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Journal of Applied Geophysics 56 (2004) 41–58
Using amplitude variation with offset and normalized residual
polarization analysis of ground penetrating radar data to
differentiate an NAPL release from stratigraphic changes
Thomas E. Jordana,*, Gregory S. Bakera, Keith Hennb, Jean-Pierre Messierc
aDepartment of Geology, University at Buffalo, Buffalo, NY, USAbTetra Tech NUS, Pittsburgh, PA, USA
cUnited States Coast Guard Support Center, Elizabeth City, NC, USA
Received 19 May 2003; accepted 12 March 2004
Abstract
Amplitude variation with offset analysis of ground penetrating radar data (AVO/GPR) may improve the differentiation of
nonaqueous phase liquid (NAPL) from stratigraphic changes. Previous controlled experiments have shown that common offset
(CO) GPR methods can detect the presence of NAPL in soil by examining amplitude and travel time (velocity) anomalies.
Unfortunately, stratigraphic changes such as the presence of a silt or clay lens or perched water table may produce similar
amplitude and velocity anomalies. Therefore, it is difficult to delineate NAPL in a terrain with unknown stratigraphy
exclusively using CO data collection methods. Forward models based on the Fresnel equations predict that amplitude responses
exist at various incidence angles that will allow for differentiating NAPL from hydrogeologic changes. Models generated as part
of this study indicate that analyzing the difference in amplitude responses from linearly polarized electric field vertically
oriented (EV) to the horizontally oriented (EH) signals at various incidence angles improves target discrimination. A case
history is presented demonstrating that collecting common-midpoint (CMP) GPR data using EH and EV polarized signals at
anomalous CO amplitude responses and analyzing the data using AVO and normalized residual polarization (NRP) methods
may improve the detection and differentiation of NAPL from stratigraphic changes in the subsurface. These results are
corroborated using a capacitively coupled resistivity instrument and subsequent intrusive sampling.
D 2004 Elsevier B.V. All rights reserved.
Keywords: NRP; AVO; NAPL
1. Introduction and background information
Over the past 10 years, many researchers have
investigated the feasibility of using ground penetrat-
0926-9851/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jappgeo.2004.03.002
* Corresponding author. Key Environmental, Inc. 1200 Arch
Street, Suite 200, Carnegie, PA 15106, USA.
E-mail address: tejordan@buffalo.edu (T.E. Jordan).
ing radar (GPR) for detection of nonaqueous phase
liquid (NAPL) in soil (e.g. Olhoeft, 1992; Brewster
and Annan, 1994; Sander and Olhoeft, 1994; Dan-
iels et al., 1995; Baker, 1998; Kim et al., 2000).
These papers have identified that amplitude anoma-
lies (bright spots and dim spots), travel time (ve-
locity) anomalies, and occasional phase anomalies
are associated with the presence of NAPL. Unfor-
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5842
tunately, the presence of stratigraphic and hydro-
geologic changes may also result in similar ampli-
tude, velocity, or phase anomalies that are indis-
tinguishable from the presence of NAPL (Brewster
and Annan, 1994). Forward models developed for
this research are based on the Fresnel equations
and indicate that stratigraphic changes may produce
anomalous GPR responses similar to NAPL
responses when data are collected at closely spaced
transmitter and receiver offsets. Therefore, this and
previous research indicates that in order to apply
GPR methods to NAPL identification and delinea-
tion, one must either characterize the stratigraphy
in advance of the survey, or have a secondary
method to determine if the anomaly is due to a
known stratigraphic change or from the presence of
NAPL.
Baker (1998) first investigated modeling ampli-
tude variation with offset (AVO) analysis of GPR
data for the purpose of differentiating NAPL from
stratigraphic changes. Work presented here
expands on those previously developed models
and presents field data demonstrating the merits
of amplitude variation with offset AVO/GPR
techniques. The forward models used here gener-
ate the reflection coefficients (R) for linearly
polarized GPR signals. The results of the forward
models indicate that amplitude and phase data
collected at a range of incidence angles can be
used to assist in differentiating stratigraphic
changes from the presence of NAPL contamina-
tion. This is the premise of the AVO/GPR
technique.
This research was initiated to determine the ap-
plicability of AVO/GPR techniques for delineation of
jet propellant-4 (JP-4) and jet propellant-5 (JP-5),
which are both a light NAPL (LNAPL). Data were
collected at a JP-4 and JP-5 release at a United
States Coast Guard (USCG) Support Center facility
located in Elizabeth City, NC. The test site was a
former fuel farm (FFF) that is approximately 180 m
from the shoreline of the Pasquotank River (Figs. 1
and 2).
The FFF is located in the Outer Coastal Plain
physiographic province. The near surface geology of
the site consists of clayey silt with varying amounts
of clay and interspersed sand lenses overlying fairly
uniform fine-grained sand. Typically, the surficial
clayey silt layer is present from ground surface to
approximately 2.0 m below ground surface (bgs).
The underlying sand is present from the bottom of
the silt to more than 30 m bgs at the site. The water
table ranges from 1.0 to 2.4 m bgs with areas of
perched ground water present above clay lenses.
Areas of perched LNAPL have not been previously
delineated and are not anticipated at the FFF site. A
conceptual model of the site geology is presented as
Fig. 3.
The USCG used the study area as a jet fuel
farm from 1942 to 1991. The FFF consisted of
numerous storage tanks, which are now understood
to be the source(s) of significant amounts of
LNAPL present in the subsurface over approxi-
mately 3 ha of the site. These storage tanks have
been removed, and the site is presently an open
field.
Based upon previous investigations at the site
(TtNUS, 2003), it was determined that free LNAPL,
residual LNAPL, and dissolved phase fuel contami-
nation are present within the near surface soil at the
FFF. A significant portion of residual and free
LNAPL is present in a smear zone (from 1.2 to
2.4 m bgs) as a result of the stratigraphy and a
fluctuating water table. The corrected thickness of
free LNAPL ranges from 0.0 to 0.6 m, as determined
from on-site recovery and ground water monitoring
well data. A map of free LNAPL thickness based on
ground water monitoring well data is presented as
Fig. 4.
The presence of free phase LNAPL indicates that
zones within the soil are at least 15% saturated with
JP-4 and JP-5 (Wiedemeier et al., 1999). The FFF
contaminants JP-4 and JP-5 are LNAPLs with pub-
lished densities ranging from 0.75 to 0.80 kg/l at 15
jC and have a published relative permittivity value of
1.7 (ASI Instruments, 2002; Environment Canada,
2001; North Carolina Division of Pollution Prevention
and Environmental Assistance, 1999; Place et al.,
2001).
2. Development of forward modeling methods
The forward models for AVO/GPR techniques
are based on Fresnel R equations for linearly
polarized electromagnetic (EM) waves (Jordan and
Fig. 1. Key map showing the location of the FFF site, Elizabeth City, NC.
T.E.Jordanet
al./JournalofApplied
Geophysics
56(2004)41–58
43
Fig. 2. Site map showing location of the former fuel farm relative to the Pasquotank River.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5844
Baker, in review). These equations account for
antenna orientation for linearly polarized waves
where the electric field vector (E) is oriented either
within a vertical plane (E-vertical or EH) or within
a horizontal plane (E-horizontal or EH) (Jordan and
Baker, 2003). Fresnel R equations describe the
amplitude and phase of the reflected waves as a
function of incident angle (h) and contrasts in the
relative permittivity (er) properties of the media.
The Fresnel equations used to model the responses
are presented in Eqs. (1a)–(2b) which describe the
Fig. 3. A conceptual hydrogeologic model showing a generalized stratigrap
hydrogeological conditions that are forward modeled in this paper.
R for an EH or EV polarized EM wave traveling
through lossless, non-ferromagnetic media.
REH ¼ Er
Ei
¼cosh �
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffier2er1
� sin2hr
cosh þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffier2er1
� sin2hr ð1aÞ
TEH ¼ Et
Ei
¼ 2cosh
cosh þffiffiffiffiffiffier2er1
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� er1
er2sin2h
r ð1bÞ
hic profile of the FFF. Annotations ‘‘A’’, ‘‘B’’, ‘‘C’’, and ‘‘D’’ refer to
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 45
REV ¼ Er
Ei
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffier2er1
� sin2hr
� er2er1
cosh
er2er1
cosh þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffier2er1
� sin2hr ð2aÞ
TEV ¼ Et
Ei
¼2
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffier1er2
coshr
cosh þffiffiffiffiffiffier1er2
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� er1
er2sin2h
r ð2bÞ
where: REH = reflection coefficient of the EH polar-
ized EM wave (dimensionless), REV = reflection coef-
ficient of the EV polarized EM wave (dimensionless),
TEH = transmission coefficient of the EH polarized
wave (dimensionless), TEV = transmission coefficient
of the EV polarized wave (dimensionless), er1 =rela-tive permittivity of near surface layer 1 (dimension-
less), er2 = relative permittivity of deeper layer 2
(dimensionless), h = angle of incidence (radians).
The forward model results were modified to con-
sider the affects of signal attenuation from low loss
Fig. 4. Map of FFF showing corrected LNAPL thickness and AV
soil by multiplying the REH and REV for each strati-
graphic layer by the following equation:
Low loss attenuation ¼ e�az ðNepers; ½Np�Þ ð3aÞ
where: a = signal attenuation constant (Np/m), z = sig-
nal travel path length (m).
The signal attenuation constant (a) is defined as:
a ¼ xere02
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ r
xere0
� �2
�1
s24
35
24
35
12
ðNp=mÞ
ð3bÞ
where: x = angular velocity (rad/s), e0 = permittivi-
permittivity of free space (constant, 8.85� 10� 12
F/m), r = conductivity (S/m), er = relative permittiv-
ity (dimensionless).
Determining the values for er for Eqs. (1a)–(3b)
required the use and modification of the complex
refractive index mixing (CRIM) equation and pub-
lished relative permittivity values for dry soil types,
water, air, and various types of NAPL (Greaves et
O/GPR CMP locations. Map modified from TtNUS (2003).
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5846
al., 1996; Ulriksen, 1982). Eq. (4) is modified from
the three-phase CRIM equation to consider fractional
saturation of air, water, and NAPL in the pore
spaces.
er ¼ /SNAPLffiffiffiffiffiffiffiffiffiffiffieNAPL
p þ ð1� /Þ ffiffiffiffiffiem
p
þ /ð1� SNAPL � SairÞffiffiffiffiffiew
p þ /Sairffiffiffiffiffiffieair
p
ð4Þ
where: eNAPL= relative permittivity of NAPL (dimen-
sionless), em = relative permittivity of soil matrix
(dimensionless), ew = relative permittivity of water
(dimensionless), eair = relative permittivity of air (di-
mensionless), / = porosity (dimensionless), SNAPL=
fractional NAPL saturation (dimensionless), Sair =
fractional air saturation (dimensionless).
The variable em can be roughly approximated
based on soil density by Eq. (5) where
em ¼ q1:93 ð5Þ
However, not having soil density values we used
published values of relative permittivity for dry soils
from Ulriksen (1982) and input them into a simple
ratio relationship to estimate the variable em where
em ¼/ � ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
er dry soilp
/ � ffiffiffiffiffiffieair
p� �2
ð6Þ
Published values of relative permittivity from
Ulriksen (1982) and estimated values from Eqs.
(4)–(6) were used to model the stratigraphy and
hydrogeology of the FFF and are presented in Table 1.
The relative permittivity values from Table 1 and
Eqs. (1a)–(3b) were used to develop four forward
Table 1
Values of relative permittivity used for the conceptual model for the
FFF
Soil type Porosity Estimated relative permittivity values
Water saturation NAPL saturation
0% 15% 100% 15% 30% 50%
Loose, mixed
grain sand
0.40 6.00 8.58 34.45 29.67 24.89 18.51
Soft, slightly
organic clay
0.66 2.40 9.47 54.35 46.46 38.57 28.05
Values of porosity are from Dunn et al. (1980). The NAPL is
assumed to be present at the water table and have a relative
permittivity value of 1.70.
AVO/GPR models for the FFF (Figs. 5–8). The
thickness for each forward model was constant and
is presented in Table 2.
The AVO/GPR forward models are based on
Eqs. (1a)–(3b), and the er values presented in Table
1. These forward models account for the reflected
versus transmitted energy at each and subsequently
deeper interfaces. The forward models also consider
the impact of signal attenuation using Eqs. (3a) and
(3b). Travel path length for the multilayered models
is calculated using Snell’s Law and the offset
distance between the transmitter and receiver. How-
ever, the AVO/GPR forward model results are
presented as incidence angle versus reflection coef-
ficient where incidence angle is based on a simple
geometric path from the transmitter to the target
layer and return.
The reflection coefficient results of the AVO/GPR
forward models were independently normalized
(REH/REHMax) and (REV/REVMax
) with care taken to
preserve phase. The independently normalized polar-
ization differences (REH�REV) were then calculated
and used to assist in data interpretation. The inde-
pendently normalized polarization differences are
henceforth referred to as normalized residual polar-
ization (NRP) and are plotted versus incidence angle
for each target reflector. The forward modeled NRP
values were plotted versus incidence angle for four
hydrogeologic scenarios common to the FFF site as
follows:
(A) Clayey soils with and without the presence of a
sand lens.
(B) Sandy soils with and without NAPL present
beneath a clay lens.
(C) Sandy soils with and without NAPL present at the
water table (without an overlying clay lens).
(D) Sandy soils with perched ground water with and
without NAPL above a clay lens.
The parameters for the four models are presented
in Table 3 and the results are presented in Figs. 5–
8. The conductivity values used are our best esti-
mate of the subsurface conditions at this site.
Conductivity values of 1 mS/m were used to repre-
sent unsaturated dry sandy or clayey soils. Conduc-
tivity values of 3 mS/m were used to represent
moist sandy soils and values of 6 mS/m were used
Fig. 5. Forward Model ‘‘A’’ compares the anticipated NRP responses of the water table with and without the presence of a sand lens in clayey
vadose zone soils.
Fig. 6. Forward Model ‘‘B’’ compares the anticipated NRP responses of a sandy water table reflector with LNAPL present versus absent. This
model considers the effect of an overlying moist clay lens in the vadose zone.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 47
Table 2
Forward model layer thickness parameters
Layer no. Thickness (m) Comment
1 1 Surface layer
2 1 Intermediate layer
3 Infinite Deep layer
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5848
to represent moist clayey soils. Conductivity values
of 10 mS/m were used to represent saturated soils.
It should be noted that varying the conductivity
values can severely impact the modeled amplitude
responses and care was taken to use realistic and
consistent values.
3. Test site methods
Field data were collected at the FFF in June 2002.
Ground penetrating radar (and resistivity data) were
collected at a 100 by 100 m area that was formerly
utilized as an above ground storage facility for the JP-
4 and JP-5. All of the above ground storage tanks and
some of the underground utilities and pipes were
removed prior to our data collection. A chain-link
fence was located along the southeastern site bound-
ary and a metallic shed housing a ground water
extraction and treatment system were present at the
north-central site boundary.
Common offset (CO) data were collected at the
FFF using a sled mounted 200 MHz antenna gener-
ating EH polarized signals. The CO GPR data were
collected by towing the GPR antenna sled 20 m
behind a truck at a constant rate of approximately 2
km/h. An odometer was interfaced to the GPR unit to
provide accurate location information during data
collection.
Table 3
Relative permittivity and conductivity values for the three layers of the fo
Layer Model A Model B
numberSand lens
absent in
layer 2
(er/r)
Sand lens
present in
layer 2
(er/r)
NAPL absent
beneath clay
lens (er/r)
NAPL present
beneath clay
lens (er/r)
1 2.4/1 2.4/1 6.0/1 6.0/1
2 2.4/1 6.0/1 9.5/6 9.5/6
3 34.5/NA 34.5/NA 34.5/NA 18.4/NA
Values of conductivity are presented in units of mS/m.
The CO GPR data from the FFF were gridded in
plan view as time/depth slices of average amplitude
responses. The average amplitude data from the CO
survey were color contoured in map view for a depth
slice of 1.1 to 1.7 m, which corresponds to the depth
to the water table and suspected LNAPL smear zone.
Anomalous amplitude responses from this depth slice
may represent the presence of NAPL, changes to
stratigraphy, or changes in soil moisture content
(Fig. 9). Anomalous amplitude responses are pre-
sented in colors other than dark blue.
Capacitively coupled resistivity data were also
collected at this site to assist in the interpretation
of the GPR data. This instrument is equipped with a
transmitter that generates an alternating current in the
ground and a receiver that detects the current and
measures voltage. This instrument is configured to
collect resistivity data in a dipole–dipole array. The
resistivity data were collected along the same profile
lines as the GPR CO data and at station spacings of
0.3 to 0.6 m. Data were collected using a 5 m
antenna with a transmitter to receiver separation of
5 m.
The apparent resistivity survey data were pro-
cessed using a kriging algorithm and spherical
semi-variogram model. The color-contoured apparent
resistivity results are presented as Fig. 10. Back-
ground resistivity data are presented in shades of
green and anomalous responses are presented in
blue, yellow, and red.
The locations of anomalous CO amplitude and
resistivity data were overlain onto the base map of
the FFF (Fig. 11). Areas where the GPR and resistiv-
ity anomalies overlapped were targeted for further
testing and evaluation using AVO/GPR methods.
Areas where GPR and resistivity anomalies were
rward models
Model C Model D
NAPL absent
in sandy
soils (er/r)
NAPL present
in sandy
soils (er/r)
Layer 2
perched
G.W. w/o
NAPL (er/r)
Layer 2
perched
G.W. with
NAPL (er/r)
6.0/1 6.0/1 6.0/1 6.0/1
8.6/3 8.6/3 34.5/10 18.4/3
34.5/NA 18.4/NA 9.5/NA 9.5/NA
Fig. 7. Forward Model ‘‘C’’ compares the anticipated NRP responses of a sandy water table reflector with LNAPL present versus absent. This
model considers the effect of overlying sandy soils in the vadose zone.
Fig. 8. Forward Model ‘‘D’’ compares the anticipated NRP responses of a clay layer reflector with an overlying perched ground water versus an
overlying perched ground water and NAPL mixture.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 49
Fig. 9. Relative GPR amplitude data results from depths of 1.1 to 1.7 m. Shades of green and yellow indicate anomalous amplitude responses.
The areas encircled in red are regions where anomalous amplitude responses were not detected within this depth slice.
Fig. 10. Capacitively coupled apparent resistivity results. Color shades of yellow, orange and red indicated high resistivities that may be
indicative of the presence of NAPL or anomalously resistive soil (e.g. sand) in the subsurface.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5850
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 51
absent appeared to correspond, suggesting that mini-
mal additional data collection were necessary in those
regions of the site.
Eight locations were selected for further analysis.
These locations were revisited using common-mid-
point (CMP) GPR methods. The CMP data were
collected using perpendicular broadside and parallel
end-fire antenna orientations to generate EH and EV
polarized signals, respectively. Common-midpoint da-
ta were also collected in suspected LNAPL and non-
LNAPL impacted areas to obtain signal velocity
information and geo-electric stratigraphic information
for this site.
4. AVO/GPR data processing and analysis
The raw CO and CMP data were processed using
commercially available software. Raw amplitude
values versus transmitter to receiver separation dis-
tances were extracted from EH and EV CMP data
using software that assisted in the selection of the
maximum relative amplitude value within a 5 ns
window for each trace. Care was taken to preserve
the original CMP amplitudes and not corrupt them
with automatic gain control (AGC) or other process-
ing/interpretation steps.
Velocity data for each reflector were extracted
from the CMP data for depth correlation purposes
through the use of the Dix equation. The Dix
equation was used to convert apparent velocity (Va)
and travel time (t) data to interval velocity (V), layer
thickness, and layer depth. The Dix equation for
determining the root mean square velocity (Vrms) is
presented as Eq. (7), and the interval velocity equa-
tion is presented as Eq. (8).
Vrms ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXni¼1
V 2aiDti
Xni¼1
Dti
vuuuuuuut ð7Þ
V1 ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV 2rms2
t02 � V 2rms1
t01
t02 � t01
sð8Þ
The velocity results were converted to layer thick-
ness and depth and then calibrated to test boring data
provided by the USCG for this experiment. The
results from the velocity analysis indicate that the
total Vrms from surface to 2.0 m varies from 0.76 to
0.11 ns/m across the site. These variable Vrms values
may be indicative of changes in moisture content
related to heterogeneous soil texture or stratigraphic
changes at the FFF.
The maximum raw amplitude data versus trans-
mitter/receiver separation distance for each reflec-
tion event were extracted from the EH and EV
CMP data and archived into a database. The EH
and EV polarization data were individually normal-
ized to account for the inability to simultaneously
collect multi-polarized GPR data. Therefore, EH
polarization data were normalized to EH data and
EV polarized data were normalized to EV data.
The NRP data (EH–EV) were then calculated and
plotted versus incidence angle. The NRP AVO/
GPR results are presented as incidence angle
versus reflection coefficient difference where inci-
dence angle is based on a simple geometric path
down from the transmitter and back to the receiver
(Figs. 12–19).
5. Results
The NRP AVO/GPR responses are presented
from the eight CMP locations (Figs. 12–19) and
are summarized in Table 4. The absence, presence,
and thickness of LNAPL were confirmed after the
completion of data interpretation using several in-
trusive approaches. Confirmation methods included
the used of a membrane interface probe/electrical
conductivity (MIP/EC) unit followed by collection
of soil samples and ground water monitoring well
installation. The MIP/EC was used to identify the
location of soil samples and the placement of
NAPL monitoring wells. Soil samples and an oil/
water interface meter were used to confirm the
presence and apparent thickness of the free NAPL.
The apparent thickness of the free NAPL was
corrected based upon accepted methods (USEPA,
1996).
The first location analyzed using AVO/GPR tech-
niques is situated adjacent to ground water monitor-
Fig. 11. Map showing resistivity and GPR anomalies, select ground water monitoring well locations, and the relative position of the CMP data
collected for the AVO analysis at the FFF.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5852
ing well 32MW17 where anomalously high values of
resistivity and CO GPR amplitude responses have
been historically observed. Maximum uncorrected
Table 4
Study summary information showing LNAPL thickness data obtained from
and AVO/GPR testing
CMP
location
Corrected LNAPL
thickness (m)
CO GPR
amplitude
anomaly
Resistivit
anomaly
32MW17 0.55 Yes Yes
32TRW1A 0.00 No No
32TRW2A 0.59 Yes No
32TRW3A 0.23 No No
100N, 30E 0.00 Yes Yes
32RW1 0.40 Yes No
32EMP6 0.20 No No
32TRW5C 0.02 Yes Yes
LNAPL thickness at this well location is 0.80 m.
The NRP AVO/GPR response at this location is
similar to Model ‘‘B’’, a free NAPL response be-
direct intrusive testing versus responses from CO GPR, resistivity,
y AVO/GPR
anomaly
Comment
Yes, negative phase NAPL thickness varies from
0.01 to 0.63 m within a 20 m
radius of this CMP location
No
Yes, negative phase NAPL thickness varies from
0.55 to 0.62 within 10 m of
this CMP location
Yes, negative phase NAPL thickness varies from
0.00 to 0.45 m within a 20 m
radius of this CMP location
Yes, positive phase
Yes, negative phase
No NAPL thickness varies from
0.00 to 0.40 m within a 10 m
radius of this CMP location
Inconclusive
Fig. 12. Residual AVO/GPR data collected near location 32MW17. The reflection events compared were from an estimated depth of 1.9 m bgs.
The results from this analysis are similar to Model ‘‘B’’ with NAPL present. This response was confirmed from test boring 32NAPL05 data
where a corrected thickness of 0.6 m of NAPL was observed at this location.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 53
neath a clay layer (Fig. 12). This interpretation was
confirmed by test boring 32NAPL05, which indicat-
ed the presence of clay and a corrected thickness of
0.55 m of free NAPL near the water table at this
location.
Fig. 13 represents the NRP AVO/GPR response
collected near ground water monitoring well
Fig. 13. Residual AVO/GPR data collected from location 32TRW1A. T
1.6 m bgs. This response is similar to Model ‘‘B’’ where NAPL is a
wells 32TRW1A and 32EMP1 indicate that NAPL is absent at this C
32TRW1A. This CMP location was selected be-
cause it was present between two resistivity
anomalies and in an area where an anomalous
CO GPR response was not observed. This AVO/
GPR response is similar to forward Model ‘‘B’’,
where NAPL is absent. Data from adjacent ground
water monitoring wells 32TRW1A and 32EMP1
he reflection events compared were from an estimated depth of
bsent. Confirmation data from adjacent ground water monitoring
MP location.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5854
indicate that free NAPL is absent at this location.
Subsurface testing was also completed approxi-
mately 20 m west of this CMP that indicated
that residual NAPL was absent in the subsurface
soils.
Fig. 14 represents the NRP AVO/GPR response
collected near ground water monitoring well
32TRW2A. This CMP is located in an area where a
CO GPR anomaly was observed and a resistivity
anomaly was not observed. This NRP AVO/GPR
response is similar to forward Model ‘‘C’’ and may
represent the presence of a free phase LNAPL at the
water table. This response was verified from soil
borings 32NAPL02 and 32NAPL05 that indicated
the presence of a corrected thickness of 0.59 m of
free phase NAPL at both locations.
Fig. 15 represents the NRP AVO/GPR response
collected near ground water monitoring well
32TRW3A. This CMP was collected between two
resistivity anomalies at a location where a CO GPR
amplitude anomaly was absent. The NRP AVO/GPR
response from this location is similar to forward
Model ‘‘C’’ and may represent the presence of a free
phase LNAPL at the water table. The NRP AVO/GPR
values indicate that an overlying clay layer is absent.
Test borings completed within 20 m of CMP location
Fig. 14. Residual AVO/GPR data collected near location 32TRW2A. The re
This AVO response is similar to Model ‘‘B’’ or Model ‘‘C’’ where NA
32NAPL05 indicate a corrected thickness of 0.6 m of free LNAPL at bot
indicate that NAPL thickness varies from 0.00 to
0.45 m.
Fig. 16 represents the NRP AVO/GPR response
from survey reference grid location 100N, 30E. This
location was selected due to its proximity to two
known NAPL locations, RW-1 and 32TRW3A to the
southeast, and the presence of both resistivity and
CO GPR anomalies. The results of the NRP AVO/
GPR analysis suggest the presence of a velocity
inversion (e.g. V1>V2) at this location. This response
is similar to forward Model ‘‘A’’ where a vadose
zone sand lens is present and NAPL is absent. The
absence of NAPL at this location was confirmed
from test boring 32SB06. The results from this
location demonstrate that anomalous responses from
CO GPR and resistivity surveys may be insufficient
to detect NAPL at sites with complex stratigraphy
and hydrogeology.
Fig. 17 represents the NRP AVO/GPR response
located north of ground water monitoring well
32EMP6. This location was selected as it was in
an area where resistivity and GPR anomalies were
absent. The results of the NRP AVO/GPR analysis
were inconclusive at this location. Test boring data
collected south of this CMP location in an area of
anomalous CO GPR responses indicated the pres-
flection events compared were from an estimated depth of 2.1 m bgs.
PL is present. Confirmation data collected from 32NAPL02 and
h locations which were near this CMP location.
Fig. 15. Residual AVO/GPR data collected near location 32TRW3A. The reflection events compared were from an estimated depth of 1.5 m bgs.
This response is similar to Model ‘‘C’’. Confirmation data from boring 32NAPL07 indicates the presence of 0.5 m (corrected) of free LNAPL in
the vicinity of this CMP location.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 55
ence of corrected thickness of 0.4 m of NAPL. Data
collected north of this location in an area of observed
resistivity anomalies indicated the absence of NAPL;
however, data from boring 32NAPL19 located to the
southeast indicated the presence of NAPL. These
Fig. 16. Residual AVO/GPR data collected from reference grid location 1
depth of 1.2 m bgs. Adjacent test borings and ground water monitoring wel
GPR and resistivity responses were observed at this location.
results may indicate that this CMP location repre-
sents the fringe of detectable NAPL using AVO/GPR
techniques.
Fig. 18 represents the NRP AVO/GPR response
adjacent to NAPL recovery well RW-1. A CO GPR
00N, 30E. The reflection events compared were from an estimated
ls indicate that NAPL is absent at this location. Note that anomalous
Fig. 17. Residual AVO/GPR data collected north of location 32EMP6. The reflection events compared were from an estimated depth of 1.7 m
bgs. These results were inconclusive. Confirmation data collected south of this CMP location indicated the presence of 0.4 m (corrected) of
NAPL and data collected north indicate the presence of approximately 0.0 m NAPL.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5856
anomaly was observed at this location and resistivity
anomalies were absent. These results are similar to
Model ‘‘B’’ and represent the presence of NAPL at the
water table with an overlying clay layer present. Data
from NAPL recovery well RW-1 indicate the presence
Fig. 18. Residual AVO/GPR data collected near location RW-1. The reflec
results are similar to Model ‘‘B’’ and may represent the presence of NAPL
the presence of greater than 0.3 m (uncorrected) of free NAPL at this loc
of greater than 0.3 m (uncorrected) of NAPL at this
location.
Fig. 19 represents the NRP AVO/GPR response
near ground water monitoring well 32TRW5C. A CO
GPR and resistivity anomalous responses were ob-
tion events compared were at an estimated depth of 1.9 m bgs. These
beneath a clay lens. Data from adjacent recovery well RW-1 indicate
ation.
Fig. 19. Residual AVO/GPR data collected near NAPL recovery well RW-1. The reflection events compared were at an estimated depth of 2.0 m
bgs. These results are inconclusive due to insufficient EV polarized data. Ground water monitoring data from 32TRW5C indicates up to 0.1 m of
free NAPL at this location.
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–58 57
served at this location. Unfortunately, insufficient EV
polarized data were available to make an interpreta-
tion so the results are considered inconclusive.
Ground water monitoring well data indicates that
0.02 m of free NAPL may be present at this location.
The response from the NRP AVO/GPR data indicates
that insufficient NAPL thickness may be present at
this location for detection using this method.
6. Discussion and conclusions
The results of this research reinforce that the use
of CO GPR and resistivity data collection methods
alone are insufficient to delineate the presence of
LNAPL in areas where sand lenses, clay lenses, and
perched ground water may be present. Hydrogeo-
logic changes, the presence of NAPL, and the
absence of NAPL can result in similar anomalous
CO GPR and resistivity responses. Therefore, anom-
alous responses from CO GPR (and resistivity sur-
veys) are insufficient to differentiate the presence of
NAPL from stratigraphic or hydrogeologic changes.
The collection of CMP data at locations where
anomalous CO GPR and resistivity responses are
observed and processing the data using NRP AVO/
GPR techniques may improve the ability to differ-
entiate NAPL responses from those due to strati-
graphic or hydrogeologic changes. Collecting both
EH and EV polarized CMP data at these anomaly
locations and examining NRP amplitude responses
also appears to improve the ability to detect NAPL or
differentiate the response from stratigraphic changes in
the subsurface.
As with all geophysical methods, prior knowledge
of the anticipated subsurface conditions and develop-
ment of forward models assist in assessing the feasi-
bility of the technique to meet the research objectives.
The forward models for this research indicate that
AVO/GPR analysis methods may be useful in differ-
entiating CO GPR amplitude anomalies due to the
presence of clay lenses from those responses that
result from the presence of NAPL. Our forward
models also indicate that AVO/GPR analysis methods
might be useful in delineating and differentiating clay
lenses with and without perched ground water present.
However, these models do not take into consideration
the impact that gradational interfaces may have on the
reflection coefficients and care should be taken in
their application.
Our field test results indicate that the most effective
field procedure for AVO/GPR data collection is to
acquire CO GPR data first, and then process it in the
field to identify areas of anomalous amplitude
responses. These anomalous responses should be
revisited using large offset CMP data collection meth-
T.E. Jordan et al. / Journal of Applied Geophysics 56 (2004) 41–5858
ods. The CMP data should be collected using antenna
orientations to generate both EH and EV polarized
signals. The CMP results should be processed using a
procedure to extract maximum raw amplitude values
for each value, which are then compared to the
incidence angle of the target reflector. The amplitude
versus incidence angle results should be independent-
ly normalized to account for the inability to simulta-
neously collect EH and EV polarized data. Subtracting
the independently normalized EV from EH polarized
amplitude signal appears to substantially improve the
interpretation process.
It was also observed that the use of a secondary
geophysical technique that measures the electrical
properties of soils improves the development of a
strategy for focusing on a specific set of anomalies.
Various geophysical techniques such as EM induction
and capacitively coupled resistivity devices are avail-
able for rapidly measuring the apparent resistivity or
conductivity of the near surface soil.
The preliminary results of this research indicate
that the NRP AVO/GPR technique is a promising
method that here is shown to assist in the detection
of near surface LNAPL and differentiate the responses
from changes in shallow subsurface stratigraphy and
hydrogeology.
References
ASI Instruments, Dielectric constant reference guide. Retrieved Au-
gust 12, 2002 from the World Wide Web: http://www.asiinstr.
com/dc1.html.
Baker, G.S., 1998. Applying AVO analysis to GPR data. Geophys-
ical Research Letters 25, 397–400.
Brewster, M.L., Annan, A.P., 1994. Ground-penetrating radar mon-
itoring of a controlled DNAPL release: 200 MHz radar. Geo-
physics 59, 1211–1221.
Daniels, J., Roberts, R., Vendl, M., 1995. Ground penetrating radar
for the detection of liquid contaminants. Journal of Applied
Geophysics 33, 195–207.
Dunn, I.S., Anderson, L.R., Kiefer, F.W., 1980. Fundamentals of
Geotechnical Analysis. John Wiley & Sons, New York.
Environment Canada, 2001. Environmental Technology Centre. Re-
trieved August 12, 2002 from the World Wide Web: http://
www.etc.ec.gc.ca.
Greaves, R.J., Lesmes, D.P., Lee, J.M., Toksoz, M.N., 1996. Ve-
locity variations and water content estimated from multi-offset,
ground penetrating radar. Geophysics 61, 683–695.
Jordan, T.E., Baker, G., 2003. Recommendation for new terminol-
ogy for linear polarized components of ground penetrating radar
waves. Journal of Environmental and Engineering Geophysics
8, 38–42.
Jordan, T.E., Baker, G.S., in review. Amplitude and Phase Variation
with Offset (APVO). Analysis of Ground Penetrating Radar
Data: Theory and Forward Modeling: Environmental and Engi-
neering Geoscience Journal.
Kim, C., Daniels, J., Guy, E., Radzevicius, S., Holt, J., 2000.
Residual hydrocarbons in a water saturated medium: a detec-
tion strategy using ground penetrating radar. American Asso-
ciation of Petroleum Geologists, Environmental Geosciences 7,
169–176.
North Carolina Division of Pollution Prevention and Environmental
Assistance, 1999. Fact Sheet, Petroleum Fuels: Basic Composi-
tion and Properties. 1639 Mail Service Center-Raleigh, NC
27699-1639.
Olhoeft, G.R., 1992. Geophysical detection of hydrocarbon and
organic chemical contamination. In: Bell, R.S.Proceedings on
Application of Geophysics to Engineering, and Environmental
Problems. EEGS, Oakbrook, IL, pp. 587–595.
Place, M.C., Coonfare, C.T., Chen, A.S.C., Hoeppel, R.E., Rosan-
sky, S.H., 2001. Principles and Practices of Bioslurping. Battelle
Press, Columbus.
Sander, K.A., Olhoeft, G.R., 1994. 500-MHz ground penetrating
radar data collected during an intentional spill of tetrachloro-
ethylene at Canadian Forces Base Borden in 1991. U.S. Geo-
logical Survey Digital Data Series DDS 25.
Tetra Tech NUS, 2003. Corrective action plan addendum 6, Former
Fuel Farm (SMMU No. 32) United States Coast Guard Support
Center, Elizabeth City, NC.
Ulriksen, P.C., 1982. Application of impulse radar to civil engineer-
ing. PhD Dissertation. Lund University.
USEPA, 1996. How to effectively recover free product at leaking
underground storage tank sites, USEPA 510-R-96-001.
Wiedemeier, T.H., Hanadi, H.S., Wilson, J.T., Newel, C., 1999.
Natural Attenuation of Fuels and Chlorinated Solvents in the
Subsurface. Wiley, New York.