United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-92/089 June 1992
Project Summary
A Study to Determine the
Feasibility of Using a Ground-
Penetrating Radar for More
Effective Remediation of
Subsurface Contamination
Dennis G. Douglas, Alan A. Burns, Charles L. Rino, Joseph W. Ma re sea, Jr.
and James J. Yezzi
A study was conducted (1) to assess
the capability of ground-penetrating ra-
dar (GPR) to identify natural subsur-
face features, detect man-made objects
burled in the soil, and both detect and
define the extent of contaminated soil
or ground water due to a toxic spill,
and (2) to determine the minimum per-
formance specification (In terms of
hardware, data collection, and signal
processing) necessary for a GPR to
achieve these goals. As a means of
addressing both aspects of the study,
several models were developed to
quantify the response of different GPR
systems to these subsurface environ-
ments. A number of conclusions
emerged from this study. The technol-
ogy for making all of the above mea-
surements already exists, but the sys-
tems most commonly found in com-
mercial use today either are not ad-
equately designed to detect and define
subsurface soil and ground-water con-
tamination or are not operated in such
a way as to make this possible. In terms
of hardware, it was found that, to oper-
ate effectively in all three generic sub-
surface environments investigated in
this study, a radar system must have a
very high figure of merit. In terms of
signal processing, it was found that for
typical GPR systems synthetic-aper-
ture-radar (SAR) processing is required;
this conclusion was based on three
reasons: (1) better horizontal resolu-
tion is achieved with SAR processing;
(2) SAR processing allows the system
to operate at lower frequencies and thus
achieve deeper penetration; and (3)
SAR processing reduces ambient noise,
which improves the detection and iden-
tification capabilities of GPR. It is rec-
ommended that simple proof-of-prin-
ciple experiments be undertaken to vali-
date the models developed in this
study. To the extent that the experi-
ments prove successful, GPR may be-
come a significant tool in rapidly iden-
tifying and cost-effectively remediating
subsurface contamination.
77i/s Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, 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
Remediation of toxic spills is often costly
and entails cumbersome procedures to
determine the horizontal and vertical ex-
tent of the contaminated soil or ground
water. The traditional method is to drill
core samples in the area where the con-
taminant is thought to be present and
then analyze these in a laboratory. The
denser the sampling grid, the more effec-
tive it is; unfortunately, it is also more
expensive to implement and more damag-
ing to the environment. Even with very
dense sampling grids, serious interpreta-
tion mistakes can be made if a fluvial
pathway is not detected by monitoring
wells. A nonintrusive method of detecting
Printed on Recycled Paper
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subsurface contamination, therefore, would
be highly desirable. Toward this end, the
use of ground-penetrating radar (GPR) to
locate and map subsurface contamination
was investigated. If GPR proves effective,
it can be combined with conventional meth-
ods to ensure better placement of drilling
sites and to reduce the number of samples
necessary.
GPRs have been proposed repeatedly
for applications involving sensing or map-
ping the location of underground contami-
nants. Some experimental evidence of the
successful use of GPR for this purpose
has been presented in a number of publi-
cations and technical presentations. How-
ever, this evidence is largely anecdotal
and lacks quantitative support. Thus, those
positive results appear as special cases
that cannot be extrapolated to general situ-
ations. This deficiency arises from the lack
of a quantitative model for the effect of
contaminants on those properties of soils
that affect GPR performance and, hence,
GPR performance specifications. The work
described in the full report shows that the
lack of success in these earlier programs
can be largely ascribed to use (or consid-
erations) of radars whose performance lev-
els are insufficient for the task and to a
need for much more intensive data pro-
cessing.
While the literature indicates that radar
measurement systems may have the po-
tential to improve remediation efforts, such
potential has yet to be realized. Three key
elements are lacking in the radar studies:
(1) a quantitative model of the effect of
contaminants on the radar-relevant prop-
erties of soils, (2) understanding and
"matching" the radar characteristics to the
environment and the target(s) to be de-
tected so as to get the best possible sig-
nals, and (3) the development of signal-
processing methods best suited to take
advantage of the signals returned from
subsurface scatterers.
Objectives
One objective of this work was to as-
sess the capability of GPR to identify natu-
ral subsurface features, detect man-made
objects buried in the soil, and both detect
and define the extent of contaminated soil
or ground water. The second objective
was to determine the minimum GPR per-
formance specification required to accom-
plish the first objective.
Report Organization
The work done in fulfilling these objec-
tives is presented in the final report, "A
Study to Determine the Feasibility of Us-
ing a Ground-Penetrating Radar for More
Effective Remediation of Subsurface Con-
tamination."
Conclusions
The consideration of alternative designs
for the radar system required the elucida-
tion of a "figure of merit" for the system
that included hardware and data collec-
tion considerations. This work considered
the relative performance of several exist-
ing radar designs: a typical short-pulse
radar, a short-pulse radar with higher trans-
mitter power, and a synthetic-pulse radar.
The results of this work showed that, al-
though the short-pulse and synthetic-pulse
radars were mathematically equivalent, the
synthetic-pulse radar offered performance
that was potentially 40 to 60 dB better
than the short-pulse radar because it could
transmit far greater power per spectral
line. Furthermore, while the improved per-
formance offered by the synthetic-pulse
system was not often needed for the usual
"hard-target" GPR applications, this im-
proved performance was essential for the
detection of small changes in dielectric
constant such as would be expected in a
situation where a small region (or thin
layer) of contamination was encountered.
Two signal-processing methods were
included in this stage of the work: real-
aperture and synthetic-aperture (SAR) pro-
cessing. The results showed that synthetic-
aperture (coherent) processing had a sig-
nificant advantage in remediation applica-
tions over the typically used incoherent
processing. Although it is computation-in-
tensive, SAR processing afforded a higher
signal-to-noise ratio and effectively sup-
pressed the clutter from adjacent reflec-
tions. To detect modest levels of most
common contaminants at depths of 10 to
15m and in moderately conducting soils,
it was estimated that a combined radar-
processing figure of merit of 200 to 220
dB is necessary. Considering the current
radar technology, it was determined that
this level of figure of merit could best be
achieved by combining a synthetic-pulse
radar with synthetic-aperture processing.
It was also determined that one can ob-
tain an additional processing gain of 35
dB by using a 3000-s observation time to
collect data from a three-dimensional vol-
ume and then SAR-processing these data;
the figure of merit could thus be effec-
tively increased by 35 dB, which would
mean better performance of the GPR in
detecting the desired target.
As part of the radar design assessment
task, a numerical model was developed
that could illustrate the nature of radar
returns from various modeled soils and
geometries and show the processing gains
obtained by different signal-processing
methods. The model described the electri-
cal characteristics of soils and of various
potential contaminant materials at radar
frequencies from about 20 MHz to about
200 MHz. This work showed that, for the
radar frequencies considered, the dielec-
tric constant and attenuation coefficient
associated with these soils (with the ex-
ception of wet clays and ionic (salt-laden)
silts and sands) were not fundamental ob-
stacles to the propagation of radar energy
through the medium. The work further
showed that, for the range of radar fre-
quencies most suitable to radar energy
penetration to a working-goal-depth of
about 10 m (in nominal soil conditions),
consistent with the bandwidth necessary
for adequate resolution, the dielectric con-
stants and attenuation coefficients were
nearly constant over the spectral range.
The second stage of the work entailed
developing geological and contamination
geometries that could describe various
contamination layers and plumes. These
geometries were needed so that the per-
formance of the radar designs could be
tested against the modeled electrical char-
acteristics of the bulk materials. This work
led to the development of three essential
geometries that represented seven of the
ten "common cases" of remediation con-
figurations described by the EPA.
It is difficult to define a "typical"
remediation site suitable for a baseline
test, because soil properties, moisture con-
tent, and contamination environment dif-
fered dramatically from site to site. An
almost limitless combination of factors
could be ascribed; this made the radar
design assessment difficult because not
every combination could be addressed.
To deal with this lack of a "typical site," a
generic soil condition (comprised of a sand-
clay mixture) was selected. The assess-
ment work entailed adding various mois-
ture and contaminant contributions to this
soil mix, appropriate to the geometry con-
sidered. This "soil" is roughly equivalent to
the "synthetic soil matrix" (SSM) devel-
oped by the EPA as representative of
Superfund sites.
The numerical model allowed the ef-
fects of various spatial sampling schemes
to be examined. This model showed that,
with the incoherent signal processing usu-
ally applied to GPR data, the wide
beamwidths associated with GPRs cre-
ated a confusing display of the subsur-
face environment; these results were con-
sistent with the images usually created by
commercial GPRs. The numerical model
also showed that when synthetic-aperture
processing was used, the background
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noise in the radar images was reduced
and discrete scatterers in the radar field
of view were tightly localized.
A significant portion of the design as-
sessment task addressed various detec-
tion strategies. That is, given a radar and
a geometry, this work sought to identify
key strategies that could be used to de-
duce the presence (or lack) of contamina-
tion in the subsurface returns from the
processed data. This was a central effort
in the work (after the basic feasibility of
using synthetic-pulse radar and SAP pro-
cessing for remediation support had been
established). This work resulted in two
novel findings. First, it appears that for
contaminants with low dielectric constants
(which include most non-ionic materials),
there can be large contrasts in volumetric
scattering (up to 7 dB) between contami-
nated and uncontaminated regions. Such
contrast ratios are well within the detec-
tion range of a system with a high figure
of merit, such as the synthetic-pulse radar
with SAR processing designed in this work
assignment. Second, it was determined
that a thin layer of contaminant "floating"
on the water table will produce a measur-
able signal if the layer is more than a few
centimeters thick; the appearance of such
a signal at some point in a survey would
indicate the presence of contaminant there.
However, the nature of this signal is such
that it might be difficult to distinguish it
from a local change in the depth of the
water table, so its overall utility is ques-
tionable, even though it is surprisingly
strong. On the other hand, thicker layers
will produce a strong and distinctive sig-
nal. Strong signals will also be produced if
abrupt transitions are induced at the
boundaries between a lighter, immiscible
contaminant and a water table with gradu-
ally increasing saturation. It was shown
that volumes of pure fine and very fine
soil particles (such as are found in silts
and clays) can greatly degrade the perfor-
mance of a GPR. Some fraction of these
soils is usually composed of larger imbed-
ded particles, such as sand grains. The
analytical model has determined that the
presence of these larger particles facili-
tates the detection of contaminated re-
gions.
Recommendations
This work assignment showed that, for
the environments and contamination ge-
ometries modeled here, a radar with a
high figure of merit combined with coher-
ent signal processing can detect contami-
nation under a wide range of conditions.
Such a radar can be achieved with exist-
ing technology.
The results of this work showed that
two-dimensional coherent processing (sur-
face distance, depth) was necessary to
detect deep, low-contrast targets. Impor-
tant performance gains, however, could
be achieved with the use of three-dimen-
sional techniques (surface area, depth).
This type of data collection (and process-
ing) will be necessary in order to enhance
weak targets in a cluttered environment,
and it is essential for providing a vehicle
for "intelligent," understandable displays.
The models developed in this work rep-
resented an idealized soil environment and
did not address an inhomogeneous propa-
gation medium. In an extreme case, an
inhomogeneous medium can lead to poor
estimates of the radar propagation veloc-
ity, which limits the ability of the coherent
processing to properly "register" the scat-
terers. It can also lead to an increase in
the radar clutter field, which decreases
the otherwise achievable high signal-to-
noise ratio. On the other hand, the detec-
tion of contaminants is facilitated by the
presence of small irregularities in an oth-
erwise "pure" soil as a result of the in-
creased volume scattering that occurs.
While modeling of more complex environ-
ments is possible in principle, such work
would not (because of the clutter) realisti-
cally or economically model any particular
environment. Therefore, unless an experi-
ment is performed to test and validate the
models and the findings resulting from
this work, the use of GPR for remediation
purposes will remain an unanswered ques-
tion.
It is recommended that simple proof-of-
principle experiments be undertaken to
validate the models and findings devel-
oped in this study. The purpose of the
initial experiments would be to calibrate a
GPR in terms of figure of merit and other
needed parameters. Once the figure of
merit of the radar has been estimated,
additional experiments would be conducted
at a quantified site. The purpose of this
second set of experiments would be to
collect a limited set of subsurface data
with which to validate the models and
better understand in real environments the
use of high-figure-of-merit radars with syn-
thetic-aperture signal processing. It is fur-
ther recommended that in the initial ex-
periments data be collected and coher-
ently processed three-dimensionally. With
theoretically attainable improvements in the
performance figure of merit, GPR may
become a significant tool in rapidly identi-
fying and cost effectively remediating sub-
surface contamination.
The full report was submitted in fulfill-
ment of Contract No. 68-C9-0033 by Vista
Research, Inc., under the sponsorship of
the U. S. Environmental Protection Agency.
•U.S. Government Printing Office: 1992 — 648-080/60035
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Dennis G. Douglas, Alan A. Burns, Charles L Rino, and Joseph W. Maresca, Jr.,
are with Vista Research, Inc., Mountain View, CA 94042. The EPA author, James
Yezzt (also the EPA Project Officer, see below) is with the Risk Reduction
Engineering Laboratory, Edison, NJ 08837.
The complete report, entitled "A Study to Determine the Feasibility of Using a
Ground-Penetrating Radar for More Effective Remediation of Subsurface Con-
tamination, " (Order No. PB92-169 382/AS; Cost: $26.00, 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:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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