United States
Environmental Protection
Agency
Hazardous Wastes Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-85/014 July 1986
Project Summary
Microwave System for Locating
Faults in
Hazardous Material Dikes
Robert M. Koerner and Arthur E. Lord, Jr.
Failure or rupture of impoundments con-
taining hazardous material is one of the
causes of release of hazardous materials
to the environment. Another area of con-
cern is the integrity of hazardous material
pond liners and of liners or diking-grouting
systems for secured waste sites. This pro-
ject identified and assessed two nonde-
structive test methods for investigating the
subsurface of impoundments containing
hazardous materials: 1) the continuous
wave microwave system and 2) the pulsed
radio frequency systems (also termed
ground-penetrating radar or GPR). The
primary focus of the project was to con-
duct a technological assessment of these
two systems for identifying dike failure
characteristics (seepage, formation of soil
stratigraphic discontinuity, voids, grouting
breakthrough). During the project, these
two systems were also evaluated for their
potential to detect related subsurface ob-
jects such as waste drums.
This exploratory study developed or
modified current systems, surveyed the
literature, verified laboratory and field ex-
periments, and defined the applicability of
each technique for determining subsur-
face phenomena.
Continuous wave microwaves success-
fully located subsurface water zones and
the tops of rock surfaces, but they had dif-
ficulty in detecting containers, voids, and
soil stratigraphy. Upgrading of the system
by an electronics manufacturing firm
should improve performance.
The ground-penetrating radar (GPR)
method is a limited tool for environmen-
tal monitoring. (Future more extensive
work might alter this conclusion.) A high-
light of the GPR testing was its successful
use in monitoring chemical grout. The
most significant feature of the system is
the realtime printout of the subsurface
detail to the limit of the signal's penetrabil-
ity. This feature is an advantage when
observing a cylindrical drum in shallow
sand, but under other circumstances, the
system records too much information and
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research 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
Failures of earthen dikes containing
reservoirs of hazardous materials have
been attributed to structural defects,
hydraulic problems, and to other, seeming-
ly unrelated, factors. Dike conditions and
subsurface integrity beneath the soil sur-
face in the foundation, reservoir, and em-
bankment areas need to be determined.
The range of conditions and variety of
situations is extremely wide.
The traditional method for investigating
the subsurface is to test material removed
from soil borings and test pits. However,
this approach presents a number of poten-
tial problems: data, soil disturbances,
danger to personnel and elevated costs. To
avoid such potential problems, this project
investigated nondestructive test (NOT)
methods.
Laboratory and Field Test
Results.
Two NOT methods, the continuous wave
microwave system and the pulsed radio
frequency (also called ground-penetrating
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radar, or GPR), were addressed in this pro-
ject. Both use electromagnetic waves. The
continuous wave microwave system was
designed and manufactured by the
authors under the auspices of this project.
This system sweeps through a range of
frequencies and displays alternating con-
structive and destructive interference
peaks. Knowing these peaks and counting
their frequency, measures the depth to the
interface. The interface seen is actually a
material with dielectric properties differ-
ing from the one above it. To evaluate the
adequacy of the continuous wave micro-
wave system, a series of laboratory and
field experiments were undertaken. Table
1 lists these tests and presents a qualita-
tive assessment of the results. The tech-
nique used shows promise in a number of
areas although it is not applicable to all
subsurface problems. It works best when
the differences between the dielectric
properties of the soil and the anomaly are
the greatest; e.g., dry soil to saturated soil
or soil to rock can be distinguished, but the
boundary between two different soil types
cannot be easily identified.
For the pulsed radio frequency system
(GPR), several commercial systems are
available. For this project, a system was
purchased and somewhat modified for a
specific site situation. The modifications
included new transceiver carriages for
rough terrain, and construction of two nar-
row torpedo antennae, a transmitter, and
a receiver for downhole monitoring. The
GPR system is basically a microwave
reflection system in which a short pulse
is transmitted into the ground, reflecting
off a dielectric anomaly, and returning to
the ground surface. Travel time is
measured, and depth can be calculated
from the propagation velocity of the elec-
tromagnetic waves in the soil. The GPR
system provides a continuous printout as
the transceiver antenna is moved across
the ground surface. The stronger the
dielectric difference is between the soil
and the anomaly, the stronger will be the
return signal, and the sharper will be the
recorder trace. These systems are ex-
tremely sensitive to all subsurface
anomalies, and numerous data are always
received. Their sensitivity requires fre-
quent tuning to assure accurate visual
interpretation.
To evaluate the adequacy of the GPR
system, a series of laboratory and field
experiments were conducted. Table 2 lists
these tests and qualitatively assesses the
results. Although the table may seem to
imply that the technique is not viable, the
reader should note that most of these very
Table 1. Summary of Continuous Wave Microwave Tests Conducted During
This Project
Test No.
Objective
Results in
Terms of
Stated
Objectives
Comments
Lab. 1 Water location
Lab. 2 Soil interface detection
Lab. 3 Void detection
Field 1 Steel plate detection
Field 2 Water table detection
Field 3 Top of rock detection
Field 4 Void detection
Excellent Depth ranged from 0 to 18".
Spatial location very good.
Requires sharp boundary.
Poor Frequency peaks not distinguishable.
Requires soil layers to have different
moisture contents.
Amplitude distinction possible but
difficult.
Doubtful Frequency peaks not sharp enough.
Amplitude distinction possible.
Needs good area to contrast against
void area for relative assessment.
Possible Depends on nature of backfill soil —
dry granular soil is good; wet, cohesive
soil is poor.
Good Has worked to 60" depth.
Must be sharp and distinct.
Good Rock probed to 12'with good correla-
tion to predicted values.
Possible Thorough tunnel survey conducted, but
no verification determined.
challenging tests had never been at-
tempted before and there have been suc-
cesses. When a sharp dielectric difference
existed between upper and lower mater-
ials, test results were most accurate; i.e.,
buried containers were easily detected,
but polluted water and small voids in dry
soils were not easily defined.
Conclusions and
Recommendations
The continuous wave microwave
system used in this project was built by
the authors. The system successfully
located subsurface water zones and the
tops of rock surfaces. Containers, voids,
and soil stratigraphy were seldom located.
To improve the system's performance in
the latter area would require further
electronic development. The current
experimental system requires a micro-
processor-based minicomputer to count
oscillloscope wave cycles and calculate
depths to the reflecting anomalies. To
greatly improve the system a real-time
printout should be developed to provide
continuous operation. For field use, the
current power pack should be made more
mobile with a built-in 110-volt electrical
system. In general, the entire system
needs further development, using more
sophisticated laboratory and field tests to
refine procedures.
The GPR method can be used for en-
vironmental monitoring, but has limita-
tions, as noted in the full report. The
highlight of the experimentation using
the GPR system was its use in monitoring
the chemical grout commonly used to halt
seepage in leaking earth dikes. Two adja-
cent boreholes with transmitting and
receiving antennae provide travel time
printouts. Comparing traces before and
after grouting allows instantaneous
assessment of the completeness and
uniformity of the grouting operation.
When an unsatisfactory area is repaired,
the GPR system can be used for actual
control of the field operation. Also there
were preliminary successes in using the
technique to detect metal drums. This
work should definitely be followed up.
A very significant feature of the equip-
ment is the real-time printout of the sub-
surface detail to the limit of the signal's
penetrability. Although this feature is ex-
cellent for observing a cylindrical drum
buried at a shallow depth in dry sand, it
often makes interpretation difficult in less
than ideal circumstances. Often the
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Table 2.
Test No.
Summary of Pulsed Radio Frequency Tests Conducted During
This Project
Objective
Results in
Terms of
Stated
Objectives
Comments
Lab. 1
Lab. 2
Lab. 3
Field 1(a)
Field Kb)
Field 2
Field 3
Field 4
Sensitivity study
Polluted water detection Poor
Small void detection Possible
Defining water table Possible
Defining soil stratigraphy Good
Buried container location Excellent
Poor
Void detection
Poor
Chemical grout monitoring Possible
A dielectric constant difference of less
than 1 results in a detectable signal.
Salt saturation was difficult to quantify.
Salt boundary not observable.
Acetone was not responsive.
Steel pipe easily detected.
Smooth hole reasonable.
Roughened hole marginal.
Good in granular soil.
No good in fine-grained soil.
Soils in different categories (sands,
silts, clays) are distinguishable.
Vertical boundaries are detectable.
Metal drums in granular soil.
Metal drums in fine-grained soil.
Insufficient dielectric contrast unless
void is water-filled.
Qualitative tool at present.
Perhaps quantifiable.
Tremendous potential if additional work
is successful.
system records, too much information, and
at times the background is overwhelming,
completely obscuring the desired trace.
Signal enchancement techniques are
definitely needed to solve this problem.
Two approaches are possible: One is a
computer-aided system, and the other is
empirical comparisons. Both approaches
should be pursued to achieve the full ad-
vantage of a GPR system.
The full report was submitted in fulfill-
ment of Cooperative Agreement No.
R80476301 by Drexel University under
the sponsorship of the U.S. Environmen-
tal Protection Agency.
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Robert M. Koerner and Arthur E. Lord, Jr., are with Drexel University, Philadelphia,
PA 19104.
John E. Brugger is the EPA Project Officer (see below).
The complete report, entitled "Microwave System for Locating Faults in Hazardous
Material Dikes," (Order No. PB 85-173 821/AS; Cost: $14.50, 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:
Releases Control Branch
Hazardous Wastes Engineering Research 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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EPA/600/S2-85/014
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