side and interpreted on a GIS platform along with aerial photos (see
Doneus & Neubauer 1998; Neubauer 2004), but alternatives do exist, and
Kvamme (2006a) has provided some guidelines to a more complete form of
data integration in which measured values obtained by different geophysical
methods can be statistically combined into something meaningful and as
such strengthen the interpretation of the surveyed area.
44
3. Development and current use of
archaeological geophysical prospection
methods
“The proton magnetometer and the computer will together, with
certainty, revolutionise field archaeology”. [Authors translation
from Swedish] Gad Rausing (1971:110).
3.1. A brief history of archaeological geophysics
The Swedish archaeologist Hjalmar Stolpe used a form of primitive
seismic investigation when searching for graves on the island of Birka,
Uppland, in Sweden in the 1870s (Stolpe 1888:6). He hit the ground with a
cane and listened to the sound differences, as areas containing graves
produced a much deeper sound. It can also be concluded from a letter written
by Stolpe to Hans Hildebrandt, Director-General of the Swedish National
Heritage Board, in 1882 that Stolpe used this method in other areas as well
(see Arbman 1941:158, and references cited therein). The same method was
also used by the English ethnologist and archaeologist Lieutenant-General
Augustus Pitt Rivers in his excavation in Cranborne Chase. He called it
“bosing” and used it to search for buried ditches (see Atkinson 1946:32;
Clark 1996:11; Gaffney & Gater 2003:13). This method and its limitations
are also discussed by Aitken (1974:203).
3.1.1. Geoelectric methods
In November 1938 a Canadian geophysicist, Mark Malamphy, working
for a company called Hans Lundberg Ltd., used the equipotential survey
method (Sundberg et al. 1923:13) to search for a buried vault at the Colonial
Williamsburg historical site (Bevan 2000). Hans Lundberg, head of the
company, was a Swedish geophysicist who emigrated to Canada in the
1930s (Lagerström 1968:609f) and became famous for his involvement in
the discovery of Tepexpan man in Mexico in the mid-1940s (de Terra
1947:41, 1949:33ff; Fig. 22).
45
Figure 22. The Swedish geophysicist Hans Lundberg carrying out electrical
measurements during the search for Tepexpan man in Mexico in 1947. Photo: ©
Juan Guzman. Picture published in Life magazine, March 31, 1947.
His interest in archaeology was shared by his younger brother Erik J.
Lundberg, a cultural historian and architect, who was involved in several
Swedish excavations: Vreta Monastery 1916-23, Nyköping 1925,
Söderköping 1925-36 and Skänninge 1928. He later became an executive at
the Swedish National Heritage Board and professor at the Royal Institute of
Art in Stockholm (Meyerson 1982-1984; Berthelson 1969). The survey in
Colonial Williamsburg, which marked the actual beginning of archaeological
geophysics, would, according to Bevan (2000:58), have been carried out by
Lundberg himself had he not been involved in a car accident.
The first archaeological-geophysical survey in Europe was carried out in
1946 at Dorchester-on-Thames, England, by Richard Atkinson (1953). He
successfully used an electrical resistivity instrument, the Megger earth
tester, to locate ditches dug into gravel (Clark 1996:13). England was from
this point on destined to take a very active part in the development and use
of several new archaeological geophysical methods, but work connected
with the early development and use of the methods was also carried out in
France, Italy and Germany (see Scollar 1959; Hesse 1966 and references
cited therein; Carabelli 1967).
3.1.2. Magnetometry
In 1958 Martin Aitken and Edward Hall built a proton magnetometer that
they used to locate buried pottery kilns at Water Newton in
Northamptonshire (Aitken 1958). Subsequent tests verified that the
instrument was also able to detect pits and ditches (Aitken 1959).
46
Interest in electrical resistivity and magnetometry increased day by day,
and during the following decades the methods were developed and refined to
be better suited to archaeological applications. New types of magnetometer
and new configurations such as the proton gradiometer (Aitken 1960), a
proton magnetometer in variometer mode (Scollar 1961), the fluxgate
gradiometer (Alldred 1964) and the optically pumped rubidium
magnetometer (Ralph 1964; Bevan 1995b) were being used for
archaeological purposes in England, Italy and Germany, for example. Later
on, other types of magnetometer, the Overhauser magnetometer (Salvi
1970), the optically pumped Caesium magnetometer (Becker 1995) and the
superconducting quantum interference device, or SQUID (see Chwala 2001,
2003), were used at archaeological sites. These magnetometers have
improved sensitivity relative to the widely-used fluxgate magnetometer,
enabling the detection of even more weakly magnetic archaeological
structures in suitable environments.
3.1.3. Electromagnetic methods
Eight years after the Water Newton survey, Marc Howell (1966)
developed the Soil Conductivity Meter (SCM), also known as “the banjo”,
which, in spite of its name, actually measured the magnetic susceptibility of
the soils at Cadbury Castle in England (cf. Tite & Mullins 1970). This was a
transmitter-receiver electromagnetic instrument based on a Swedish
invention, the Slingram (Werner 1937). This type of electromagnetic device
was developed over the following years and led to instruments such as the
Geonics EM38 and EM31 (McNeil 1980b) and the SH3 (see Parchas &
Tabbagh 1978). Important developments, refinements and research projects
were carried out by Tabbagh (1986a, 1986b), and the method was used as an
archaeological prospecting tool in the USA as early as the mid-1970s (Bevan
1983).
The metal detector, an electromagnetic instrument similar to the above
devices, was first used in 1945 to investigate an archaeological site in
Switzerland (von Bandi 1945), although tests with a simple version of a
metal detector had been carried out in England even earlier (Passmore &
Jones 1930). These early surveys are described more thoroughly by Hesse
(2000).
The use of GPR in archaeological research began in the 1970s, and one of
the earliest surveys was conducted in Chaco Canyon, New Mexico, USA, in
1974 (Vickers & Dolphin 1975; Vickers et al. 1976). This was followed by
GPR surveys in Philadelphia, USA (Bevan & Kenyon 1975), and at the
pyramids of Giza, Egypt (Barakat et al. 1975).
47
3.2. Use of archaeological geophysical prospection
methods in other countries
To gain a better understanding of the development of archaeological
geophysics in Sweden it is necessary to form an overview of the popularity
of the methods, their frequency of use, the opportunities for training and
archaeologists’ attitudes towards the methods in other countries. As this
information is difficult to come by and only a few publications containing
such details are available (e.g. Bevan 2000, 2002; Gaffney & Gater 2003;
Herbich 2011; Visser et al. 2011; Lowe 2012) the author sent e-mail
messages to all members of the International Society of Archaeological
Prospection (ISAP)6 asking them to evaluate the discipline of archaeological
geophysics in their respective countries. The answers enabled me to describe
schematically the situation in a number of the countries concerned. This is
admittedly an incomplete list, as information is missing for many countries,
and the short descriptions should only be regarded as a brief guide to the
international situation. One must also be aware that some of the information
presented is anecdotal and represents a single researcher’s view. It has
likewise been difficult to obtain updated information on the frequency of the
use of archaeological geophysics in England, for example, and the data
presented here may not be up to date in this respect.
3.2.1. Austria
According to the account by Neubauer (2001:29ff) of the development of
archaeological geophysics in Austria, the first survey using electrical
methods was carried out by Hampl and Fritish in 1959 (see Neubauer
2001:29 and references cited therein). The department of geophysical
prospection, geodesy and photogrammetry at the University of Vienna offers
training in archaeological geophysics and is currently collaborating with the
recently established Ludwig Boltzmann Institute for archaeological
prospection and virtual archaeology (LBI), located at the Central Institute for
Meteorology and Geodynamics in Vienna and engaged in research in the
above-mentioned disciplines all over Europe together with partner
organisations in Austria, Britain, Germany, Norway and Sweden7.
3.2.2. Denmark
The first archaeological survey using an Askania Gfz vertical intensity
magnetometer was published in 1965 (Abrahamsen 1965). This was
followed by several publications on archaeomagnetism. A book describing a
6
7
http://www.bradford.ac.uk/archsci/archprospection/menu.php?0
http://archpro.lbg.ac.at/
48
number of geophysical methods and containing information on many of the
earliest geophysical surveys in the country was published in 1984 (Möller et
al. 1984). An estimated 4-8 surveys are currently carried out annually in
Denmark, but apart from a few lectures at Aarhus University each year, for
example, there are no opportunities for specialised education in
archaeological geophysics in the country (Bevan, B. pers. comm. 31.5.2012).
3.2.3. England
Gaffney & Gater (2003:23) report that c. 450 surveys were being carried
out in England each year in the early 2000s. England is often considered to
be the country where archaeological geophysics is most popular. This has
partly been explained by the inclusion of archaeological geophysics in
Planning Policy Guideline 16, which briefly mentions geophysical surveys
as a possible aspect of archaeological assessments8. This seems to have led
to a wider awareness of the possibilities of using such methods among
English archaeologists and subsequently to their more frequent inclusion in
archaeological projects (for an extended discussion of the situation in
England, see Gaffney & Gater 2003:22f). An evaluation of the use of
archaeological geophysical prospection methods in northern England has
been provided by Jordan (2009). Many of his conclusions are also valid for
Sweden and will be discussed below.
Archaeological geophysics is taught at many universities across the
country and the M.Sc. in Archaeological Prospection-Shallow Geophysics at
Bradford University is one of the best-known university programmes in the
world.
3.2.4. Finland
The first geophysical survey at an archaeological site in Finland was
carried out in 1983 in a joint project between archaeologists from the
University of Turku and geophysicists from the University of Oulu (Lavento
1992 and references cited therein). A list of sites surveyed between 1983 and
1991 is presented by Lavento (1992). Since the first survey in 1983, the use
of geophysical prospection in archaeology has been very limited and the
methods are only used sporadically today (Pertolla, W. pers. comm.
26.5.2012). There is currently only one University in Finland that provides
any training in archaeological geophysics and that is at the University of
Helsinki (Pertolla, W. pers. comm. 30.5.2012).
8
http://www.communities.gov.uk/archived/publications/planningandbuilding/ppg16
49
3.2.5. Germany
The early work by Scollar in the late 1950s marked the beginning of the
discipline in Germany (Clark 1996: and references cited therein). Although
accurate statistics about the use of geophysical prospection in Germany are
difficult to find on account of the federal system, the country has many
groups that are regularly involved in archaeological projects. Archaeological
geophysics is carried out prior to 80% of all rescue excavations in the
German Palatinate region (Zeeb-Lanz, A. pers. comm. 28.5.2012), while in
Bavaria, south-eastern Germany, aerial photography has aided in the
discovery of some 30,000 new archaeological sites, approximately 700 of
which, c. 24 surveys annually, have been investigated using magnetic
methods, electrical resistivity and GPR by the Institute of Geophysics,
University of Munich and the Bavarian State Department of Monuments and
Sites since the early 1960s (Fassbinder, J. pers. comm. 29.5.2012). For
further information regarding the development of archaeological geophysics
in Bavaria and Anatolia, see Fassbinder (2011). There are currently two
universities in Germany offering courses on geophysics with special focus
on archaeology: the Ludwig Maximilian University in Munich and the
Christian-Albrecht University in Kiel.
3.2.6. Iceland
The use of geophysics on Iceland has been very sporadic and the first
systematic surveys were carried out as late as 1999 (Horsley 2004).
Although the igneous bedrock of Iceland makes magnetic prospection more
challenging, results have shown that magnetometry and resistivity combined
can provide reliable evidence for archaeological structures such as long
houses, pit houses, churches etc. (Horsley 2004:367ff).
3.2.7. The Netherlands and Flanders
The development of archaeological geophysics in the Netherlands has
been reviewed by Kattenberg (2008), who mentions that the first two
successful geophysical prospection projects in the country were carried out
in 1968 and 1970 (2008:12). According to a recent report by Visser et al
(2011:27), a total of 273 surveys took place between 1996 ̶ 2010, of which
89 were electrical resistivity surveys and 38 magnetometer surveys, with
between 10 and 30 surveys carried out annually since 1996 (Visser et al.
2011:28f, fig. 13). The only place which currently teaches archaeological
geophysics in the Netherlands is the Institute for Geo- and Bioarchaeology in
the Faculty of Earth and Life Sciences of the VU University, Amsterdam.
In Flanders the use of the methods has been quite marginal, but the
frequency has increased since 2003. Research is currently conducted at the
50
University of Gent, which has been important for the development of
archaeological geophysics in the area (Visser et al. 2011:35).
3.2.8. Norway
The use of archaeological geophysics is still very limited in Norway. The
first survey was carried out in 1972, and recent statistics indicate that
approximately 29 surveys took place between 2005 and 2009 (Anderson
Stames 2010:14f). Geophysical education is available at the Norwegian
University of Science and Technology, although there is currently no
academic course that covers archaeological geophysics. Norway has recently
been involved in collaboration with LBI (see above) and several surveys
have already been carried out at Larvik, Vestfold9.
3.2.9. Poland
The first application of archaeological geophysical methods in Poland
was a magnetic survey carried out on 19-21 April 1961 (Dabrowski 1963;
Herbich 2011 and references cited therein), and current estimates imply that
c. 10-20 surveys are carried out annually (Wroniecki, P. pers. comm.
26.5.2012). There are nowadays several active Polish groups carrying out
geophysical prospection nationally and internationally. There is an
introductory course available at the University of Warsaw for anyone
interested in magnetic and electrical resistivity methods, and it is also
possible to learn about the magnetic method at the University of Poznan
(Herbich, T. pers. comm. 30.5.2012).
3.2.10. Scotland
Geophysical measurements were first carried out in Scotland in 1972 by
Haddon-Reece and Clark10, and a paper dealing with magnetic measurements
at the Standing Stones of Stenness, Orkney, was published by Clarke (1975).
Since then, archaeological geophysics has only been used sporadically.
Similar attitudes towards the usefulness of geophysics as noted in Sweden
have also been mentioned by archaeological geophysicists in this country
(e.g. Ovenden 2010). The number of companies that do archaeological
geophysics has increased, however, and an increased frequency of use can be
noted as a consequence (Cuenca-Garcia, C. pers. comm. 10.7.2012).
Education in archaeological geophysics is currently available at the
University of Glasgow.
9
http://archpro.lbg.ac.at/larvik-vestfold/larvik-vestfold
http://sdb2.eng-h.gov.uk/Maps/britain.asp?206,31
10
51
3.2.11. Turkey
The first archaeological-geophysical survey in Turkey was carried out as
early as in 1953 (see Drahor 2011:1 and references cited therein). The
current frequency of use is increasing and the educational situation is
improving thanks to the opening of the Center for Near Surface Geophysics
and Archaeological Prospection at the Dokus Eylül University in 2004
(Drahor 2011:2f).
3.2.12. North and South America
The current level of use in this area is difficult to estimate, but one
account provided by Bruce Bevan suggests that roughly 620 geophysical
surveys were carried out between 1938 and 1999 (Bevan 2002), 6.6% of
which were in Canada, 6.3% in Central and South America and the
remaining 87.1% in the USA. This does not, of course, reflect the current
level of use, but it still shows that an average of 223 surveys were carried out
in North and South America between 1990 and 1997 (Bevan 2000:53).
Bevan also estimates that about 2% of all archaeological projects carried out
in this area since 1980 made use of geophysical prospection methods (Bevan
2000:52). This figure has most likely increased, however, as current
estimates for the state of Ohio alone suggests a frequency of 70-80 surveys a
year (Burkes, J. pers. comm. 25.5.2012). The educational situation for
archaeological geophysics in the USA is better than in many European
countries, and several universities such as Minnesota State University
(Moorhead), the University of Denver, the University of Georgia and the
University of Arkansas provide courses in its use. In addition, a short course
in archaeological geophysics has been taught annually by the National Park
Service (DeVore 2001).
Apart from the numbers provided by Bevan (2002), the current frequency
of use of archaeological geophysics in Canada can only be conjectured upon,
and it is estimated that these prospection methods are currently being used in
less than 1% of the archaeological projects being carried out in the country
(Jeandron, J. pers. comm. 25.5.2012).
3.2.13. Australia
Archaeological geophysics was used in Australia for the first time in the
1970s (Connah et al. 1976), but the method was not employed very
frequently after these initial trials (Lowe 2012). The frequency of use is now
increasing, and several Australian universities and their departments of
archaeology possess their own geophysical equipment and provide a few
short courses in the subject for archaeology students (Lowe 2012).
52
3.3. Archaeological geophysics in Sweden
The historical development of archaeological geophysics in Sweden is
described in Paper I and will be summarised below.
Apart from a few early metal detector surveys (Hagberg 1961; Nylén
1972), the first real geophysical prospection in Sweden with an explicit
archaeological focus was carried out in 1977 at Frillesås in Halland (Fridh
1982:25f) when electrical resistivity and magnetometry surveys were
performed before archaeological excavations at a site dating back to the PreRoman Iron Age. The idea was to compare the results with the excavated
features, but as the geophysical surveys were never properly georeferenced,
no such comparison was possible. An additional early test survey using a
proton magnetometer was carried out in the late 1970s/early 1980s in
Rappesta, Östergötland, but the results were invalid as plastic-coated metal
measuring tapes had been used during the prospection and these had had
quite a strong impact on the data (Hedman, A. pers. comm.4.11. 2008). After
the initial tests, magnetometry was used sporadically in archaeological
projects during the 1990s (see Johansson & Nylund 1990; Kresten &
Ambrosiani 1992; Kresten & Kresten 1994; Sträng 1995; Hedberg et al.
1996; Masters 1998; Wedmark 1999; Larsson 2000; Lorra et al. 2001;
Mercer & Schmidt 2001).
The end of the 1970s also saw the first use of GPR in Swedish
archaeology, the first experiments being conducted at Ystad and Malmö in
the province of Scania (Wihlborg & Romberg 1980). Successful surveys at
the late Mesolithic site of Skateholm I was carried out during the spring of
1980 (Bjelm & Larsson 1980, 1984), and several other GPR surveys were
subsequently carried out during the 1980s in search of graves (Burenhult
1984; Sundström 1985; Burenhult & Brandt 2002), settlement remains
(Löfroth 1987), a buried wooden causeway (Wahlström 1993) and buried
walls (Carelli 2003), for example.
Very few electrical resistivity surveys have yet been carried out in
Sweden, as there are only a few examples available from the 1990s and
2000s (Sträng 1995; Hedberg et al. 1996; Larsson 2000; Dahlin 2001;
Misiewicz 2007; Sanmark & Semple 2008). Two of these (Larsson 2000;
Dahlin 2001) used the Continuous Vertical Electrical Sounding method
(CVES), producing electrical pseudo-sections (Aspinall & Crummett 1997),
while in the other cases the data were collected either in single traverses or
using what Gaffney and Gater (2003:34f) call an “area survey approach”,
where data is collected at fixed intervals inside a survey grid.
Even though the electromagnetic Slingram instrument was invented in
Sweden as early as 1936, the first archaeological application was not until
the early 1990s, when Kjell Persson of the Archaeological Research
Laboratory (ARL) at Stockholm University tested the EM31 and EM38
instruments by Geonics near the famous boat graves at Vendel, Uppland
53
(Persson & Olofsson 1995). The method later became an integrated part of
the teaching provided at the ARL, and the EM38 has, along with soil
phosphate analysis, been part of many undergraduate student projects and
other geophysical surveys carried out there (e.g. Sanglert 1995; Kristiansson
1996; Lundberg 1997; Mattsson 1997; Stavrum 1997; Wåhlander 1997;
Englund 1999; Stålberg 2000; Vaara 2004; Persson 2005a; Sabel 2006;
Viberg 2007; Wesslén 2011). Another electromagnetic method used
sporadically for archaeological prospection is VLF. This was used in a trial
survey at Vendel during the 1990s, but the EM16 by Geonics, provided
limited resolution, so that further surveys were cancelled (Persson 2005a).
Many laboratory measurements of the magnetic susceptibility of soil
samples were carried out in Sweden starting in 1980 (Freij 1980), and the
MS2B by Bartington Instruments is regularly used at the Environmental
Archaeology Laboratory (MAL) in Umeå (see Paper 1 for further
references), where field measurements are also conducted with the MS2D/F
for educational purposes (Linderholm, J. pers. comm. 4.6.2012).
To date only one seismic refraction survey has been reported as a part of a
Swedish archaeological project. This was carried out on the island of Björkö
in 1992 (Andrén & Lindeberg 1997) and was intended to assess the
thickness of the cultural layers at the Viking Age site of Birka.
The use of dowsing as a mean of detecting buried archaeological
structures has been advocated by several authors both in Sweden (e.g. Silver
1991, 2004; Andersson 1998) and internationally (e.g. Bailey et al. 1988),
but many authors have been critical of the scientific evidence for the validity
of the approach (e.g. Aitken 1959; van Leusen 1998; Lagerlund 2000;
Lindström 2000). As no scientific test has yet verified the reliability of the
divining rod or been able to reveal its underlying principles of operation, the
author strongly advises against its use as an archaeological prospecting tool.
The total number of geophysical surveys on archaeological targets
between 1977 and 2008 was 280 (cf. Fig. 23). The statistics should not be
considered complete, however, as it is impossible to be certain that every
survey has been included in the list. Many geophysical surveys are never
published properly, and in some cases they are carried out by a private
contractor and might never become available to the public. There is also a
risk that many unsuccessful surveys have remained unpublished, making the
statistics even more uncertain. Note that the numbers presented in Fig. 23
differ from those in Paper I, as several additional surveys have been
published and have become available since the publication of that paper.
This has not altered the overall trends, however, and the conclusions drawn
in Paper I are still valid. Since the first Swedish archaeogeophysical
investigations in 1977, the methods have been used at an increasing rate,
with a clearly visible rise from the early 2000s onwards (Fig. 23). Some 137
surveys were carried out between 2005 and 2008, which corresponds to
about 34 surveys a year. This number is probably rather high, since a large
54
number of surveys were carried out in 2006 and 2007, when several new
archaeological prospection companies, such as the geophysical unit at the
Swedish National Heritage Board, were established in Sweden and a lot of
test surveys were arranged. This establishment triggered an important
renewal of the discussion regarding the use of archaeological geophysics in
Sweden, appropriate data collection routines and the visualisation and
interpretation of geophysical data. Even though complete statistics for the
period from 2009 to 2012 are not available, it is estimated that perhaps 10-20
surveys are now being carried out in Sweden each year. This does not
include the metal detector investigations that form part of various
archaeological projects, which, if included, would probably at least double
the estimates for 2005-2008 (cf. Fig. 26).
When
investigating
the
geographical distribution of
these surveys, it is clear that the
majority were carried out in a
few Swedish provinces, namely
Uppland, Scania and Västergötland (Fig. 24), the vast
majority (62) in Uppland, reflecting in part the geophysical
proximity of the region to the
Swedish Geological Survey
(SGU), Uppsala University and
the Archaeological Research
Laboratory
at
Stockholm
University,
who
are
responsible
Figure 23. Graph showing the approximate
number of archaeological geophysical for many such surveys. Includprospection surveys carried out in Sweden ed in the Uppland statistics are
between 1977 and 2008.
surveys conducted at the Viking
Age site of Birka on the island
of Björkö, which is one of the more intensely surveyed sites in Sweden.
Several surveys in Uppland have also been carried out by the prospection
unit of the Swedish National Heritage Board and the archaeological
prospection company GeoFysica, which is situated in the Stockholm area.
Scania, where many of the early GPR surveys were carried out, is in second
place (35) and in addition to its geographical proximity to Lund Institute of
Technology, part of Lund University, has seen a lot of geophysical
prospections being carried out at the Viking Age settlement and trading site
Uppåkra. The most recent prospections at the Uppåkra site, as at Birka in
Uppland, has consisted of large-scale GPR and magnetometer surveys
carried out by LBI. In third place is the province of Västergötland, where 25
surveys have been carried out. Several surveys (14) have also taken place in
55
Södermanland. When analysing the geographical distribution, one must be
aware that the data also reflect to some extent the development activity that
has been going on in different parts of the country. The areas that have seen
the least amount of prospection over the years are the provinces of northern
Sweden. This reflects not only their geographical location far away from any
Swedish institutions carrying out archaeological geophysical surveys, with
the exception of the occasional surveys carried out by Luleå University of
Technology in Norrbotten, Malå Geoscience in Lapland and Radarteam
Sweden in Norrbotten, but also the relatively low property development
activity, with fewer rescue excavations. It is interesting that the geographical
distribution in many respects mirrors the distribution of population between
the provinces (Fig. 25).
Figure 24. Graph showing the approximate number of archaeological geophysical
prospection surveys carried out in Sweden between 1977 and 2008.
56
Figure 25. Population of Sweden by province. Data from Statistiska centralbyrån
(SCB) 2000 (cf. Fig. 31).
The most popular geophysical instrument is clearly the metal detector.
The relative ease with which anyone can obtain good results and the low cost
of the instrument are factors that have contributed to its popularity. The
problem, however, is that given the intense use of metal detecting, it is
almost impossible to keep an accurate record of all the surveys ever carried
out. My own list currently exceeds 400, and as the number performed on
Gotland alone has recently been estimated at c. 700 since 1972 (Fig. 26), the
metal detector can be said to be indisputably the most popular geophysical
instrument in Sweden. Included in the statistics for Gotland are 662 surveys,
but it should be noted that many more surveys have been carried out but
have been omitted from the diagram as the year of surveying is not
indicated. The data on metal detecting do not include the many illegal metal
detector surveys that are carried out each year, as the results of these are not
published.
57
Figure 26. Graph showing the approximate number of metal detection surveys
carried out on the island of Gotland between 1972 and 2010.7ote that the first
yearly interval is larger than the following ones. Published with permission by 7y
Björn Gustafsson, 2012.
The use of other geophysical methods varies a lot depending on the
province in question, but it is clear that, apart from metal detection, GPR is
the most frequently used method for prospection (Fig. 27). The high
frequency of other methods used in Uppland, and in Sweden in total, can be
explained by the fact that the EM38 by Geonics has been used frequently by
the Archaeological Research Laboratory during many years. The annual use
made of four of the more common geophysical prospection methods in
Sweden is presented in Fig. 28.
Figure 27. Use of GPR, Magnetic Resistivity and other prospection methods such
as EM surveys for archaeological surveying in the Swedish provinces of Uppland,
Scania and Västergötland. Data for Sweden as a whole presented in the lower
right-hand picture.
58
The majority of the surveys performed in Sweden have used only one
method or instrument, and when an integrated approach is adopted, the most
popular combination is without doubt GPR and magnetometry. Other
popular combinations are GPR and EM38 or earth resistance and
magnetometry.
There are currently five Swedish companies or academic institutions
working with archaeological geophysics: GeoFysica (GPR & Slingram),
SAGA-geofysik (Magnetometry), Modern arkeologi (GPR and Magnetometry), the prospection unit at the Swedish National Heritage Board (GPR
and Magnetometry) and the Archaeological Research Laboratory (GPR,
Magnetometry and Slingram).
The educational situation for archaeological geophysics is currently very
limited. The only place in Sweden where you can learn about the methods
and have an opportunity to use them in archaeological projects at the
bachelor’s or master’s level is the Archaeological Research Laboratory at
Stockholm University, but for several years the only instrument available for
training purposes was the EM38. Several other universities and university
colleges such as Uppsala University, Luleå University of Technology, the
University of Gothenburg, the Royal Institute of Technology and the
department of applied geophysics at Lund University offer the possibility of
studying applied geophysics, and occasionally also the possibility of
working on archaeological case studies (e.g. Sträng 1995; Hedberg et al.
1996; Masters 1998; Wedmark 1999; Larsson 2000; Persson 2005; Karlberg
& Sjöstedt 2007).
Figure 28. Comparison between GPR, EM methods, magnetic methods and
electrical resistance or resistivity methods in terms of usage in Sweden between
1977 and 2008.
59
3.4. Concluding remarks
When comparing Sweden to the other countries it is clear that the
Swedish situation is by no means unique. Most countries started to use
geophysical prospection during the 1960s, but the continued use of the
methods after these early surveys has varied greatly. In some countries such
as England and Germany the methods are being used very frequently and are
an integrated part of many archaeological projects. In England this has partly
been explained by the inclusion of geophysical prospection in the PPG16
document, while in Germany the geological and pedological preconditions
for geophysical prospecting are very suitable in many areas and this has
resulted in very convincing results, contributing to the popularity of the
methods. The character of the archaeological remains in these countries,
including the often manifest remains dating from the Roman period, may
also have been a possible factor contributing to the success of the methods.
These countries also provide education in archaeological geophysics, and
this has definitely had a positive impact on the acceptance of the methods. In
other countries the process has been slower. In the U.S.A., for example,
acceptance and use of the methods has increased gradually over many years,
while in the other Nordic countries, in Scotland and in Australia the process
of adopting archaeological geophysics has been slow. This could be
explained in some cases by unsuitable geology and pedology, making
geophysical surveys ineffective or problematic in those countries, while in
other cases the character of the archaeological remains, perhaps with a
predominance of postholes, hearths and pits, may have affected the success
of geophysical surveys. These factors and their implication for the likely
success of archaeological geophysics in Sweden will be investigated in
greater detail in the following chapters.
60
4. Geological and pedological conditions in
Sweden and their implication for
archaeological geophysics
4.1. Geology, pedology and land use
The bedrock in Sweden can be divided into three principal formations.
The first is the parent rock (Sw. urberget) which is a part of the larger Baltic
Shield (Fig. 29), also including parts of Norway, Finland and Russia. This is
roughly 3 billion–500 million years old (Lundqvist & Bygghammar
1994:16ff; Lundqvist 2000:29ff). This accounts for the major part of the
Swedish bedrock falls and includes many different rock types, although
predominantly igneous and metamorphic rocks such as granites and
gneisses. The second formation, the Scandinavian Caledonides, situated in
the north-western part of the country (Fig. 29), was created between 510-400
million years ago when two tectonic plates, Baltica and Laurentia, collided
(Stephens et al. 1994:22; Lindström 2000a:209ff). As a consequence of this
collision the geology of the area is highly diverse, including rock types such
as quartzite, limestone, sandstone, dolomite, amphibolite, greenschist etc.
The third bedrock formation consists of sedimentary rocks such as limestone
and sandstones created mainly during the Phanerozoic period, from c. 530 to
1.4 million years ago (Norling 1994:25; Lindström 2000b:230ff). This
sedimentary bedrock is found on the islands of Öland and Gotland in the
Baltic Sea, but also exists in the Swedish province of Scania and in smaller
areas elsewhere in Sweden (Fig. 29).
The magnetic susceptibility of Sweden’s various rock types is highly
diverse, and the magnetic variations caused by the bedrock can be mapped
by means of aerial measurements with a magnetometer despite the overlying
soils and water (Fig. 30). The strong positive anomalies clearly visible in the
northern, western and south-eastern part of the country (Fig. 30) are most
often caused by the presence of the highly magnetic mineral magnetite in the
bedrock (Eriksson & Henkel 1994:76ff). As the sedimentary bedrock on
Gotland has a very low magnetic susceptibility, the high readings visible on
the map may be surprising. These are created by the more magnetic parent
bedrock situated beneath the sedimentary rocks at depths of between 300m
and 800m (Erlström et al. 2009).
61
Figure 29. Principal bedrock formations of Scandinavia: Scandinavian
Caledonides (dark green), Baltic Shield (light green, orange, pink and yellow) and
the sedimentary bedrock outside the Scandinavian Caledonides (blue). © Swedish
Geological Survey (SGU).
Figure 30. Magnetic anomaly map of Sweden with the geomagnetic reference field
subtracted. The data colour scale is: -464 dark blue to +1058 dark red. © Swedish
Geological Survey (SGU).
62
The quaternary deposits are dominated by glacial till, which covers
roughly 75% of the country’s area (Fredén 1994:105; blue and purple in Fig.
32). This is a highly heterogeneous material that broke loose from the
bedrock beneath the continental ice sheet during the various glaciations. The
ice sheet would often transport this material far away from its area of origin,
and consequently one can find igneous rocks deposited by the retreating ice
margin on the island of Gotland even though such rocks are not native to this
sedimentary environment. The melting ice has also created landforms such
as eskers, moraines and drumlins which are highly visible in the landscape.
Clay and deposits of sand and gravel are other commonly occurring soils
in Sweden (yellow and orange in Fig. 32). These are found predominately in
the central parts of southern Sweden and along the east coast of northern
Sweden. The clay was deposited both during the melting of the ice sheet and
after the glaciations due to the flushing and redeposition of sediments, and
these types are distinguished as glacial and postglacial clays respectively.
Clays can be found in areas previously covered by water and are frequently
used for farming because of their high fertility.
Peat soils with a high percentage of
organic matter are to be found in many
areas of Sweden but to an increased
extent in the northern parts (brown in
Fig.32). Sweden also has large areas in
the northwest and in the eastern and
western parts of southern Sweden
where the soil cover is very thin, thus
partly or completely exposing the
underlying bedrock (red in Fig. 32).
In terms of land use the Swedish
landscape is dominated by forest,
covering roughly 53% of the total area
(see Table 4 and Fig. 33). The figure
can in fact be even higher locally, as
shown by the statistics for the counties
of Gävleborg, Västernorrland and
Värmland (cf. Fig. 31). The average
proportion of arable land in Sweden as
a whole is only 7.6%, but in Scania it
reaches 47.6%. The land-use map of Figure 31. The 21 counties of
the Swedish counties presented in Fig. Sweden.
33 reveals the dominance of marshland
and open heathland in the northern
parts of the country.
63
Figure 32. Quaternary deposits in Sweden.
64
Figure 33. Land use in Sweden, by county (cf. table 4).
65
Table 4. Land use in Sweden (data in percent), by county. Data from
Statistiska centralbyrån (SCB) 2005 (cf. Fig. 33).
66
4.2. Implications for archaeological geophysics
Geology, pedology and land use factors are important if one is to
understand the pitfalls and possibilities entailed in carrying out geophysical
measurements in Sweden. Given that most prehistoric remains found during
archaeological excavations in Sweden have a small physical contrast, a
knowledge of the properties of the surrounding soils is vital when predicting
the likely success of geophysical prospection methods.
If the soil cover is thin, as in the mountain tundra region of north-western
Sweden, magnetic bedrock can affect magnetometry results and soil
magnetic susceptibility measurements, effectively obscuring signals from the
often weakly magnetic archaeological remains (Viberg et al. 2009; Paper II).
This effect can nevertheless differ from site to site, because of the
geologically diverse nature of the Scandinavian Caledonides, which means
that an initial assessment of the magnetic effect of the bedrock should be
carried out before planning any geophysical survey. As suitable maps are
provided by the Swedish Geological Survey (SGU), such an evaluation
could be made reasonably quickly. Most types of sedimentary bedrock are
suitable for archaeological geophysics (see Gaffney & Gater 2003:79; David
et al. 2008:15) and the limestone bedrock on Öland and Gotland provides a
good example of an environment in which magnetic measurements would
be appropriate and even recommendable (cf. Papers III and IV).
Because of the heterogeneous nature of glacial till, magnetic rock
fragments deposited in the landscape by the melting ice sheet could have a
harmful effect on geophysical data, causing spurious anomalies that can
overshadow signals from the archaeological remains of interest (see Aspinall
et al. 2008:174f; Paper I). These anomalies might also be misinterpreted as
archaeological features. When used for the construction of prehistoric
structures such as the stone foundations of houses, however, such anomalies
could be an asset and make detection much easier (cf. Paper IV).
The attenuation of radar waves in highly conductive soils has been
discussed in previous chapters, and in this respect the clay soils of Sweden
could be potentially problematic for GPR surveys if the archaeological
features are situated at some depth. The presence of water is also important,
as this reduces the penetration depth of the GPR. Areas with both clay soils
and high precipitation during some parts of the year,11 such as some parts of
south western Sweden, could be potentially problematic.
Regarding land use, the most obvious difficulty for geophysical surveys is
the high density of forests and forest plantations. It is difficult and timeconsuming to carry out surveys in such areas and there are often problems in
covering a large enough area or carrying the survey out on a strictly regular
grid. It is also important to recognise the time factor, too, as this will
11
http://www.smhi.se/polopoly_fs/1.4159!image/p117.png_gen/derivatives/fullSizeImage/p117.png
67
increase the cost of the survey. The situation is similarly complicated by the
fact that tree roots are often visible in GPR data, thus limiting the
possibilities for detecting archaeological remains in such areas (see Fig. 34).
A magnetometer should not be affected by roots, however (Kvamme
2001:360), and can be a practical solution if the increased time required for
data collection and the negative effect of the trees on data quality are
acceptable. The local topography with uneven surfaces that is common in
mountainous and forested areas can also have a negative impact on the data
and/or prevent the use of GPR.
Figure 34. The effect of root systems on GPR data as visible in the top and mid-part
of the data from Fornsigtuna. The time slice was taken at an approximate depth of
1dm (1ns thick). Data collected with 0.1m transect spacing.
The farmlands of Sweden are very suitable for measurement purposes.
Their large open areas, at least during some seasons of the year, provide
practically optimal conditions for many geophysical methods (e.g.
magnetometry, cf. Paper IV). One must remember, however, that a large
proportion of the country’s farmland is situated on clay soils, which may
restrict the applicability of GPR in certain circumstances.
If the prehistoric remains are buried deep in the soil, detection by
geophysical methods could be challenging, and consideration should be
given to the information on the depth sensitivity and possible penetration
depths of different geophysical instruments already provided in the previous
chapters. One environment where one could expect deeply buried features is
wetlands and peat bogs, but the only method that is known to produce usable
results here is GPR (Clarke et al. 1999; Conyers 2004a:46). Only a few
successful examples exist of GPR surveys aimed at locating archaeological
remains in Swedish peat bogs (e.g. Henkel 2006:84), but since a significant
portion of the country is covered by peat, GPR could be recommended for
future studies.
68
5. Data collection, interpretation and the
character of Swedish archaeological
remains
5.1. Sampling density, data collection and
georeferencing survey grids
The importance of the density of the data collected cannot be
underestimated, as the sampling density governs the detectability and
interpretability of archaeological features of various sizes. The inline
sampling density is the density of data points along the survey transects,
which is governed by the speed with which the instrument can collect data.
This is rarely a problem as, apart from electrical resistance and magnetic
susceptibility instruments such as the MS2D, where any increase in inline
sampling density is a factor to consider carefully, modern instruments are
generally very fast. The crossline sampling density, however, i.e. the
distance between the transects, is very important, as it has far-reaching
implications for the speed of the survey, and thereby the costs, and also on
the sizes of archaeological features detectable by the geophysical method.
The benefits of a small crossline distance and its implications for the
detection of small archaeological features have been discussed by many
researchers (e.g. Clark 1996:81; Neubauer 2002:140f; Gaffney & Gater
2003:70, 95; Grasmueck et al. 2003; Leckebusch 2003:216; Dennis 2004;
Kvamme 2006b:214; Aspinall et al. 2008:110f; Novo et al. 2008; Ernenwein
& Hargrave 2009:79; Jordan 2009) and it has been shown beyond reasonable
doubt that an increased sampling density will enable the detection of smaller
features or features buried more shallowly in the soil. This important
conclusion has led the geophysical unit of the Swedish National Heritage
Board to recommend 0.25m as a standard distance between GPR profiles in
order to detect many of the smaller archaeological features common in
Sweden (e.g. Trinks 2005). There are various ways of thinking about the
necessary density of measurements. One rule of thumb is that in order to be
able to interpret the data correctly, the sampling density must be governed by
the expected size of the buried archaeological features. The archaeological
feature needs to be sampled at least twice and preferably even more
(Kvamme 2003:437, 2006b:214f). This is obviously true for both the inline
69
and crossline sampling distance, but as data are as a rule collected in a much
denser fashion inline than crossline, it is primarily the crossline distance that
is of concern in this respect. One can expect as such to be able to detect and
correctly interpret archaeological features equal to or larger than twice the
size of the chosen crossline sampling distance. For archaeological purposes
this means that if an archaeological feature is expected to be two metres in
diameter the survey transects needs to be one metre apart at most. Similarly,
for a 0.5m archaeological feature a crossline sampling distance of 0.25m is
needed. Another rule of thumb states that the distance between transects
should be less than the distance to features located below the instrument
(Bevan, B. pers. comm. 23.7.2012). This implies that if an object is situated
at a depth of 0.5m below the instrument, a crossline sampling spacing of less
than 0.5m would be desirable.
This theoretical view on sampling density obviously needs to be nuanced
and weighed against the aim of the survey. In many cases 100% detection of
every archaeological feature may not be necessary, and if the purpose of the
survey is to delineate possible prehistoric activity areas from areas not
containing such remains, a wider transect spacing may be used. The
sampling density, both inline and crossline, per unit area is as such a very
important measure to consider. Obviously very dense measurements have an
impact on the cost of the survey, as halving the crossline distance could
double the time spent in the field and increase the costs of the survey. When
surveying for previously known features in restricted areas this may not
present an economic limitation, and the cost of a dense survey may be
considered reasonable. If the purpose is to survey larger areas where no
archaeological remains are previously known, however, a different approach
is needed. One way of dealing with this could be to survey a larger area with
a wider sampling density. When the data have been analysed, one can carry
out more detailed measurements in limited parts of this area. This approach
and the whole question of how to design a geophysical survey have been
discussed by Sambuelli and Strobbia (2002).
Another way of increasing the sampling density without increasing the
costs is to use multiple sensors simultaneously or mounted next to each other
in arrays (Fig. 35). These arrays can also be motorised for even faster data
acquisition. The sampling density can then be adapted according to the
desired resolution, common crossline distances, i.e. typical distances
between the mounted probes in magnetic surveys, being 0.5m and 0.25m.
One of the earliest examples of a successful array investigation was the 1997
magnetic survey at Uppåkra, situated between Malmö and Lund in southern
Sweden, by Lorra et al. (2001). The survey was carried out using a Foerster
fluxgate gradiometer array system consisting of five probes with very dense
inline and crossline sampling densities of 0.05 x 0.2 m. The sampling
densities of two of the currently available motorised GPR arrays are 0.075m
x 0.075m (3D radar) and 0.08m x 0.08m (Malå MIRA system), thus
70
providing an unsurpassed data density combined with fast data acquisition
for large-scale prospecting operations. The arrays are also dependent on
broad, smooth expanses of terrain in order to operate well, however, and are
most suitable for use in flat, open areas such as farmland. A single
instrument survey as such will always be needed, especially in a country like
Sweden. A motorised system for electrical measurement is also available
(see Dabas 2009). It should be noted that the cost of buying any motorised
array system is high and only a few are available for archaeological
prospection in Europe. The price per unit area and the resulting data density
of a motorised survey must also be compared with the corresponding figures
for a single instrument survey when deciding between the two. When
collecting high density data over large areas with motorised systems the time
needed to interpret the collected data also increases substantially, as the
resulting data files are very large.
An additional benefit of arrays mounted onto rigid frames is that noise
introduced into the data during magnetometer surveys, i.e. heading errors,
can be kept to a minimum, thus producing clearer pictures of the subsurface.
Heading errors are introduced into the data when the magnetometer cannot
be kept in the correct upright position or when it is carried in different ways
(see Aspinall et al. 2008:120). This can be caused by strong, buffeting
winds, for example (cf. Papers IV and V). One must be aware, however, that
noise may also be introduced by the motorised transport and the frame itself.
Arrays, both motorised and non-motorised, have been used in Swedish
archaeology for several years and have produced excellent magnetic and
GPR data (see Lorra et al. 2001; Gustafsson & Alkarp 2007; Trinks et al.
2007, 2010b; Biwall et al. 2011; Trinks & Biwall 2011b).
General recommendations regarding the sampling density of geophysical
surveys performed using different methods have been published in England
(e.g. David et al. 2008). These guidelines suggest a magnetometer sampling
spacing of 0.25 x 1m for evaluation purposes and a sampling spacing of
0.25m x 0.5m for a more thorough characterisation of the archaeological
remains. Similarly, the suggested sampling distance for GPR surveys is
0.05m x 1m for evaluation purposes and 0.05m x 0.5m for more detailed
measurements, and that for surveys with EM instruments such as the EM38
is 1m x1m for evaluation and 0.5m x 1m or 0.5m x 0.5m for characterisation
(David et al. 2008:8). These guidelines function well as general
recommendations, but with regard to the Swedish situation and the nature of
the Swedish geology, pedology and the sizes of the commonly occurring
archaeological remains it is clear that the sampling densities need to be
adjusted according to the desired resolution and archaeological questions to
be answered.
71
Figure 35. Example of arrays used in archaeological prospection. Using the MIRA
system manufactured by Malå Geoscience (left; photo: MALA internt,) and the 3channel gradiometer system manufactured by Foerster mounted on a non-magnetic
cart (right; photo: Siska Williams).
Since the sizes of Swedish archaeological features determine the sampling
density strategy, I made a small review of the lengths and widths of 984
hearths reported at archaeological sites all over the country to try to establish
their mean size. The information was compiled from excavation reports were
selected at random from the web pages of various Swedish National Heritage
Board excavation units (UV) (Lindman & Stibeus 2000; Streiffert 2000;
Ängeby 2001; Strömberg 2001; Andersson et al. 2003; Gustafsson et al.
2004; Grön & Sander 2005; Häringe Frisberg & Renck 2005; Karlenby et al.
2005; Seiler 2005; Björck & Appelgren 2006; Edenmo 2006; Kraft 2006;
Lindman 2006; Lindkvist & Westin 2006; Molin 2006; Westin 2006a, b;
Edlund 2007; Granlund & Karlenby 2007; Holm & Thorsberg 2007;
Stenvall 2007; Sundberg et al. 2007; Bergold 2008; Björck et al. 2008;
Dunér & Vinberg 2008; Edlund 2008; Hamilton & Östlund 2008; Hamilton
et al. 2008; Hagberg 2009; Helander 2009; Bäck et al. 2010; Eriksson &
Häringe Frisberg 2010; Stenvall 2010)12. For northern Sweden, where largescale excavations are rare, the sizes of hearths interpreted and compiled by
Hedman (2003) were added. The dates of the hearths ranged from the
Mesolithic up to modern times. The median sizes of the hearths (see Table
5), indicate that the sampling density when searching for hearths should be c.
0.45m, and that a crossline distance exceeding 0.5m will not succeed in
mapping the median hearth and will render a substantial number of hearths
difficult to interpret correctly or miss them altogether. This recommendation
is also valid for EM measurements made using the EM38.
12
http://www.arkeologiuv.se/cms/arkeologiuv/publikationer/rapporter.html
72
Table 5. Mean and median lengths and widths, in metres, of hearths discovered
during excavations in Sweden (see text for further details).
Hearth southern and middle Sweden
Hearth northern Sweden
Hearth entire country
Average
Average
length
1.05
1.37
1.17
width
0.95
0.96
0.96
Median
length
Median
width
0.96
1.30
1.20
0.86
0.90
0.90
Apart from the size of the archaeological feature, the sampling density for
a GPR survey is dependent on the frequency of the antenna, in addition to
which, in order to avoid any aliasing or interpolation of the data, the
sampling distance in all directions should in optimal circumstances and
when aiming at a true 3D resolution be no more than a quarter of the
wavelength in the soil (see Novo et al. 2008, 2010). For a 500MHz antenna
this is only c. 5cm. This resolution will probably be obtainable in future
versions of motorised GPR arrays and will most certainly be feasible for
detailed research surveys of smaller areas, but despite the impressively clear
data and the possibility to detect very small archaeological features, it is
perhaps too optimistic to think that this will become a standard survey
procedure. The general recommendations that the transect spacing should
not be larger than 0.5m when aiming to detect larger structures using
400MHz and 500MHz antennas (see Neubauer et al. 2002; David et al.
2008) can be considered reasonable. When aiming at smaller structures this
distance should be smaller, possibly 0.25m, the spacing suggested as a
standard by several authors (Leckebusch 2003; Trinks 2005). As noted
above, however, this must still be governed by the questions to be answered
in the research.
To exemplify the effect of different crossline sampling spacings on GPR
data, a small simulated comparative study was performed (Figs. 36 and 37).
The first example is from the Dominican convent in Sigtuna (Paper V; Fig.
36), obtained using crossline distances of 0.25m, 0.5m and 1m, the results
for 0.5m and 1m being simulated by removing profiles from the original data
set. The time slices in all the examples below are 2ns thick and are situated
at depths of 18.7ns (left-hand slices) and 25.0ns (right-hand slices).
The simulated data with 1m transect spacing are clearly substandard and
fail to image many of the walls correctly, and many other features visible in
the two denser examples, such as the electric cable and many of the smaller
walls, are completely missing from the data or have an erroneous spatial
distribution. A dramatic improvement is visible when reducing the transect
spacing to 0.5m. At this point the electric cable can be interpreted correctly
and the majority of the walls are visible. When the spacing is reduced further
to 0.25m, the image sharpens up even more and it becomes possible to
interpret some of the smaller walls, such as the one situated in the top right
73
corner of the cloister garth. The lavatorium, extending from the southern
range into the cloister garth, is also easier to interpret.
Figure 36. Effects of different crossline transect spacings, as exemplified by the
data set from the Sigtuna Dominican Priory (Paper V). The slices are 2ns thick and
the lefthand pictures originate from a depth of 18.7ns and the right-hand pictures
from a depth of 25ns. Data collected with 0.25m transect spacing (top), a simulated
data set with 0.5m transect spacing generated by removing profiles from the
original data set(middle), and a simulated data set with 1m transect spacing
generated by removing further profiles from the original data set (bottom).The size
of the survey area is 50 x 25m and references points are indicated by letters a and
b. 7ote the electrical cable visible in the middle and bottom lefthand picture.
The effect of crossline spacing can also be exemplified by the GPR
results from a Migration Period house terrace at Fornsigtuna, north of
Stockholm. Seasonal measurements were conducted at this site in order to
evaluate the impact of soil moisture on GPR data (see section 3.2.2.). The
house terrace, no. 6, had previously been excavated during the 1980s
(Damell 1991:48f; Fig. 13) and the GPR survey was carried out in August
2010 with a profile spacing of 0.1m. The time slices are 1ns thick and are
situated at a depth of c. 3ns (Fig. 37). By removing profiles from the original
data set, the effect of an increase in transect spacing can be simulated. The
spacings presented below are 0.1m (Fig. 37: bottom picture), 0.3m (Fig. 37:
74
second picture from the bottom), 0.5m (Fig. 37: second picture from the top)
and 1m (Fig. 37: top picture).
The data with 1m transect spacing is not informative enough and its
interpretation would lead to many erroneous conclusions. The most dramatic
improvement is visible when the crossline spacing is reduced from 1m to
0.5m, but a smaller spacing is necessary if correct assumptions are to be
made regarding the size of the house as well as the correct identification and
interpretation of some of the possible postholes. The tree roots in the top
centre of the slices also become clearer and the smaller spacing allows these
to be differentiated from any archaeological features visible in the data. The
curved wall of a longhouse along with one of the gables (in the left of the
picture) becomes clearer with a decrease in transect spacing.
Figure 37. Effect of different crossline transect spacings, as exemplified by the data
set from Fornsigtuna. The slices are 1ns thick and originate from a depth of 3ns.
Data collected with 0.1m transect spacing (top), a simulated data set with 0.3m
transect spacing generated by removing profiles from the original data set (second
from top), a simulated data set with 0.5m transect spacing generated by removing
further profiles from the original data set (second from bottom), and a simulated
data set with 1m transect spacing generated by removing still more profiles from
the original data set (bottom). The size of the survey area is 37 x 11m and
references points are indicated by letters a and b.
75
The examples prove that the crossline transect spacing has an impact on
the interpretability of the data. When investigating a large stone or brick
structure a crossline sampling spacing of 0.5m or less is recommended in
order to produce interpretable data. If the smaller walls and features of the
convent building are to be mapped correctly, a spacing of 0.25m is
necessary, for the smaller and more elusive remains of the longhouse 0.25m,
and an even smaller transect spacing is needed if small postholes are to be
mapped and interpreted correctly.
Equally important is the routine of georeferencing the data grids
collected. This is crucial if subsequent excavations are to target geophysical
anomalies. In order to achieve the desired high precision it is recommended
that the grids that are surveyed should be tied to the national coordinate
system used in Sweden, SWEREF99, by means of either a total station or an
RTK-GPS, with which an accuracy of 0.001m is achievable in favourable
circumstances. These high precision instruments could also be used when
establishing the survey grids. The use of low-cost handheld GPS systems,
sometimes with an accuracy of several metres, or routine of georeferencing
grids by means of fixed landmarks using measuring tapes, is in most cases
not recommended. The quality of the data is also dependent on accurate
positioning within the survey grids, in that grids that are georeferenced
properly could still suffer from discrepancies caused by the surveyor failing
to walk in straight lines or to maintain a constant pace. The use of guide
ropes placed on the surface at regular intervals within the grid is thus
recommended in order to increase the quality and precision of the data as
collected.
5.2. Interpretation
After collection, the data should be filtered properly before the process of
interpretation begins. There are several basic overviews of the data
processing of magnetic (Aspinall et al. 2008) and GPR data (Conyers
2004a), and the art of interpreting geophysical data has been described by
several authors (see e.g. Gaffney & Gater 2003; Conyers 2004a, 2006b;
Aspinall et al. 2008; David et al. 2008). The method for interpreting GPR
data is presented by Poscetti et al. (2011) and should be regarded as a stateof-the-art description of the process. Different parts of the interpretation
process as described in this paper could also be adopted when interpreting
results obtained using other geophysical measurement techniques. As this is
a fairly well covered subject it is not my purpose to present a thorough
explanation of interpretation, but some crucial points need to be mentioned.
A geophysical data set can be interpreted in different ways and on
different levels. At the lowest level, geophysical maps are viewed and the
spatial distribution of the anomalies is described, e.g. as linear or round
76
(visual inspection level). At this level of interpretation only certain very
basic information can be extracted from the data, and the data may need to
be interpreted further to become meaningful to archaeologists. The next level
is that of technical interpretation (parametric interpretation level), which
according to Bevan (2003:3) will consider such parameters as the location of
the source of the anomaly, the depth of the feature, the estimated mass of
the material and an evaluation of what material is causing the anomaly. The
technical interpretation could also have the purpose of singling out the
anomalies that are most likely to be caused by geological features, or by
shallow metal artefacts when these are unwanted. Following or
accompanying such a technical interpretation, the archaeological evaluation
or interpretation of the data should be added (archaeological interpretation
level). This interpretation would ideally be carried out by someone with a
thorough knowledge of archaeology and of the geophysical method or
methods employed, possibly in cooperation with site specialists/excavators
who have a comprehensive knowledge of the archaeological conditions at
the site. The archaeological interpretation must depend on a thorough
knowledge of earlier land use at the site as obtained by evaluating historical,
geological and pedological maps and maps of previous archaeological
investigations and an evaluation of the impact of agriculture. This
interpretation and comparison of data from different sources is best carried
out using a GIS platform (see Neubauer 2004), as the resulting
georeferenced interpretation maps can provide valuable information for the
planning and execution of subsequent excavations.
5.3. Swedish archaeological remains
The majority of the prehistoric remains found during excavations in
Sweden are pits, postholes and hearths, and as they are often quite small and
have a weak physical contrast with the surrounding matrix, their
identification by means of geophysical measurements can be problematic.
This has also been a problem in France, where, precisely because of such
limitations, archaeological geophysics has recently been recommended only
as a complement to more traditional archaeological preinvestigation methods
such as trial trenching (Hulin & Simon 2012). A few examples are presented
below of typical remains that might be encountered during archaeological
excavations and their likely detectability using geophysical methods.
5.3.1. Graves
Graves are very common structures encountered during many
archaeological excavations and have from the early days of geophysical
prospection in Sweden been valuable discoveries (Bjelm & Larsson 1980,
77
1984). Despite occasional comments to the contrary (e.g. Alkarp & Price
2005:268), this very heterogeneous category is notoriously difficult to map
(see David et al. 2008:15), but successful surveys have been carried out in
the U.S.A. primarily by using GPR, electrical conductivity and electrical
resistance to map graves in both historical and modern cemeteries (see
Bevan 1991; King et al. 1993; Conyers 2006a; Fiedler et al. 2009).
Magnetometer surveys in England have in a few cases also been successful
for mapping prehistoric Roman and Anglo-Saxon inhumations (see Linford
2004). As stated by Gaffney and Gater (2003:136), it is important to
understand that geophysical techniques rarely detect the actual buried body,
but rather graves are mostly identified on the strength of constructions
associated with the burial. Historical graves are often located on the grounds
of interruptions in soil layers visible in the GPR radargrams, which are in
many cases evidence of grave shafts dug through these soil strata and thus
provide indirect evidence of the actual burial. A few surveys have been
carried out in Sweden with the explicit purpose of mapping graves, the
earliest successful ones having involved the use of GPR for detecting
Mesolithic graves at the cemeteries of Skateholm I and II in Scania (Bjelm &
Larsson 1980, 1984). As the detection of graves is dependent on a contrast
between the fill and the surrounding soils, it becomes clear why geophysical
methods are rarely able to map such features. As the grave is dug and the
body placed in it, the same soil that was removed from the hole is often used
for refilling it and only a small physical contrast can be expected, unless
there was a marked stratification in the natural soil. In recent years a
magnetometry survey aimed at detecting graves in the Viking Age
inhumation cemetery at Broby bro (RAÄ 42) in central Sweden was
unsuccessful, highlighting the problems involved (Andersson 2011). A more
successful survey, however, was carried out at the Franciscan Convent in
Krokek, mapping graves dating from medieval times and onwards using a
GPR (Trinks et al. 2008a). Clear evidence of graves aligned in an east-west
direction is visible in the time-sliced data, acting as a valuable complement
to the already existing map of burials at the site.
GPR and magnetometer surveys in Uppåkra, Scania, as also at Borre,
Norway, have been successful at identifying an overploughed grave mound
(see Trinks et al. 2010a; Biwall et al. 2011). In the Borre survey detection
was possible because the stones covering the central part of the grave were
magnetic. In addition, the overploughed mound was surrounded by a small
ditch detectable by both the magnetometer and the GPR.
It has been shown on Öland that Early Iron Age cemeteries typically
consist of circular stone settings, predominantly of limestone, while the outer
kerb ring of such a grave often consists of granite or diorite stones (Hagberg
1979:14). As these rock types generally have a high magnetic susceptibility,
they would most likely produce a pattern detectable by a magnetometer.
Deposited artefacts or coffin nails could also aid in the discovery of certain
78
kinds of burials when using a metal detector (David et al. 2008:16). When
searching for cremation burials, which are common in Sweden, the graves
could produce a magnetic response identifiable by a magnetometer if the
cremation had occurred in situ. A strong response should also be produced in
the GPR data if the grave contains void spaces.
Larger mounds have been surveyed by GPR on a few occasions in
Sweden and Norway13 (e.g. Persson & Olofsson 2004) and while such
surveys are feasible, it is important to correct the resulting data for tilt and
elevation of the antenna in any direction if the results are to give a correct
representation of the internal features of the mound (see section 2.2.3. and
Goodman et al. 2007).
5.3.2. Prehistoric settlement remains
Prehistoric settlement remains such as postholes, pits, ditches, middens,
long houses, stone foundation houses etc. have been successfully detected by
geophysical methods on many occasions. While some of these features are
easier to detect than others, some are small and only provide a weak physical
contrast with the surrounding soil matrix. One small problematic
archaeological feature is the post hole. These come in a variety of sizes and
in their basic form may consist merely of a post inserted into the ground.
This post may or may not be burned or lined with stone, and this means that
their detectability may vary greatly. A burned post will most likely have an
enhanced magnetic contrast, just as a stone-lined posthole, as the magnetic
properties of the stones may enhance the possibility of detection by a
magnetometer or GPR. One must also consider the possibility that the post,
in most cases, may have been removed or have decayed completely, which
will have negative implications for their detectability.
There are a few examples of successful detections of postholes, including
the instance at Borre in Norway when the geophysical unit of the Swedish
National Heritage Board produced a convincing geophysical map with
several postholes forming the typical shape of a prehistoric long house14. In
this case the postholes were quite large and could be identified accurately
with a crossline sampling spacing of 0.25m.
At the Viking Age settlement site of Birka (see above), small postholes
were identified as likely parts of a Viking Age building within the Black
Earth area using the MIRA system manufactured by Malå Geoscience
(Trinks et al. 2010). The data were collected at a density of 8cm, both inline
and crossline, enabling the mapping of postholes of roughly 0.16m in size.
13
http://www.vfk.no/doc/kulturminnevern/geofysikk/Borre/web/Borre_web_files/frame.htm
http://www.vfk.no/doc/kulturminnevern/geofysikk/Borre_oct2007/web/Borre_Oct_2007_web_files/fra
me.htm
14
79
Postholes identified by GPR have also been confirmed at an Iron Age
settlement site in the parish of Fresta in Uppland (Viberg 2007). Here roofbearing postholes, part of an Iron Age longhouse, were visible as
depressions. Despite the difficult terrain and a coarse crossline sample
spacing, a positive correlation could be made between the anomalies in the
GPR data and known postholes. Stone-lined postholes have also been found
by magnetic methods in archaeological excavations at Skuttunge, Uppland
(Seiler & Östling 2008:61f).
At some sites entire houses have burned down and have produced distinct
magnetic anomalies. One example is the Iron Age settlement site of Uppåkra
in southern Sweden, where magnetic measurements indicated a burned
house (Voss & Smekalova 2007) and the subsequent archaeological
excavation confirmed this interpretation (Larsson, L. pers. comm. 2009).
Stone foundation houses typical of the Migration Period on the Baltic
islands of Gotland and Öland have recently been shown to produce a
characteristic magnetic response, as they are generally constructed of
magnetic stones of igneous origin (Paper IV). At Stånga in the parish of
Lyrungs, Gotland, the data contained evidence of stone boundary walls (sw.
stensträngar) constructed of the same material which opens for the detection
of overploughed prehistoric agrarian field systems in other areas. Remains
from partly overploughed prehistoric agrarian field systems and clearance
cairns have also been detected with a magnetometer in Västergötland
(Berglund 2007). Other stone foundation houses, such as those constructed
of limestone typically found within the ringforts of Öland, have also been
shown to be identifiable by GPR and to some extent with a magnetometer
(cf. Paper III).
Filled-in ditches can be detected by geophysical methods, as shown by
the clear examples available from magnetic and GPR surveys at Nibble in
Uppland and Skänninge in Östergötland (Trinks et al. 2009a, b). Filled-in
pits, such as cooking pits, are equally suitable targets for primarily magnetic
prospection. Magnetic investigations carried out at Ryssgärdet by Gerhard
Schwartz of the Swedish Geological Survey, for example, successfully
detected c. 70% of the cooking pits discovered during subsequent
excavations despite a coarse crossline transect spacing of 1m (Schwartz
2005). Cooking pits have also been detected and confirmed by excavations
in Uppåkra (Lorra et al. 2001:46; Larsson, L. pers. Comm. 2009) and a
midden or heap of fire-cracked stones was successfully detected at RAÄ
1372 in Lapland (Paper II).
5.3.3. Fortified sites
Only a few fortified sites have been investigated geophysically, one of
which is the Öland ringfort of Sandbyborg (see Paper III). This is currently
the only geophysical survey that has produced clear evidence of houses
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within a fort on Öland. Another Öland ringfort, Gråborg, has been surveyed
with a magnetometer, but the results and interpretations have recently been
questioned and intensively debated (Trinks & Biwall 2011a; Danielsson
2012). The argument primarily concerns the interpretation of a linear
anomaly outside the fort which was originally said to indicate a moat (e.g.
Tegnér 2008), whereas Trinks and Biwall (2011) prefer to see this as alightning induced magnetic anomaly. The easiest and fastest way to resolve
the issue would be to dig in a small trench over the suspected feature, or to
carry out a resistivity survey.
5.3.4. Fired structures
The detection of fired structures is one of the more common applications
for magnetic and electromagnetic methods internationally and in Sweden.
Successful identification of such structures has been achieved when using
geophysical methods in archaeological projects in Sweden. These include
hearths (Fridh 1982, 1983; Furingsten 1984, 1985; Persson 2005a; Schwartz
2005; Seiler & Östling 2008), kilns, ovens and blast furnaces for bronze,
glass, brick and iron production (Fridh 1982, 1983; Furingsten 1984, 1985,
Kresten & Kresten 1994; Riisager 2003; Persson 2005a; Seiler & Östling
2008).
5.3.5. Medieval buildings
Many structures of medieval age are suitable targets for geophysical
surveys. GPR have been shown to produce some outstandingly clear results
in cases of medieval structures, and some of these will be referred to below.
Many of these structures are made of brick and thus produce a significant
reflection in GPR data. Bricks, even though randomly oriented (Bevan
1994), can produce a strong magnetic response when surveyed with a
magnetometer, but many buried medieval brick structures are situated in an
urban environment, so that a magnetometer will not always be suitable
because of interference from surrounding structures and passing cars.
The use of geophysical methods for mapping churches is nowadays
common in Sweden, and GPR investigations of churches, monasteries and
convents or parts thereof have produced clear results; see, for example, Old
Uppsala Church (Persson 2005a), Skänninge Dominican Convent (e.g.
Trinks 2005), Krokek Franciscan Convent (Trinks et al. 2008a), Ås
Cistercian Monastery (Löwenstein 2010), grave chambers in Kalmar
Cathedral (Ohlsson 2010), the porch or weapon house of Alunda Church in
Uppland (Kjellberg et al. 2010), Sigtuna Dominican convent (Paper V) and
St. Lars Church in Sigtuna (Viberg et al. manuscript). Other medieval
buildings of a non-ecclesiastical nature would of course also be detectable
(see Sträng 1995; Lück et al. 2003; Trinks et al. 2009b, d).
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5.3.6. Gardens
Historical gardens have been a popular target over the years, as many of
them have been rebuilt from time to time and contain the buried remains of
previous layouts, such as fountains and paths, which are traceable by
geophysical methods (see Persson 2005a; Trinks 2006; Winroth et al. 2011).
5.3.7. Detection of archaeological remains in present-day urban
environments
Another important factor to consider is whether the surveys are conducted
in an urban or rural environment. Because of interference from passing cars,
electromagnetic sources and often a complicated stratigraphy, the only
geophysical instrument that seems to have been really successful in Swedish
urban environments is the GPR (see Trinks et al. 2008b, 2009c, 2009e;
Trinks & Biwall 2008). Surveys of urban environments have also been
carried out in Jönköping, but as the quality of the pictures in the reports
available online are poor, it is difficult to evaluate the data (Petterson &
Winroth 2011a, b).
5.3.8. Ferrous objects
Ferrous objects are easy to detect using magnetometers, GPR or other
electromagnetic devices. Many are of modern origin, however, and as such
are unwanted, as they disturb the identification of prehistoric structures. But
in some cases a ferrous response in magnetometer data may originates from
prehistoric ferrous objects or artefacts (see Kresten & Kresten 1994). During
interpretation of the magnetic data collected by Lorra et al. (2001) in
Uppåkra, Scania, an anomaly was interpreted as being caused by either iron
slag or an accumulation of metallic or magnetic objects. Subsequent
excavations proved this interpretation right when a large deposit of Iron Age
swords was discovered at that point (Larsson, L. pers. Comm. 2009).
5.3.9. Other non-archaeological features
Other more modern features are also detectable by geophysical methods,
the more common ones being electric cables (Trinks et al. 2009a, f; Paper
V), metal pipes (Fennö & Dyhlén Täckman 2002), refilled archaeological
trenches (Trinks et al. 2009a) and drainage pipes in farm land (Sträng 1995;
Trinks et al. 2009a, f; Gustafsson & Viberg 2010). Geophysical prospection
has also been used in Swedish forensic archaeology (Persson 1996).
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6. Discussion
Despite being successfully deployed in many archaeological projects,
geophysical methods have generally been used to a quite limited extent and
have not enjoyed great popularity among the Swedish archaeological
community. One common issue that is repeatedly mentioned in
archaeological reports evaluating the success of geophysical surveys is the
fact that the results are difficult to understand as they don’t seem to correlate
well with the archaeology of the sites (see Eklund 2008; Steineke 2008;
Bennström & Helgesson 2010; Lindberg 2010; Andersson 2011). The
examples mostly concern magnetometer surveys, but similar comments have
also been expressed in connection with GPR surveys (e.g. Holback et al.
2004; Ohlsén 2006). Some archaeologists even go as far as to suggest that
the methods cannot be regarded as reliable enough. A magnetometer, for
example, failed to identify 77% of the hearths discovered at one site during
subsequent excavations (Hylén 2007). A few other comparisons between
large-area geophysical measurements and archaeological excavations have
been carried out, and the results and success rates of the geophysical
methods differ greatly. Geophysical measurements with GPR and a
magnetometer detected c. 35% of the archaeological remains at Lövstaholm,
Uppland (Larsson 2004), but also provided clear evidence of houses that
were not visible during the subsequent excavations. In a similar manner,
measurements made using the EM38 at Stensborg in the parish of Grödinge,
Södermanland, seemed to indicate archaeological remains that were not
found during the subsequent excavations (Viberg 2008). In rare cases the
correlation between the geophysical measurements and the archaeological
excavations is zero, as in a survey conducted at Skänninge (Trinks et al.
2010c). Such poor results are somewhat difficult to understand, as previous
surveys in the area, in a similar geological setting and carried out by the
same staff using the same instrument had correlations between 60% and
100% (Trinks 2005; Trinks et al. 2009b). The only difference seems to be
that the unsuccessful survey was carried out in December as opposed to
May, June and July for the successful ones. It is unknown to what extent this
influenced the results but it might be considered as a possible explanation.
It must, however, be strongly emphasised that positive examples do exist,
e.g. the results presented in papers III and V and those from Ryssgärdet and
Skuttunge in Uppland, which also show an excellent correlation between the
geophysical measurements and the excavation results (Schwartz 2005; Seiler
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& Östling 2008). Note that the magnetic survey at Ryssgärdet succeeded in
detecting c. 70% of the cooking pits but only 42% of the hearths. According
to the prospector, this was due to the coarse crossline transect spacing (1m)
combined with a complicated geology (Schwartz 2005:79). Other positive
examples are also listed in chapter 5.
It can be concluded that in the majority of the more problematic surveys
geological features, modern ferrous objects or magnetic stones were
erroneously interpreted by the prospector as archaeological remains. It is
thus not surprising that the correlation with the data from the subsequent
archaeological excavations was poor (see Hylén 2007; Steineke 2007). Overoptimistic magnetometer interpretations have previously been noted in the
case of a number of reports (e.g. Envall 2007b; Henriksson 2011). These
surveys tend to fail to pass beyond the Visual Inspection Level discussed in
chapter 5, and thus disregard the geophysical signature of the anomalies.
They also fail to translate these observations into a reasonable archaeological
interpretation. It is not good praxis to leave interpretation of the geophysical
data to the archaeologists, as they generally have little or no experience
regarding such a procedure. As stated in the literature: “It is the
archaeological interpretation that is the common and acceptable end point
for all archaeological prospecting” (Aspinall et al. 2008:115) and as such
uninterpreted data is of little or no value.
A possible problem with the attempts to correlate geophysical data and
archaeological excavations is that features or objects in the plough zone
might have been removed using a bulldozer or excavator before the results
could be compared. If this is true, there is a risk that the cause of some of the
anomalies visible in the geophysical data had been removed with the
ploughed soil. This could easily be estimated by looking at the geophysical
data and using the half-width rule, for example, to estimate the probable
depth of some of the magnetic anomalies that are missing from the
excavation record. An additional factor is the georeferencing of the
geophysical grids. In many of the above examples the methods used for
georeferencing are not known, and it is likely that some of the features
causing the geophysical anomalies could have been situated within a buffer
zone around the mapped anomaly. This buffer zone would differ in size
depending on the chosen sampling density, the accuracy of the established
grids and the accuracy of the subsequent georeferencing.
Regarding the geophysical measurements at Broby bro in Uppland
(Andersson 2011), another method such as GPR could probably have been
more successful in detecting the inhumation burials, as magnetometers
generally only produce weak responses to such features. Graves, especially
those without any related stone constructions, are notoriously difficult to
map, however (cf. section 5.3.1.) and a change of instrument would not
necessarily yield success, but it does highlight the possible benefit of using
multiple methods at a site.
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There are also a few reports available on magnetometer prospections
without any archaeological interpretations, which makes it impossible to
evaluate their quality in this respect (e.g. George 2010, 2011). Many
magnetic results are also presented without basic map elements such as an
amplitude scale showing the magnitude of the magnetic anomalies. Such
maps are almost impossible to evaluate correctly (Hylén 2007; Steineke
2008; Bennström 2009; Bennström & Helgesson 2010; George 2010; Ní
Chíobháin 2010; George 2011). Some of the reports mentioned above have
such scales, but they only give the approximate magnitude of the
geophysical response on a scale ranging from high to low without stating the
actual nT values, and this similarly makes the results difficult to evaluate.
This could in part be due to the fact that many of the surveys are reproduced
in archaeological reports, so that this information might still be present in the
original survey report (e.g. Envall 2007a, b). Thus, archaeologists who
summarize geophysical survey results should, if possible, publish this
information together with the maps obtained from the geophysical
prospection. It is also good practice, if possible, to present both the original
geophysical data and the interpretation in both geophysical and
archaeological reports.
If one compares the success rates in Sweden with a few international
examples (e.g. Linford & David 2001; Ibsen, T. pers. comm. 2012), it can be
stated that in Sweden it should be reasonable under normal circumstances to
expect a correlation of at least 50% between the geophysical data and the
subsequent excavations. Clearly the surveys presented above that produced a
correlation of less than 30% are problematic for the general reputation of
archaeological geophysics. When this is combined with high expectations
from archaeologists, often caused by unrealistic promises made by the
surveyor, it becomes even more problematic. Some of the surveys are still
conducted with a very coarse crossline transect spacing, which might explain
some of the difficulties, but even more problematic is the fact that the
surveyor seems in some cases to be lacking in knowledge of how to interpret
the data correctly. This is at least partly caused by the limited opportunities
for education in archaeological geophysics in Sweden, so that instead of
going abroad to get a proper training, it has been easier to buy an instrument
and learn the procedure by trial and error. Unfortunately many poorlyexecuted surveys are the result of such a strategy and disappointed
archaeologists faced with these measurements are now hesitant about using
the methods again. This is not a new problem, but it does mean that all
unsuccessful archaeological geophysical surveys should be re-evaluated
thoroughly, if possible, in order to understand the reason behind the lack of
results (cf. Conyers 2004a:8).
Not all archaeological remains, however, are likely to produce a physical
response detectable by geophysical methods. One example of this has been
reported by Maki (2006), in which hearths fired at high temperatures for a
85
long period of time lost the initial increase in magnetic susceptibility and
inverted back from maghemite to haematite. These hearths may be readily
identifiable during excavations but invisible in the geophysical data. In other
cases a lack of magnetic contrast between the hearth and the surrounding
soils might be caused by firing at very low temperatures so that no magnetic
anomaly was created (Trinks et al. 2009a). The area in Nibble, Uppland, was
nevertheless identified as a hearth during the subsequent excavation and did
produce an echo in the GPR data. In other cases, the geophysical data have
shown evidence of archaeological structures which were not visible in the
excavation (e.g. Becker 1995; Conyers 2004a:158f; Dalan 2008:7).
Consequently, it must also be acknowledged that archaeological features
may be visible in geophysical data but not to the naked eye during
excavations. These features may only be detectable when using very
sensitive magnetometers in magnetically quiet areas, as the increased
magnetic susceptibility in one such case (Becker 1995) and was most likely
caused by the presence of biogenic magnetite within the remains of micronsized magnetic bacteria living off decaying wood. In areas where the
magnetic susceptibility of many soils is high, such as Sweden or northern
England, such features would most likely not be detected even with sensitive
instruments. David Jordan, having evaluated the use of archaeological
geophysics in northern England, came to the conclusion that the varied
geology of this area, with the presence of glacial remains such as stony till,
will not benefit from using more sensitive instruments but rather from
increasing the sampling density according to the size and nature of the
expected archaeological remains to be detected (Jordan 2009). As the major
part of Sweden is covered by glacial till soils, these conclusions are also
valid for the Swedish situation. Sweden has many areas, however, that are
very suitable for geophysical surveys and valuable results have been
produced in a wide range of geographical, geological and pedological
environments.
Prior to 2005 the vast majority of geophysical areal surveys carried out in
Sweden were collected in grids very rarely exceeding 50x50m with a
crossline transect spacing of between 0.5m and 5m, with 1m being the most
common. The inline sampling spacing differed depending on the choice of
method, but varied between 0.03m and 0.05m for GPR surveys, 0.5 and 5m
for conductivity and magnetic susceptibility measurements using the EM38
by Geonics and 0.02 and 5m for magnetic surveys. Given the review of
hearth sizes conducted here, such crossline sampling spacings are not
sufficient for the detection of the average size of hearth present in the data
set. As almost 1000 hearths were included in the data set, it is likely that this
calculation can be used as a reasonable estimate for future surveys and
archaeological investigations. The sampling density needs to be governed by
the sizes of the archaeological features. This means that in a majority of
surveys carried out before 2005 the sampling distance was too large to allow
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correct characterisation and interpretation of the archaeology, and as a result,
many of the smaller features at the sites were most likely missed or proved
difficult to interpret. This is equally true for the sizes of the areas surveyed,
an important consideration for the correct evaluation of noise levels in the
survey area and for achieving better interpretations (see Gaffney & Gater
2003:91ff). The situation has only partially improved since 2005.
The founding of a geophysical unit at the Swedish National Heritage
Board has been important for the development of archaeological geophysics
in this country and for the debate on the importance of proper routines
regarding data collection, interpretation and the importance of densely
sampled data, but many surveys are still performed with too coarse a
sampling density and this limits the detectability of smaller archaeological
remains. The amount of geophysical work carried out in larger areas has
increased and this is a positive factor for the interpretability of the resulting
data.
The difficulty of making correct interpretations of GPR radargrams was
highlighted early on by Wihlborg & Romberg (1980) and Wahlström (1993),
among others, and even though the practise of using time slices to increase
the interpretability of GPR data was introduced into archaeological
geophysics in the early 1990s (Goodman & Nishimura 1993; Goodman et al.
1995), it did not become commonplace in Sweden until around 2005. This
was also noted by Alkarp and Price (2005:264), who strongly advised
against the exclusive use of radargrams except in situations when timeslicing is impossible or inappropriate or as a complement to time slices in
order to strengthen the interpretation of a data set. To my knowledge, the
first application of time slicing in Swedish archaeological research was by a
team from Kiel University surveying in Uppåkra in 1997 (Lorra et al. 2001),
and there have been a few examples of GPR time slicing from 2003
onwards, using software such as GPR Slice and Easy 3D (e.g. Persson 2003;
Persson 2005a) and GPR process (e.g. Persson 2004, 2005b, c). It is obvious
that the adoption of time-slicing have increased the reliability of
interpretations of GPR data and made it easier for archaeologists to involve
themselves in the interpretation process. The use of time slices to visualise
GPR data must be regarded as standard today, and this has been important
for the production of the many convincing sets of GPR results published in
recent years. There are, however, still many geologists and geophysicists
without experience in carrying out archaeological surveys using GPR who
continue to work exclusively in a 2D environment when interpreting the data
they have collected.
It is also interesting to note that the magnetometer, which has been one of
the most popular instruments for archaeogeophysical research abroad, was
only used sporadically in Sweden before 2005. Magnetometers have
probably been available at the geophysical departments of many Swedish
universities, but as collaboration between archaeologists and geophysicists
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has been sporadic, they have had a limited impact on archaeological
research. The same is true for geophysical equipment in general, but as
magnetometers have not been available for rental in past years, this may
have been an extra factor restricting the use of the method. As the lack of
geophysical instruments has been an obvious problem, it has also been
difficult to educate archaeologists in the use of these methods. Students with
an ambition to learn about archaeological geophysics have had to attend
courses elsewhere in order to achieve the proper competence. Education in
geophysical prospection methods has been available at the geophysical
departments in Uppsala and Luleå, and collaborations with archaeologists
has taken place in the past 10-20 years (see Sträng 1995; Masters 1998;
Wedmark 1999; Karlberg & Sjöstedt 2007), but in many cases the sampling
strategies, sizes of the areas surveyed and the visualisation methods in these
reports have not been adapted to the archaeological application and the
authors have failed to produce any easily interpretable or clear results (e.g.
Hedberg et al. 1996; Wedmark 1999; Karlberg & Sjöstedt 2007). At present,
the only section of a Swedish academic archaeological department offering
the possibility of working with archaeological prospection methods such as
GPR, magnetometry and the EM38 is the Archaeological Research
Laboratory at Stockholm University, where the availability of
instrumentation has recently been improved partly through collaboration
with the Royal Institute of Technology in Stockholm.
The necessary level of education is also a recurring question when
discussing archaeological geophysics in Sweden. As Sweden lacks any
guidelines regarding the qualifications of survey personnel, one has to look
at the requirements in other countries. A primary source of information
regarding the use of archaeological geophysics is the English Heritage
Guidelines (David et al. 2008), where this issue is dealt with under
subsection 2.8, which states that the project leader, i.e. the archaeological
geophysicist, must have: “competence in basic metric survey procedure;
experience in a supervised capacity of at least 30 different site surveys, or a
minimum of three full years’ supervised experience of archaeological
geophysics, and a degree in archaeology and/or an appropriate science (e.g.
MSc in Archaeological Prospection).” (David et al. 2008:5).
This topic is also discussed by Ernenwein and Hargrave (2009): “The
least effective and least responsible approach is to attempt to learn
geophysics oneself, relying exclusively on the manual, while doing one’s first
“real” survey. Everyone’s initial surveys are likely to include some poor
quality data, sub-optimal data processing, and possibly ill-founded
interpretations. It is likely to be detrimental to a customer’s view of
geophysics in general and oneself in particular to try to “learn on the job”
without the support of a competent instructor.” (Ernenwein and Hargrave
2009:98).
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It is thus recommended that anyone planning to commission a geophysical survey should make sure that the company selected for the job can
provide the necessary credentials, as this will be a guarantee that the survey
and its interpretation are carried out in the best possible way. Novice users
are advised to get a proper education before starting to collect data. In this
regard the concept of the Learning Curve presented by Schurr (1997) could
be an important starting point alongside the introduction to archaeological
geophysics presented by Ernenwein and Hargrave (2009).
6.1. The Swedish situation and beyond
It can be stated as a result of e-mail correspondence with many
archaeologists who have used archaeological geophysics in projects in
Sweden that the majority of them are interested in the possibilities offered by
the methods and in seeing more examples of geophysical surveys carried out
in different Swedish environments and geological settings before deciding if
and when to re-evaluate the usefulness of the methods. Many also believe
that Sweden is unsuited in its geology and pedology for geophysical
prospection and this being the case, it was my hope that the present thesis,
with examples of successful surveys performed using a range of methods
and in different geological and pedological settings, could put forward some
additional evidence to archaeologists who may be hesitant about using
geophysics. From a geophysical point of view, the effort of trying to prove
the method’s reliability in different areas of Sweden might seem strange, as
the methods have been shown to work abroad and one would assume that a
quick glance at a geological and pedological map would indicate areas that
are suitable for prospection. However, as most Swedish archaeologists were
relatively unexposed to the use of geophysical prospection methods before
the middle of the last decade, and as several of the archaeologists’ first
contacts with the methods may have been disappointing, it is reasonable to
think that acceptance will take some time and that more positive examples
are needed. This is similar to the experience in other countries as well, so
that in the U.S.A., for example, there has been a gradual increase in the use
of these methods in archaeological projects over a number of years. On a
positive note, one can see that the use of geophysical prospection methods,
as a valuable tool for the initial assessment of archaeological sites, is
recommended in the Swedish National Heritage Board’s guidelines for
contract archaeology15. It is to be hoped that such statements will lead to a
more frequent involvement of archaeological geophysics during
archaeological investigations in Sweden, as has been the case in England.
Another factor influencing the likely use of the methods is cost. The extra
cost will in some cases be a limiting factor. When discussing this with some
15
http://www.raa.se/publicerat/varia2012_30.pdf
89
of the field archaeologists that I have been in contact with during my time as
a PhD student, the price of the surveys has been a matter of constant
concern. The change from a crossline transect spacing of 0.5m to 0.25m, for
example, or from a crossline transect spacing of 1m to 0.5m, will increase
both the time spent in the field and the cost of the survey. Are Swedish
archaeologists ready to pay that extra money to get more accurate and easily
interpretable results? I would like to think that this cost, when compared
with the cost of an archaeological excavation, would turn out to be quite
small, and would be bearable if the geophysical results provided were clear
and accompanied by a solid interpretation that is readily understandable and
usable for archaeologists.
All things considered, is the Swedish situation unique? The answer to this
question is definitely no, as the small survey of the use of the methods in
different countries provided in chapter 2 has shown. Many countries are in
the same situation as Sweden and facing the same challenges, and Scotland
and the other Nordic countries share many of the same geological and
pedological characteristics that sometimes make geophysical surveying
challenging. The first surveys and trials using archaeological geophysical
prospection in Sweden were carried out quite late relative to the situation in
other countries, and this may have had an impact on the development of the
discipline here. It should be remembered, however, that although countries
like Denmark and Norway started using the methods earlier than Sweden,
they only use them sporadically nowadays, so that other reasons for their
limited use must be sought. In this regard, the geology and pedology of the
countries concerned and the character of their archaeological remains may
have been two of the major causes of the failure of many surveys in the past.
When no specialised education is available for surveyors, who have to learn
to use the methods on the job and without a full understanding of the
underlying principles of operation or of the interpretation of geophysical
data, the differentiation between anthropogenic remains and traces caused by
the geology and pedology of the area concerned becomes difficult. This
might explain the lack of success and ultimately the cause of a lot of the
disappointment felt among archaeologists involved in such projects. The
solution is to initiate specialised courses of university-level training,
combined with shorter courses for archaeologists interested in learning about
the basic principles of the methods and how to make the best use of them in
different situations.
6.2. Brief guidelines for archaeologists
Taking the above-mentioned factors into consideration, archaeologists
commissioning geophysical surveys need to make sure that:
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• The archaeological remains are expected to give measureable
contrasts with the surrounding soil matrix in terms of physical
properties.
If such as contrast is not expected, a geophysical survey should not be
recommended.
• The appropriate geophysical methods are chosen for detecting the
presumed archaeological remains.
As shown in chapter 3, many of the methods measure different geophysical
parameters, some of which may be relevant to the question in hand and some
not. Often companies selling these services only use, or own, one of the
many available methods and instruments, and as a consequence the
archaeologist should seek quotations from different companies to make sure
that the optimum method or methods will be chosen for the specific project.
Many methods provide complimentary information, and some projects could
benefit from an integrated approach. An especially successful combination
of methods seems to be GPR and magnetometry, but earth resistance or
resistivity can provide an alternative to GPR where the terrain or other
factors restrict the use of the latter. Other combinations could definitely be
valid, as seen in the case of the mountain tundra survey presented in Paper
II, and care should therefore be taken to choose suitable methods depending
on the archaeological, geological and pedological preconditions at each site.
• The data should be collected at a sufficient sampling density for the
detection and description of the archaeological remains in question.
Let the sizes of the expected features govern the sampling density and make
sure that this density allows the smallest archaeological features to be
resolved to be covered by at least two consecutive transects. This also
involves choosing the appropriate antenna frequency when surveying with
GPR. As GPR results are a trade-off between depth and resolution, this is an
important decision to make in order to be able to reveal features of a certain
size at certain depths.
• The results should include a thorough interpretation of the data and
should be interpreted by someone with training in archaeology and
experience in the geophysical methods used and the expected
archaeological situation, or through collaboration between an
archaeologist and a geophysicist.
Many reports of geophysical surveys that are available online do not contain
realistic geophysical or archaeological interpretations of the data and are of
91
limited value. The goal of any geophysical survey should be the
archaeological interpretation, as mentioned above, and it is only at this point
that the data will become meaningful to the archaeologists involved in the
project.
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7. Conclusions and future prospects for
archaeological geophysics in Sweden
Archaeological geophysics can provide answers to a range of archaeological questions. As the studies from Gotland and the mountain tundra region presented in Papers II and IV show, it can be used as a means of detecting the presence or absence of archaeological structures and features in
an area. But it can also be used for detailed studies of both Iron Age and
Medieval archaeological remains, as a knowledge of the spatial layout of
buried constructions is valuable for answering specific archaeological questions. It has also been shown that geophysical prospection, as at the
Dominican Convent in Sigtuna, can connect information from widely
separated trenches and put these results into a meaningful context.
Furthermore, it is possible in surveys of large areas, such as those carried out
by the Ludwig Boltzmann Institute at Björkö, Gamla Uppsala and Uppåkra,
to connect archaeological information on an even wider scale and thereby
enable more far-reaching conclusions about the archaeology of entire
landscapes. Especially suitable methods for geophysical prospection in
Sweden are magnetometry and GPR, but electrical resistance or conductivity
measurements could be an alternative to GPR on steep slopes or in highly
conductive soils.
Geophysical surveys cannot provide the answer to all archaeological
questions, however, and use of these methods might be limited in some areas
by the geology, pedology and/or former land use, for example. Factors such
as a lack of properly educated surveyors and the unavailability of
geophysical instruments, in combination with sceptical archaeologists, may
also have contributed to the low frequency of use of archaeological
geophysics in Sweden. Many areas exist in Sweden, however, that are
eminently suitable for geophysical surveys, and by making careful
predictions as to the likely success of certain methods, based on a thorough
knowledge of geological, pedological and land use factors and the character
of the expected archaeological remains, the information extractable from
such surveys can be increased dramatically. This includes tailoring the
sample density of each geophysical survey according to the sizes of the
expected archaeological remains.
Some areas in Sweden and certain kinds of archaeological remains are not
suitable for the use of geophysical surveys, and an equally important task for
a geophysical prospector must be to give the archaeologists responsible
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advice on looking for other solutions where appropriate. This would make it
possible to avoid many disappointing results. In some cases a combination of
trial trenching and archaeological geophysics might be a sensible solution,
and in other cases the use of one of these methods might suffice. The nondestructive character of a geophysical survey makes it an especially
attractive possibility at sensitive sites.
In order to increase the use of geophysical methods in Sweden,
professional archaeologists should be made aware of the pitfalls and
possibilities of the methods, which implies an even more urgent need to
address the educational situation in the country. More specialised education
in archaeological geophysics, and shorter and more general courses in the
subject aimed at archaeologists working with various excavations
companies, county boards and universities, would be desirable. An
additional and important step towards increasing the general awareness of
geophysical methods and how to implement them in archaeological projects
might be to formulate official Swedish guidelines, similar to those presented
by English Heritage, on how to use archaeological geophysics. This would
ensure that Swedish archaeologists constantly have updated
recommendations to turn to. It is also suggested that a website should be
constructed where companies working with archaeological geophysical
surveys in Sweden can get present data from their past surveys together with
their CVs, publication lists and contact information, as this would make it
easier for archaeologists or contractors interested in commissioning
geophysical surveys to make educated estimates of the expected quality of
the results, the price of the survey, the expected data density and the quality
of the subsequent interpretations. In the meantime, it is my hope that this
thesis could provide a point of departure for a renewed discussion on
archaeological geophysics in Sweden, a discussion which would place the
remnant echoes of the past as collected by geophysical methods in a
meaningful archaeological context.
94
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