United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Las Vegas NV 89114
Research and Development
EPA/600/S7-84/064 May 1985
SER& Project Summary
Geophysical Techniques for
Sensing Buried Wastes and
Waste Migration
J. Jeffrey van Ee
Descriptions of the use of six geo-
physical techniques are presented to
provide a broad understanding of the
application of these techniques for
sensing buried wastes and waste migra-
tion. Technical language and jargon are
avoided as much as possible so that
those with limited technical background
can acquire a general understanding of
current techniques and sufficient back-
ground to define project requirements,
select professional support, and mon-
itor and direct field programs.
Emphasis on cost-effective investiga-
tions at hazardous waste sites requires
an integrated, phased approach: (1)
preliminary site assessment involving
the use of aerial photography, on-site
inspections, and readily available infor-
mation to approximate site boundaries
and locations of waste concentrations,
as well as probable site geology; (2)
geophysical surveys to pinpoint buried
wastes, estimate quantities, and delin-
eate plumes of conductive contami-
nants in groundwater; and (3) confirma-
tion of groundwater contamination
through monitoring well networks de-
signed on the basis of plumes and
subsurface stratigraphy defined by the
geophysical surveys.
The six geophysical techniques de-
scribed include metal detection, mag-
netometry, ground penetrating radar,
electromagnetics, resistivity, and seis-
mic refraction (Figure 1). Metal detec-
tors and magnetometers are useful in
locating buried wastes. Ground pene-
trating radar can define the boundaries
of buried trenches and other subsurface
disturbances. Electromagnetic and re-
sistivity methods can help define plumes
of contaminants in groundwater. Re-
sistivity and seismic techniques are
useful in determining geological strat-
igraphy.
This Project Summary was developed
by EPA's Environmental Monitoring
Figure 1. Geophysical sensing techniques.
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Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The cost-effective investigations of haz-
ardous waste sites involve an integrated,
phased approach: (1) preliminary site
assessment, involving the use of aerial
photography, onsite inspections, and
readily available information to approx-
imate site boundaries and locations of
waste concentrations, as well as probable
site geology; (2) geophysical surveys to
pinpoint buried wastes and estimate
quantities, and to delineate plumes of
conductive contaminants in groundwater;
and (3) confirmation of groundwater
contamination through monitoring well
networks designed on the basis of plumes
and subsurface stratigraphy defined by
the geophysical surveys.
Geophysical Sensing
Figure 2 diagrams the operation of
simple metal detectors, which respond to
changes in electrical conductivity caused
by the presence of metallic objects, both
ferrous and nonferrous. Figure 3 is a
schematic of a simple magnetometer,
which can be used to detect perturbations
in the earth's geomagnetic field caused
by buried ferromagnetic objects such as
drums, tools, or scrap metal. Magnetom-
eters can sense ferrous objects at greater
depths than metal detectors, and can be
used to locate objects even in the pres-
ence of interferences created by fences,
power lines, and buried pipes and cables.
Figure 4 is a schematic showing the
various components of a ground-pene-
trating radar (GPR) system which radiates
short-duration electromagnetic pulses
into the ground from an antenna near the
surface. These pulses are reflected from
interfaces in the earth (such as trench
boundaries) and are picked up by the
receiver section of the antenna.
Figure 5 shows the basic principle of
operation of electromagnetic (EM) con-
ductance measuring devices, which yield
a signal proportional to the conductivity of
the earth between the transmitter and
receiver coils. Figure 6 shows EM profiles
from a hazardous waste site investigation
over a 25-acre site, together with the
locations of monitoring wells which were
installed without benefit of the geophys-
ical measurement data. In this instance,
use of the geophysical surveys would
Transmitter
and
Receiver
Circuits
Primary Field
Emitted by Transmitter
Distorted by Buried
Metallic Objects
Figure 2. Metal detector.
have enabled a more efficient monitoring
well network to be designed and could
have reduced the total number of wells
installed. Figure 7 shows the traces
resulting from passes across a barrel-
filled trench with a metal detector, mag-
netometer, electromagnetic induction
instrument, and GPR.
Figure 8 shows a common electrode
configuration, called a Wenner array, for
resistivity soundings. In this configura-
tion, a current is injected into the ground
by a pair of surface electrodes, and the
resulting potential field is measured
between a second pair of surface elec-
trodes. The subsurface resistivity is cal-
culated from the electrode separation,
applied current, and measured voltage.
The measured resistivity is a function of
the total geohydrologic section, including
soil, rock, and groundwater. Interpreta-
tion of resistivity measurements provides
information on layering and depths of
subsurface horizons as well as lateral
changes in the subsurface.
Figure 9 shows a schematic view of
seismic waves traveling through a multi-
layered system of soil and rock strata; at
each interface between layers energy is
reflected back to the sensor. Using know-
ledge of travel times in rock strata, the
approximate depths of the various strata
can be estimated.
Metal Detectors
At hazardous waste sites, metal de-
tectors are primarily used to determine
the presence, location, and definition of
trench boundaries. They can also be used
to assist in the process of selecting a site
for drilling, so that metallic containers
and underground utilities are not acci-
dentally struck during the drilling opera-
tions. Buried tanks and pipes which may
be sources of leaks can be located, and in
addition, the location of utilities may serve
to define areas representing more per-
meable passageways in which contam-
inants may flow.
Metal detectors will detect any kind of
metallic material, including ferrous met-
als, such as iron and steel, and nonferrous
metals, such as aluminum and copper. (In
contrast, the magnetometer responds
only to ferrous metals.)
Metal detectors have a relatively short
range. They can detect quart-sized con-(
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Amplifiers
and
Counter
Circuits
/^\\
^Vl^
Chart and
Mag Tape
Recorders
0?$&?»^
"'
Distortion in
Earth's Magnetic Field
Due to Iron in
Buried Drum
Figure 3. Magnetometer.
Graphic Recorder
Antenna
Sampler
Circuits
Bow\Tie
5 to 300 Meten
Cable
Controller
0 Radar £
Waveform
o
'^X-?^
o
Tape Recorder
oo
a a a a
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Figure 4. Ground-penetrating Radar.
meter. The response of a metal detector
increases with the target's surface area;
therefore larger objects, like 55-gallon
drums, may be detected at depths up to 3
meters, and massive piles of metallic
materials may be detected at depths up to
6 meters. Specific performance is highly
dependent upon the type of metal detector
used. Generally, most metal detectors are
incapable of responding to any targets, no
matter how large, at depths much greater
than 6 meters.
An experienced operator can usually
make a reasonably accurate estimate of
target size and depth. However, any
attempt at detailed calibration will likely
be useless, because of the many variables
involved.
Metal detectors are very susceptible to
noise caused by some natural soil condi-
tions, unwanted metallic debris, pipes,
fences, vehicles, buildings, etc.
There are many different types of metal
detectors available commercially, each
with its own advantages and limitations.
The choice of a metal detector should be
determined by the type of targets to be
located, their depth, the nature of the soil,
the size of the search area, site conditions,
and other project requirements.
Magnetometer
A magnetometer responds to the pres-
ence of buried ferrous metals. At hazard-
ous waste sites, magnetometers may be
used to:
• Locate buried 55-gallon drums;
• Define boundaries of trenches filled
with ferrous containers;
• Locate ferrous underground utilities,
such as iron pipes or tanks, and the
permeable pathways often associated
with them;
• Aid in selecting drilling locations that
are clear of buried drums, underground
utilities, and other obstructions.
While several factors influence the
response of a magnetometer, the mass of
a buried target and its depth are the most
important. A magnetometer's response is
directly proportional to the mass of fer-
rous metal present and varies by one over
the distance cubed (1/d3) for total field
measurements. If a gradiometer is used,
the response falls off even faster, as one
over the distance to the fourth power
(1 /d4). With sensors of equal sensitivity,
the total field system provides the greater
working range. Typically, a single drum
can be detected at distances up to 6
meters, while massive piles of drums can
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Phase
Sensing
Circuits
and
Amplifiers
/^^N
VI^
Chart and
Mag Tape
Recorders
— ' rn;i "»»
Secondary Fields
from Current Loops
Sensed by
Receiver Coil
Figure 5. Electromagnetic measurement of conductance.
tainers at a distance of approximately one
be detected at distances up to 20 meters
or more. There is a wide variety of
magnetometers available commercially;
specific performance is highly dependent
upon the type of magnetometer and the
field conditions. While the number and
depth of buried drums may be calculated,
such results should be considered only
approximations because of the number of
variables associated with target, site
conditions, and calculations. Actual re-
sults may vary considerably.
A magnetometer with continuous re-
cording capabilities can be used to pro-
duce a strip chart of the field data. A strip
chart is helpful in assessing signal-to-
noise ratio, anomaly shape, and target
location, and provides a means of exercis-
ing quality control over field data. This
continuous coverage is much more suit-
able for high-resolution requirements and
the mapping of extensive areas.
The effectiveness of a magnetometer
can be reduced or totally inhibited by
noise or interference from time-variable
changes in the earth's field and spatial
variations caused by magnetic minerals
in the soil, or iron and steel debris, ferrous
pipes, fences, buildings, and vehicles.
Many of these problems can be avoided
by careful selection of instruments and
field techniques.
Ground Penetrating Radar
In areas where sufficient ground pene-
tration is achieved, the radar method
provides a powerful assessment tool. Of
the geophysical methods discussed in
this document, radar offers the highest
resolution. The method provides contin-
uous spatial sampling and can be carried
out very rapidly at traverse speeds from
0.5 to 8 km/h. Its continuous graphic
format permits rapid semi-quantitative
interpretation for in-field analysis.
Radar performance is highly site-spec-
ific. Depth of penetration is primarily
dependent upon soil properties and pore
fluids which influence electrical conduc-
tivity. In the wide range of natural soil/
rock conditions found throughout the
United States, ground penetrating radar
penetration varies from less than a meter
to more than 30 meters. Typical maximum
penetrations at any given site are 1 to 10
meters.
Interpretation of radar data is relatively
straightforward if site conditions are
simple and a strong dielectric contrast
exists between the features of interest
and the surrounding soil. As subsurface
conditions increase in complexity, inter-
pretation of the data becomes difficult,
and more elaborate interpretation and
processing may be necessary. High qual-
ity radar data is not commonly obtained in
the field; however, experienced interpret-
ers are usually able to cope with field data
of lower quality.
A radar system is a complex instrument.
The results of a radar survey are depend-
ent on many interacting system controls,
various field procedures, site conditions,
and interpretation. Therefore, the suc-
cessful application of the radar method
requires personnel with an understand-
ing of electronics, physics, and basic
earth sciences. The more complex the
site problem, the greater the amount of
training and experience required.
Figure 6.
= Well
Three dimensional representation of conductivity data, showing location of buried
hazardous materials.
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EM Conductivity
Magnetometer
Metal Detector
Loess -
Hardpan
Cherty
Clay
Figure 7. Four geophysical profiles.
Electromagnetics (EM)
Although the EM technique can be
used for profiling or sounding, profiling is
the most effective use of the method.
Profiling makes possible the rapid map-
ping of subsurface conductivity changes,
and the location, delineation, and assess-
ment of spatial variables resulting from
changes in the natural setting or from
many contaminants.
EM is a very effective reconnaissance
tool. The use of qualitative non-recorded
data can provide initial interpretation in
the field. If site conditions are complex,
the use of a high-density survey grid,
continuously-recording instruments, and
computer processing may be necessary
in order to properly evaluate subsurface
conditions. When continuously-recording
instruments are used, total site coverage
is feasible. More quantitative information
can be obtained by using conductivity
data from different depth ranges. At
present, three different systems must be
used to acquire data from 0.75 meters to
60 meters. Very often, however, data
from two standard depths, e.g., 6 and 15
meters, is adequate to furnish depth
information.
Resistivity
The resistivity method provides a means
of measuring one of the electrical proper-
ties of the geohydrologic section including
soil, rock, and groundwater. These meas-
urements may be used to assess lateral
changes and vertical cross sections of the
natural geohydrologic settings. Since the
resistivity of soils and rocks is predom-
inantly controlled by porosity, permeabil-
ity, amount of water, and concentrations
of dissolved solids in the water, the
method provides a tool to evaluate con-
taminant plumes and to locate buried
wastes at hazardous waste sites.
The resistivity technique may be used
for "profiling" or "sounding." Profiling
provides a means of mapping lateral
changes in subsurface electrical proper-
ties. This field technique is well-suited to
the delineation of contaminant plumes
and to the detection and location of
changes in natural geohydrologic condi-
tions. Profile lines and contour maps can
be used to locate geologic variations or
contaminant plumes. The apparent resist-
ivity values are typically used becausethe
primary objective is to use the data for
location purposes. Relative trends and
semi-quantitative analyses are often im-
mediately obvious to the experienced
interpreter from a plot of sounding data.
Sounding provides a means of deter-
mining the vertical changes in subsurface
electrical properties. Interpretation of
sounding data provides the depth and
thickness of subsurface layers having
different resistivities. Commonly 3 to 4
layers may be resolved with this tech-
nique. The resistivity sounding method is
in general a more effective method than
the EM sounding method described. The
analysis of resistivity sounding data re-
quires that the interpreter be knowledge-
able about the resistivity method, the
conditions under which the data are
obtained, the geohydrologic conditions,
as well as the specific techniques, com-
puter models, or curve matching.
The operator must insure that adequate
space is available at the site and that it is
relatively clear of buried pipes and fences.
Finding sufficient space for a long profile
array with an overall length nine to twelve
times the depth of interest can sometimes
be a problem.
Although resistivity sounding methods
are primarily intended for use in uniformly
layered geological conditions, useful data
may be obtained from the complex sub-
surface conditions often found at haz-
ardous waste sites. With both profiling
and sounding techniques, inhomogene-
ities in the nearsurface soils may intro-
duce noise in the data. Some surface
conditions such as dry surface materials,
concrete roads, or parking lots may
preclude the use of the resistivity method.
The resistivity method is inherently
limited to station measurements, since
electrodes must be in physical and elec-
trical contact with the ground. This re-
quirement makes the resistivity method
slower than a non-contact method such
as EM.
Seismic Refraction
The seismic refraction method can be
used to aid in defining natural geohydro-
logic conditions, including thickness and
depth of soil and rock layers, and depth to
bedrock or water table. Generally two- or
three-layer systems can be analyzed in
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Battery
Current Meter
Figure 8. Resistivity measurement.
the field by the use of nomograms and
simple calculations. More complicated
sites having three to four layers with dip
will require a programmable calculator or
a small computer to solve the seismic
equations.
Since seismic wave velocity is directly
related to the material properties of the
layer such as density and hardness,
lateral variations in composition or an
irregular interface between layers will
show up as geologic scatter on a time/
distance plot. This is a valuable indicator
of variations in site conditions. The anal-
ysis of this data requires that the inter-
preter be knowledgeable about the meth-
od, the conditions under which the data
was obtained, and the geohydrologic
conditions.
The seismic line must be three to five
times the maximum depth of interest.
Lateral resolution in the data is deter-
mined by the geophone spacing.
Depending on site conditions, a ham-
mer source is useful for obtaining seismic
data to depths of 10 to 15 meters, while a
500-pound drop weight is required for
depths of 50 to 100 meters. Explosives or
projectile sources may be used to obtain
deeper data.
Since the seismic method measures
small ground vibrations, it is susceptible
to vibration noise from a variety of natural
and cultural sources.
The seismic method is inherently a
station measurement because geophones
must be implanted in the surface of the
ground. This makes the method relatively
slow when compared to the other contin-
uous techniques.
Conclusions
Geophysical methods cannot be ex-
pected by themselves to provide all the
information that an investigator may need
in the assessment of a hazardous waste
site. Nevertheless, the information gath-
ered can be helpful in completing the
picture that is developed solely from
monitor well data. Similarly, no one
geophysical method can be expected to
provide all the answers. The authors of
this document have sought to show that
six existing geophysical methods in ex-
perienced hands can be effectively em-
ployed in the study of hazardous waste
sites provided the capabilities and limita-
tions are understood.
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Control and Recorder
Unit
Surface
Siesmic
Source
'*^':':^
Boundary
Boundary
, R2
R1
\
R2
\
R3
I
Trace 1
I I
Trace 2 (1st Enhancement)
\ I
Figure 9. Basic seismic reflection technique.
EA Trace 3 (2nd Enhancement)
L/*.-*,,,- '"V^.—.-_- •
U. S. GOVERNMENT PRINTING OFFICE: 1985/559 111/10849
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Richard C. Benson, Robert A. Glaccum, and Michael R. Noel are with TECHNOS.
Inc., Miami, FL 33142.
J. Jeffrey van Ee is the EPA Project Officer (see below).
The complete report, entitled "Geophysical Techniques for Sensing Buried
Wastes and Waste Migration,"(Order No. PB84-198 449; Cost: $22.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89114
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
OCOC329 PS
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