WHY GROUND PENETRATING RADAR
FOR PLUME DETECTION?
By
Gennaro Dicataldo
To
Dr. Jones
CEEN 540
1
Table of Contents
Abstract
1. Introduction
1
1.1 Background
1
2. Why GPR?
2
3. How GPR Works
2
3.1 Basic Principles
3
3.2 How to Create an Image
7
3.3 Examples of GPR Images
8
4. Stepped-FM versus Pulse Radar GPR Systems
10
5. GPR Applications for Plume Detection
11
6. Case Histories of Plume Detection with GPR
13
Middletown AFB
13
Wurtsmith AFB
13
7. Conclusion
14
References
15
2
Abstract
This report discusses the deployment of ground penetrating radar (GPR) as a
non-destructive technique to detect and locate contaminated plumes. The basic
principles of this technique include the transmission and reception of electromagnetic
waves to and from the ground. The electromagnetic waves reflected form the
subsurface are traduced by a GPR control unit into profile images. Strong images are
created at the boundaries of two materials with different dielectric properties (i.e. clean
sand and contaminated sand).
Stepped-FM and pulse radar systems differ in ways similar to FM and AM radio
frequencies. GPR FM systems have the advantage to produce better resolutions for
close objects, to be unaffected by the surrounding radio waves (i.e. cell phones, and
radios), and to have simpler data acquisition operations. Several applications and two
case histories in which GPR was successfully used are discussed.
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WHY GROUND PENETRATING RADAR FOR PLUME
DETECTION?
1. Introduction
The purpose of this paper is to discuss the basic principles, the creation of an
image, the difference between pulse radar and stepped-FM systems, and some
successful applications and case histories of plume detection by ground penetrating
radar (GPR).
1.1 Background
The first surveys performed using ground penetrating radar were
reported in Austria in 1929 to sound the depth of a glacier. In 1934 the
word RADAR was introduced as an acronym of RAdio Detection And
Ranging. Later, in the 1950s the U.S. Air Force used the radar to detect the
depth of the ice as the airplanes were trying to land in Greenland. In 1967
the radar was used for space missions on the moon. In 1972 Morey and
Drake began to construct and sell commercial ground penetrating radar
systems with Geophysical Survey Systems Inc. Since then there has been
an explosion of applications and research and publications, that were
encouraged mainly by the Geological Survey of Canada, the U.S. Army,
and the Cold Region Research and Engineering Laboratory (CRREL).
Currently there are 300 patents registered at the Patent Office that are
somewhat related to the original GPR invention (Olhoeft).
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2. Why GPR?
As stated by Benson, GPR is a useful tool in mapping the subsurface of the
ground and groundwater contaminants. GPR surveys can help environmental
engineering as well as geophysicists to identify the boundaries of contaminant plumes
and provide other useful geological information. Mellett reported that “ The ability to
see through, below, and, into solid materials using non-invasive techniques has
important applications in a variety of fields where investigations may otherwise require
intrusive or destructive methods.”(1995). The conventional approach to investigate a
contaminated site has been mostly destructive. Soil borings and groundwater wells
have been used for decades as the only techniques to gain information about the status
of the contamination. Although destructive methods are actually capable of providing
data about the amount and characteristics of the contaminant at single points, they are
very costly and ineffective in determining the extent and the location of a plume (Van
der Roest et al.). Therefore GPR is in large a technique that can effectively reduce the
cleanup costs of a contaminated site by increasing the quality of the investigation.
3. How GPR Works?
Ground penetrating radar also known as ground probing radar, earth sounding
radar or georadar, transmits electromagnetic waves into the earth with a transmitter and
picks up the reflected waves with a receiver. Reflected waves are caused by changes in
the magnetic properties of a material. Therefore if there is a large change in material
(i.e. a buried metallic object) it will cause an increased amount of waves to be reflected
producing a good image. Reflection of part of the propagating waves can occur at the
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boundary of two geological layers, with different densities, or in the presence of voids,
contaminated plumes, hazardous waste, variations in water content or density in the
same material (i.e. abrupt change in sand density), and buried objects.
3.1 Basic Principles
GPR consists of a radar system, which includes a radio transmitter,
two antennas
and a
receiver. A
signal is
transmitted a
short
Figure 1. Schematics of a GPR System.
distance into
Source: GeoRadar, Inc.
the ground and part of it is reflected back, and part of it propagates into the
soil surface. Any reflection is caused by a change in material properties
with respect to the host material (i.e. dirt). The greater the contrast between
the properties of materials, the stronger the reflected signal. Usually the
strongest signals occur at the boundaries of two materials that have very
different electrical properties (Conyers and Goodman, 1997). The
parameters affecting the penetration of the waves generated by GPR are the
characteristics of the material through which the waves travel and the
frequency of the waves. The characteristics of a material that affect the
radar waves are the electrical conductivity and the magnetic permeability.
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Electrical conductivity is the ability of a material to transport charge
through the process of conduction (Olhoeft). The magnetic permeability of
a medium is defined as the ability of a medium to become magnetized when
an electromagnetic field is imposed upon it (Conyers and Goodman,1997).
Soil and rocks are dielectric or have low magnetic permeabilities. This
means that soil and rocks will allow the passage of most electromagnetic
energy without dissipating it. On the other hand iron-rich materials or
materials that contain magnetite, have high magnetic permeabilities (or low
dielectric) therefore transmitting radar energy poorly. The more dielectric a
material is the less electrically conductive it is. To achieve the maximum
radar penetration a medium should have low electrical conductivity and
high dielectric (or low magnetic permeability). The standard unit used to
measure radar propagation is the Relative Dielectric Permittivity (RDP).
RDP is defined as the “capacity of a material to store, and then allow the
passage of, electromagnetic energy when a field is imposed upon it.”
(Conyers and Goodman, 1997). RDP is determined as a ratio of a material’s
electrical permittivity to the electrical permittivity of a vacuum, which is
one. The lower a material’s RDP is, the higher the radar velocity will be of
the wave passing through the material. Table 1 shows the typical relative
dielectric permittivities of several common geological materials.
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Table 1. RDPs of Common Geological Materials (with 100 MHz Antenna).
Material
RDP
Material
RDP
Air
1
Dry Silt
3-30
Freshwater
80
Saturated Silt
10-40
Ice
3-4
Clay
5-40
Seawater
81-88
Permafrost
4-5
Dry Sand
3-5
Average Surface Soil 12
Saturated Sand
20-30
Dry, Sandy
Volcanic
Coastal Land
10
Ash/Pumice
4-7
Limestone
4-8
Forested land
12
Rich Agricultural
Shale
5-15
Land
15
Granite
5-15
Concrete
6
Coal
4-5
Asphalt
3-5
Source: Conyers and Goodman, 1997.
RDP is also related to the velocity of the radar waves by the
following equation:
(K)1/2 = C/V
(1)(Conyers and Goodman,1997)
K = Relative Dielectric Permittivity (RDP) of the material through which
the radar energy passes
C = Speed of Ligth (0.2998 meters per nanosecond)
V = Velocity of the radar energy as it passes through a material (meters per
nanosecond)
The greater the difference between the RDPs of materials, the larger
the amplitude of reflection generated. Therefore, the reflection generated at
the boundaries of two materials can be expressed by the following equation:
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R = [ (K1) ½ - (K2) ½] [(K1) ½ + (K2) ½] (2)(Conyers and Goodman,
1997)
R = Coefficient of reflectivity at a Buried Surface
K1 = RDP of the Overlying Material
K2 = RDP of the Underlying Material
In order to produce a good reflection, the difference in dielectric
permittivities of two materials must occur over a short distance. In fact, if
RDPs change gradually over a long distance, small changes in reflectivity
will occur and very weak reflections will be generated. For example, if a
metallic drum is buried in the ground and the propagation waves strike it,
they will be reflected 100% and will shadow anything that is directly
beneath it.
The depth of an object can be determined by knowing the dielectric
constant of the soil. GPR can measure the transit time of a signal very
accurately, however, the propagation velocity can change considerably with
the soil type. The velocity of propagation is determined by solving equation
(1) for V, knowing the RDP of the soil. The depth of an object can be
calculated by the following equation:
Depth = ½ V Tr
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(3)(Daniels, 1996)
V = Velocity of the radar energy (meters per nanosecond)
Tr = Transit Time to and from the Target (nanosecond)
The second parameter affecting penetration of a GPR system is the
frequency of the waves. Commercial GPR antennas frequencies range from
10 to 1000 megahertz (MHz). As a rule, the greater the depth of
investigation, the lower the frequency of the antenna needs to be. Also, the
lower the frequencies of the waves the larger the antennas are. Therefore, a
1000 MHz antenna is about 15 centimeters and can be moved around easily
in almost any space, while a 10 MHz antenna is 15 meters long and needs a
much larger space in order to operate.
3.2 How To Create An Image
As the radar moves along a transect it transmits signals, picks up
their echo, and plots the results on a computer-like display. Images are
created by
plotting
every signal
received
side-by-side.
Figure 2
Figure 2. Creation of a GPR Image.
shows the
Source: GeoRadar, Inc.
schematics of the data acquisition of a buried object. The image created in
this example is a hyperbola because the object is ahead of the radar. As the
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radar moves closer to the target it will take less time to pick up the signal
whereas moving away from it will take longer. This effect generates an
image that will have the shape of a hyperbola (see Figure 2). By experience
a GPR operator knows that a hyperbola’s shaped image represent a small
object (like a buried pipe). Sometimes images can be ambiguous. For
example the diagram in Figure 3 shows the image of a buried pipe with the
biggest side
parallel to
the transect.
The same
image could
be
Figure 3. Example of a GPR Image of a Buried Pipe.
Source: GeoRadar, Inc.
interpreted
as the
boundary between two layers of different density, the groundwater table, or
a horizontal pancake-like plume. A way to overcome this problem is to take
some readings at 90°degrees of the previous transect direction.
3.3 Examples of GPR Images
The following are examples of images produced by a GPR
GeoRadar ® Model 1000. Figure 4 shows a test pit study performed by
Lockheed Martin Corporation. Several pipes were buried at different
depths. The pipes have different diameters and have been used to create a
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3D model as shown in Figure 5. The 3D model was acquired from a 2D
model and then processed on a Silicon Graphics workstation at Lockheed
Martin Corporation.
Figure 4. GPR Image of Buried Pipes.
Source: GeoRadar, Inc.
Figure 5. 3D Model of a GPR Image of Buried Pipes.
Source: GeoRadar, Inc.
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4. Stepped-FM versus Pulse Radar GPR Systems
The majority of ground penetrating radars are of a type called pulse video
systems. Pulse systems transmit narrow pulses to the subsurface which are bounced
back from different materials or objects underground. This technology is similar to the
radar system used originally during World War II. As reported by GeoRadar, Inc. there
are many similarities between stepped-FM compared to pulse radar. Stepped-FM and
pulse radar compare in much the same way that FM and AM radio compare to each
other. Daniels stated that it is increasingly more difficult to design pulse radar(AM)
systems with narrow bandwidths than it is to design FM systems with wide bandwidths
(1996). The Stepped-FM, or more properly called Frequency-modulated continuoswave (FMCW), system has several advantages over the pulse radar system:
• Objects close together can be resolved
• Images resemble the actual objects instead of hyperbolas
• No complicated operation for data acquisition are needed
• FMCW works well indoor and around surface metal objects
• Interference with other radio transmitters is negligible
The following images were
recorded by using a conventional pulse
radar system and a FMCW system.
Figure 6 shows the drawing of a test pit
Figure 6. Drawing of a Test Pit.
where seven plates were buried each at
Source: GeoRadar, inc.
one foot apart by one foot deeper than
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the previous one. Figure 7 shows an image created by using a conventional pulse radar
system, while Figure 8 shows the same image recorded with a stepped-FM GPR system.
The conventional pulse radar image overlaps each plate making it difficult to interpret
the image. A stepped-FM has a much better resolution and each plate’s relative
positions in the soil are visible.
Figure 7. Pulse Radar GPR Image.
Source: GeoRadar, Inc.
Figure 8. Stepped-FM GPR Image.
Source: GeoRadar, Inc.
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5. GPR Applications for Plume Detection
GPR has been deployed to investigate the extent and location of contaminated
plumes throughout the United States and Europe. As reported by EPA, since 1986 GPR
has been succefully used to assess the relative concentrations and extent of hydrocarbon
contamination in 11 large petrouleum storage facilities, two airports, and one pipeline
section in Massachusetts, New Jersey and California (1991). Case studies from sites in
Utah and Arizona have shown that GPR is an effective tool to identify approximate
boundaries of contaminated plumes (Benson, 1995). Brewster and Annan, conducted
studies on DNAPLs contamination in a natural sandy aquifer using a 200 MHz GPR
system. GPR was found to be very successful in monitoring DNAPLs movements in
the subsurface (1994). The Airborne Environmental Surveys Division of Era Aviation,
Inc. for the past three years has conducted several surveys to locate subsurface
contaminated plumes by using the Ground Penetrating Radar technique (Cameron et
al.).
Delft Geotechnics in Netherlands used GPR to successfully create a contour
map showing the level of contamination of a site. The site contained high concentrated
hydrocarbon plumes that were identifies with GPR (Daniels, 1996). Merin conducted
an integrated study using GPR and historical aerial photography to locate buried wastes
and relative leachate at a manufacturing facility. GPR showed that several illegal
landfills partially or not showing on aerial photographs were located on the site (1990).
Saarenpaeae et al. in Finland detected contaminated plumes in groundwater caused by
landfill lechate successfully using GPR techniques (1997).
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6. Case Histories of Plume Detection with GPR
In this section of the paper two case histories will be presented in which
ground penetrating radar techniques were heavily deployed to detect and locate the
extent of contaminated plumes. The case histories include the Middletown Air
Force Base (AFB) in Harrisburg, Pennsylvania and Wurtsmith AFB in Oscoda,
Michigan.
Middletown AFB
Since 1947 The Middletown Air Force Base has been in operation in
Harrisburg, Pennsylvania. Activities at the base include: warehousing and
supply of parts, equipment, general supplies, oil and lubricants and complete
aircraft overhaul (i.e. stripping, repainting, reassembly, and engine overhaul).
Also, the site has been sold to a manufacturing company of truck trailers which
performed activities on the site such as painting, foaming and welding. Since
1983 studies have been conducted on the site (300 acres of land) for suspected
contamination of trichloroethylene (TCE). In 1984 ground penetrating radar and
magnetometer surveys have shown the presence of several TCE contaminated
plumes in various location and successfully identified buried drums.
Wurtsmith AFB
Wurtsmith Air Force Base (AFB) is located near Oscoda, Michigan. It is
located in an area in which a well known site (FT-02) was being investigated by
ground penatrating radar and is currently being biormediated. A plume was
accidentally discovered at Wurtsmith AFB because of a background
contaminant variability study performed at FT-02. This study included an
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extension of the GPR grid to the neighboring areas, which encompassed
Wurtsmith. GPR profiles showed areas of high-conductivity ‘shadow’at the top
of the aquifer similar to those discovered at FT-02. Additional GPR surveys
were conducted by a group of students at the Western Michigan University to
locate the approximate extension of the plume.
7. Conclusion
Ground penetrating radar is without a doubt a cost-effective solution for many
fields of study including environmental engineering, to explore the subsurface. Besides
the advantage of being a non-destructive technique, other benefits of GPR include the
capability to locate the position and extent of contaminated plumes quickly and
economically. The implementation of this technique as a preliminary tool for
remediation strategies of contaminated plumes is highly encouraged. However, GPR
needs skilled operators when interpreting the results and a general knowledge of the soil
investigated. Also, GPR works well when used in profiling shallow contaminated
plumes or other anomalies in the subsurface.
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References
Benson, Alvin .K. 1995. Applications of ground penetrating radar in assessing some
geological hazards: examples of groundwater contamination. Journal of Applied
Geophysics, 33, 1-3.
Brewster, Michael, and Annan, A. Peter. 1994. Ground-penetrating radar monitoring of
a controlled DNAPL release: 200 MHz radar. Geophysics, 59, 8, 1211-1221.
Cameron, Robert M, Stryker, Tony, Mitchel, Dave L, and Halliday, Wayne
S. 1993. Development and application of airborne ground-penetrating radar
for environmental disciplines. PROC SPIE INT SOC OPT ENG, SOCIETY OF
PHOTO-OPTICALINSTRUMENTATION ENGINEERS, 1942, 21-33.
Conyers, Lawrence B., and Dean Goodman.1997. GROUND-PENETRATING RADAR:
An Introduction for Archeologists. Walnut Creek, Cal.: AltaMira Press.
Daniels, D.J. 1996. Surface-penetrating radar. London : Institution of Electrical
Engineers.
EPA, RODS Information,
http://search.epa.gov/s97is.vts?action=Vi… t%3D1%26ResulCount%D10&)&HLNavig
ate=ALL
EPA Office of Research and Development Alternative Treatment Technology
Information Center (ATTIC), http://www.epa.gov/bbsnrmrl/attic/a2/RM00017.html
GeoRadar Inc. Home Page, http://www.georadar.com/index.shtml
Hoeloft, GR, GRORADAR™ , http://www.g-p-r.com/em.htm
Mellett, Jamas S. 1995. Ground penetrating radar applications in engineering,
environmental management, and geology. Journal of Applied Geophysics, 33, 1-3,157166.
Merin, I.S.. 1990. Identification of Previously Unrecognized Waste Pits Using Ground
Penetrating Radar and Historical Aerial Photography. Superfund '90. Proceedings of the
11th National Conference, 314-319.
Saarenpaeae, J., Korkealaakso, J., Rossi, E., and Ettala, M. 1997. Investigation of
groundwater contamination from waste landfills using ground penetrating radar surveys.
Environmental Impact, Aftercare and Remediation of Landfills, Environmental Sanitary
Engineering Center, 173-180.
Van der Roest, P.B., Brasser, DJ S., Wagebaerst, A.PJ, and Stam, P.H.. 1997. Zeroing
in on hydrocarbons. Environ. Prot., 8, 5, 44-46.
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