The Use of GPR in Road Rehabilitation Projects

The Use of GPR in Road Rehabilitation Projects
The Use of GPR in Road
Rehabilitation Projects
Mara Nord Project in cooperation with:
European Commission, Finnish Transport Agency,
Swedish Transport Administration, Norwegian Road
Administration, ELY center, Lapin Liitto, North Calotte
Council, Rovaniemi University of Applied Sciences, Oulu
University of Applied Sciences, Roadscanners, Road
Consulting,
Carement,
Ramböll
Sweden,
Malå
Geoscience, SwedIsh National Road and Transport
Research Institute, SINTEF, 3D Radar, NCC Roads
The Use of GPR in Road Rehabilitation
Projects
Contents
1.
0BIntroduction .......................................................................................................................................... 5
2. Ground Penetrating Radar (GPR) Technology.......................................................................................... 6
6B2.1 General ................................................................................................................................................ 6
7B2.2 Electrical properties affecting GPR wave propagation ....................................................................... 6
2.2.1 Dielectric value ............................................................................................................................. 6
8B2.2.2 Electrical conductivity .................................................................................................................. 6
2.3 Principles of different GPR systems .................................................................................................... 7
9B .................................................................................................................................................................. 7
2.3.1 Impulse radar ............................................................................................................................... 7
10B2.3.2 Stepped frequency radar principles ............................................................................................. 8
1B2.4 Principles of GPR image ...................................................................................................................... 8
1B2.4.1 Reflection and polarity ................................................................................................................. 8
12B2.4.2 Depth penetration, resolution and interface depth .................................................................... 9
13B2.5 Ground Penetrating Radar in road surveys......................................................................................... 9
2B3. Survey Equipment ................................................................................................................................... 10
14B3.1 General .............................................................................................................................................. 10
15B3.2 Ground penetrating radar quality requirements .............................................................................. 14
3B4. Survey Planning and Performance .......................................................................................................... 15
16B4.1 Measurement timing, planning and information ............................................................................. 15
17B4.2 Implementation of measurements ................................................................................................... 16
18B4.3 Digital image capture during GPR road surveys................................................................................ 21
19B4.3.1 In general ....................................................................................................................................... 21
34B4.3.2 Digital video or still image capture equipment and data collection..................................... 21
20B4.3.3 Video data processing and linking to software projects ................................................................ 23
21B4.4 GPR surveys and reference coring .................................................................................................... 23
4B5. GPR Data Processing and Interpretation ................................................................................................ 24
2B5.1 In general .......................................................................................................................................... 24
23B5.2 Preprocessing .................................................................................................................................... 24
24B5.3 Air coupled antenna data processing and interpretation ................................................................. 25
25B5.4 The ground coupled antenna data processing and interpretation ................................................... 29
35B5.4.1 400 – 600 MHz ground coupled antenna data ....................................................................... 29
26B5.4.2 50 – 200 MHz ground coupled antenna data ................................................................................ 31
5B6. Reporting and Delivery of Results ........................................................................................................... 32
27B6.1 General .............................................................................................................................................. 32
28B ................................................................................................................................................................ 32
29B6.2 Printing to an image file .................................................................................................................... 32
30B ................................................................................................................................................................ 33
3BLITERATURE ............................................................................................................................................. 33
PREFACE
Ground Penetrating Radar (GPR) is a non-destructive ground survey method that can be used in
assessing roads, railways, bridges, airports, tunnels and environmental objects. Its main advantage is the
continuous profile it provides over the road structure and subgrade soil. GPR technique is becoming an
increasingly important tool especially for planning structural evaluation of roads. Another important
advantage of GPR in road surveys is that it does not essentially disrupt the flow of traffic during the
survey.
The Nordic countries have reached good quality of skills and accumulated the knowhow of GPR
applications on roads over the last 15 years. However Finland, Sweden and Norway have slightly
different practices in the use of GPR and that is why there is a need for common procedures now when
companies are more and more operating across the borders. Also the level of knowledge, awareness and
experience regarding the use of GPR in the Road Administrations vary in all three countries and there is a
need to share the knowledge and develop procedures to ensure better quality of GPR services. To
respond to these recognized needs Mara Nord, international cooperation project financed by Interreg IV A
Nord, has been initiated among Finland, Sweden and Norway. In this project one goal was to produce
common guidelines that can be used as a reference in procurement processes in all three countries.
Johan Ullberg from Swedish Transport Administration has been in charge of the guidelines. Mara Nord
project lead partner has been Rovaniemi University of Applied Sciences and experts participating have
been Anita Narbro and Janne Poikajärvi. Other key experts have been Katri Eskola from Finnish
Transport Agency, Kalevi Luiro from ELY-centre of Lapland, Finland, Per Otto Aursand and Leif Bakløkk
from Norwegian Public Roads Administration, Rauno Turunen from Oulu University of Applied Sciences,
Finland. This recommendations for the guidelines has been written by Timo Saarenketo and Pekka
Maijala from Roadscanners Oy, Finland.
Rovaniemi April 25th 2011
Timo Saarenketo
Pekka Maijala
1.
Introduction
0B
Nordic countries have been pioneers in ground penetrating radar (GPR) applications on roads. The first
applications have been reported already in early 1980´s from Denmark and Sweden. In Finland subgrade
soil and road structures have been analyzed with GPR already for almost thirty years and the method is
a routine tool in Finnish Traffic Agency’s rehabilitation projects.
This GPR method description guideline for road rehabilitation projects has been made due to new
procurement policies in Nordic road administrations which means that road condition surveys will be
ordered through open competition. That is why it is essential that current practice is described precisely
to make it also possible for new entrepreneurs to know how and at which level the ground penetrating
radar surveys should be done and what is the default quality level for the results. There is also a need to
specify separately the procedure that should be followed in surveys projects made in 2 dimensional (2D)
and 3 dimensional (3D) GPR projects.
This method description guideline should be used when doing data collection with the 2D and 3D GPR
systems prior to rehabilitation planning. It should also be used when modifying output data of the
interpretation and problem diagnostics on a compatible format that can be read with GPR data viewer
software packages. A description of the testing arrangements and antennas accreditation is given as
well. In this publication, there is also an exact description of how ground penetrating radar data
collection should be done, how the collected data should be processed and interpreted and how the
outcome should be linked to road registry data bases or other GIS systems used in Nordic Road
Administrations. Pertaining to the parts and methods not described in this publication, the other
national publications and guidelines related to road rehabilitation projects must be followed.
Digital video or digital images collected during the GPR data collection have proven to be essential
during GPR data analysis and interpretation and in quality assurance of GPR projects. In order to
emphasize the importance of this the description how to collect the images or video(s) and the required
quality of these are described in this publication.
This publication presents also a short introduction to GPR theory and principles. Guidelines are written
to meet the general guidelines and practices in Nordic region but this document can be also used
elsewhere in the world. The use and guidelines of GPR technology in other road and bridge applications
is described in other documents published by MaraNord project.
2. Ground Penetrating Radar (GPR) Technology
2.1 General
6B
Ground penetrating radar method is based on the use of radiofrequency electromagnetic (EM-) waves.
Used frequency range is 30 – 3000 MHz. Inside of this frequency range, it is said that EM- waves can
propagate in a low electrical conductivity medium. Physical parameters affecting the wave are the
medium’s conductivity, dielectricity and magnetic susceptibility. In Nordic countries, magnetic
susceptibility does not have a significant effect on ground penetrating radar signal’s behavior.
2.2 Electrical properties affecting GPR wave propagation
7B
2.2.1 Dielectric value
Dielectric value describes substance’s ability to charge or polarize from influence of an electric field.
After the electric field’s effect ends, the substance returns to its’ initial state. If the material’s structure
is such that its’ initial state does not return completely, its polarization is partly lossed. In such cases,
dielectric value can be considered as a complex quantity, where the real part describes reversible
polarization and loss is in imaginary part. The most important element in molecular polarization in road
materials and subgrade soil is the water molecule. The extent of dielectric value depends most on how
much free water there is in the material because the dielectric value of water is more than 10 times
higher than the dielectric value of other road materials. Therefore increasing water content increases
the dielectric value of the material.
8B2.2.2 Electrical conductivity
A medium’s electrical conductivity describes the ability of free charges to move in the medium. External
electric field’s move charges from place to place. The more free charges, ions and electrons there are,
the higher the conductivity of material and ground penetrating radar attenuation.
2.3 Principles of different GPR systems
9B
GPR hardware systems used in road can be divided into two categories; impulse radars and stepped
frequency radars. The principles of these system are described in the following.
2.3.1 Impulse radar
Impulse radar is the most used ground penetrating radar type. The working principle is described in the
following. A pulse, generated in a transmitter antenna, is sent into the medium. The length of the pulse
is from under a nanosecond up to tens of nanoseconds, depending on the frequency. When propagating
in road structures and subgrade soil a part of the pulse energy is reflected from surfaces of different
electrical properties; some of them are propagating through the interface and are reflected back from
the following interfaces. How the signal attenuates is a result of geometric attenuation, signal
scattering, reflections and thermal losses. GPR system records the two way travel time and amplitudes
of signal reflections as a function of travel time is presented. When measurements are made rapidly in
sequential survey points, it can be compiled as a GPR profile (radar image).
In the Nordic ground penetrating radar projects , the gray scale tone and a reflector polarity in the
printouts should be like in Figure 1.
Figure 1. Ground penetrating radar profile, which is measured with the air coupled antenna and its individual pulse.
The profile has reflections from interfaces of two mediums with different dielectric properties (ε). Image structure’s
layer 1 describes the pavement, layer 2 describes base course, layer 3 sub-base and layer 4 filter course. The figure
shows that dielectric value of materials (moisture) grows when going down from the road surface with an
exception that dielectric value of filter course (ε4), is less than in the sub-base and its’ reflection has inverted
polarity (black line in the middle of two white lines).
10B2.3.2 Stepped frequency radar principles
In a stepped-frequency radar the radar waveform consists of a series of sine waves with stepwise
increasing frequency. The radar measures the phase and amplitude of the reflected signal on each
frequency and used an inverse Fourier Transform of these data to build a time domain profile. Thus, the
step-frequency radar collects data in the frequency domain and converts the data to time-domain data
through computer processing. The resulting radargram is similar to impulse radar data, and they can be
processed and interpreted the same way as impulse rada data. However, since stepped-frequency data
are collected in the frequency domain, they allow advanced filtering and signal processing to be applied
directly to the raw frequency domain data.
2.4 Principles of GPR image
1B
1B2.4.1 Reflection and polarity
When GPR signal propagates from medium 1 to medium 2 and medium dielectric values are E1 and E2,
the reflection coefficient will be:
R
1   2
1   2
On the basis of the formula, polarity of the reflection changes if E1 is smaller than E2, which is usually the
default situation in road and soil structures (moisture content is getting higher when getting deeper). If
E1 is bigger than E2, then the polarity of the reflected wave remains the same as the progressive wave’s
polarity at the interface. In road radar measurements, however, it is common practice that the surface
reflection is recorded as positive, even though the reflection coefficient is negative. Similarly, the other
layers, where ε(upper) < ε(lower), are recorded as positive reflections. In the gray scale they should be
presented in such a way that the white reflection is in the middle (see Figure 1. ε1 < ε2). Correspondingly,
if the dielectric value of the lower layer is smaller than the upper, for instance in the structure of Figure
1 ε3 > ε4 , the reflection is so-called negative and then the black reflection is in the middle. GPR signal
polarity, leaving the antenna and progressing in the medium, can be changed 180 degrees easily by
changing the positions of the transmitter- and receiver antenna or when doing GPR data post processing
by multiplying the signal with a factor -1.
12B2.4.2 Depth penetration, resolution and interface depth
Achievable depth penetration with ground penetrating radar depends on what antenna frequency is
used and therefore the wave length of signal. The attenuation increases when GPR central frequency
increases. A high conductivity of medium results in an increase in the amount of energy scattering
objects, when the length of wave gets shorter. Similarly, the penetration depth gets smaller while the
frequency gets higher. On the other hand the resolution gets better at the same time. The resolution
gets better also when dielectric value increases.
Resolution refers to how close interfaces can be from each other, when they can still be identified as
separate interfaces. This applies to both directions, horizontally and vertically. The vertical dimension’s
resolution of the pulse can be calculated from the following formula:
h
 c
2  r
c

εr
,
where
=
=
=
the speed of light in vacuum (0.3 m/ns)
the pulse length (ns)
medium’s relative dielectricity
The depth to an observed interface can be calculated from this formula (for monostatic antennas):
s  vt 
0.5  twt  c
twt
r
=
, where
two way travel time of the wave.
2.5 Ground Penetrating Radar in road surveys
13B
In Nordic countries there are at the moment tens of people employed or involved with GPR, working for
universities, consulting companies and for contractors. The use of ground penetrating radar has spread
from subgrade investigations and structural layer and thickness measurements to the structural layers’
and subgrade quality investigations. One of the biggest advantages of 2D ground penetrating radar
surveys is its ability to produce continuous line data from the investigated target. And when using 3D
GPR system the changes in road structures and their properties can be measured also in road cross
section.
During last few years ground penetrating radar technology has been applied to other types of research
as well. Pavement quality assurance and structure thickness quality assurance surveys have been
applications that are coming increasingly popular. Bridge deck condition assessment and monitoring has
developed rapidly in recent years.
3. Survey Equipment
2B
3.1 General
14B
Most of GPR equipment used in road investigations are with pulse radar principles as previously
described. Equipment for road investigations consists of several components (Figure 2). Antennas
consist of a transmitter, which transmits the pulse to the medium and a receiver that receives the
reflected signals. The antennas are controlled by a control unit, where the wavelength and strength of
pulse are determined. In digitizers the received pulses are changed into digital format. Digitizer can be
located in the antenna or in control or central processing unit. The data collection setup is controlled
with a data collection software. Some setup parameters include scans per time or distance unit (for
instance scan/sec, scan/m), measuring time window (ns), the number of the samples per scan ( for
instance 512, 1024 samples/scan) and the data format (for instance 8, 16, 32 bit). Calibrated optical
encoders (distance measuring instruments) are used to trigger the control unit as the systems moves
over a distance. Nowadays GPS is also an essential part of GPR data collection and this also applies with
digital video and still image capture process. A power supply is of course necessary for operation of the
equipment.
Figure 2. Typical instrument configuration of the ground penetrating radar van, which is used in road
U
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surveys.
Ground penetrating radar antennas can be divided roughly into two categories: air coupled antennas
and ground coupled antennas (Figure 2). These in turn can be either monostatic, when the same
antenna is a transmitter and a receiver, or bistatic, when transmitter and receiver are different antennas
units. Most road survey antennas are bistatic, but the antenna elements are built in the same box.
The frequency of ground coupled antennas in road rehabilitation surveys vary from 50 MHz to 2500
MHz. Their advantage compared to air launched antennas is better depth penetration, though ringing
(caused mainly by strong coupling ), may cause interferences. The ground coupled antennas have better
resolution of individual objects than the air launched antennas. A summary of different antenna
qualities is presented in Table 1.
Table 1.Different ground penetrating radar antenna qualities in road rehabilitation surveys. The table depicts
U
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general characteristics and it must be remembered that depth penetration and resolution depends on electrical
properties of a road structure and sugrade and it should also be noted that the resolution of wide band antennas
close to the surface is significantly higher than the antenna center frequency would suggest.
A n t e n n a C e n t r a l D e p t h - Resolution A p p l i c a t i o n s
Frequency
penetration (m) (mm)
(MHz)
Ground
(εr 6)
coupled
antennas
1 high frequency
900 – 2500
0,4 – 1,0
40 – 65
pavement, base course, sub base,
steel net
2 medium frequency
400 – 600
1,5 – 4,0
80 – 150
base course, sub base, overall
thickness of structure layers, low
embankments, subgrade soil <
3m
3 Low frequency
50 - 200
3 - 30
250 - 500
overall
structure
thickness,
embankment, subgrade soil < 20
m (excl. clay and silt)
Air coupled antennas
1 high frequency
2000 – 2500
0,4 – 0,6
15 – 25
gravel road wearing course. ,
pavement, steel net, base course,
subbase
2 medium frequency
900 – 1200
0,5 – 1,0
35 – 50
pavement, base course, sub base,
steel net
3 low frequency
400 - 600
1,0 – 3,0
60 - 100
base course, sub base, overall
(not yet in use in
structure thickness, subgrade soil
Finland)
< 3m
air- 200 – 30001
Broadband
1,0 – 3,0
30 - 100
gravel road wearing course. ,
coupled
antennas
pavement, steel net, base course,
(for
stepped-
subbase,
frequency radar)
overall
structure
thickness, subgrade soil < 3m
Air coupled antennas are mainly horn antennas, but lately also other antenna types have been
developed. Frequencies vary from 400 MHz to 2.5 GHz, but the most commonly used central frequency
is 1.0 GHz. Also 2.0 - 2.2 GHz air coupled antennas are also available in the market. These antennas have
proven to be effective, for instance, in detecting individual layers in bitumen bound layers, in gravel road
wearing course thickness surveys and in locating steel nets. Normally the depth penetration of air
coupled antennas is 0.5 – 0.9 m and that is why they are mainly used in pavement structure and in
bridge surveys. During the measurements, air coupled antennas are typically 0.3 – 0.5 m above the
pavement surface. Since pulse radar air coupled antennas are suspended in the air, the antenna’s
coupling does not change when electrical properties of the road surface change. That is why these
antennas can also be used in repetitive measurements, where the ground penetrating radar data quality
should not change if and when the properties of the road surface are changing. This makes possible to
calculate quality parameters on the basis of reflection amplitude and frequency response. In addition,
air coupled antenna data collection can be done using survey speed up to 80-100 km/h and thus
without distracting the other traffic.
At the moment there are several device manufacturers, which produce GPR equipment suitable for road
surveys. The biggest ones are Geophysical Survey Systems, Inc. (GSSI), IDS (Italy), Mala Geoscience
(Sweden), 3d-Radar A/S (Norway), Sensors&Software (Canada), Utsi Electronics (U.K), Penetradar
(USA), Radarteam Sweden Ab (Sweden), Geoscanners AB (Sweden). In addition, there are also several
smaller companies involved in the business.
1
Frequency range for stepped frequency radar is specified as minimum and maximum edge frequencies instead of
centre frequencies. This means that a 100 – 3000 MHz antenna spans approximately the same frequency range as
a combination of a 400 MHz, 900 MHz, and 1500 MHz antenna.
3.2 Ground penetrating radar quality requirements
15B
In Europe all Ground Penetrating Radar hardware systems sold have to be approved by ETSI and their
use requires further ECC approval. At the moment in Finland all GPR units have to be registered and
licensed in Ficora before they are allowed to be used. Ficora licensing provides also some extra
instructions and restrictions for the use of GPR in Finland. For instance GPR survey log book has to be
kept where each survey time, place and survey equipment info is documented. Similar GPR registration
and licensing system will be most likely applied also in Sweden and in Norway within the next few years.
In addition, when doing GPR measurements for the Finnish Transport Agency pavement quality control
surveys, only tested and approved equipment can be used. If the equipment is not annually approved, it
cannot be used. Measurements performed with improperly working equipment must be repeated with
the properly approved equipment. The annual quality tests for air coupled antennas (1.0 GHz) are
described by Scullion et al. (1996). These tests must be passed every year. Tests are done in approved
laboratories, which are impartial to parties involved. The Finnish Road Agency publishes a list of the
approved laboratories every year. Measurement licenses are issued to companies with approved
equipment.
After ground penetrating radar tests, the antennas are classified in categories A and B and the
requirements are presented in Table 2. If antennas are in category A, they can be used as is. If it is in
category B, they can only be used in specified surveys and with specified settings mentioned in the
license. When surveying category B equipment, the complete straight pulse amplitude should be
recorded, making it possible to track interferences and noise level. In these tests, time linearity needs to
be done only once for each antenna, all other tests must be performed annually.
Table 2. Mandatory Ground Penetrating Radar system quality requirements in the Finnish Road Agency
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asphalt quality control surveys.
Tests
Classification A.
Classification B.
No restrictions in FINRA projects
Can be used following specific rules and conditions
signal/noise -ratio
5%
10 %
short time amplitude stability
1%
3%
long time amplitude stability
3%
6%
long time stability
5%
10 %
linearity of time
5%
7,5 %
Ground coupled antennas will also have their own quality standards in the future.
4. Survey Planning and Performance
3B
4.1 Measurement timing, planning and information
16B
Surveys with ground coupled antenna systems can typically be made any time of the year, although
there are some restrictions. In areas where there is frost, measurements should be made either before
or after the frost has melted from the subgrade soil, but preferably not during thawing periods. It is not
recommended that gravel surface roads are measured in the summer without client’s permission,
because salt used to reduce dusting attenuate the signal. Winter surveys provide additional information
about the frost depth. If the client demands, the surveys can be done in the summer, but then the client
is also responsible if data quality is poor.
When using air coupled antennas in the winter, measurements should be when the frost front is deeper
than 0, 8 m from the road surface. On paved roads caution should be taken with winter measurements
as when deicing salt is causing signal attenuation.
In pavement and pavement structure surveys it is recommended to use air coupled antennas in the
summer time especially when the interest is in bituminous pavement thickness and its quality, unbound
base course and its quality, wearing course of the gravel roads and its quality. In this case the dielectric
value can be used to determine the water sensitivity of the wearing course.
Before the surveys, a revised plan should be sent to the client´s contact person, describing when and
how the measurements are intended to be done (longitudinal sections and cross sections),
measurement equipment and personnel. The note can be sent via mail, e-mail, or fax or if this is not
possible, then taken care of over the phone.
Safety considerations for the ground penetrating radar measurements are described in a road works
safety plan required by each Nordic road agency. But in general survey crews have to have accepted
national road work safety course documents and also each country require traffic safety plan and
advance notice for GPR works especially on high volume roads.
Nordic government authorities may require also radio license for the use of each GPR equipment in the
country. These national guidelines may define also special geographical areas, such as airports,
astronomical survey stations, hospitals, prisons or defense force area, where the use of GPR is
prohibited or a special license is required.
GPR equipment, if working properly, will not cause
interferences but broken equipment can possibly cause false alarms.
4.2 Implementation of measurements
17B
The ground penetrating radar hardware must be safely and securely mounted to the survey vehicle and
the measurements should be conducted in a way not to endanger the workers, road users or anyone in
close vicinity of the road. The survey vehicle must be equipped with nationally accepted warning lights
and safety signs. To ensure GPR data quality and for traffic safety reasons there should be always two
persons in the survey car, one focusing on driving and the other running and controlling the
measurement devices. The driver can make comments from the road’s condition, if necessary. The
measurement staff should have completed necessary local road safety courses (Finland: Tieturva I,
Sweden: Arbete på Väg, Norway: Kurs i arbeidsvarsling).
2D GPR surveys can be made with a multi-channel system where it is possible to measure with air
coupled and ground coupled antennas at the same time. In this case, the data can be collected for
example with a 1.0 – 2.5 GHz air coupled antenna and a 300-600 MHz ground coupled antenna. Ground
penetrating radar antennas can be sequentially or side by side. The basic principle is that the
measurement is done in outer wheelpath in the main direction of the road. If the antennas are side by
side, the air coupled antenna should travel on the right wheel path and ground coupled antenna in the
middle of the lane. If the measurements are done with two separate runs but with the exact same start
and end point, it must be ensured that the data later is scaled so that the lines are exactly the same
length in case there are differences in the driving path. Different measurement profiles can be matched
with each other using culverts and/or bridges as a marker. The maximum longitudinal difference is 2.5 m
presented in the data with individual objects.
On special occasions, such as roads with geotechnical problems, the client can order separate GPR
surveys with 100 – 200 MHz antennas. Usually there is need for better depth penetration, for example
to map the bottom of a peat layer.
Pavement surveys made with the air coupled antennas gather information from the pavement’s and
base course’s dielectric values. When using air coupled antenna, antenna bouncing (distance from
pavement surface) can provide some information about the roughness of the road. Ground coupled
antennas should be used to for example, in road cross section measurements.
To minimize the changes in coupling of the ground coupled antenna, the distance from the road surface
should not change. With 300 – 600 MHz antennas, distance from the road surface may not exceed 8 cm
and with 1.0 GHz or higher frequency antenna the distance may not exceed 5 mm. In practice this
means that the antenna should dragged along the pavement surface.
For scan interval, a very accurate and calibrated pulse encoder should be used. Time-based sampling
should not be accepted, except in cross section measurements, where lines have to be painted on the
road and data has to be matched to them using markers. In final result delivery, no time-based data is
allowed; all data has to be distance based.
The amount of survey longitudinal survey lines depends on the information needed from the road.
Table 3 presents different types of GPR surveys on road that can be used in different kinds of projects.
Table 3. Different kinds of 2D and 3D surveys on roads and their characteristics.
*) depends on the frequency
The sampling interval in GPR surveys should be at least 10 scan / m in longitudinal survey profiles and at
least 40 scans / m in cross section profiles. All the measurement data should be recorded in minimum 16
bit format and the smallest sampling density is 512 samples / scan. One-point gain (so called flat gain)
must be used with the air coupled antenna. Ground coupled antennas allow smooth multipoint gaining,
avoiding any visible differences in time window. Signal clipping is not allowed in any data.
Filtering can be done according to the manufacturer’s recommendation. These values should be the
same as used in antenna tests. Horizontal filtering should be avoided because it has a tendency to
remove also reflection information of horizontal structures from the road.
When defining time windows for ground penetrating radar surveys, the following principles should be
used:

Antennas (1.0 – 2.0 GHz): The time window should be at least 20 ns and the time window below
surface at least 12ns. Correspondingly antennas greater than 2.0 GHz, the measuring time
window should be at least 10 ns.

Antennas (300 – 600 MHz): time window below the surface must be at least 60ns.

Antennas (100-200 MHz): the measuring time should be agreed with the client case by case.
Here the expertise of GPR provider must be used to estimate the depth penetration of the
antenna and the depth of the wanted information. Table 3 shows the required time windows
(two way travel time) for different target depths and different soil types (different Er).
Table 3. The required time windows for different soil types and different target depths.
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target depth
twt (ns)
twt (ns)
twt (ns)
sand, gravel;
moraine;
peat;
Er = 5
Er = 9
Er = 40
3m
130
6m
100
120
250
10 m
150
200
450
20 m
300
400
Road number, road section or distance, direction and the survey personnel should be marked into the
header file. In addition, there should be a logbook in the survey vehicle, where time of the survey,
survey equipment, weather, possible problems and other possible notes are documented. In addition in
Finland Ficora requires that the following survey information is documented: the location, date, time,
unit serial number, antenna types and serial numbers used in each survey. In practice each central unit
has a logbook, where all the surveys are marked with required information.
Before starting a survey, the correct location must be ensured. Usually the surveys start and end in road
registry’s start and end points or nodes, even though the interpretations can be made from a shorter
section. The GPR files should be cut from the key points such as road data base knot points or road
section breaks . If the survey does not follow the road registry, the start and end point should be marked
with paint on the road, unless they are simple and unmistakable locations such as culverts or bridge
joints. When surveying new roads and pavements it is sometimes easier to follow project distance and
chainage.
The GPR operator can add markers in locations of intersections, bridges, culverts, etc. These markers
can be used to link the GPR file and video, unless linking has been done already during the survey.
Markers are also used, when two or more lines or lanes are surveyed and there is no GPS from all survey
lines.
Surveys should not be done when it is raining or when the road is wet. Survey speed should be constant
and stopping minimized.
When using air coupled antennas, after every survey session and before the GPR unit is switched off, a
metal pulse should be taken. It is recommended that metal reflection is done also before the start of
the measurement. In addition, if a height correction file is not available, a “bouncing file” should be
recorded. Figure 3 presents basic calibration tests that should be done when GPR data is collected.
When measuring with air coupled antenna, so-called direct pulse (from transmitter antenna to receiver
antenna) must be completely visible all the time. Correspondingly when measuring with the ground
coupled antenna, the surface pulse must always be visible. With ground coupled antenna a lift test files
or similar other test can be recorded to locate the pavement surface in the scan.
Figure 3. GPR antenna calibration tests before or after data collection: photo A. presents the lifting test
for a ground coupled antenna, photo B. the metal plate test for a horn antenna and photo C. the antenna
bouncing test for a horn antenna.
4.3 Digital image capture during GPR road surveys
18B
4.3.1 In general
19B
When the road authorities make a purchase order for GPR surveys, simultaneous digital video or still
images is strongly recommended to be required because it has proven to improve the quality of GPR
interpretations and avoid mistakes. If digital video is collected it should have an audio track, where oral
commentary of the road condition done by surveyor is recorded. Digital video or digital images help
road engineers, designers and contractors to get a better understanding of the road condition and its
surroundings and based on this data road surface conditions and the condition of the road assets can be
documented reliably.
4.3.2 Digital video or still image capture equipment and data collection
34B
The digital image(s) from the road can either be performed with a video or high class digital camera.
The mionimum requirements when using video camera should be at least 140*480 resolution, 12 fps
and progressive scan must be used. When collecting still images the maximum distance between images
should be 10 meters and minimum resolution 1024 *768 pixels.
The camera optics should be of high quality and the lens should be wide-angle. The lens should be clean.
Focusing should be made so that the area of interest is as precise as possible. Autofocus systems should
not be used.
The camera should be mounted horizontally and firmly to minimize shaking. It should be on the roof of
the survey vehicle, or as high as possible. Recording through car window should be avoided. The camera
angle should be such that on flat, open road section, the image is mostly road surface – the sky can be
seen only as a thin stripe in the top of the image. It is preferable that part of the antennas, would be
visible in the bottom of the picture, making exact location analysis possible.
The camera angle depends whether the measurement is ordered from one or both lanes. If images are
recorded from only one lane, the camera should be mounted in the left (centerline) side of the car and
the camera angle adjusted to show the whole road and surroundings widely. If the measurements are
made from both lanes, the camera should be in the outer side, so that the surveyed lane is visible, but
also the grass and soil verges and ditches.
Figure 4. Example of the two video camera setup. Camera A is describing a situation, when there is only
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one video from the road. B and C are describing situations where there are videos taken in both lanes.
The image capture should always be done in good light, so that details can be seen as well as possible. If
the ground penetrating radar measurements are made at night, for example, because of the traffic, the
video can be taken separately in the daytime.
The survey speed should be such that the picture quality is accurate, the recommended speed on an
average is 20 – 40 km/h, where there is also enough time to make reliable oral observations. In good
conditions slightly higher speeds can be used remembering the limitations of observation times.
During the recording, survey personnel should make comments about the roads condition and the
surroundings. The comments should be focused on the following subjects:

Is the road in a cut, embankment, side sloping ground or 0 level

Drainage conditions

Observed bedrock

Visible damages in the road surface, above all shoulder deformation, which is sometimes hard
to see from the video.

Estimates of the soil type

Other road design factors
The principle is that the commentary is continuous, but observations should be done at least every 200
meters. In the beginning of each recording, a short summary of the road, road section and for what
project must be announced.
The image capture should always be attached to a location (GPS coordinates, chainage), which allows
the data to be linked to software projects and other data.
4.3.3 Video data processing and linking to software projects
20B
The collected video will be linked to software project. In addition, if required, a subtitle including road
number, section and corrected road register address can be added. Video should be compatible with
Microsoft Windows and the data compression system must be such that the video compatible with
common media player software (for example Media Player 10).
Linking the video should be in a way that the clear GPR reflections match the video view and the
difference must not be more than 1 meter. Linking should match also with GPS coordinates. Preferred
linking method is that the video’s frame number is linked into radar data’s scan number, but also other
sufficiently precise linking methods are allowed. The principles for video linking should be presented in
the quote.
4.4 GPR surveys and reference coring
21B
In most Nordic GPR survey projects on roads, it is required that reference samples are taken to support
and confirm the interpretations. Sampling can be used also to define properties of pavement structure
layers in laboratory analysis.
There are three basic types of reference coring:
A. Pavement and base course
B. Pavement, bound and unbound base course
C. Pavement, bound and unbound base course, other structure layers and their total thickness and
the sub grade soil quality.
Normally reference coring should be made at 2-3 km intervals, with a minimum 1 / 10 km road section.
More samples can be taken if accepted by the client. Representative sampling points are decided after
preliminary analysis of GPR data. The preferred sampling point is the outer wheelpath, but exceptions
can be made, if accepted by the client, especially on high traffic roads. The exact reference core location
must be described in sampling documents.
From the reference cores, the thickness of pavement layers and bound layers are determined with an
accuracy of 4-5 mm, thickness of the unbound layers with 2 cm accuracy and thickness of other
pavement structure layers with 5 cm accuracy. If the sample is needed for laboratory analysis, it is
written in the quote. However, it is recommended, that at least the drill core samples from bound layers
are photographed and the photographs delivered to the client.
5. GPR Data Processing and Interpretation
4B
5.1 In general
2B
The major goal for the GPR surveys is to provide information for rehabilitation and pavement design. In
this case the most important thing is to produce precise information about a) the total thickness of the
pavement b) the total thickness of the base course, c) total thickness of the unbound layers, d) thickness
of the embankment or depth to the bottom of the embankment, e) the quality of subgrade soil and f)
road assets, such as culverts. In addition, the interpreter can produce other information needed in the
rehabilitation design as much as possible, such as reasons for damages and distress, other road
furniture, etc.
If the data collection has been carried out properly with appropriate equipment, the information
described above can be produced with careful and professional processing and interpretation. The
following chapters describe the general guidelines for data processing and interpretation.
5.2 Preprocessing
23B
In this case, preprocessing means GPR data editing and combing other road testing data, acquired in
different ways. Data editing includes operations which do not change the original information content of
the data. This means distance scaling, joining and splitting of different lines and reversing directions (if
needed).
If the surveys are ordered from the both lanes, they must be arranged in same direction with the lines
matching each other (for instance the culvert is in coinciding locations). If the lines do not match, they
must be scaled to match with markers and culverts.
Linking the distance coordinates means that the survey has a starting point and all the data is scaled to
this starting point with equal scale. This is suitable for situations, where surface coordinates are not
important. Even in these cases, it is useful if the altitude information is available. It is strongly
recommended to scale the GPR surveys to the road register length. If there is a big difference (>15m /
5km) between the announced registry length and the length of ground penetrating radar survey, it
should be discussed what is the possible reason for this and if the survey lines must be scaled to an
fictive length.
If the coordinates at certain points on a line are known, these points can be linked to the data.
Commonly the coordinates are collected simultaneously during the surveys, using real time GPS
systems. Data processing software combines all the information.
5.3 Air coupled antenna data processing and interpretation
24B
Data processing of air coupled antenna is described in Figure 4. More detailed process is described for
instance in GPR text books and software manuals made by different GPR manufacturers. In the first
stage, air pulse (scan 2) should be removed from raw data (scan 1), if there abnormal ringing below the
zero level (P-level). (Important notice: According to ECC/ETSI guidelines, when measuring air pulse,
antenna must not be pointed up to the sky) If the system and the antenna is high quality, the pulse is
practically straight after the surface level, it is not necessary to remove the air pulse and the processing
is made only using metal reflection data (scan 3). After the metal reflection removal, so-called surface
leveling (elimination of antenna bouncing) should be done and at the same time the dielectric value of
the first layer is calculated. At this stage, in many GPR software, the surface resolution is improved by
using different techniques. The use of these techniques is allowed, even desirable, because the
pavements in Nordic countries are mainly thin.
In the following, the basic principles for air coupled antenna data interpretation are shown. Instructions
apply to all other surveys except pavement quality (void) measurements.
Figure 5. The pulses used in the processing of air coupled antenna. S is so-called direct pulse and it is
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used as a reference level. P is the reflection from the surface. Pulse 2 is so-called air pulse, which
describes the reflections caused by the antenna elements themselves. Pulse 3 is a metal reflection pulse,
where the air pulse (background, pulse 2) has been already removed. Pulse 4 is the resulting pulse used
in the final interpretation. Pulse 4 is obtained by reducing the pulse 1 and part of the pulse 2 from the
pulse 3. Removing the pulse 3 will improve the surface resolution.
The maximum amplitude technique should be used in all interpretations, observing also the polarization
variations.
In the pavement antenna interpretation, the minimum interpretation (basic package) is as follows

the total thickness of bound layers (pavement or pavement + bound base) (code 21)

total thickness of unbound base course (code 41)
If the layer interfaces are weak and the interpretation is uncertain, the interpreter must use interface
quality codes published in “Ground Penetrating Radar” (1992), a booklet by Geotechnical Society of
Finland. These interface codes are:
a) a clear, strong and reliable interface (solid line)
b) unclear, but probable interface (a line longer than the interval)
c) unclear interface (interval longer than line)
If the interpreter cannot interpret the required interfaces, a separate report should be written to the
client clarifying the reasons of not succeeding in the task. For instance “frost fatique” is not a reason to
leave layer interfaces to be left uninterpreted. In unclear situations, the client can commission a
comparative survey of the work.
Besides the minimum interpretation, there many other interface and objects that can be interpreted
and /or calculated from the GPR data. Their identification and reporting requires extra work and have to
be ordered separately. Some of the “extra packages” are listed in the following:
1) Extra package A: Dielectric value of asphalt and base course (if reliable), antenna elevation info
2) Extra package B: Identification of other unbound layers such as sub base and filter course
3) Extra package C: Identification and detecting of steel reinforcements, geotextiles and frost
insulation boards from the data
4) Extra package D: Identification of damages inside pavement, such as debonded bound layers or
stripping inside the pavement
5) Extra package E: Identification of bitumen bound layers located in the middle of unbound layers
(sandwich structure) less than 1.0 m from the surface. At least the upper surface should be
interpreted from these.
Horn antenna interpretations should always be recorded at least in one meter intervals. So-called “few
points” interpretation (the thickness is interpreted only from changing points) is not allowed, because it
substantially impairs the accuracy.
The most common errors can be avoided with the following checklist:

Preprocessing must be carried out correctly

The data must be scaled correctly, and it has to match with the other data such as videos,
maps and profilometer data

Different antennas must match the depths of reflection, taking into account the fact that
antennas might be side by side. Ground coupled antenna’s 0-level can be roughly matched
to the data by comparing the response of the air antenna data. Reflection travel times differ
only in strongly dispersive dielectric materials.

There should be always polarity control in the interpretation. Especially the interface
between sub base aggregate (higher dielectric value) and the filtering sand below (lower
dielectric value) has inverted polarity and the interface should be interpreted in the middle
of the “black” line.

Dielectric values have been calculated and updated. Especially the dielectric value of the
pavement should always be visible. When presenting the results, a line with max 1 m
running average is used. Also deviation of dielectric value of bituminous pavement is a good
indicator of the quality of pavement. If fixed dielectric value is used it should be reported.

If there are sections with so-called sandwich-structure – unbound layer between bound
layers – the interpretation line should be drawn directly up to the bottom of the upper
bound layer (Figure 6).
Figure 6. Example how to interprete bound and unbound base layer thicknesses in the case of sandwich
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structure. The bound layer interface picking should be forced to be lifted up in the point where sandwich
structure starts and back to old bound base bottom where new pavement ends.
5.4 The ground coupled antenna data processing and interpretation
25B
5.4.1 400 – 600 MHz ground coupled antenna data
35B
The 400 - 600 MHz center frequency antennas’ basic pre-processing usually includes only the definition
of the surface level and “background removal” filtering operation. If needed, vertical high pass and low
pass filtering and gaining can be used.
Ground coupled antenna interpretations must be recorded at least in one meter intervals.
Interpretation should include at least the following interfaces and reflections (included in the same
basic package described in chapter 5.3):

Bottom of the unbound structure layers / overall structure thickness (code 71)

Bottom of the embankment (code 81) (when the road is on embankment). If the new road
is built on an old and poor quality road (gravel road), it should be marked as embankment.

Culverts. Found culverts should always have road register distance marked in them. Also
culverts transition wedges should be interpreted if they exist and they are detectable.
As with air coupled antennas, there many other interface and objects that can be detected and
interpreted from the ground coupled GPR data. Examples of the ground coupled “extra packages” are
listed in the following:
6) Extra package F: The quality and type of subgrade soil, and clear interfaces in it (soil type
changes
7) Extra package G: Bedrock surface (code 201), if possible to be seen The information from the
falling weight deflectometer (FWD) and “depth to stiff layer” calculation-algorithm information
can and should be used to support the interpretation.
8) Extra package H: The old road structures inside the unbound structures (code 35). This layer is
usually also the bottom of old base course or sub-base layer and it should be marked as text
notification
9) Extra package I: big boulders in road structure and subgrade soil
10) Extra package J: Soil replacement structures and other special structures in the road
Reliability of the interpretation is described in chapter 5.2 using Ground Penetrating Radar book
evaluation method.
The most common errors can be avoided with the following checklist:

Data is scaled to the length mentioned in Section 5.2 so that the culverts chainage match
the road register chainage.

Verification of the 400-600 MHz data to match other antennas data (culverts are a good
control and they must be precisely at the same point in all data)

Interpreted interfaces must not cross with any air coupled antenna interpretations

Interpretation should always include:
o
Total thickness of structure (code 71), and it can be:
a. subgrade soil interface
b. top of the embankment or
c. old road structure, if it can be classified only as embankment (for instance old
gravel road)

All embankments should be interpreted, if the interface is inside the time window. To help
the interpretation, video control should be used to confirm embankment starting and
ending points.

Old pavement or other bound layers inside the unbound layer and the other significant
structures (for example bound base) must be interpreted and marked as a text annotation
at least every 250 meters.

The quality/type of the subgrade soil must be marked often enough to help the design. It is
recommended to use the falling weight deflectometer information in the interpretation if it
is available. Geological control must acknowledged (for instance silt and peat does not
appear under the moraine in Finland)

When interpreting subgrade soil structures and road structures the following codes should
be used: transition wedges 55 (are often interpreted with code 71), bottom of a soil
replacement 75 (often interpreted with code 71), bottom of peat 95, peat layers 96,
mud/gyttja bottom 97, the top of the frost layer 98, and the bottom of the frost layer 99.

The following point-like objects should be interpreted: culverts (if possible, quality
mentioned (steel, concrete, plastic)), bridge (beginning and end), cable, pipeline, boulders
(large).

If ground coupled antenna winter data is collected, it is useful as follows:
o
Estimating the quality of subgrade soil
o
Total thickness of the structural layers.
o
The presence of thin peat layers below the road (Er = 30, if there is not any other
information)
o
Bedrock above the frost line
o
Frost lenses, marked as a text annotation
o
The depth of a frost line (if it is ordered separately)
5.4.2 50 – 200 MHz ground coupled antenna data
26B
If the client has ordered also the low frequency antenna (50 – 200 MHz) survey, the following interfaces
and notes must be observed:

Different soil types

Bottom of the peat

Silt layers (are located in low-lying areas and in eskers vicinity)

Alternating silty sand and fine sand layers. These are located mostly near rivers and eskers

Different kinds of moraine (glacial till) layers. Note that moist/wet moraines are visible as
strong reflections and interfaces in GPR profile, they should be marked.

Bedrocks visible with two types of black/white color schemes. If the bedrock interface is
unclear, ”uncertain” interface quality code should be used. Note that it is important to try to
make observations of bedrock on high elevation areas (hills, hummocks etc) to help to
design of possible deep road cut. If possible, clear shear zones in bedrock should be marked.

Big boulders
6. Reporting and Delivery of Results
5B
6.1 General
27B
When delivering Ground Penetrating Surveys in Nordic Countries, the results and collected data should
always be delivered also as scaled raw data (data that has been used in interpretations), bitmap of the
GPR profile is not enough. The ownership of the data also stays in the client if not otherwise agreed.
The ground penetrating radar results and printouts are made on a case by case basis, following the
client's instructions. All the data is given in digital format copied or zipped to a memory stick, DVD or
external hard disk drive. There are three different ways to deliver the data and the client can ask one or
several of them:
1. Printing data and interpretations to digital image files
2. Giving the data as ready made software project views (for instance RD-projects), that can be
watched using viewer software.
3. Printing the results as text files in a format defined by the client. However, the outputted results
should always include at least a) location as register distance, b) layer identification core or
name, if multiple layers in the same file, c) layers´ depth, thickness or elevation level as meters
or millimeters and d) Er-values used for calculations. Additional information could be the xyz
coordinates and interpretation quality numbers of points.
GPR survey consultant should also release a brief report to the client, where the surveys are described.
28B
6.2 Printing to an image file
29B
The image files should be delivered in png-format. 4-size horizontal page size should be the minimum of
width=1096 x height=756 pixel. The client determines how long distance should be printed on a single
page. If the size is not specified, 1000 meters on one page in 20cm/m scale. In this case, the picture’s
width is c. 2040 pixels. The image files should be named so that the road number, section (if road
sections are used) and distance are in the title and also numbered. For instance: 21_216.001 PNG,
21_216.002 PNG.
30B
LITERATURE
3B
Ground Penetrating Radar, 1992. Geotechnical Society of Finland
Rakenteen parantamista edeltävät tutkimukset / Tiehallinto, tie- ja liikennetekniikka, 1999, TIEL
2140015, ISBN 951-726-515-8
Saarenketo, T. 2008. NDT Transporation. Chapter 13 in text book book “Ground Penetrating Radar:
Theory and Applications” Ed- Harry M. Jol. Publisher Elsevier, 524 p.
Scullion, Tom; Lau, Chun Lok; Saarenketo, Timo 1996. Performance specifications of ground penetrating
radar. In: GPR '96 : 6th International Conference on Ground Penetrating Radar, September 30 - October
3, 1996, Sendai, Japan. Proceedings. Sendai: Tohoku University, 341-346.
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