Field-Based Geophysical Technologies Online Seminar - CLU-IN

Field-Based Geophysical Technologies Online Seminar - CLU-IN
Field-Based Geophysical Technologies Online Seminar
Field-Based Geophysical
Technologies Online Seminar
Presented
by the
U.S. Environmental
Protection Agency’s
Technology
Innovation Office
1
EPA
Instructor:
Explain to participants that this seminar examines geophysical
characterization techniques and is excerpted from EPA TIO’s FieldBased Technologies Training Program.
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Field-Based Geophysical Technologies Online Seminar
Objective of the Seminar
‹ Introduce geophysical methods that can be used
to support the development of a systematic plan
for site restoration through the development of a
Conceptual Site Model (CSM)
‹ Demonstrate through the use of case studies
how geophysical methods and CSMs can
promote efficiency in sampling and analysis
programs
2
EPA
Notes:
C
The objective of this seminar is to introduce participants to various types of geophysical
methods that can be used to assist in the development of systematic plans used for
managing site restorations. A systematic plan based on a well-defined conceptual site
model (CSM) will assure site managers that projects are performed in the most efficient
and defensible way possible.
C
During the online presentation, a series of case studies will be presented to demonstrate
how geophysical methods described in this seminar are applied at some real hazardous
waste sites. The complete case studies will be available through the links to additional
resources section at the beginning and end of the seminar.
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Field-Based Geophysical Technologies Online Seminar
Geophysics and the Triad Approach
‹ Geophysical surveys offer high information value
for a limited cost particularly where contaminant
source distributions are complex and geological
data is limited
‹ The “Triad Approach.” which focuses on
performing site restorations cheaper, better, and
faster, relies heavily on CSMs developed using
geophysical methods
3
EPA
Notes:
C
Geophysical methods offer project managers high information values at a reasonable
cost, particularly where contaminant distributions are complex and where geologic
information is limited. In fractured media, preferred pathways can go undetected when
standard drilling methods are used. Geophysical surveys can assist in such cases to
ensure that preferred pathways are identified and wells and other monitoring and
measurement methods are focused where needed.
C
The Triad Approach, or the use of systematic planning, dynamic work plans, and fieldbased measurement technologies (including Geophysics) is an initiative that focuses on
performing site restorations cheaper, better, and faster. Paramount to this approach is the
use of a well-defined systematic plan based on a CSM. Early in a project’s life,
geophysical methods can ensure that funds allotted for monitoring and measurement
activities are expended wisely. Pincushion sampling at complex sites usually is not
possible because of economic constraints. Geophysical techniques offer an alternative
approach that will provide the broad data coverage required to ensure that project
decisions are defensible at a reasonable costs.
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Field-Based Geophysical Technologies Online Seminar
Systematic Plan Using the Triad
Approach and Geophysical Surveys
‹ Collect and evaluate
existing data
‹ Conduct
Geophysical
surveys
‹ Optimize intrusive
monitoring and
measurement
activities
4
EPA
Notes:
C
When using the Triad Approach it is recommended that project managers use existing
data to the maximum extent possible. Gathering available information will ensure that a
data collection scheme is focused efficiently on areas where uncertainty is the highest
relative to project decision-making. Geophysical surveys should then be considered at
sites where little or no geologic information is available or where the complexity of site
conditions suggests that an intrusive sampling technique will be inefficient or
economically unfeasible unless well directed.
C
Based on the geophysical results obtained, a project manager then can refine a sampling
and analysis scheme that targets most efficiently those areas with the highest uncertainty
and most bearing on the project decisions attempting to be made.
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Field-Based Geophysical Technologies Online Seminar
Geophysics Overview
‹ Geophysics methods that will be reviewed
include:
» Magnetics
» Resistivity
» Conductivity
» Ground Penetrating Radar (GPR)
» Borehole
» Seismic
5
EPA
Notes:
C
In this seminar, we will focus on several of the commonly used methods that can assist
project managers in developing systematic plans for site restorations.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Physical Basis
‹ Magnetic susceptibility
‹ Remanent magnetism and susceptibilities of earth
materials
‹ Magnetic field
of the earth
6
EPA
Notes:
C
A body placed in a magnetic field acquires a magnetization that typically is proportional
to the field. The constant of proportionality is known as the magnetic susceptibility. For
most natural materials, susceptibility is very low. However, ferromagnetic and
ferromagnetic materials have relatively high magnetic susceptibilities. The susceptibility
of a rock typically depends only on its magnetite content. Sediments and acid igneous
rocks have relatively low susceptibilities, while basalts, gabbros, and serpentinites
usually have relatively high susceptibilities.
C
Ferromagnetic and ferromagnetic materials have permanent magnetic moments in the
absence of external magnetic fields. An object that exhibits a magnetic moment is
characterized by a tendency to rotate into alignment when exposed to a magnetic field.
C
The magnetic field of the earth originates from electric currents in the liquid outer core.
Earth’s magnetic field strengths typically are expressed in units of nanoTesla (nT).
Sunspot and solar flare activity can create irregular disturbances in the magnetic field.
Such changes are referred to as magnetic storms.
Instructor:
Explain that the sun can affect the measurement of the magnetic field of
the earth. Solar activity may strongly alter the field for brief periods of
time, or cause slow variations in the measured field.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Field
‹ Found in the vicinity
of a magnetic body
or electrical medium
that is carrying
current
NORTH
Ambient
Net
Anomaly
Body
Body
Anomaly
7
EPA
Notes:
C
The earth’s magnetic field induces a magnetic moment per unit volume in buried
ferromagnetic debris (bottom), causing a local perturbation (anomaly) in total magnetic
field (top).
C
The total magnetic field measured is a vector sum of the ambient earth’s magnetic field,
plus local perturbations caused by buried objects.
Instructor:
Use the graphic to illustrate why the shape of the characteristic
anomaly occurs. It is simply a process of vector addition. Ask the
participants how their strategy for locating buried ferrous objects
would differ if they were making gradient, rather than total intensity,
measurements. The key concept to illustrate is that objects typically are
located at maximum gradients, but not maximum total field anomalies.
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Field-Based Geophysical Technologies Online Seminar
Electric Field
‹ Causes charged
bodies to be
attracted to or
repelled by other
charged bodies;
associated with
electromagnetic
wave or changing
magnetic field
8
EPA
Notes:
C
The graphic depicts field lines and contours of equal field strength around two static,
oppositely charged objects. The electric field intensity of a small charged object has a
magnitude proportional to the charge and inversely proportional to the square of the
distance from the charge. Note the similarities and differences to the magnetic field
depicted in the previous slide.
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Field-Based Geophysical Technologies Online Seminar
Diurnal (nT)
Magnetic Surveys — Diurnal
Variation
40
30
20
10
0
00
06
12
18
24
Time (hrs)
9
EPA
Notes:
C
The field varies during the day because of changes in the strength and direction of
currents circulating in the ionosphere; those changes are referred to as diurnal variation.
Instructor:
Explain that the graphic depicts normal daily diurnal variation in the
magnetic field at a single point. Ask the participants to identify which
hours of the day appear to be most affected by the rate of change of
diurnal variation. Compare those hours (afternoon) with typical survey
work hours to emphasize the need to account for diurnal variation.
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Field-Based Geophysical Technologies Online Seminar
gammas
Deviation of Magnetic Field Caused
by Ferrous Metal
10
EPA
C
Magnetometers are used to measure the earth’s magnetic field.
C
Deviations of magnetic field intensity are caused by ferrous minerals in the soil or
ferrous metals.
Instructor:
Explain to the participants that this illustration shows lines of equal
magnetic intensity. A ferrous object, such as a drum will change the
intensity causing an anomalous measurement of the field.
C
This diagram shows a typical response when a magnetometer passes over a ferrous
object. In this case, the anomaly is associated with a single drum.
C
This profile illustrates the change in the magnetic field as the sensor passes over a ferrous
object.
Instructor:
Explain to the participants that the measured anomaly caused by a
ferrous object can vary. An anomaly may be positive or negative. In
fact, there is a possibility that an object may be oriented within the
earth’s magnetic field such that no anomaly will be detected.
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Field-Based Geophysical Technologies Online Seminar
Uses of Magnetic Surveys
‹ Can be used for the location and mapping of
buried ferrous metals such as drums,
underground storage tanks, and utilities
‹ Provide rapid delineation of subsurface ferrous
metal objects
11
EPA
Notes:
C
Magnetic surveys are used to map subsurface ferrous metal objects such as tanks, pipes,
and utilities.
Instructor:
Point out to the participants that magnetic surveys are often a rapid
method to delineate the location of suspected buried drums, define the
location of underground storage tanks and locate isolated buried steel
water lines.
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Field-Based Geophysical Technologies Online Seminar
Types of Magnetometers
Cesium
Proton
12
EPA
Notes:
C
This slide shows a proton and a cesium magnetometer.
C
Proton precession, fluxgate, and cesium vapor magnetometers used for surface
geophysical surveys are portable, self-contained units that require a single operator.
Most modern proton precession, fluxgate, and cesium vapor magnetometers support
recording and retrieval of data.
C
Proton precession magnetometers can be rented for approximately $300 per month or
$10 per day, with a mobilization fee of about $95. The cesium vapor magnetometer can
be rented for approximately $1,770 per month or $59 per day, with a mobilization fee of
about $95. A fluxgate magnetic locator can be rented for approximately $180 per month
or $6 per day, with a mobilization fee of about $55.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Survey
Practice
‹ Survey grids
‹ Monitoring of diurnal variation
40
30
20
N
Drum
10
0,0
10
20
30
50
13
EPA
Instructor:
40
Inform the class that proton precession magnetometers require more
time to sample the magnetic field and surveys are usually on a regularly
spaced grid point. The Cesium vapor magnetometer is more commonly
set up on a regularly spaced time interval (for example, every 5 feet for
proton precession and every 0.1 seconds with cesium vapor
magnetometers).
Notes:
C
Proton precession instruments usually are used to obtain measurements along regularly
spaced grid points. The surveys are also conducted along regularly spaced lines. This
survey procedure is common for most surface geophysical surveys. Fluxgate
magnetometers are used in serpentine search or clearance patterns, in addition to the
grid-based data acquisition approach. Cesium vapor magnetometers are used extensively
today because they support rapid acquisition and storage of data, and are usually set up
on a time interval for measurement intervals along equally spaced lines.
C
When total field measurements are being obtained, as is the case with long baseline
instruments, a separate stationary magnetometer should be used to measure diurnal
changes in the ambient magnetic field, as well as possible effects of magnetic storms.
Magnetic measurements are susceptible to noise related to ferrous content of buildings,
fences, vehicles, and utility fixtures.
Instructor:
Ask participants whether they think it is important to select a data
collection spacing and why?
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Interpretation
‹ Contouring-based interpretation
14
EPA
Notes:
C
The illustrated magnetic data in this slide is from a survey to delineate the presence of
disposal trenches. This data was provided from a paper presented at the annual meeting
of the Engineering and Environmental Geophysical Society (EEGS) symposium. The
data was collected at Tinker Air Force Base. Various means of spatial predictions can be
used to prepare contour maps to present the magnetic properties across an area of
interest. Contours of total field or magnetic gradient commonly are used as
interpretation tools. Other advanced processing can be done to minimize the variety of
anomaly shapes encountered in magnetic surveys.
C
Survey design for magnetic surveys, and other “grid based “ geophysical methods is an
important consideration. Line spacing and sample intervals are important
considerations. These parameters are based upon the expected size of the object or
feature being investigated, which could range from defining a single tank or drum or
delineating the extent of a landfill or plume.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Advantages
and Limitations
‹ Advantages
» Detects objects at a greater distance from the sensor
» Can assist in delineating geologic formations
containing ferrous minerals
‹ Limitations
» Detects only ferrous materials
» Anomaly shape often is complex
» Interferences from nearby ferrous objects may mask
objective
15
EPA
Notes:
C
Magnetometers are used for the detection of magnetic or ferrous objects and minerals.
Magnetometers are not effective in sensing other metals such as aluminum or brass.
Nearby objects such as fences, steel posts, automobiles, steel well casings will mask
subsurface ferrous objects that may be the focus of an investigation. Careful
consideration should be conducted to determine if a magnetometer survey can be
conducted at the site. Other methods may be better suited for subsurface investigations.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Costs
‹ Daily rental rate for magnetometer = $60
‹ Daily rental rate for gradiometer = $90
‹ Base station = $20 to $60
‹ Shipping weight = 70 to 150 pounds
‹ Contractor crews and equipment = $1,450 per
day
‹ Productivity = 3 to 6 acres per day
16
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provided later in this discussion. Typical rates for magnetometer equipment are
about $60 to $90 per day. Shipping weights vary and should be considered in costing a
project. If a contractor were hired to perform the work, costs can vary from about $1,450
per day plus mobilization and reporting.
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Field-Based Geophysical Technologies Online Seminar
Magnetic Surveys — Summary
‹ Used for mapping buried ferrous metal items
‹ Rapid survey method
‹ Detects large, deeply buried ferrous metal objects
‹ Results can be difficult to interpret
‹ May not be effective near buildings, vehicles, or
areas with reinforced concrete
17
EPA
Notes:
C
Magnetometers are used to detect buried ferrous metal objects such as drums, tanks and
pipes.
C
Magnetometers allow rapid characterization of a site.
C
Magnetic surveys can detect large, deeply buried ferrous metal objects. However,
smaller objects at depth may not be detected by this method.
C
The results from magnetic surveys can be difficult to interpret, but recent developments
in data analysis software have made data interpretation easier and more effective.
C
Magnetometers may not be effective near buildings, vehicles, power lines or in areas
with reinforced concrete, such as parking lots.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Physical Basis
‹ Electrical resistivities of
common materials (a)
‹ Contact resistances (b)
‹ Apparent resistivity (c)
(continued)
EPA
18
Notes:
C
The resistivity of a rock is roughly equal to the resistivity of the pore fluid divided by the
fractional porosity. In general, soils have lower resistivity than rock, and clay soils have
lower resistivity than coarse-textured soils.
C
Measurement of resistivity in the earth is defined as apparent resistivity because it is
unlikely that the material into which electrodes are inserted, and of which measurements
are taken, is homogenous.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Physical Basis
‹ Electrical resistivities of common materials
‹ Contact resistances
‹ Apparent resistivity
Current
source
v
Measured
potential
Lines of
equal potential
EPA
Current flow
lines
19
Notes:
C
Resistance (R) is measured in ohms when current (I) is in amps and potential (V) is in
volts. The resistance of a unit cube to current flowing between opposite faces is termed
resistivity. Resistivity is measured in units of ohm-meters (S-m).
C
In geophysics, resistivity involves applying a current into the subsurface and measuring
the resulting potential between two other electrodes.
C
The resistivity of a rock is roughly equal to the resistivity of the pore fluid divided by the
fractional porosity. In general, soils have lower resistivity than rock, and clay soils have
lower resistivity than coarse-textured soils.
C
Measurement of resistivity in the earth is defined as apparent resistivity because it is
unlikely that the material into which electrodes are inserted, and of which measurements
are taken, is homogenous.
Instructor:
Describe the use of electrodes placed into the subsurface using metal
rods. Explain that there are current electrodes that apply a direct
current into the ground. Two additional electrodes are used to measure
the resultant potential voltage. Information about the applied current
and the resultant measured voltage provide information about the
subsurface apparent resistivity. The next page provides ranges of
resistivities of earth materials.
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Field-Based Geophysical Technologies Online Seminar
Typical Values of Electrical Resistivity of Earth Materials
Resistivity Range (S-m)
Earth Material
Saline groundwater
0.01 – 1
Clay soil
1 – 30
Fresh groundwater
2 – 50
Calcareous shale (or Chalk)
10 – 100
Sand (SP1, moderately to highly saturated)
20 – 200
Shale (mudstone/claystone)
1 – 500
Shale (siltstone)
50 – 1,000
Limestone (low-density)
100 – 1,000
Volcanic flow rock (scoriaceous basalt)
300 – 1,000
Lodgement (dense, clayey, basal) till
50 – 5,000
Sandstone, uncemented
30 – 10,000
Ablation (dry, loose, cohesionless) till
1,000 – 10,000
Fluvial sands and gravels (GW1, unsaturated)
1,000 – 10,000
Loose, poorly sorted sand (SP, unsaturated)
1,000 – 100,000
Metamorphic rock
50 – 1,000,000
Crystalline igneous rock
100 – 1,000,000
Limestone (high-density)
1
1,000 – 1,000,000
GW and SP above are Unified Soil Classification terms for well-graded gravel and poorlygraded sand, respectively.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Instruments
‹ Electrodes and arrays
20
EPA
Notes:
C
The device that applies a measured current in a resistivity survey is known as the
transmitter. Transmitters usually are designed to reverse the direction of the current,
with a cycle time from 0.5 to 2 seconds. The reversal of the current helps minimize
electrode polarization effects. Power is provided by a battery or a generator.
C
Voltage measuring devices often are referred to as receivers.
C
In newer instruments, the transmitter and receiver equipment generally are housed in a
single unit, with microprocessor control and internal data storage.
C
Earth resistivity meters can be rented for approximately $300 per month or $10 per day,
with a mobilization fee of about $65.
Instructor:
C
C
This slide shows a typical resistivity layout using multiple wires and
electrodes. Note that the operator has control of both the transmitter
and the receiver.
Current electrodes typically are metal stakes. Voltage electrodes can be metal, but more
often consist of a “pot” of nonpolarizing material, such as porcelain or unglazed ceramic.
Inside the pot is a copper rod surrounded by copper sulfate solution. Contact with the
ground is made through the solution, which leaks into the base of the pot.
Resistivity profiling is a technique in which transects are used to detect lateral changes in
subsurface resistivity. The geometric perimeters of the array are kept constant, and the
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Field-Based Geophysical Technologies Online Seminar
depth of penetration therefore varies only with changes in subsurface materials and
variations in layering of geologic materials.
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Field-Based Geophysical Technologies Online Seminar
Field Arrays for the Resistivity Method
WENNER ARRAY
~
surface
CE
PE
a
V
PE
a
CE
a
SCHLUMBERGER ARRAY
~
surface
CE
A
PE
M
V
PE
N
CE
A
DIPOLE-DIPOLE ARRAY
~
surface
CE
V
CE
PE
a
EPA
PE - Potential electrode
CE - Current electrode
a to 5a
EXPLANATION
- Voltmeter
V - Current source
~
PE
a
a - Electrode “a” spacing
A,M,N,B - Electrode locations
21
Notes:
C
Electrodes are placed in well-defined geometric patterns known as arrays. Commonly
used arrays include:
S
S
S
S
Wenner
Schlumberger
Dipole-Dipole
Gradient
Shown are three of the commonly used DC resistivity arrays.
S
The Wenner and Schlumberger arrays are commonly used for vertical electrical
soundings.
S
The Dipole-Dipole array is used to produce a two-dimensional cross sectional
profile over a site.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Survey Practice
‹ Resistivity depth-sounding
‹ Resistivity profiling
‹ Resistivity Cross-sections
22
EPA
Notes:
C
The resistivity depth-sounding technique uses arrays in which the distances between
some or all of the electrodes are increased systematically. Apparent resistivities are
plotted against changes in the geometry of the array. With this technique, information
about the change in resistivity as a function of depth is inferred.
C
Soundings provide a single point vertical profile of subsurface electrical properties.
C
Some limitations and interferences include: (1) contact resistance effects; (2) effects of
cultural features, such as buried utilities; and (3) noise levels at the site. Data
interpretation is another potential limitation because it is fairly subjective and requires an
expert. Resistivity surveys also are relatively slow because of the need to move
electrodes and cables between each measurement.
C
Resistivity cross sections commonly are generated from the depth-profiling data. The
calculation is made by entering site information about depths and soil or rock units. The
data may be available from borehole information or general geologic maps of an area.
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Field-Based Geophysical Technologies Online Seminar
Vertical Electrical Sounding
Examples
Electrode Spacing, AB/2, in feet (ft)
10
20
50
100
200
500
1000
2000
5000
10,000
1000
500
200
3
100
Schlumberger
Apparent
Resistivity, P,
in Ohm-Meters
50
1
2
20
10
40
5
8
40
200
EPA
Instructor:
1
2
3
26
130
Depth in meters
2
1
26
8
40
1
2
5
10
20
50
100
200
500
Electrode Spacing, AB/2, in meters
1000
2000
5000 10,000
23
Ask the participants what a background or representative value of
resistivity might be for areas not affected by landfilling. Explain that, in
the field, media values often are used as an approximation. Ask the
participants which measure—resistivity or resistivity gradient—seems to
be a more effective means of locating the boundary of the landfill.
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Field-Based Geophysical Technologies Online Seminar
Example of Electrical Profiling
100
a = 30 feet
90
b = 60 feet
70
50
40
0
10
20
30 Meters
0
100 Feet
Horizontal Scale
VES 4
0
Sand and Gravel
20
0
10
Clay
EPA
Depth in meters
60
Depth in feet
P , IN OHM-METERS
APPARENT RESISTIVITY,
80
40
20
24
Notes:
C
The data shown at the top of the figure are taken at two electrode spacings. Wider
spacing usually provides information about deeper subsurface materials.
C
The information at the bottom of the figure is an interpretation of the data. “VES 4” is a
vertical electric sounding. Likely, the data from VES 4 indicate the presence of three
geologic layers within the depth of investigation of the sounding.
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Field-Based Geophysical Technologies Online Seminar
Resistivity Profiling
(continued)
EPA
25
Notes:
C
This graphic illustrates the resulting measuring point from a single electrode position
measurement.
C
Each move of the current and potential electrodes results in a particular location and
depth of measurement.
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Field-Based Geophysical Technologies Online Seminar
Resistivity Profiling
26
EPA
Notes:
C
Resistivity profiling or soundings can provide information including depth of fill and
lateral extent of landfills.
C
This cross-sectional view was created using a dipole-dipole resistivity array. The data
was processed using a recently developed software program Res2Dinv. The software
uses the calculated apparent resistivity data and enter it into a modeling program. The
program calculates and interpolates depths of resistivity layers and produced the crosssectional view.
Instructor:
Discuss the cross section stressing the contrasting resistivities from
background shales to the foreign municipal landfill.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Advantages and
Limitations
‹ Advantages
» Surveys are relatively inexpensive
» Provides information about multiple soil layers in the
subsurface
» The method is effective in shallow areas of
investigation
‹ Limitations
» Sufficient space is required to lay out electrode array
» Difficult to place electrodes in rocky soils
» Lateral variations in soil may affect result
27
EPA
Notes:
C
Resistivity surveys are a cost effective way to obtain information about subsurface soil
and rock units. The method is better than time domain electromagnetic soundings for
the analysis of shallow layers such as a thin clay cap over a landfill. The method does
however require adequate space in order to lay out an array. Lateral variations in soils
may affect the result of sounding surveys.
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Field-Based Geophysical Technologies Online Seminar
Resistivity Profiling — Costs
‹ Daily rental rate = $50
‹ Accessories = $25
‹ Shipping Weight = 75 to 150 pounds
‹ Contractor crews and equipment = $1,400 to $1,800 per
day
‹ Productivity generally in number of soundings per day (2-
20) or line mile coverage per day (one-forth mile to 1 mile
per day). These productivity rates vary because of site
conditions, depth objectives, and required resolution.
28
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provided later in this discussion. Typical rates for near surface resistivity equipment
are about $75 per day. Shipping weights vary and should be considered in costing a
project. If a contractor were hired to perform the work, costs can vary from about $1,400
to $1,800 per day plus mobilization and reporting.
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Field-Based Geophysical Technologies Online Seminar
Resistivity — Conclusions
‹ Determine lateral variations of resistivity
‹ Determine depth of fill material
‹ Determine depth to bedrock
‹ Advantages and limitations
29
EPA
Notes:
C
Direct current resistivity can be a useful tool for mapping lateral variations of earth
resistivity for site characterization.
C
The method is useful for determining depth of fill and depth to bedrock in certain
geologic and site environments.
C
Direct current resistivity is better suited than seismic refraction for determining depth of
fill.
C
The direct current method requires good ground contact. Rocky surfaces may limit
effectiveness. At landfill sites, metals in the subsurface may interfere with the data.
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Field-Based Geophysical Technologies Online Seminar
Electromagnetic Conductivity
Surveys
‹ Active
electromagnetic
induction
techniques
‹ Applications
Phase
sensing
circuits and
amplifiers
Chart and magnetic
tape recorders
PRIMARY
FIELD
Coil
Transmitter
» Profiling
Receiver
Coil
GROUND SURFACE
Induced
current
loops
» Sounding
SECONDARY FIELDS FROM
CURRENT LOOP SENSED BY
RECEIVER COIL
30
EPA
Notes:
C
Conductivity methods also are known as active electromagnetic induction techniques,
and can be used to detect both ferrous and nonferrous metallic objects.
C
These methods use a transmitter coil to establish an alternating magnetic field that
induces electrical current flows in the earth. The induced currents generate a secondary
magnetic field that is sensed by a receiver coil. The character and magnitude of the
secondary field are governed by the frequency of the transmitted current and the
distribution and magnitude of the electrical properties in the nearby subsurface.
C
Profiling is accomplished by making fixed-depth measurements along a traverse line.
C
Sounding is accomplished by making measurements at various depths at a fixed location
by varying the coil orientation or separation of the transmitter and receiver.
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Field-Based Geophysical Technologies Online Seminar
Conductivity Surveys — Survey
Practice
‹ Data acquisition
‹ Conductivity is influenced by soil and rock
properties
‹ Effects of cultural features
31
EPA
Notes:
C
Approaches to conductivity surveys are similar to those used in resistivity surveys.
Profiles involve continuous or station-based conductivity measurements obtained along a
transect. Measurements can be obtained along a grid of transects to support contouring
of the data. Data usually are recorded digitally for rapid transfer to a computer for
processing.
C
Most soil and rock minerals have low conductivities when dry. A unique conductivity
value cannot be assigned to a particular material because of variations in composition
and structure of soils and in pore fluids. A range of conductivities, based upon field and
laboratory measurements has been determined.
C
The presence of surface conductors (for example, railroad tracks) must be noted carefully
in the survey log.
C
The ranges of conductivity aid in the interpretation for determining or inferring the
subsurface geologic properties.
Instructor:
State that the next slide provides some ranges of conductivity of
common earth materials.
33
Field-Based Geophysical Technologies Online Seminar
Range of Electrical Conductivities
in Natural Soil and Rock
Conductivity (milliS/m)
103
102
101
1
10-1
10-2
10-3
Clay and Marl
Loam
Top Soil
Clayey Soils
Sandy Soils
Loose Soils
River Sand and Gravel
Glacial Till
Chalk
Limestones
Sandstones
Basalt
Crystalline Rocks
32
EPA
Notes:
C
This illustration shows the range of conductivity that can be encountered in various
terrain materials from a variety of climatic zones. The ranges have been compiled for
different terrain materials from a variety of survey and laboratory measurements.
C
In general, clays or fine-grained materials have high conductivity values, gravels, and
sand have moderate conductivity values, and consolidated bed rock has low conductivity
values.
34
Field-Based Geophysical Technologies Online Seminar
Conductivity Surveys Interpretation
‹ Data contouring
33
EPA
Notes:
C
Various means to obtain spatial representations, such as magnetic surveys, also can be
applied to conductivity measurements. Color contour maps usually are represented using
available software packages that generate these types of maps. This illustration is of a
the same site illustrated in the magnetometer example (Tinker Air Force Base,
Oklahoma) from the proceedings of the EEGS. The objective of the survey was to
determine the locations of waste pits and also areas where waste pits were absent.
35
Field-Based Geophysical Technologies Online Seminar
Electromagnetic (EM) Surveys
‹ The EM method
provides a means
of measuring the
electrical
conductivity of
subsurface soil,
rock, and
groundwater
EM31
34
EPA
Notes:
C
The electromagnetic method (EM) measures electromagnetic properties of subsurface
media and can determine subsurface variations of conductivity.
C
This slide shows an EM31 terrain conductivity meter, discussed in more detail later in
this section.
C
This slide shows the components of an EM31 terrain conductivity meter. The EM31 is a
fixed geometry instrument. The transmitter is located on one end and the receiver is on
the other. In the center, near the operator, the electronics and data logger can be found.
C
The EM31 can be used for bulk conductivity measurements up to about 18 feet in depth.
A focused bulk conductivity measurement to about 7.5 ft can be obtained by rotating the
instrument such that the transmitter and receiver coils are in the horizontal dipole
position.
Instructor:
Ask the class why it may be useful to obtain both horizontal or vertical
dipole measurements, or why one measurement may be favored over the
other. The standard operation is the vertical dipole mode, for maximum
depth of exploration.
36
Field-Based Geophysical Technologies Online Seminar
Uses of EM
‹ Detect and map contaminant plumes
‹ Map buried wastes, metal drums and tanks, and
metal utilities
35
EPA
Notes:
C
The EM terrain conductivity method can be used for mapping contaminant plumes. The
method can be used to detect geologic areas that may be preferential pathways for
contaminant migration.
C
This method is also useful for detecting buried waste, drums, tanks, and utilities.
37
Field-Based Geophysical Technologies Online Seminar
EM31 — Costs
‹ Daily rental rate = $50
‹ Shipping Weight = 80 pounds
‹ Contractor crews and equipment = $1,300 per
day
‹ Productivity = 4 to 8 line miles per day
36
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provided later in this discussion. Typical rates for EM31, and other conductivity
instrument equipment, are about $50 per day. Shipping weights vary and should be
considered in costing a project. If a contractor were hired to perform the work, costs can
vary from about $1,300 per day plus mobilization and reporting.
38
Field-Based Geophysical Technologies Online Seminar
Geonics EM34 Terrain Conductivity
Electromagnetic System
37
EPA
Notes:
C
This slide shows a typical layout of an EM34 terrain conductivity meter. The transmitter
and operator are in the background, and the receiver and operator are in the foreground.
C
The EM34 is a variable geometry instrument. The transmitter and receiver can be
separated at 10, 20 and 40 meters. Depth of exploration increases with increased coil
separation. Maximum depth of exploration is about 100 ft.
Instructor:
Ask the class if they can provide examples where the EM31 or EM34
may be appropriate for geophysical surveys. Can they provide any
example site?
39
Field-Based Geophysical Technologies Online Seminar
EM34 Data Presentation
‹ Landfill
Landfill
‹ EM34 survey
20
‹ 20 meter spacing
12
2
‹ 15 meter depth
6
12
6
Fa
rm
ing
ton
Riv
er
2
2
0
100
Meters
EM34 Survey
20 meter spacing
Contour 2 mmho/m
38
EPA
Notes:
C
This graphic illustrates a simplified electromagnetic terrain conductivity survey over a
landfill. The data was collected using a 20 meter coil separation using the horizontal
dipole mode of operation. This results in a bulk apparent conductivity measurement to
about 15 m or 50 feet in depth.
C
This example illustrates an EM34 survey of a landfill.
Instructor:
Ask the class if they can determine what material is represented by the 2
millisieman measurements. Have the class look back to the chart on
Slide SG-17 “Range of Electrical Conductivities in Natural Soil and
Rock.”
40
Field-Based Geophysical Technologies Online Seminar
EM34 — Costs
‹ Daily rental rate = $70
‹ Shipping Weight = 100 pounds
‹ Contractor crews and equipment = $1,350 per
day
‹ Productivity = 1 to 3 line miles per day
GT-39
39
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provided later in this discussion. Typical rates for EM34 conductivity instrument
equipment are about $70 per day. Shipping weights vary and should be considered in
costing a project. If a contractor were hired to perform the work, costs can vary from
about $1,350 per day plus mobilization and reporting.
41
Field-Based Geophysical Technologies Online Seminar
Uses of Metal Detectors
‹ Used to locate
buried metal
containers, drums,
tanks, and utilities
‹ Includes time
domain (EM61) and
frequency domain
metal detectors
EM61
40
EPA
Notes:
C
The Geonics EM61 is a time domain metal detector used to detect subsurface metals of
all types.
C
The system transmits a time varying electromagnetic pulse in the subsurface. The
receiver measures secondary signals in the subsurface created in metallic objects. The
measured quantity is the millivolt. Higher value measurements are related to either near
surface or large objects.
C
The Geonics EM61 is a cart mounted system with two coils. The lower coils acts as a
transmitter and receiver. The upper coils acts as a secondary receiver used to estimate
depth of burial. The operator carries a back pack and a data logger. The wheel system
contains an odometer that triggers data collection at about 0.6 feet per measurement.
The position is automatically recorded along with the signal measurement.
42
Field-Based Geophysical Technologies Online Seminar
EM61 Survey
Sidewalk
(0,125)
7000
Elevated asphalt
area surrounded
by concrete
curb
Telephone
Pole
Light Pole
0
Response
(mV)
Building
Water
Meter
Cover
Telephone
Pole
Telephone
Pole
Pump Island
Light Pole
(0,0)
Underground
Storage Tanks
EPA
Pump Island
Probable
Buried
Tanks
Traffic Signal
Control Box
Light Pole
(110,0)
Sidewalk
30
0
Scale
Abandoned Gas Station, East St. Louis, Illinois
41
Notes:
C
This map shows EM61 metal detector data from an underground storage tank survey at a
gas station. The bar graph on the upper right shows the range of measured millivolt
signal. The contours clearly increase in magnitude in the vicinity of the underground
storage tanks.
43
Field-Based Geophysical Technologies Online Seminar
EM61 — Costs
‹ Daily rental rate = $70
‹ GPS option = $50
‹ Shipping Weight = 130 pounds
‹ Contractor crews and equipment = $1,400 per
day
‹ Productivity = 1 to 3 acres per day
42
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provided later in this discussion. Typical rates for EM61 equipment is about $70 per
day. A GPS option is about $50 per day. Shipping weights vary and should be
considered in costing a project. If a contractor were hired to perform the work, costs can
vary from about $1,400 per day plus mobilization and reporting.
44
Field-Based Geophysical Technologies Online Seminar
Electromagnetic Surveys —
Advantages and Limitations
‹ Advantages
» Do not require placement of electrodes
» EM31 surveys are useful in determining lateral variations in soil
conductivity
» EM31 survey can be conducted rapidly and interpreted quickly
» Time domain soundings provide excellent vertical resolution
» EM61 surveys detect all types of metal
‹ Limitations
» EM31 surveys provide only a bulk conductivity measurement
» The equipment is somewhat cumbersome
» Survey data may be affected by utilities
43
EPA
Notes:
C
Electromagnetic surveys can be a cost effective way of determining the physical
properties of the subsurface. Many methods are available including the EM31 terrain
conductivity method, time domain electromagnetic sounding surveys, and EM61 metal
detection, and other methods such as the GEM electromagnetic system. Limitations of
electromagnetic surveys exist. These include effects from other electromagnetic sources
such as power lines and other utilities. Some of the equipment may be cumbersome and
require frequent rest during surveys.
45
Field-Based Geophysical Technologies Online Seminar
Case Study: Taylor Lumber, Wood
Treatment Facility, Sheridan,
Oregon
‹ Problem Statement:
» Identification of preferential flow paths for
dissolved phase pentachlorophenol and
cresote
» What is the configuration of the bedrock
aquitard surface below the site and what
impact will it have on the accumulation of
dense non-aqueous phase liquids (DNAPL)?
44
EPA
Notes:
C
At this active wood treatment facility, pentachlorophenol and cresotes are known to have
impacted groundwater. Both of these contaminants also can occur as dense non-aqueous
phase liquids (DNAPLs). This study uses both time domain electromagnetic soundings
and inductive conductivity measurements to identify preferential flow paths (alluvial
channels) and the topography of bedrock (a dense clayey siltstone) to focus follow-on
monitoring and measurement activities.
46
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — Map
45
EPA
Notes:
C
The slide presents a site map of the wood treatment site showing geophysical lines, TEM
profiles, and sounding numbers.
47
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — EM31
Conductivity Profile 1 and TEM
Resistivity Sections
46
EPA
Notes:
C
The slide presents an EM31 data plot and TEM approximate depth calculations versus
resistivity section along Profile 1.
48
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — EM 31
Conductivity Profile 2 and TEM
Resistivity Sections
47
EPA
Notes:
C
The slide presents an EM31 data plot and TEM approximate depth calculations versus
resistivity section along Profile 2.
49
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — EM 31
Conductivity Profile 3 and TEM
Resistivity Sections
48
EPA
Notes:
C
The slide presents an EM31 data plot and TEM approximate depth calculations versus
resistivity section along Profile 3.
50
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — EM31
Conductivity Contour Plot
49
EPA
Notes:
C
The slide presents an EM31 terrain conductivity map for the entire wood treatment site.
51
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — TEM Bedrock
Modeling Results, Sounding 1A
50
EPA
Notes:
C
The slide presents simultaneous inversion modeling results for the sounding TEM1A.
The data fits are shown for (a) resistivity data, and (b) TEM data. TEM values are latetime apparent resistivities. Modeling results are presented in (c).
52
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — TEM Bedrock
Modeling Results, Sounding 11
51
EPA
Notes:
C
TEM 11 modeling results are presented in this figure. Note the following, (a) represents
field data values using all time apparent resistivity, (b) are the results using late time
apparent resistivity, and (c) is the resulting layered earth parameters and 95%
confidence interval.
53
Field-Based Geophysical Technologies Online Seminar
Taylor Lumber Site — Summary
‹ Time domain electromagnetic geoelectric sections clearly
identified paleochannels
‹ Correlation of TEM and conductivity data provide
confidence in areas where TEM could be collected or
was unreliable
‹ Boreholes were recommended downgradient based upon
geophysical interpretation to optimize the sampling
design
‹ Bedrock was found to consistently dip gently to the
northeast suggesting few locations for DNAPL
accumulation to occur
52
EPA
Notes:
C
The combined use of the two techniques employed at the site were used to focus well
placements downgradient of the facility within the paleochannels on the site. Coarse
grained deposits most likely to act as conduits to transport the contaminants were
localized within the southeastern portions of the facility.
54
Field-Based Geophysical Technologies Online Seminar
GPR Surveys — Physical Basis
‹ Principles of GPR
‹ Generation of the electromagnetic wave
‹ Propagation and scattering of the
electromagnetic wave
‹ Applications
‹ Conclusions
53
EPA
Notes:
C
GPR technologies use the transmission of pulses of electromagnetic energy (radar waves)
into the ground. The signals transmitted travel into the ground and are reflected by
buried objects. Reflected signals travel back to the receiving unit, are recorded, and are
processed into an image.
C
GPR transmitter antennas are designed to radiate a broadband pulse of only a few
nanoseconds’ duration when excited. For optimal performance, the GPR antenna should
be positioned perpendicular to the ground surface.
C
The propagation of electromagnetic waves in the subsurface is primarily dependent on
the frequency of the wave, conductivity of the ground, soil moisture content, and relative
permittivity of the subsurface. Abrupt changes in conductivity or relative permittivity
create interfaces in the subsurface where reflection or refraction can occur.
C
Under optimal conditions, GPR technologies are capable of detecting both metallic and
nonmetallic objects.
C
The main limitation of GPR is its reduced effectiveness in highly conductive soils (such
as wet clay soils).
55
Field-Based Geophysical Technologies Online Seminar
GPR Surveys — Instruments
‹ Instrumentation
‹ Costs
POWER
SUPPLY
GRAPHIC
RECORDER
TRANSMITTER
RECEIVER
TRANSMITTERRECEIVER SWITCH
ANTENNA
TRANSMITTED
IMPULSES
TAPE
RECORDER
GROUND
SURFACE
REFLECTED
IMPULSES
LAYERED
MATERIAL
ISOLATED
REFLECTOR
54
EPA
Notes:
C
GPR instruments consist of a control unit, antennas and cables, a printer or digital data
recorder, and a power supply. The control unit generates timing signals to key the
transmitter on and off and synchronize the keying with the receiver. The unit controls
the scan rate, the time range over which echoes are compiled, and the gain applied to the
echoes. The transmitting antenna is excited by a semiconductor device (therefore, it is a
transducer). A separate, but identical, receiver is used because echoes can return from
near-surface targets before the transmitting antenna has stopped radiating. Transmitting
antenna frequency ranges of 80 megahertz (MHz) to 500 MHz are used most often in
environmental applications.
C
GPR equipment can be rented for approximately $3,000 to $7,000 per month or $150 to
$350 per day, depending on accessories. Typical mobilization fees are approximately
$300.
Instructor:
Explain that the graphic provides a schematic illustration of a typical
GPR system. Most instruments provide either digital or strip-chart
recording of processed data. Data recorded digitally sometimes are
enhanced through additional signal processing.
56
Field-Based Geophysical Technologies Online Seminar
Uses of GPR
‹ Obtain cross-section of natural geologic and
hydrogeologic conditions
‹ Locate buried man-made objects and structures
‹ Detect and map some contaminant plumes and
buried wastes
‹ Best resolution of all surface geophysical
methods
‹ Borehole
55
EPA
Notes:
C
The typical display of GPR data shows a cross-sectional view of received reflections of
the subsurface. Data processing and interpretation can provide a cross sectional
representation of the subsurface in some cases.
C
GPR is often useful in detecting man-made objects and structures including tanks, pipes
and foundations.
C
In site-specific instances, GPR has been effective in detecting contaminant plumes and
buried waste.
C
GPR, when it is applied in the optimum geologic or site setting, can provide the best
resolution of subsurface conditions of all surface geophysical methods.
57
Field-Based Geophysical Technologies Online Seminar
Complete GPR System
Printer
56
EPA
Notes:
C
This is a photograph of a complete, portable GPR system, the Subsurface Interface Radar
(SIR) 2 system developed by Geophysical Survey Systems Inc. (GSSI). The system
consists of a real-time color display and controller that is carried by the operator, a
battery pack, a thermal printer (for hard copies of data), and a GPR antenna. In this case,
a high-frequency antenna is used to detect shallow targets — for example, to determine
the thickness of reinforced concrete pavement.
58
Field-Based Geophysical Technologies Online Seminar
Low-Frequency Bistatic GPR
Antenna
57
EPA
Notes:
C
This illustration shows a pair of 100 MHz GPR antennae. The antennae are designed for
deeper exploration. An operator is shown pulling the antennae across the ground
surface. Also shown in the foreground is the cable that supplies power to the antennae
and transmits signals back to the controller-receiver.
59
Field-Based Geophysical Technologies Online Seminar
GPR Surveys — Interpretation
‹ Profiles and qualitative signal for pattern recognition
0
10
Distance (Ft.)
20
30
0
1
Approximate Depth (Ft.)
2
Fill
Fill
3
4
Tank
Tank
Tank
5
6
Diffraction Patterns
EPA
(continued)
Radar Profile – 500 mHz antenna
58
Notes:
C
The interpretation of unprocessed GPR data is relatively subjective and yields only
approximate information about the shape and location of buried objects. The depth of
investigation typically is no greater than 10 to 15 meters and may be considerably less in
soils that exhibit relatively high electrical conductivity. For example, a near-surface steel
underground storage tank (UST) buried in sand could be completely invisible to GPR if
it were covered by a 6-inch layer of highly conductive, clay-rich soil. Further, the
equipment is relatively bulky, and its operation requires a power source (such as a car
battery).
Instructor:
C
Use the “invisible UST” example noted to stress the utility of using
more than one method to support the investigation. In that example, an
inexpensive magnetometer or EM61 metal detector could be used to
locate such a tank quickly.
This slide shows the interpretation of the data presented in the previous slide.
60
Field-Based Geophysical Technologies Online Seminar
GPR — Advantages and Limitations
‹ Advantages
» GPR can provide a good cross-sectional
representation of the subsurface
» GPR is useful for a number of applications
‹ Limitations
» The method is not effective in conductive soils
» Processing of data into a “plan map” is difficult
» Depth interpretation may be difficult
59
EPA
Notes:
C
GPR surveys can provide a good cross-sectional representation of the subsurface. The
method is useful for a number of applications such as detecting drums, utilities, shallow
bedrock, fractures, and so on. The GPR method is very site specific. The method is
effective only in low conductivity soils such as sands. Clay and silt (conductive soils)
attenuate GPR signals and limit the depth of investigation. Lateral variations in soil
affect the performance of the system. These lateral variations will also affect the
interpretation about depth of measurement.
C
The project team should carefully consider the limitations of GPR when considering the
method for a site.
61
Field-Based Geophysical Technologies Online Seminar
GPR — Costs
‹ Daily rental rate = $105 to $225
‹ Accessory antenna = $25
‹ Shipping Weight = 100 to 200 pounds
‹ Contractor crews and equipment = $1,400 per
day
‹ Productivity varies from 1,200 feet per day to
more than 20 miles per day depending upon
logistics and objectives
60
EPA
Notes:
C
There are a number of vendors that rent geophysical equipment. Links to these vendors
are provides later in this discussion. Typical rates for GPR equipment are about $105 to
$225 per day. Shipping weights vary and should be considered in costing a project. If a
contractor were hired to perform the work, costs can vary from about $1,400 per day plus
mobilization and reporting.
62
Field-Based Geophysical Technologies Online Seminar
GPR — Summary
‹ Highly site-specific method
‹ Very effective in sandy soils
‹ Ineffective in clay soils
‹ Useful for detection of tanks and drums
‹ May be used for mapping subsurface geology if
favorable conditions exist
61
EPA
Notes:
C
GPR is useful only under certain site conditions.
C
The method requires relatively low conductivity soils such as sand.
C
The GPR method is not effective in clay soils or in areas where reinforced concrete is
present.
C
GPR can be useful for the detection of subsurface tanks, drums or for mapping
subsurface geology, if favorable conditions exist.
Instructor:
Inform the class that it is important to have significant site knowledge
before selecting GPR for a site investigation.
63
Field-Based Geophysical Technologies Online Seminar
Borehole Geophysical Methods
‹ Direct measure of
heterogeneity
‹ Geologic control and rapid
interpretation of data
‹ In situ analysis of physical
parameters
‹ Site-specific and inter borehole
applications in combination
with surface method results
Pad
EPA
(continued)
62
Notes:
C
Borehole geophysics can be used to obtain geologic data, including information about
geologic control and in situ analysis of physical parameters, especially in heterogeneous
conditions.
C
With borehole geophysical data, rapid interpretation is possible. When combined with
surface geophysics, application of borehole geophysical methods offers a threedimensional understanding of conditions.
C
Selection of a logging program should be considered carefully. Factors such as project
goals, geophysical information desired, instrumentation, and surface and subsurface
conditions will affect the logging program.
64
Field-Based Geophysical Technologies Online Seminar
Borehole Geophysical Methods
‹ Electrical and
magnetic
‹ Nuclear (gamma
and neutron)
‹ Caliper
‹ Sonic
‹ Video
‹ Nuclear magnetic
resonance
‹ GPR
63
EPA
Notes:
C
Electrical and magnetic borehole logging techniques generally are used for (1)
identifying general lithology, (2) performing stratigraphic correlation studies, and (3)
performing water quality studies.
C
Nuclear borehole logging techniques generally are used for (1) identifying clay and shale
layers, (2) performing stratigraphic correlation studies, and (3) measuring bulk density,
porosity, and moisture content.
C
Caliper borehole logging techniques generally are used in conjunction with other
borehole methods. The caliper log identifies changes in the diameter of the borehole as a
function of depth.
C
Sonic borehole logging techniques generally are used for (1) performing lithologic
characterization, (2) measuring porosity, and (3) identifying fractures and solution
openings.
C
Video borehole logging techniques generally are used for (1) inspecting the integrity of
monitoring well casings, (2) identifying fractures and solution openings, and
(3) performing lithologic characterization.
C
Nuclear magnetic resonance techniques generally are used for evaluating porosity,
permeability, moisture content, and water content.
65
Field-Based Geophysical Technologies Online Seminar
C
Other borehole logging techniques include (1) dipmeter surveying that identifies the
correlation of sedimentary structures and fractures and (2) directional surveying that
identifies the position of the borehole.
C
It is important to understand that application of such methods as neutron and gammagamma (nuclear techniques) requires a radioactive source. Use of such sources generally
requires a special license and may not be allowed in some states.
Instructor:
Inform participants that borehole logging techniques are traditional oilfield characterization tools that, in the environmental field, are not used
as frequently as in situ and surface geophysical techniques.
66
Field-Based Geophysical Technologies Online Seminar
Stratigraphic Correlation Based On
Gamma and Resistivity Logs
20
0 40 80 120 0 40 80
0
0 40 80 120 0 40 80
20
0
0 40 80 120 0 40 80
-20
-20
-20
-20
-40
-40
-40
-40
-40
-40
-60
-60
-60
-60
-60
-60
-80
-80
-80
-80
-80
-80
-100
-100
-100
-100
-100
-100
-120
-120
-120
-120
-120
-120
-140
-140
-140
-140
-160
-160
-160
-160
-20
-20
64
EPA
Notes:
C
This illustration shows three boreholes spaced approximately 100 feet apart. The vertical
differences are related to changes in surface elevation. Notice the similarities in the
measured gamma and resistivity from borehole to borehole in this example of
stratigraphic correlation of geologic units. The gamma data are shown on the left, while
the apparent resistivity is shown on the right of each borehole. Low resistivity and high
gamma count are likely related to clay zones or fine-grained geologic materials.
67
Field-Based Geophysical Technologies Online Seminar
Overview
Acoustic Velocity
Caliper
Lithology
Fine
Alluvium
Coarse
‹ Instrument
Quartz
sandstone
packages
and
coincident
grid surveys
Limestone
Sandstone
Coal
Spontaneous Long-Normal
Potential
Resistivity
10-inch
casing
Neutron
6-inch
open hole
Gamma
Shale
Gypsum
Freshwater
Salinewater
Sandstone
Fractured
Limestone
6-inch
open hole
Anhydrite
Arkose
Arkosic
conglomerate
Granite
Single-Point
Resistance
EPA
Temperature
65
Notes:
C
It is common practice to combine several instruments (for example, caliper, gamma ray,
and neutron) in one “package” and obtain measurements simultaneously in a single
downhole logging run. Similar approaches are used for surface geophysics when two or
more sensors are mounted to a survey vehicle in a package format. Both downhole and
surface geophysics approaches use multiple sensors or instruments to obtain
measurements during a single run or transect. As an alternative, two or more sensors can
be used in the same borehole or along the same transect, but not simultaneously. That
approach often is referred to as coincident surveying or the multisensor approach.
68
Field-Based Geophysical Technologies Online Seminar
Borehole — Advantages and
Limitations
‹ Advantages
» Assists in determining details missed in
geologic lithologic logs
» Assists in the selection of surface geophysical
tools and interpretation
‹ Limitations
» Most probes can only be used in open holes
66
EPA
Notes:
C
Borehole geophysical surveys are useful for the determination of specific details about a
geologic formation that may be missed in some borehole situations. The borehole tools
can provide detailed information about the physical properties of the subsurface. These
physical properties can assist in the selection of the proper geophysical tool to use for
surface geophysical surveys. Consideration of borehole techniques should be conducted
in advance of construction of monitoring wells or well completion. Uncased holes can
be used by a variety of borehole tools. PVC-cased holes can be surveyed using natural
gamma and electromagnetic induction conductivity. Steel-cased holes can be used by
limited borehole techniques.
69
Field-Based Geophysical Technologies Online Seminar
Borehole — Costs
‹ Daily rental rates
‹
‹
‹
‹
» Winch electronics cable (>500 feet) = $100
» Natural gamma, resistivity probe = $70
» Induction conductivity probe = $50
» Sonic probe = $175
» Flow rate = $100
Contractor crews and equipment = $1,150 per day
Logging fees = $0.30 to 3 per foot
Shipping weight = 100 to 400 pounds
Productivity varies with probe selection, number of runs or probes,
hole depth, and speed of logging. Usually about 10 to 20 feet per
minute, plus mobilization time.
67
EPA
Notes:
C
Daily rental rates are wide ranging for borehole systems. A common system includes the
winch, basic Natural Gamma - Resistivity - Self Potential probe and costs about $170 per
day. Additional probes are from about $50 up to about $175 per day. Shipping costs are
a consideration due to the length of probes and weight of the system. A contractor may
charge about $1,150 per day plus logging fees per foot. Typical logging rates are about
10 feet per minute depending on resolution and probe used.
70
Field-Based Geophysical Technologies Online Seminar
Case Study: University Of
Connecticut (UConn) Landfill
‹ Problem Statement:
» Locate disposal trenches
» Identify geologic features and distinguish them from
leachate
» Locate preferential pathways for leachate migration in
a fractured-rock system
‹ Geophysical Techniques:
» Surface (DC-resistivity, EM-conductivity, GPR)
» Borehole (caliper,gamma,conductivity,EM, opticalteleviewer,acoustic-televiewer, GPR)
68
EPA
Notes:
C
An integrated suit of geophysical methods were used to characterize the hydrogeology of
a fractured bedrock aquifer to identify contamination or pathways for contamination
migration near a former landfill at the University of Connecticut (UConn). Surface
methods were used to identify the dominant direction of fracture orientation and to locate
potential leachate plumes.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill — Survey and
Borehole Locations
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Notes:
C
The UConn campus is located in Storrs, Connecticut, in the northeastern part of the state.
The study area occupies a north trending valley with highlands to the northeast and
southwest. The 5-acre landfill is situated over a minor groundwater divide that drains to
the north and south along the axis of the valley. Surface runoff flows north through a
wetland towards Cedar Swamp Brook and south toward Eagleville Brook through a
seasonal drainage. The study area is bounded on the east by a steep hill and on the west
by minor hills. Bedrock is folded, faulted and fractured schist and gneiss with sulfide
layers. The bedrock aquifer is overlain by glacial till and unconsolidated deposits from
zero to six meters thick.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site —
Conductivity Profiling
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Notes:
C
The slide presents the inductive terrain-conductivity measurements collected using the
Geonics EM34 for three lines at the north end of the landfill study area with 10-meter
spacing on the left and 20-meter spacing on the right.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site —
Contoured Conductivity and Model
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Notes:
C
The slide presents the contoured terrain conductivity from the EM34 using vertical
dipole orientation with 20-meter separation from the grid. The bottom figure represents
the conductivity response curve generated by a forward modeling program and using the
data from the grid at the UConn Landfill.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site — Direct Current
Resistivity Results
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Notes:
C
The slide presents inverted resistivity sections generated from a modeling program.
From top to bottom, the dipole-dipole array, schlumberger array, resistivity model,
dipole-dipole array inverted synthetic resistivity sections, and the inverted schlumberger
array inverted synthetic resistivity sections. Data were collected with 5-meter spacing
between electrodes.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site — Borehole
Logs and Borehole Radar MW 121R
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EPA
Notes:
C
The data presented in this slide is from borehole MW121R. It includes caliper,
temperature, conductance, and conductivity information along with Transmissive
Fractures on the left and borehole radar on the right.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site — Borehole
Logs and Borehole Radar MW 105R
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EPA
Notes:
C
The data presented in this slide is from borehole MW105R. It includes caliper,
temperature, conductance, and conductivity information along with Transmissive
Fractures on the left and borehole radar on the right.
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Field-Based Geophysical Technologies Online Seminar
UConn Lanfill Site —
Specific Conductivity
– Pump Test
Fractures at 22 meters
less conductive then ones
at 34 meters
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EPA
Notes:
C
Shown in this slide is the specific conductivity in MW105R, in response to pumping at a
rate of 3.7-liters per minute. Individual logs shown are for elapsed time in minutes from
the start of the pumping.
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Field-Based Geophysical Technologies Online Seminar
UConn Landfill Site — Summary of
Geophysical Investigations
‹ Surface geophysical methods were used to
identify extent of disposal trenches and most
likely potential contaminant pathways for further
study
‹ Borehole results constrain interpretations of
surface methods allowing for the distinction
between geologic features and leachate plumes
‹ Without the use of geophysical methods
identification of preferred pathways would have
been extremely difficult
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EPA
Notes:
C
Surface geophysical surveys were used to identify potential contamination pathways at
the UConn landfill using electromagnetic and electrical geophysical methods.
Additional borehole geophysical tools were used to characterize hydrology of the
bedrock. Measured high electrical conductivity zones confirmed the presence of sulfiderich layers in the bedrock. Borehole results constrained the interpretation of surface
investigations and the overall survey demonstrates effectiveness of combined methods to
evaluate an electrically conductive fracture system that could represent a preferred
pathway for the migration of chlorinated solvent known to be present at the site.
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Field-Based Geophysical Technologies Online Seminar
Seismic Surveys — Interpretation
Types of Seismic Surveys
‹ Seismic Refraction
‹ Seismic Reflection
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EPA
Notes:
C
There are two types of seismic surveys employed for environmental investigations,
seismic refraction and seismic reflection.
Instructor:
Inform the class that the next slides will discuss some basic information
about the mechanics of seismic refraction and reflection followed by
some example field data results.
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Field-Based Geophysical Technologies Online Seminar
Seismic Surveys — Physical Basis
‹ Types of elastic waves
‹ Seismic velocities
‹ Interface effects
Shot Point
Recording Geophones
Direct waves and ground roll
Reflected waves
V1
Refracted waves
V2
EPA
Overburden
Bedrock
Where velocity, V2 > V1
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Notes:
C
When a sound wave travels in air, the molecules oscillate backward and forward in the
direction of energy transport. This type of wave propagates as a series of compressions
and expansions. It is referred to as a P-wave.
C
P-waves propagate in a solid matrix as do S-waves, which are oscillations at right angles
to the direction of energy transport. P and S wavefronts expand throughout the matrix
and are termed body waves.
C
In both refraction and reflection surveys, information about subsurface conditions is
derived by evaluating the travel times of various wavepaths between sources and
receivers.
C
As is the case with EM and sonic waves, scattering phenomena (including refraction and
reflection) and energy partitioning occur when a seismic wave encounters an interface
between two different types of rock.
Instructor:
Explain that the graphic illustrates some of the various wave
partitioning phenomena that can occur in a two-layer stratigraphic
system.
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Field-Based Geophysical Technologies Online Seminar
Refractive Seismic Survey
SEISMOGRAPH
GEOPHONES
ENERGY SOURCE
DIRECT WAVE
REFRACTED WAVE
SOIL
BEDROCK
EPA
79
Notes:
C
Refracted waves travel down through the overburden, are critically refracted below the
interface of the overburden and the bedrock, and then travel upward through the
overburden to the geophone. Along those paths, refracted waves travel at bedrock
velocities below the interface of the overburden and the bedrock and at overburden
velocities along the upward and downward paths.
C
This slide illustrates a simplified example of refractive seismic survey.
C
A generalized refraction seismic survey uses 12 geophones as a minimum. Typical
seismic sources include a sledge hammer, mechanical elastic wave generator or
explosives.
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Field-Based Geophysical Technologies Online Seminar
Travel Time (milliseconds)
Time/Distance Plot
Depth
Layer1
L2
Layer2
L1
Xc
Source to Geophone Distance
Xc
D=
L1 = Layer 1
2
L2 = Layer 2
V1 = Velocity of Layer 1 = 1/Slope of L1
V2 = Velocity of Layer 2 = 1/Slope of L2
Xc =Critical Distance
V2 -V1
V2+V1
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EPA
Notes:
C
This slide shows a graphical representation of refraction data and calculation of geologic
material velocity.
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Field-Based Geophysical Technologies Online Seminar
Refraction Analysis
1000
DEPTH (ft)
0
500
DISTANCE (ft)
0
-500
1250 ft/sec
3500 ft/sec
50
5000 ft/sec
100
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EPA
Notes:
C
The profile shows variations in velocity among three layers. Low velocity likely
represents unconsolidated sediments, while the high velocity represents bedrock.
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Field-Based Geophysical Technologies Online Seminar
Uses of Seismic Refraction
‹ Depth and thickness of geologic strata
‹ Depth to bedrock
‹ Depth to water table
‹ Determine fracture orientation
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EPA
Notes:
C
Seismic refraction may be used to determine the depth and thickness of geologic strata,
the depth to bedrock, the depth to the water table, and fracture orientation.
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Field-Based Geophysical Technologies Online Seminar
Seismic Refraction — Advantages
and Limitations
‹ Advantages
» Determination of depth and soil/rock velocity
» Infer soil competency, weathering, fractures
» Acquisition and processing less expensive than
reflection
‹ Limitations
» Resolution less than reflection surveys
» Large impact source may be required
» Increased rock velocity with depth required
» “Hidden layers” may be detected, but possibly not
interpreted
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EPA
Notes:
C
Seismic refraction surveys are less costly than reflection surveys. Many parameters
about the subsurface may be derived from the data including competency, “rippability,”
degree of weathering and fractures. Acquisition and processing costs are less than
seismic reflection surveys. The method does however require an increased soil or rock
velocity with depth. Hidden layers, layers imbedded within a low velocity layer, may be
hidden, within the data, but may not be defined.
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Field-Based Geophysical Technologies Online Seminar
Seismic Reflection
Seismograph
shot point
Geophone
V1
V2
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EPA
Notes:
C
Reflected waves travel down to the interfaces of the overburden and the bedrock and are
reflected back up at the same angle to the geophone. Reflected waves travel at
overburden velocities along their entire paths.
Instructor:
C
Inform the class that the following slides deal with interpretation of
seismic reflection and refraction data. The slide shows a recording
truck on the surface. Notice the seismic record indicates early arrivals
near the truck and later (apparently deeper) arrivals at a distance from
the truck. This phenomenon is likely due to the source near the truck
location. Also shown in the illustration is a geophone. Perhaps discuss
how the geophone is designed.
Seismic reflection surveys usually require additional equipment including an increased
number of recording geophones and a good energy source for generating signal. This
may include a vibration source.
Instructor:
Inform the class that the following slide shows simple processing and
interpretation of seismic reflection data.
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Field-Based Geophysical Technologies Online Seminar
Processed Reflection Record
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EPA
Notes:
C
The final product of a seismic reflection survey is obtained after significant processing of
data. Distance versus time plots are converted into distance versus depth plots. Often,
layers or structure is plotted on the interpretation map.
Instructor:
Inform the class that the seismic reflection method is useful for
obtaining detailed information about subsurface geologic layering and
structure.
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Field-Based Geophysical Technologies Online Seminar
Seismic Reflection — Advantages
and Limitations
‹ Advantages
» Resolution greater than seismic refraction
» Geometry between impact source and geophones
smaller than refraction
» Details within a soil or bedrock can be mapped
‹ Limitations
» Site conditions dictate data quality
» Rock properties are not derived
» Expensive acquisition cost for near surface objectives
» Processing cost greater than refraction
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EPA
Notes:
C
Seismic reflection surveys can provide a great resolution of the subsurface. The method
is somewhat site specific in that data quality may be in question in some situations such
as urban environments, areas where the impact source has problems such as in loose soils
or high water table. Acquisition and processing can be costly.
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Field-Based Geophysical Technologies Online Seminar
Seismic Survey (Refraction) —
Costs
‹ Daily rental rate = $135
‹ Accessories = $100 to $300
‹ Shipping Weight = 150 to 600 pounds
‹ Contractor crews and equipment = $2,000 to
$2,500 per day
‹ Productivity varies from about 1,200 linear feet
per day to several miles depending upon
logistics, objectives, and depth of investigation
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EPA
Notes:
C
Daily rental rates range from about $135 for seismograph and $100 to $300 for
geophones, cables and impact source. Several vendors are available to provide
equipment. Shipping weighs are a major consideration due to cost factors. A contractor
crew may charge about $2,000 or more per day plus the processing and report costs.
Productivity varies due to site conditions.
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Field-Based Geophysical Technologies Online Seminar
Seismic Survey (Reflection) — Costs
‹ Daily rental rate = $150 to $300
‹ Accessories = $100 to $1,000
‹ Shipping Weight = equipment often is
transported
‹ Contractor crews and equipment = $3,500 to
$5,000 per day
‹ Productivity varies from about 1,200 linear feet
per day to several miles depending upon
logistics, objectives, and depth of investigation
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EPA
Notes:
C
Daily rental rates range from about $150 to $300 for seismograph and $100 to $1,000 for
geophones, cables and impact source, including hammer, elastic wave generator and
vibrator sources. Several vendors are available to provide equipment. Shipping weighs
are a major consideration due to cost factors. A contractor crew may charge about
$2,000 or more per day plus the processing and report costs. Productivity varies due to
site conditions.
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Field-Based Geophysical Technologies Online Seminar
Seismic Survey Conclusions
‹ Method useful for determining depth to bedrock
and determining geologic formations
‹ Can be only method useful at some sites, such
as urban environment
‹ Labor intensive and may be costly
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EPA
Notes:
C
Seismic surveys can be useful for determining depth to bedrock and mapping geologic
formulations.
C
The seismic method may be the only geophysical method that can provide subsurface
information in some urban environments.
C
The method can be relatively labor intensive and the seismic reflection method can be
costly, particularly with respect to costs to mobilize equipment.
Instructor:
Inform the class that in urban environments, electrical power and
subsurface utilities can affect electrical and electromagnetic geophysical
methods. Magnetometers will be affected by ferrous objects such as
automobiles and reinforced concrete. Seismic surveys are not affected as
greatly by urban environments, except for vehicle motion and other
vibration noise sources.
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Field-Based Geophysical Technologies Online Seminar
Case Study: Imaging DNAPL Using
Seismic Refection Techniques at Two
Department of Energy (DOE) Sites
‹ Problem Statement:
» Can seismic methods be used to identify the
nature and extent of DNAPL?
‹ Methods Used:
» Standard high quality seismic reflection data
collected
» Amplitude of arrival time of seismic waves
versus offset (AVO) methods to detect DNAPL
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EPA
Notes:
C
This study was conducted using data from both the Department of Energy’s (DOE)
Hanford, Washington Site and The Savannah River Site in South Carolina. The study
used complex techniques to evaluate seismic data by modeling expected responses and
using high quality seismic field data. The technique used amplitude of arrival time of
seismic waves versus offset (AVO) methods to detect hydrocarbon bearing strata. This
technique commonly is used in oil and gas exploration.
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Field-Based Geophysical Technologies Online Seminar
Savannah River Site — Location
Map of M-Area Seepage Basin
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EPA
Notes:
C
This figure shows the location of all 2-D seismic lines acquired for AVO analysis at the
Savannah River Site M-Area seepage basin. Well MSB-3D is adjacent to MSB-22.
Well MSB-3D has free-phase DNAPL at the bottom of the well.
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Field-Based Geophysical Technologies Online Seminar
Savannah River Site — Graph of
Modeled Substrata - Reflection
Coefficients
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EPA
Notes:
C
The graph shown represents reflection coefficients versus angle of incidence using the
Zoeppritz equations for water saturated sand overlying TCE saturated sand. The middle
graph is for water saturated sand overlying the green clay. The lower graph is the
reflection coefficient versus angle of incidence for water saturated overlying the sand.
C
This slide is provided to the participant to illustrate how the interpretation was derived.
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Field-Based Geophysical Technologies Online Seminar
Savannah River Site — Offset Range
Limited Stacks
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EPA
Notes:
C
The slides shows offset range limited stacks and fluid factor stacks for profile M-1. Near
the offset section (top), far offset (middle), and fluid factor (bottom). The middle section
was generated by stacking offsets greater than 17.68 meters. High amplitudes that occur
only at far offsets should denote presence of DNAPL. The bottom section is a fluid
factor stack of M-1 Well MSB-3D that is adjacent to MSB-22. The water table occurs at
a depth corresponding to approximately 100 milliseconds time. The amplitude envelope
(magnitude of Hilbert transform) is displayed.
C
This slide is presented to show that the fluid factor stack can provide information about
the location of the contaminant bearing areas within the formation. It is not expected
that a full understanding of the processes to derive this data are obtained. A highly
trained seismic geophysicist is required for this type of data processing.
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Field-Based Geophysical Technologies Online Seminar
Hanford Site — Location Map
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EPA
Notes:
C
This figure shows the location of all 2-D seismic lines acquired for AVO analysis at the
Hanford study area site.
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Field-Based Geophysical Technologies Online Seminar
Hanford Site — Seismic Reflection
Line
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EPA
Notes:
C
This figure shows the seismic reflection line Z-9-1 at 200 West area Hanford Site. The
line direction is south to north, left to right. The upper black line is the top of the
Hanford Fine. The lower black line is the top of the Plio-Pleistocent, and the blue line is
the top of the caliche layer. The concave features are channels in the Hanford Formation.
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Field-Based Geophysical Technologies Online Seminar
Hanford Site — Plio-Pleistocene
Contour Map
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EPA
Notes:
C
This map shows the amplitude values for the Plio-Pleistocene surface.
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Field-Based Geophysical Technologies Online Seminar
Hanford Site — Caliche Contour Map
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EPA
Notes:
C
This map shows contours of amplitude values for the top of the caliche layer.
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Field-Based Geophysical Technologies Online Seminar
Hanford Site — Carbon
Tetrachloride Isoconcentration Map
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EPA
Notes:
C
This map shows the carbon tetrachloride isoconcentratins at the top of the caliche layer.
The contour intervals are in parts per million.
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Field-Based Geophysical Technologies Online Seminar
Savannah River Site and Hanford
Site — Summary
‹ Savannah River Site DNAPLs can be imaged using high
resolution seismic data
‹ Hanford Site seismic modeling agree with measured
amplitude anomalies
‹ AVO methods when modeled correctly can be used to
map DNAPL
‹ This is a somewhat costly study and caution must be
exercised in applying the technique to other sites. Site
information and modeling are necessary before a seismic
survey is conducted.
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EPA
Notes:
C
This study shows that, in these two cases, DNAPL can be imaged using high resolution
seismic reflection data. The Hanford site model agrees with measured amplitude
anomalies. This method and study is quite costly (costs are not known as of this
writing). Caution must be exercised before full seismic surveys are conducted. Site
information and modeling are necessary before conducting this type of investigation.
C
The participant should realize that the processing techniques used in this case study were
used by a highly trained seismic geophysicist. The participant is not expected to fully
understand the detailed processing, but rely on a geophysical contractor team for the data
processing and interpretation.
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Field-Based Geophysical Technologies Online Seminar
Seminar Summary
‹ In fractured or other complex geologic settings,
geophysics can increase the information content
available at a reasonable cost upon which a
systematic plan can be established for a site
‹ Geophysical methods can result in substantial
cost savings by helping to focus monitoring and
measurement activities
‹ Using both surface and subsurface methods with
differing capabilities is the best means for
assuring the defensibility of project decisions
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EPA
Notes:
C
When applying the Triad Approach at complex sites or where little is known about the
geology and hydrogeology at a site, geophysical methods offer high information value at
a reasonable cost. Intrusive sampling activities can be focused and remedial goals
achieved more quickly and effectively when some information is used to guide follow-up
activities. It is important to have a good handle on the potential interference that can
impact the results before selecting and implementing a geophysical program to help
guide systematic planning and development of a CSM. Expending more time and money
during the front end-planning portions of a program and considering the use of
geophysical methods will, in the long run, prove to be a wise use of time and funds.
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Field-Based Geophysical Technologies Online Seminar
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EPA
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