U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006–5191

Use of Borehole-Radar Methods to

Monitor a Steam-Enhanced Remediation

Pilot Study at a Quarry at the Former

Loring Air Force Base, Maine

Prepared in cooperation with the U.S. Environmental Protection Agency,

Office of Solid Waste and Emergency Response, Office of Superfund

Remediation and Technology Innovation
Cover. Borehole-radar data collection at the steam-enhanced remediation site,

former Loring Air Force Base, Maine, September 2002. (Photo by Peter Joesten.)
Use of Borehole-Radar Methods to Monitor 

a Steam-Enhanced Remediation Pilot Study 

at a Quarry at the Former Loring Air Force 

Base, Maine

By Colette Grégoire, Peter K. Joesten, and John W. Lane, Jr.

Prepared in cooperation with the U.S. Environmental Protection Agency, 

Office of Solid Waste and Emergency Response, Office of Superfund 

Remediation and Technology Innovation

Scientific Investigations Report 2006–5191

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation: Grégoire, Colette, Joesten, P.K., and Lane, J.W. Jr., 2007, Use of borehole-radar methods to monitor a steam-enhanced remediation pilot study at a quarry at the former Loring Air Force Base, Maine: U.S. Geological 

Survey Scientific Investigations Report 2006–5191, 35 p.
iii

Acknowledgments

This work was supported by the USEPA Office of Solid Waste and Emergency 

Response, Office of Superfund Remediation and Technology Innovation, and by the USGS 

Toxic Substances Hydrology Program. Colette Grégoire received support from Katholieke 

Universiteit Leuven, Belgium. The authors gratefully acknowledge the assistance of Mike 

Nalipinski and Eva Davis of the USEPA; Greg Crisp, formerly of SteamTech Environmental 

Services, Inc.; and Peter Forbes of the U.S. Air Force Center for Environmental Excellence. 

The authors also acknowledge and appreciate assistance from the following USGS employees: 

Chris Kochiss, Eric White, and Marcel Belaval, for field work; Fred Day-Lewis for providing 

Matlab codes for the tomographic inversions; and Carole Johnson and Fred Day-Lewis for 

contributions and technical assistance.
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v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................3

Theory of the Borehole-Radar Method......................................................................................................3

Reflection Mode....................................................................................................................................4

Tomography Mode................................................................................................................................5

Theoretical Modeling of the Temperature Dependence of Radar-Frequency EM-Wave 

Propagation.......................................................................................................................................6

Constitutive Parameters ......................................................................................................................6

Effect of Heating on Radar-Frequency EM Wave Velocity and Attenuation ..............................7

Effect of Heating on EM-Wave Reflections from Fractures ..........................................................8

Conclusions of the Theoretical Modeling.......................................................................................14

Collection, Processing, and Interpreted Results of Geophysical Data...............................................14

Description of Monitoring Boreholes..............................................................................................14

Conventional Borehole Geophysical Logs......................................................................................15

Fluid-Temperature Logs ............................................................................................................15

Electromagnetic-Conductivity Logs........................................................................................15

Deviation Logs............................................................................................................................16

Single-Hole Radar Reflection ...........................................................................................................16

Data Acquisition.........................................................................................................................17

Data Processing and Interpretation .......................................................................................17

Analysis of the Radar Profiles ........................................................................................17

Power Correction..............................................................................................................20

Spectra Analysis...............................................................................................................22

Reflector Analysis.............................................................................................................23

Reflection Results......................................................................................................................24

Crosshole Radar Tomography...........................................................................................................24

Data Acquisition.........................................................................................................................24

Data Processing and Interpretation .......................................................................................26

Traveltime Data .................................................................................................................27

Amplitude Data..................................................................................................................27

Tomography Results ..................................................................................................................29

Summary and Conclusions.........................................................................................................................32

References Cited..........................................................................................................................................33
vi

Figures

1. Map showing location of the boreholes at the former Loring Air Force Base quarry

site, Limestone, Maine.................................................................................................................2

2. Diagram showing (A) transmitter and receiver antenna arrangement for single-hole 

radar reflection logging, and (B) typical reflection patterns from planar 

and point reflectors ......................................................................................................................4

3. Diagram showing configurations for collection of crosshole radar tomography data. (A) 

Crosshole“levelrun” data collection. (B) Tomography data collection...............................5

4-12. Graphs showing:

4. (A) Relative dielectric permittivity , and (B) electrical conductivity of water as 

a function of temperature...................................................................................................7

5. Model results for the (A) relative dielectric permittivity, and (B) electrical

conductivity of fractured limestone, and the resultant (C) velocity, and (D)

attenuation of 100-megahertz radar waves as a function of temperature and

conductivity (σ) in siemens per meter (S/m) ...................................................................9

6. Theoretical results for radar-wave traveltimes as a function of temperature 

and conductivity (σ), in siemens per meter (S/m) for an antenna-separation 

distance of 7.5 meters.........................................................................................................9

7. Effects of the (A and B) conductivity of limestone, (C and D) conductivity of 

water, and (E and F) relative dielectric permittivity of water on radar-wave 

velocity and attenuation ...................................................................................................10

8. Reflectivity of a 10-millimeter-thick fracture as a function of temperature, and 

the ratio of the reflectivity at temperature to the reflectivity at 20 degrees 

Celsius (°C) for formation conductivities of (A and B) 0.01 siemens per meter 

and (C and D) 0.03 siemens per meter............................................................................11

9. (A) Reflectivity of a 5-millimeter-thick fracture as a function of temperature, 

and (B) the ratio of the reflectivity at temperature to the reflectivity at 20 

degrees Celsius (°C) for a formation conductivity of 0.01 siemens per meter........11

10. (A) Reflectivity of a 2-millimeter-thick fracture as a function of temperature, 

and (B) the ratio of the reflectivity at temperature to the reflectivity at 20 

degrees Celsius (°C) for a formation conductivity of 0.01 siemens per meter........12

11. (A) Reflectivity of a 1-millimeter-thick fracture as a function of temperature, 

and (B) the ratio of reflectivity at temperature to the reflectivity at 20 degrees 

Celsius (°C) for a formation conductivity of 0.01 siemens per meter........................12

12. Effect of water salinity on the radar reflection. (A) Reflectivity of a 

10-millimeter water-filled fracture. (B) Reflectivity of a 10-millimeter-thick 

water-filled fracture. Concentration of the saline solution is 0.5845 g/L NaCl. 

(C) Ratio of the reflectivity with 0.5845 g/L NaCl to the reflectivity with 

0.2923 g/L NaCl ....................................................................................................................... 13

13–15. Logs showing:

13. Temperature data collected in boreholes (A) JBW-7816 and (B) JBW-7817A 

during August, September, and November 2002 at the former Loring Air Force 

Base quarry, Limestone, Maine.......................................................................................16

14. The 100-megahertz omni-directional borehole-radar reflection data collected 

in borehole JBW-7816 during August, September, and November 2002 at the 

former Loring Air Force Base quarry, Limestone, Maine............................................18

15. The 100-megahertz omni-directional borehole-radar reflection data collected 

in borehole JBW-7817A during August, September, and November 2002 at the 

former Loring Air Force Base quarry, Limestone, Maine............................................19
vii

16–29. Plots showing:

16. Peak-to-peak amplitude of the direct radar wave of single-hole radar 

reflection data collected during August, September, and November 2002 at 

the former Loring Air Force Base quarry, Limestone, Maine.....................................20

17. Power term calculated for selected single-hole radar reflection data sets 

from boreholes JBW-7816 and JBW-7817A during August, September, and 

November 2002 at the former Loring Air Force Base quarry, Limestone, Maine....21

18. Mean frequency spectra calculated for each single-hole radar reflection data 

set from boreholes JBW-7816 and JBW-7817A during August, September, and

November 2002 at the former Loring Air Force Base quarry, Limestone, Maine....22

19. Example of reflector analysis of single-hole radar reflection data: 

(A) reflector in section of radar profile, (B) mean signal in the time domain, 

and (C) amplitude spectrum in the frequency domain ...............................................23

20. Amplitude spectra calculated for the selected reflectors from single-hole 

radar reflection data collected during August, September, and November 2002 

at the former Loring Air Force Base quarry, Limestone, Maine.................................25

21. Ratio of the maximum amplitudes in the frequency domain of selected 

reflectors in single-hole radar reflection data from boreholes JBW-7816 and 

JBW-7817A collected during August, September, and November 2002 at the 

former Loring Air Force Base quarry, Limestone, Maine............................................26

22. Calibrated crosshole radar levelruns. Differences in traveltimes: 

(A) September-August 2002 and (B) November-August 2002 at the former 

Loring Air Force Base quarry, Limestone, Maine.........................................................27

23. (A) Traveltimes of crosshole radar tomography data collected during August,

September, and November 2002; and (B) radar-wave velocity in August,

September, and November 2002 at the former Loring Air Force Base quarry, 

Limestone, Maine ..............................................................................................................28

24. Differences in crosshole radar tomography traveltimes: (A) SeptemberAugust-2002 and (B) November-August 2002; at the former Loring Air Force 

Base quarry, Limestone, Maine.......................................................................................28

25. Results of the difference of the traveltime-difference tomography data: 

(A) September-August 2002 and (B) November-August 2002 at the former 

Loring Air Force Base quarry, Limestone, Maine.........................................................29

26. Peak-to-peak amplitude of the crosshole radar levelrun data: 

(A) unprocessed data and (B) calibrated data collected during August, 

September, and November 2002 at the former Loring Air Force Base quarry, 

Limestone, Maine ..............................................................................................................30

27. Attenuation measurements from calibrated crosshole radar tomography 

data collected during August, September, and November 2002 at the former 

Loring Air Force Base quarry, Limestone, Maine.........................................................30

28. Attenuation differences in crosshole radar tomography data: 

(A) September-August 2002 and (B) November-August 2002 at the former 

Loring Air Force Base quarry, Limestone, Maine.........................................................31

29. Results of the inversion of the attenuation-differences in crosshole radar

tomography data: (A) September-August 2002 and (B) November-August 

2002 at the former Loring Air Force Base quarry, Limestone, Maine........................31
viii

Conversion Factors and Abbreviations

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North Geodetic Vertical Datum of 1929 

(NGVD 29).

Electrical conductivity in millisiemens per meter (mS/m) may be converted to millimhos per 

meter (mmho/m) as follows: 1 mmho/m = 1 mS/m.

Electrical conductivity in siemens per meter (S/m) may be converted to electrical resistivity in 

ohm-meters (Ωm) as follows: Ωm = 1/(S/m).

List of Acronyms

AFB Air Force Base

dB/m Decibel per meter

o Degree

EM Electromagnetic

g/L Grams per liter

GPR Ground-penetrating radar

m/μs Meter per microsecond

mg/L Milligram per liter

MHz Megahertz

NaCl Sodium chloride

NAPL Non-aqueous phase liquid

ns Nanosecond

PCE Tetrachloroethene

PVC Polyvinyl chloride

Tables

1. Power correction for selected reflectors seen in single-hole radar reflection 

data collected during 2002 in boreholes JBW-7816 and JBW-7817A at the former 

Loring Air Force Base study area, Limestone, Maine ..........................................................24

2. Amplitude ratios calculated at central frequency for the selected reflectors 

identified in single-hole radar reflection data collected during 2002 in boreholes 

JBW-7816 and JBW-7817A at the former Loring Air Force Base study area, 

Limestone, Maine .......................................................................................................................26
ix

SER Steam-enhanced remediation

SIRT Simultaneous iterative reconstruction technique

S/m Siemen per meter

TCE Trichloroethene

μs Microsecond

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
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Use of Borehole-Radar Methods to Monitor a SteamEnhanced Remediation Pilot Study at a Quarry at the 

Former Loring Air Force Base, Maine

By Colette Grégoire, Peter K. Joesten, and John W. Lane, Jr.

Abstract

Single-hole radar reflection and crosshole radar 

tomography surveys were used in conjunction with 

conventional borehole-geophysical methods to evaluate 

the effectiveness of borehole-radar methods for monitoring 

the movement of steam and heat through fractured 

bedrock. The U.S. Geological Survey, in cooperation with 

U.S. Environmental Protection Agency (USEPA), conducted 

surveys in an abandoned limestone quarry at the former 

Loring Air Force Base during a field-scale, steam-enhanced 

remediation (SER) pilot project conducted by the USEPA, the 

U.S. Air Force, and the Maine Department of Environmental 

Protection to study the viability of SER to remediate nonaqueous phase liquid contamination in fractured bedrock.

Numerical modeling and field experiments indicate 

that borehole-radar methods have the potential to monitor 

the presence of steam and to measure large temperature 

changes in the limestone matrix during SER operations. 

Based on modeling results, the replacement of water by steam 

in fractures should produce a decrease in radar reflectivity 

(amplitude of the reflected wave) by a factor of 10 and a 

change in reflection polarity. In addition, heating the limestone 

matrix should increase the bulk electrical conductivity and 

decrease the bulk dielectric permittivity. These changes result 

in an increase in radar attenuation and an increase in radarwave propagation velocity, respectively.

Single-hole radar reflection and crosshole radar 

tomography data were collected in two boreholes using 

100-megahertz antennas before the start of steam injection, 

about 10 days after the steam injection began, and 2 months 

later, near the end of the injection. Fluid temperature logs 

show that the temperature of the fluid in the boreholes 

increased by 10°C (degrees Celsius) in one borehole and 40°C 

in the other; maximum temperatures were measured near the 

bottom of the boreholes.

The results of the numerical modeling were used to 

interpret the borehole-radar data. Analyses of the singlehole radar reflection data showed almost no indication that 

steam replaced water in fractures near the boreholes because 

(1) no change of polarity was observed in the radar reflections; 

(2) variations in the measured traveltimes were unsubstantial; 

and (3) most of the observed decreases in reflectivity were 

too small to have resulted from the replacement of water by 

steam. Analyses of the crosshole radar tomography data also 

support the conclusion that steam did not replace water in the 

fractures around the boreholes because traveltime-difference 

and attenuation-difference tomograms showed only small 

decreases in velocity and small increases in attenuation 

accompanying the steam injection.

The radar data are consistent with an increase in the 

conductivity of the limestone as a result of heating of the 

limestone matrix near the boreholes. Single-hole radar 

reflection data collected near the end of the steam injection 

near the bottom of the borehole with the largest temperature 

increase showed substantial attenuation. Also, reflector 

analysis showed small decreases in the amplitudes of radarwave reflections in data collected before injection and data 

collected near the end of the collection period. In the crosshole 

radar tomography data, decreases in velocity and small 

increases in attenuation also are consistent with temperature 

increases in the matrix.

Introduction

The Loring Air Force Base (AFB) in Limestone, 

Aroostook County, Maine (fig. 1), opened in 1953 as part of 

the U.S. Air Force Strategic Air Command. The base, which 

supported operations of long-range bombers, air-refueling 

tankers, and airborne command centers during the cold war, 

was closed in 1995. An abandoned limestone quarry on the 

base was used for storage of more than 400 drums of used 

solvents, mostly tetrachloroethene (PCE). Ground water 

in the fractured limestone bedrock underlying Loring AFB 

was found to be contaminated with chlorinated solvents and 

petroleum products. Chemical analyses from the site indicate 

PCE concentrations as high as 30 mg/L (milligrams per liter) 
67o57'30"

47o00'

67o55' 67o52'30"

46o55'

46o57'30"

67o50'

67o54'23.2" 67o54'20.7"

46o

57'

19.9"

46o

57'

18.7"

JBW-7817A

JBW-7816

I-4

I-6

I-5

I-8

VEA-5

I-7

I-3

EX-4

I-2

EX-2

EX-3 I-9

EX-1

VEA-9

VEA-7

JBW-7815

JBW-7814

JMW-0201

I-1

VEA-2

VEA-6

VEA-1

TC-1

JBW-7817B

VEA-8

VEA-4

LIMESTONE

N

MAINE

Radar borehole and

 identification number

Steam-injection borehole

 and identification number

Extraction borehole and

 identification number

Combined steam-injection/

 extraction borehole and 

 identification number

Other monitoring borehole

 and identification number

EXPLANATION

 0 5 10 15 20 METERS

 0 20 40 60 FEET

Upper quarry wall

Lower quarry wall

EX-3

I-8

I-6

JBW-7816

VEA-6

Base from U.S. Geological Survey topographic quadrangles, 

 Fort Fairfield NW (1978), Limestone (1978), Doyle Ridge (1986), and

 Hamlin (1986), Maine (1:24,000). National Geodetic Vertical Datum of 1929.

FORMER

LORING

AIR FORCE

BASE

QUARRY SITE

Figure 1. Location of the boreholes at the former Loring Air Force Base quarry site, Limestone, Maine. The single-hole radar 

refection and crosshole radar tomography surveys were carried out in boreholes JBW-7816 and JBW-7817A.

2 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
(SteamTech Environmental Services, Inc., 2001). Smaller 

amounts of trichloroethene (TCE), vinyl chloride, carbon 

tetrachloride, benzene, toluene, naphthalene, and other diesel 

and gasoline products also were detected.

Remediation efforts at the site include a field-scale 

pilot study to evaluate the effectiveness of steam-enhanced 

remediation (SER) for removal of non-aqueous phase liquid 

(NAPL) contamination from fractured bedrock. SER is an 

innovative remediation technology designed to capitalize on 

thermodynamic changes resulting from steam injection and 

to promote the removal of volatile and semi-volatile organic 

contaminants from the unsaturated and saturated zones. Steam 

injection and the resulting underground heating promote 

(1) an increase in NAPL volatility and therefore vaporization; 

(2) a decrease in NAPL adsorption to rock or soil particles; 

(3) a decrease in interfacial tension between NAPLs and 

ground water; (4) a decrease in NAPL viscosity; and (5) an 

increase in NAPL solubility (Stewart and Udell, 1988; 

Udell, 1996; Davis, 1997). Field applications of SER have 

been documented by Newmark and Aines (1994), Itamura 

and Udell (1995), and SteamTech Environmental Services, 

Inc. (1999).

The U.S. Geological Survey (USGS), in cooperation 

with the U.S. Environmental Protection Agency (USEPA), 

conducted a study to evaluate the effectiveness of boreholeradar methods to monitor the movement of steam and transport 

of heat in the subsurface during the SER pilot study. Results of 

theoretical calculations were used to assess the effectiveness 

of radar methods in monitoring SER operations and to support 

interpretation of the radar field data.

Purpose and Scope

The purpose of this report is to (1) present numerical 

modeling of the effects of heat and replacement of water by 

steam on radar-wave velocity and attenuation, (2) describe 

the borehole-geophysical methods used in this study, and 

(3) interpret the results of single-hole radar reflection and 

crosshole radar tomography surveys.

Numerical modeling was used to predict the changes in 

radar-wave velocity and attenuation that could be expected 

from heating the fluid in fractures and the limestone matrix, 

from steam replacing water in the fractures, and from changes 

in the conductive properties of the fluid in fractures. The effect 

of heating on the reflectivity from fractures also was modeled. 

The modeling results provided a basis for theoretically 

assessing the effectiveness of radar methods to monitor SER 

operations, and for interpreting the radar field data.

Borehole-radar data were collected in two boreholes at 

the study site. Single-hole radar reflection and crosshole radar 

tomography data were collected in August, September, and 

November 2002, prior to steam injection, about 10 days after 

steam injection started, and near the end of steam injection, 

respectively. Conventional borehole-geophysical data were 

collected to supplement the radar data. Fluid temperature and 

electromagnetic (EM) conductivity were used to monitor the 

changes in conditions in and near the boreholes. Deviation 

data also were collected in the boreholes, which was critical 

for the accuracy of the tomographic inversions.

The radar data were processed and examined for changes 

in radar velocity, attenuation, and reflectivity. The results of 

the numerical modeling were used to interpret the changes 

in the radar data in terms of changes in material properties 

caused by the steam and heating.

Description of Study Area

The SER pilot study at the former Loring AFB 

was conducted at an abandoned limestone quarry in the 

northwest corner of the base (fig. 1). The quarry is divided 

topographically into three areas, or tiers, at approximate 

elevations of 216, 225, and 236 m (meters). The lowest tier 

is covered by water that drains to a small stream flowing to 

the west of the quarry. Contamination has been detected in 

monitoring boreholes on all three tiers.

Bedrock at the quarry is the Carys Mill Formation, 

principally a limestone of Paleozoic age (Stephenson and 

Novakowski, 2003). The Carys Mills Formation consists 

of interbedded layers of dark to light gray, laminated and 

non-laminated micritic limestone and light gray, massive 

calcareous slate and pelites in which calcite veins are abundant 

(Pavlides, 1968; Roy, 1980; Osberg and others, 1985; Lane 

and others, 1996). Stephenson and Novakowski (2003) 

identified a north-south trending anticline with a shallow 

plunge to the north and a steeply dipping normal fault trending 

northwest-southeast through the area. The dominant bedding 

dips east-northeast at 20–50o (degrees).

Three primary fracture sets were identified in the 

quarry: (1) bedding-plane fractures having an aperture of 

approximately 0.15 mm, (2) northeast-trending joints with 

apertures of 0.05 mm, and (3) axial planar fractures with 

apertures of approximately 0.05 mm. At depths shallower than 

25 m below ground surface, fractures and joints are flanked 

by visible weathering zones that extend up to 12 mm into 

the surrounding rock (SteamTech Environmental Services, 

Inc., 2001). These fractures were considered to be the 

preferential pathways for the steam. The efficiency of the 

SER method depends on the fracture characteristics and their 

interconnectivity. The porosity of the unweathered matrix is 

about 0.02–0.5 percent (SteamTech Environmental Services, 

Inc., 2001).

Theory of the Borehole-Radar Method

Ground-penetrating radar (GPR) methods use highfrequency EM waves to image the subsurface. GPR waves 

have frequencies ranging from 10 to 2,000 MHz (megahertz), 

with corresponding wavelengths ranging from meters to 

centimeters in earth materials. In borehole GPR surveying, 

Theory of the Borehole-Radar Method 3
a radar pulse is transmitted from the borehole radar antenna 

radially outward into the rock surrounding the borehole. When 

the EM wave encounters a contrast in electrical properties 

associated with the presence of a fracture or lithologic contact, 

part of the wave energy is reflected back toward the borehole 

while the remainder of the wave energy is transmitted further 

into the bedrock. Radar methods can be used in numerous 

transmitter-receiver configurations. Information about 

subsurface structure and properties is obtained by analyzing 

the characteristics of the EM wave recorded by the receiving 

antenna, including direct-arrival traveltime, direct-arrival 

amplitude, reflection traveltimes, reflection amplitudes, and 

reflection polarity. Two borehole-radar methods were used 

in this study: single-hole radar reflection and crosshole 

radar tomography.

Reflection Mode

Single-hole radar reflection methods have been used 

for fracture characterization at nuclear waste repositories 

(Olsson and others, 1988, 1992a; Sandberg and others, 1991; 

Olsson and others, 1992b) and in ground-water investigations 

(Haeni and others, 1993; Lane and others, 1994; Lane and 

others, 2001).

In single-hole radar reflection logging, radar transmitter 

and receiver antennas are connected together as one logging 

tool (fig. 2). The two antennas are separated by a fixed 

distance and moved vertically inside the borehole. EM waves 

propagate from the transmitter antenna radially outward, and 

reflected waveform data are recorded at the receiver antenna as 

a function of time.

Radar reflection logging can be conducted with either 

omni-directional or directional receiver antennas. Omnidirectional receiver antennas provide information regarding 

the depth and dip of reflectors, but do not allow interpretation 

of the strike orientation of the reflector. Directional receiver 

antennas can indicate reflector orientation using dual-loop 

antennas, which act as four different orthogonal receivers 

coupled with a three-component magnetometer, and a plunge 

sensor which monitor the orientation of the tool in the 

borehole. The sensitivity and effective radial penetration of 

directional receiver antennas, however, are inferior to those of 

omni-directional dipole receiver antennas (Falk, 1992; Lane 

and others, 2001).

An important strength of radar reflection logging is the 

ability to identify features that do not intercept the borehole. 

A second strength is the high resolution; in some cases, 

centimeter-scale features may be resolved. The effectiveness 

of the method is limited by several factors related to the 

Reflection from

planar feature

Reflection from

point object

Reflection from

planar feature not

intercepting the borehole

Direct wave

Time

Depth

A. B.

Reflected

energy

Transmitted

energy

Point

reflector

Borehole

EXPLANATION

Transmitted energy

Reflected energy

Fracture

Fracture

Transmitter

Receiver

Figure 2. (A) Transmitter and receiver antenna arrangement for single-hole radar reflection logging, and 

(B) typical reflection patterns from planar and point reflectors.

4 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
antennas and the properties of the rock. The frequency, f, of 

waves propagated by the transmitter antenna, and the electrical 

conductivity, σ, of the rock are especially important. The depth 

of penetration of radar waves varies inversely with frequency 

and electrical conductivity; thus, for high frequencies or 

electrically conductive rocks, the radial penetration distance 

is lower. The resolution is related to the EM wavelength, λ, 

which is controlled by the frequency and the velocity, v, of EM 

waves in the rock medium (for example, Daniels, 1989):

λ = v / f , (1)

The minimum size of a detectable target is proportional to 

the wavelength of the transmitted signal; thus, in selecting the 

antenna frequency, there is a trade-off between resolution and 

the penetration depth. In this study, 100-MHz radar antennas 

were used; the wavelength was approximately 1 m and the 

radius of penetration was about 8 m from the borehole.

Tomography Mode

A tomogram is an image designed to estimate EM 

properties in the plane between two boreholes. Tomographic 

images are created from radar data collected in tomography 

(or transmission) mode between the boreholes (fig. 3). When 

collecting radar data in tomography mode, the transmitting 

antenna is moved vertically to successive positions in one 

borehole and the receiver is moved vertically to successive 

positions in a second borehole.

A crosshole survey called a “levelrun” represents a 

simplified configuration where both antennas are located at 

the same depth (fig. 3A). Levelrun surveys are faster than full 

tomography surveys because fewer raypaths are collected; 

however, they provide less information than tomographic 

surveys about the spatial variation of rock properties and can 

only detect changes with depth.

The tomography configuration (fig. 3B) covers the entire 

plane between the boreholes with raypaths; the number of 

rays depends on the length of the boreholes, distance between 

boreholes, and the separation distance between the antenna 

positions. Tomography data can be interpreted to provide 

information about both horizontal and vertical variations 

in the EM properties of the medium. The resolution of the 

method is limited by the number and orientations of raypaths, 

the offset distance between boreholes, the data quality, and 

regularization criteria applied in the data inversion (Bregman 

and others, 1989a, 1989b; Williamson and Worthington, 1993; 

Rector and Washbourne, 1994; Day-Lewis and others, 2002; 

Day-Lewis and Lane, 2004).

A. B.

Raypaths Raypaths

Transmitter

antenna

positions

Receiver

antenna

positions

Receiver

antenna

positions

Transmitter

antenna

positions

Figure 3. Configurations for collection of crosshole radar tomography data. (A) Crosshole “levelrun” data 

collection. (B) Tomography data collection.

Theory of the Borehole-Radar Method 5
The traveltime and amplitude of waveform traces are 

recorded for each raypath. These correspond to a mean 

velocity and a mean attenuation, respectively, along a given 

path. The inter-borehole region is discretized as a pixelated 

grid, and the lengths of the raypaths in each cell are calculated. 

Using tomographic-inversion methods (for example, Dines 

and Lytle, 1979; Stewart, 1992), cross-sectional images of EM 

velocity and attenuation are estimated from traveltime and 

amplitude measurements, respectively.

At high frequencies (short wavelengths) and low-loss 

(low attenuation) mediums, the radar wave velocity and 

attenuation are approximated and related to the electric and 

dielectric properties by (for example, Davis and Annan, 1989; 

Reynolds, 1997):

 , (2)

and

 , (3)

where

v is the velocity in meters per second,

c is the velocity of light in meters per second,

εr is the relative permittivity,

α is the attenuation in decibels per meter,

μ is magnetic permeability in henries 

per meter,

and

ρ is the resistivity in ohm-meters.

The attenuation of EM waves is strongly dependent on 

the electrical conductivity of the medium; thus, penetration 

distance is low in electrically conductive materials, such 

as clay or shale, whereas penetration distance is high in 

low-conductivity materials, such as clean limestone or 

granitic rocks.

The quality of resolution of radar-tomography methods 

depends on good raypath coverage of the plane between the 

boreholes. The reliability of tomographic estimates of velocity 

or attenuation varies over the image plane, with the largest 

errors (artifacts) expected at the top and the bottom of the 

image, where the raypath coverage is poor.

Theoretical Modeling of the 

Temperature Dependence of RadarFrequency EM-Wave Propagation

The physical properties that affect radar frequency 

EM-wave propagation are temperature dependent. Analytic 

theoretical models were developed to predict the changes in 

EM-wave propagation velocity and attenuation that could be 

v

c

r

≈ 

 

 ≈ 4.343 v

expected from heating limestone bedrock and fracture fluid, 

from steam replacing water in the fractures, and from changes 

in the conductive properties of the fluid in fractures. The 

effect of heating on EM-wave reflectivity of fractures was 

also modeled. The modeling results were used as a basis for 

interpreting the results of radar reflection and tomography 

surveys collected at Loring AFB.

Constitutive Parameters

Measurable characteristics of radar-wave propagation 

include EM-wave velocity, attenuation, polarity (phase), 

and reflectivity. These characteristics are determined by 

the constitutive parameters (physical properties) of the host 

material. For radar-frequency EM-waves, these physical 

properties are the dielectric permittivity, ε, in farads per meter; 

the electrical conductivity, σ, in siemens per meter; and the 

magnetic permeability, μ, in henries per meter. In most nonferromagnetic earth materials, the magnetic permeability is 

equal to that of a vacuum; therefore, the effect of magnetic 

permittivity on EM-wave propagation is assumed to be 

insignificant and can be ignored.

In practice, it is difficult to differentiate the currents 

resulting from polarization effects (dielectric permittivity) 

and those resulting from conduction effects (electrical 

conductivity). For this reason, effective parameters need to 

be defined–effective permittivity and effective conductivity–

which are complex numbers. In the case of radar, the effective 

permittivity, εe, is more commonly used (for example, 

Grégoire, 2001):

(4)

where

εe' is the real part of the effective dielectric 

permittivity,

εe'' is the imaginary part of the effective dielectric 

permittivity,

ε' is the real part of the dielectric permittivity,

σ'' is the imaginary part of the electrical 

conductivity,

ω is the frequency,

ε'' is the imaginary part of the dielectric 

permittivity, 

and

σ' is the real part of the electrical conductivity.

In the expression for the real part of the effective 

dielectric permittivity, the term σ′′/ω can be neglected for most 

radar applications, where the electrical conductivity is low and 

frequency is high, resulting in εe' ~ ε'.

  

 





 





e e e

e

e

= − i

= +

= +

' ''

' ' ''

'' '' ' ,

6 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
The wave number, k, depends on the effective dielectric 

permittivity. The real part is the frequency-dependent phase 

number, β, and the imaginary part is the attenuation, iα (for 

example, Knight, 2001):

 , (5)

Taking into account the effective parameters, the wave 

velocity, v, is expressed (for example, Knight, 2001):

 , (6)

Effect of Heating on Radar-Frequency EM-Wave 

Velocity and Attenuation

The velocity and attenuation of radar-frequency 

EM-waves propagating in a medium are related to the 

electric properties–the dielectric permittivity and the 

electrical conductivity. Theoretical calculations were used to 

examine the effect of heating on the material properties of 

the material in the fractures and of the rock matrix. Mixing 

formulas were used to calculate the apparent permittivity 

and conductivity as a function of the medium porosity and 

the fracture-filling fluids, and these results were used to 

develop theoretical predictions of the effect of heating on the 

measurable quantities–EM-wave traveltime (velocity) and 

amplitude (attenuation).

The relative dielectric permittivity, εr, of a rock formation 

is a function of the permittivity of the rock matrix, the 

k i = =    e − 

v = 



porosity, and the permittivity of the pore fluid (water, or steam 

if steam replaces the water). To predict the relative permittivity 

of the formation, we used the complex refractive index method 

(CRIM) (Birchak and others, 1974; Wharton and others, 

1980), assuming electrical conductivity effects are negligible 

and EM-wave propagation velocity is controlled by the real 

components of the relative dielectric permittivity of the rock 

matrix and water (for example, Grégoire, 2001; Knight, 2001):

 , (7)

where

 is the porosity,

εw is the relative dielectric permittivity of water 

or steam, depending on temperature, 

and

εm is the relative dielectric permittivity of the 

rock matrix.

The permittivities of rock and water are functions of 

temperature. The dielectric permittivity of water decreases 

with increasing temperature, T (in degrees Celsius) (Helgeson 

and Kirkham, 1974), increasing EM-wave velocity (fig. 4A):

 . (8)

The bulk electrical conductivity, σ, of a water-filled 

porous medium depends on the electrical conductivities 

of the water and host matrix, both of which increase with 

temperature. For this study, a simple conductivity mixing 

model was used:

    r w m

0 5 0 5 0 5 1 . . . = + ( ) −

 w = −0. . 3972 × + T 88 7

TEMPERATURE, IN DEGREES CELSIUS

0 50 100 150

RELATIVE DIELECTRIC

 PERMITTIVITY

ELECTRICAL CONDUCTIVITY,

 IN SIEMENS PER METER

TEMPERATURE, IN DEGREES CELSIUS

A. B.

0.10

0.08

0.06

0.04

0.02

0.16

0.14

0.12

0 20 40 60 80 100

0

20

40

60

80

100

Figure 4. (A) Relative dielectric permittivity, and (B) electrical conductivity of water as a function of temperature. 

The conductivity curve was extrapolated for temperatures above 35 degrees Celsius.

Theoretical Modeling of the Temperature Dependence of Radar-Frequency EM-Wave Propagation 7
 , (9)

where

σm is the electrical conductivity of the matrix,

and

σw is the electrical conductivity of the water

The pore water was modeled as a solution of sodium 

chloride (NaCl), and the conductivity was modeled as a 

function of NaCl concentration and temperature following 

the methods described by Olhoeft (G.R. Olhoeft, Colorado 

School of Mines, written commun., 2002). This curve was 

extrapolated to a temperature of 90°C (fig. 4B). For a solution 

with a concentration of 0.2923 g/L (grams per liter) NaCl 

(G.R. Olhoeft, Colorado School of Mines, written commun., 

2002), the linear extrapolation of the best-fit line is:

σw = 0.001229 T + 0.03011. (10)

For a solution with a concentration of 0.5845 g/L NaCl 

(G.R. Olhoeft, Colorado School of Mines, written commun., 

2002), the linear extrapolation of the best-fit line is:

σw = 0.0024 T + 0.0587. (11)

Because steam is generated, the electrical conductivity of 

steam is negligible.

The relative dielectric permittivity and bulk electrical 

conductivity of the fractured limestone were calculated using 

equations (7) and (9). It is difficult to predict the change 

in conductivity of the rock matrix that will result from the 

heating associated with the steam injection. The change in 

conductivity will probably be heterogeneous and localized 

depending on the fracture network. For this reason, two 

values were considered for the conductivity of the limestone 

matrix, 0.01 and 0.02 S/m (siemens per meter), to estimate 

the effect of change in the conductivity on EM-wave velocity 

and attenuation. The first value for the conductivity of the 

limestone matrix is based on background EM conductivity 

measurements, and the second value is an estimate based on 

the assumption that the conductivity might increase in regions 

that are heated because the solubility of ionic compounds 

can increase with heating. For the water in the fractures, the 

relative dielectric permittivity and electrical conductivity 

of water were calculated using equations (8) and (10). The 

total porosity was simulated as 0.02 percent (SteamTech 

Environmental Services, Inc., 2001). The relative dielectric 

permittivity of limestone was simulated as 7.2, which is a 

reasonable value for limestone.

The predicted permittivity and conductivity of the 

fractured limestone are shown in figure 5. Assuming a 

frequency of 100 MHz and a constant value of 0.01 S/m for 

the limestone matrix conductivity, the velocity and attenuation 

of the EM waves were calculated using equations (2) and (3). 

For temperatures between 20 and 100o C, the effect of heating 

on the velocity and attenuation of the EM waves is low. For an 

increase of temperature of the limestone from 20 to 100°C, the 

  = − ( ) 1   m w +  predicted increase in the attenuation is about 1 dB/m (decibel 

per meter), less than a 15-percent change (fig. 5D). For a 

transmitter-receiver distance of 7.5 m, the EM-wave traveltime 

would decrease about 1 ns (nanosecond) from background 

measurements, increasing EM-wave velocity about 1.3 percent 

(fig. 6). The predicted magnitudes of these changes are small 

because of the low primary and secondary (fracture) porosity 

of the limestone.

Replacement of water by steam at temperatures above 

100°C results in a substantial increase in EM-wave velocity 

and a small decrease in attenuation (figs. 5C and D). The 

theoretical modeling predicts that, for a transmitter-receiver 

distance of 7.5 m, the traveltime would decrease about 3 ns, 

increasing EM-wave velocity about 4 percent, whereas 

EM-wave attenuation would decrease by less than 1 dB/m 

(figs. 5D and 6).

Doubling the electrical conductivity of the limestone 

matrix from 0.01 to 0.02 S/m at a given temperature produces 

a large increase in EM-wave attenuation (about 6 dB/m) 

(figs. 5C and D) and a small decrease in EM-wave velocity of 

about 1.5 percent (fig. 6).

The sensitivity of EM velocity and attenuation to the 

effects of heating on the electrical conductivity and the 

dielectric permittivity of water and the electrical conductivity 

of limestone also was simulated. The permittivity of the 

limestone matrix was simulated as a constant of 7.2. The 

initial values for the conductivity and permittivity of the water 

were assumed to be 0.05 S/m and 80.75 at 20oC, respectively. 

The initial value for the conductivity of the limestone matrix 

was assumed to be 0.01 S/m.

An increase in the conductivity of the limestone matrix 

from 0.01 to 0.10 S/m results in a 25-percent decrease in

EM-wave velocity (from 106 to 80 m/μs (meters per 

microsecond)) and an eight-fold increase in EM-wave 

attenuation (40 dB/m) (figs. 7A and 7B, respectively). An 

increase in matrix conductivity of 0.020 S/m results in a 

decrease in EM-wave velocity of about 5 percent or 5 m/μs.

An increase in the conductivity of the formation 

water from 0.05 to 0.20 S/m results in a small decrease in 

EM-wave velocity of less than 0.5 percent (about 0.5 m/μs) 

(fig. 7C) because of the low value assumed for the limestone 

porosity. The corresponding increase in EM-wave attenuation 

is approximately 1.5 dB/m (fig. 7D). Model results for a 

decrease in the permittivity of water from 80 to 50 as a result 

of heating show a small increase in EM-wave velocity of about 

1.5 percent (from about 106 to about 107.5 m/μs) and a slight 

increase in EM-wave attenuation (about 0.10 dB/m) (figs. 7E 

and 7F, respectively).

Effect of Heating on EM-Wave Reflections 

from Fractures

Theoretical analytic calculations were used to predict 

changes in EM-wave reflections that would be expected as 

the result of heating the fluid in a fracture. The reflection 

8 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
coefficient (reflectivity) of a water-filled fracture in limestone 

(with εwater = 7.2, σ water = 0.01 S/m) was calculated as a 

function of temperature. Above a temperature of 100oC, 

steam is assumed to replace the water in the fracture and the 

electric properties will change (εsteam = 1.013, σsteam = 0.0 S/m). 

Fracture openings of several apertures were considered—

10 mm (fig. 8), 5 mm (fig. 9), 2 mm (fig. 10), and 1 mm 

(fig. 11).

When the water in the fracture is heated, a decrease in 

the reflection coefficient is predicted, which will decrease 

the amplitude of a reflected wave relative to background 

measurements at 20oC. The ratio between the reflectivity at a 

given temperature and the reflectivity at 20oC was calculated. 

For a frequency of 100 MHz, this ratio is approximately equal 

to 0.9 at 40oC, 0.8 at 60oC, and 0.65 at 100oC, independent 

of the aperture (figs. 8B, 9B, 10B, and 11B). Analysis of the 

reflectivity phase indicates that a change of polarity occurs 

when steam replaces water. For fractures of aperture 5 mm 

or less, the reflection coefficient resulting from a water-filled 

fracture (less than 100oC) is low–around 0.1 or smaller; for 

a 1-mm fracture, the predicted reflectivity is about 0.02. 

In the case of such small-aperture fractures, the effect of 

heating will probably be difficult to detect because of the low 

signal-to-noise ratio common to these measurements.

TEMPERATURE, IN DEGREES CELSIUS

40 60 80 100 120

20 40 60 80 100 120

TEMPERATURE, IN DEGREES CELSIUS

0.035

0.030

0.025

0.020

0.015

0.010

0.005

0

ELECTRICAL CONDUCTIVITY, IN S/m

20 40 60 80 100 120

TEMPERATURE, IN DEGREES CELSIUS

ATTENUATION, IN DECIBELS

 PER METER

VELOCITY, IN METERS

 PER MICROSECOND

20 40 60 80 100 120

TEMPERATURE, IN DEGREES CELSIUS

RELATIVE DIELECTRIC

 PERMITTIVITY

7.0

7.2

7.4

7.6

8.0

115

110

105

100

95 4

6

8

10

12

14

16

 A. B.

 C. D.

20

7.8

S

S

S

S

S

S = 0.01 S/m

= 0.02 S/m

TEMPERATURE, IN DEGREES CELSIUS

TRAVELTIME, IN NANOSECONDS

S

S = 0.01 S/m

= 0.02 S/m

66

67

68

69

70

71

72

73

20 40 60 80 100 120

Figure 5. Model results for the (A) relative dielectric permittivity, and (B) electrical conductivity of fractured 

limestone, and the resultant (C) velocity, and (D) attenuation of 100-megahertz radar waves as a function of 

temperature and conductivity (σ), in siemens per meter (S/m).

Figure 6. Theoretical results for radar-wave traveltimes as a 

function of temperature and conductivity (σ), in siemens per meter 

(S/m), for an antenna-separation distance of 7.5 meters.

Theoretical Modeling of the Temperature Dependence of Radar-Frequency EM-Wave Propagation 9
VELOCITY, IN METERS

 PER MICROSECONDS

VELOCITY, IN METERS

 PER MICROSECONDS

ATTENUATION, IN DECIBELS

PER METER

ATTENUATION, IN DECIBELS

PER METER

CONDUCTIVITY OF LIMESTONE, IN S/m CONDUCTIVITY OF LIMESTONE, IN S/m

CONDUCTIVITY OF WATER, IN S/m CONDUCTIVITY OF WATER, IN S/m ATTENUATION, IN DECIBELS PER METER

VELOCITY, IN METERS

 PER MICROSECONDS

RELATIVE DIELECTRIC PERMITTIVITY OF WATER RELATIVE DIELECTRIC PERMITTIVITY OF WATER

 A. B.

 C. D.

 E. F.

El

 = 7.2, Sl = 0.01 S/m, Sw = 0.05 S/m

El

 = 7.2, Ew = 80.75, Sw = 0.05 S/m El = 7.2, Ew = 80.75, Sw = 0.05 S/m

El

 = 7.2, Ew = 80.75, Sw El = 7.2, Ew = 80.75, Sw = 0.01 S/m = 0.01 S/m

El

 = 7.2, Sl = 0.01 S/m, Sw = 0.05 S/m

80

85

90

95

100

105

110

0 0.02 0.04 0.06 0.08 0.10 0 0.02 0.04 0.06 0.08 0.10

0

10

20

30

40

50

106.0

105.9

105.8

105.7

105.6

105.5

105.4

0.05 0.10 0.15 0.20 0.25 0.05 0.10 0.15 0.20 0.25

6.0

6.5

7.0

7.5

8.0

107.5

107.0

106.5

106.0

105.5

90 80 70 60 50 90 80 70 60 50

6.24

6.26

6.28

6.30

6.32

6.34

Figure 7. Effects of the (A and B) conductivity of limestone, (C and D) conductivity of water, and (E and F) relative dielectric 

permittivity of water on radar-wave velocity and attenuation. εl is the relative dielectric permittivity of limestone, εw is the relative 

dielectric permittivity of water, σ1 is the conductivity of limestone, in siemens per meter (S/m), and σw is the conductivity of water, 

in S/m. 

10 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

REFLECTIVITY REFLECTIVITY

RATIO OF REFLECTIVITY RATIO OF REFLECTIVITY

A. B.

C. D.

o

o

o

o

o

o

0

0.1

0.2

0.3

0.4

50 75 100 50 75 100

50 75 100 50 75 100

0

0.1

0.2

0.3

0.4

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

20oC

40o

C

60o

C

80oC

100oC

120oC

20oC

40oC

60o

C

80oC

100o

C

120oC

Figure 8. Reflectivity of a 10-millimeter-thick fracture as a function of temperature, and the ratio of the 

reflectivity at temperature to the reflectivity at 20 degrees Celsius (°C) for formation conductivities of (A and B) 

0.01 siemens per meter and (C and D) 0.03 siemens per meter.

FREQUENCY, IN MEGAHERTZ

REFLECTIVITY

FREQUENCY, IN MEGAHERTZ

RATIO OF REFLECTIVITY

A. B.

20oC

40oC

60o

C

80o

C

100oC

120oC

50 75 100 50 75 100

0

0.2

0.4

0.6

0.8

1.0

0

0.05

0.10

0.15

0.20

0.25

Figure 9. (A) Reflectivity of a 5-millimeter-thick fracture as a function of temperature, and (B) the ratio of the reflectivity 

at temperature to the reflectivity at 20 degrees Celsius (°C) for a formation conductivity of 0.01 siemens per meter.

Theoretical Modeling of the Temperature Dependence of Radar-Frequency EM-Wave Propagation 11
For all fracture openings that were considered, the 

reflection coefficient of the fracture filled with steam is a 

factor of 10 lower than the reflection coefficient of the same 

fracture filled with water. If steam replaces water in a fracture, 

the reflectivity will become so low that the reflection will 

likely disappear. Comparison of pre- and post-steam injection 

surveys to identify locations where reflection amplitudes 

decrease substantially or disappear completely should 

delineate zones where steam has replaced water in a fracture.

To summarize, if the water in a fracture is heated (in 

the absence of replacement by steam), the following changes 

are expected:

•	 A decrease of the reflectivity; if the heating is high 

enough, the decrease will be substantial, by a factor of 

about 0.6.

•	 No change of polarity.

•	 If the fracture has an aperture of 5 mm or less, then 

the decrease of reflectivity will probably be negligible 

compared to the level of the noise.

On the other hand, if steam replaces water, the following 

changes are expected:

•	 A decrease of the reflectivity amplitude by at least a 

factor 10.

•	 A change of polarity of the radar reflection.

The effect of the limestone matrix conductivity on the 

reflection coefficient also was considered. An increase in 

the limestone conductivity of 0.03 S/m produced a small 

decrease in the reflectivity (fig. 8). Although this increase of 

FREQUENCY, IN MEGAHERTZ

REFLECTIVITY

FREQUENCY, IN MEGAHERTZ

RATIO OF REFLECTIVITY

 A. B.

50 75 100 50 75 100

0

0.2

0.4

0.6

0.8

1.0

20oC

40oC

60o

C

80o

C

100o

C

120oC

0

0.02

0.04

0.06

0.08

0.10

0

0.02

0.04

0.06

0.08

0.10

0

0.2

0.4

0.6

0.8

1.0

FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

REFLECTIVITY

RATIO OF REFLECTIVITY

 A. B.

50 75 100 50 75 100

20oC

40oC

60o

C

80oC

100oC

120oC

Figure 10. (A) Reflectivity of a 2-millimeter-thick fracture as a function of temperature, and (B) the ratio of the reflectivity 

at temperature to the reflectivity at 20 degrees Celsius (°C) for a formation conductivity of 0.01 siemens per meter.

Figure 11. (A) Reflectivity of a 1-millimeter-thick fracture as a function of temperature, and (B) the ratio of reflectivity at 

temperature to the reflectivity at 20 degrees Celsius (°C) for a formation conductivity of 0.01 siemens per meter.

12 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
conductivity did affect the reflection coefficient, it affected the 

amplitude of the signal by attenuation, resulting in a reflected 

wave with lower amplitude.

The effect of water salinity on radar reflections also was 

considered, and the results of those theoretical calculations are 

shown in figure 12. The reflectivity of a 10-mm thick fracture 

filled with a 0.2923 g/L NaCl solution is compared to the 

reflectivity of a 10-mm thick fracture filled with a 0.5845 g/L 

NaCl solution. In both cases, the conductivity of the limestone 

rock is 0.01 S/m. The reflectivity of the water-filled fracture 

increases with the salinity of the water filling the fracture, 

especially at lower frequencies. This result was previously 

observed in experiments where a saline solution was injected 

in a fracture (Lane and others, 1998, 1999, 2000).

It is important to note that the analytical modeling 

performed for this study was simple, and should be viewed 

as the initial effort required to predict the effects of heating 

and steam on EM wave reflection behavior in fractured rock 

environments. Fundamentally, the models predict reflection 

behavior changes resulting from physical properties changes 

of the rock matrix and fracture fluid on a single, isolated 

fracture. The modeling does not address the constructive 

and destructive reflection interference effects induced by 

multiple, closely spaced, sub-parallel fractures (expected to 

exist in a fracture zone), does not account for the weathered 

zone surrounding a fracture, nor do the models address the 

complicated effects of a partial change in fracture fluid that 

could be induced by sub-wavelength scale fracture channeling 

or preferred pathways of heated water or steam within a 

given fracture or fracture zone. Assessment of EM reflection 

behavior for more complicated and realistic scenarios 

would be facilitated by the use of full wave-form numerical 

modeling methods.

FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ

REFLECTIVITY

REFLECTIVITY

RATIO OF REFLECTIVITY

 A. B.

C.

20oC

40o

C

60o

C

80o

C

100o

C

50 75 100 50 75 100

50 75 100

0

0.1

0.2

0.3

0.4

0

0.1

0.2

0.3

0.4

0

0.5

1.0

1.5

2.0

Figure 12. Effect of water salinity on the radar reflection. (A) Reflectivity of a 10-millimeter water-filled fracture. 

Concentration of the saline solution is 0.2923 grams per liter (g/L) NaCl. (B) Reflectivity of a 10-millimeter-thick water-filled 

fracture. Concentration of the saline solution is 0.5845 g/L NaCl. (C) Ratio of the reflectivity with 0.5845 g/L NaCl to the 

reflectivity with 0.2923 g/L NaCl. Temperature is in degrees Celsius (°C).

Theoretical Modeling of the Temperature Dependence of Radar-Frequency EM-Wave Propagation 13
Conclusions of the Theoretical Modeling

Results of theoretical analytical modeling indicate that 

increases in temperature will induce changes in EM-wave 

velocity, attenuation, and reflectivity, and these changes are 

predominantly controlled by physical property changes in 

water-filled fractures. 

With heating of the matrix, EM-wave velocity:

•	 Increases slightly with temperature if the conductivity 

of the matrix is assumed to be constant (fig. 5C)

•	 Increases substantially if steam replaces water in 

the fractures and if the conductivity of the matrix is 

assumed to be constant (fig. 5C)

•	 Decreases substantially if the conductivity of the 

limestone matrix increases (fig. 7A)

•	 Decreases negligibly if the conductivity of the water 

increases, (because of the low primary and fracture 

porosity of the limestone) (fig. 7C)

•	 Increases slightly if the dielectric permittivity of the 

water decreases as a result of heating (fig. 7E)

With heating of the matrix, EM-wave attenuation:

•	 Increases slightly with temperature (about 1 dB/m over 

a temperature range of 20–90 oC) if the conductivity of 

the matrix is assumed to be constant (fig. 5D)

•	 Decreases slightly if steam replaces water and the 

conductivity of the matrix is assumed to be constant 

(fig. 5D)

•	 Increases substantially if the conductivity of the 

limestone matrix increases (fig. 7B)

•	 Increases if the conductivity of the water increases 

(fig. 7D)

•	 Increases only slightly if the dielectric permittivity of 

the water decreases as a results of heating (fig. 7F)

With heating of the matrix, the reflection coefficient 

(reflectivity) of a thin fracture:

•	 Decreases because of the decrease in dielectric 

permittivity of the water with increasing temperature 

(figs. 8–11)

•	 Decreases substantially (by a factor of 10) if steam 

replaces water (figs. 8–11)

•	 Increases as the salinity of water increases, because 

of the increase in electrical conductivity of the water 

(fig. 12)

Collection, Processing, and Interpreted 

Results of Geophysical Data

This section describes the collection and processing 

of geophysical data and presents the interpretation of the 

borehole radar data collected at the former Loring AFB 

study area. Conventional borehole geophysical logs were 

collected to augment the processing and interpretation of the 

radar-reflection and tomography data, which are the focus of 

this study.

The data were collected to evaluate the effectiveness 

of radar methods to monitor the movement of steam and 

the transport of heat in the subsurface during the SER. For 

the SER pilot study, the USEPA, the Maine Department of 

Environmental Protection, and the U.S. Air Force contracted 

with SteamTech Environmental Services, Inc., to design and 

perform the steam injection. Steam was injected in several 

wells on the northeastern side of the quarry site, and water 

and vapor were extracted by pumping a number of wells 

across the entire site (fig. 1). The steam was injected at 

depths from 20 m to the bottoms of the boreholes (about 30 

to 40 m), except in borehole I-6, where steam was injected 

into three separate intervals, including one as shallow as 

10 m. Steam injection was initiated September 1, 2002, and 

continued until November 19, 2002. Extraction of vapor 

and fluid was initiated August 30, 2002, and continued until 

November 26, 2002.

Description of Monitoring Boreholes

The geophysical logging was conducted in boreholes 

JBW-7816 and JBW-7817A (fig. 1) at the former Loring AFB 

study area. Borehole JBW-7816 extends to a depth of 46 m 

below top of casing, and borehole JBW-7817A extends to 

a depth of 30 m below top of casing. The two boreholes are 

7.34 m apart, and the top of casing of JBW-7817A is 0.59 m 

higher in elevation than the top of casing of JBW-7816. 

Approximately 7 m of steel casing were installed at the 

top of each borehole when they were originally drilled for 

characterization studies. Each borehole was completed with 

solid fiberglass casing over the entire length. This prevented 

exposure of personnel and equipment to direct steam and 

contaminants while allowing radar and EM conductivity 

logging to survey the formation surrounding the borehole. 

The radar signal cannot penetrate steel casing, and polyvinyl 

chloride (PVC) casing can deform at high temperatures. The 

casings were capped at the bottom and sealed over the entire 

length of the boreholes using class G high-temperature grout 

to prevent flow and mixing between fractures intersecting 

the boreholes. The casings were filled with clean water. 

The installation and grouting of the fiberglass casings were 

completed before any of the geophysical logs were run in 

these boreholes. This precluded collecting many types of 

borehole logs that require an open borehole, such as fluid 

resistivity and televiewer data.

14 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
Conventional Borehole Geophysical Logs

Borehole fluid-temperature and EM-conductivity logs 

were collected at the start of each day of field work. Deviation 

logs were collected to provide accurate borehole location 

profiles, which were needed for processing the crosshole radar 

tomography data.

Fluid-Temperature Logs

Fluid-temperature logs are used to measure the water 

temperature in boreholes and to identify areas of temperature 

change (Williams and Conger, 1990). Fluid-temperature 

logs were collected August 21, 2002, September 12–14, 

2002, and November 5–8, 2002. Because the boreholes are 

lined with fiberglass casing and sealed with grout, accurate 

measurements of formation temperatures could not be 

made; however, the borehole temperature information was 

important for the safety of the equipment and its operators, 

and could provide general information about heating at 

the site. The largest temperature changes were expected at 

depth because the steam was injected in the interval between 

about 20 and 40 m below land surface. A Mount Sopris 

2PFA-1000 temperature-fluid resistivity tool was used to 

collect the August and September data, and a Mount Sopris 

2PCA-2SFB-1000 caliper-temperature tool was used to collect 

the November data. The background temperature logs were 

collected on August 21, 2002, about a week after the fiberglass 

casing was installed. Because the casing was sealed and the 

boreholes were filled with water at a temperature equivalent 

to ambient air temperature, the water in the boreholes may 

not have had enough time to fully equilibrate to ambient 

background temperatures of the formation.

The August and September field temperature data were 

recalibrated after the field work was completed because 

of an error in the original calibration file. In November, 

the temperature data were collected with a hybrid calipertemperature tool. The caliper arms did not stay closed, 

so the November temperature data were logged from the 

bottom of the borehole upwards, instead of from the top of 

the borehole downwards. This may not have allowed water 

to flow properly through the tool, and the differences in the 

temperature data may be underrepresented. The water in the 

borehole also may have been mixed by the upward log. As 

shown in figure 13, however, the November data in boreholes 

JBW-7816 and JBW-7817A generally are consistent with 

temperature data collected by the USGS using a hand-operated 

YSI Incorporated Model 3000 T-L-C (Temperature-LevelConductivity) meter and with thermocouple data collected 

by SteamTech Environmental Services, Inc. (Gregg Crisp, 

formerly of SteamTech Environmental Services, Inc., written 

commun., 2002)

Temperature data were collected in September 2002 on 

3 consecutive days, about 2 weeks after the steam injection 

began. In JBW-7816, the temperature was unchanged from 

the background values (fig. 13A). In JBW-7817A, small 

daily increases in the temperature were observed; the highest 

temperature was about 12°C at depths ranging from about 

20 to 25 m (fig. 13B).

Temperature data also were collected daily from 

November 5 to 8, about 9 weeks after the steam injection 

began. In JBW-7816, the maximum temperature was about 

10°C higher than the August background measurements. The 

highest temperature, about 18°C, was recorded at a depth 

of about 22 m. In JBW-7817A, the temperature increased 

as much as 40°C above the background measurements. 

The temperature increased with depth, with a maximum 

temperature of about 47°C recorded at a depth of about 29 m, 

near the bottom of the borehole. Greater temperature changes 

were expected in JBW-7817A than in JBW-7816, because 

JBW-7817A is closer to steam-injection well I-4.

Electromagnetic-Conductivity Logs

EM-conductivity logs were collected to detect 

conductivity changes in the rock and fluids surrounding the 

boreholes (Williams and others, 1993). The EM-conductivity 

measurements were collected using a Mount Sopris 

2PIA-1000 poly-induction probe. The tool detects changes 

in the conductivity over radial distances of 40 to 100 cm 

from the borehole. The EM-conductivity tool should not be 

strongly affected by the fiberglass casing and grout that line 

the boreholes. The EM-conductivity tool was not calibrated at 

the site because of the large amount of metal associated with 

the steam injection equipment. Steel casing also precluded 

logging in the upper 7 m of the borehole.

Temperature changes in the EM-conductivity probe 

result in small tool-dependent changes in the orientation of 

the coils, which changes the measured conductivity. Because 

the tool calibration has a thermal sensitivity, and because of 

the inability to calibrate the tool at the site, the calibration 

values were set at full scale for all logs so that all results were 

collected using the same calibration numbers.

In both boreholes, the variations in bulk formation 

conductivity with depth are quite similar for all three 

logging dates. In both boreholes, when the data sets are 

overlain using conductivity values for the upper part of the 

borehole as a guide, the values measured in November 2002 

appear to decrease with depth. The change is greatest at the 

depths having the largest temperature changes, and is more 

pronounced in borehole JBW-7817A, which had a much 

larger temperature increase than in borehole JBW-7816. These 

results are inconsistent with theoretical and laboratory results 

that show the conductivity of rocks increases with temperature 

(Llera and others, 1990), but might be owing to the effect of 

thermal instability of the EM tool.

Collection, Processing, and Interpreted Results of Geophysical Data 15
Deviation Logs

Borehole deviation measurements were collected 

to determine the exact location of the borehole in three 

dimensions (Keys, 1990). Accurate deviation data are 

critical for interpretation of the radar tomography data. 

Borehole deviations were measured using an Advanced 

Logic Technology (ALT)/Mount Sopris ABI-40 acoustic 

televiewer (ATV) tool, which collects deviation information 

using a three-component magnetometer for interpretation of 

televiewer data. Measurements were collected in November 

2002 at 0.003-m intervals along each borehole. The logs for 

boreholes JBW-7816 and JBW-7817A show deviations at 

the bottom of the boreholes of 1.8 and 0.8 m, respectively, to 

the west.

Single-Hole Radar Reflection

Borehole-radar reflection surveys were collected 

to identify and monitor changes in individual fractures 

surrounding the boreholes. Data were collected 

August 13–16, 2002, as background data before the steam 

injection; September 11–16, 2002, about 2 weeks after the 

steam injection began; and November 4–8, 2002, about 

9 weeks after the injection began and about 10 days before the 

end of the injection. The data were processed to correct for 

power variations, and the results of the theoretical modeling 

were used to interpret changes in the data as a result of heating 

from the steam injection.

September 12

recalibrated

TEMPERATURE, IN DEGREES CELSIUS

5 10 15 20 25

5

10

15

20

25

30

35

45

 A. JBW-7816 B. JBW-7817A

40

0

TEMPERATURE, IN DEGREES CELSIUS

DEPTH, IN METERS BELOW TOP OF CASING

0 10 20 30 40 50

30

25

20

15

10

5

0

August 21 

(background)

recalibrated

November 8

SteamTech

November 7

DEPTH, IN METERS BELOW TOP OF CASING

YSI 

November 7

SteamTech 

November 7

August 21 

(background)

recalibrated

September 12

recalibrated

November 8

Figure 13. Temperature data collected in boreholes (A) JBW-7816 and (B) JBW-7817A during August, September, and 

November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. The YSI measurements on November 7 in 

borehole JBW-7817A were made for this study using a hand-operated YSI Incorporated temperature-level-conductivity meter.

16 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
Data Acquisition

The radar-reflection logs were acquired in single-hole 

reflection mode using Malå GeoScience RAMAC electric 

dipole antennas having a central frequency in air of 100 MHz 

and directional dual-loop antennas having a central frequency 

in air of 60 MHz. The 100-MHz electric dipole antennas offer 

superior resolution, but the omni-directional data cannot be 

used to determine the strike of reflectors.

The 100-MHz omni-directional reflection data 

were collected at 5-, 10-, and 20-cm intervals. A 1.00-m 

spacer was used between antennas for a total transmitterreceiver separation of 2.73 m. The sampling frequency was 

1,200 MHz, and 512 samples were collected per trace. As a 

result of antenna loading, the effective central EM frequency 

propagating in the formation is about 60 MHz. The boreholes 

were logged over their complete length by manually lowering 

the antennas and using a digital encoder on the tripod to 

trigger data collection at the desired intervals.

The 60-MHz directional reflection data were collected 

at 20-cm intervals, the smallest interval allowed by the 

system. A 1.00-m spacer was used between antennas for a 

total transmitter-receiver separation of 6.39 m. A sampling 

frequency of 611.24 MHz was used, and 512 samples were 

collected per trace. The directional receiver antenna can 

indicate reflector orientation using dual-loop antennas, which 

act as four different orthogonal receivers, coupled with a threecomponent magnetometer and a plunge sensor that monitor 

the orientation of the tool in the borehole. The sensitivity 

and effective radial penetration of directional dual-loop 

receiver antennas are inferior to those of omni-directional 

dipole receiver antennas (Falk, 1992; Lane and others, 2001). 

Because of the severe rotation of the antennas in the borehole 

during acquisition, the results were difficult to interpret and 

are not included in this report.

Data Processing and Interpretation

The omni-directional radar reflection profiles are 

shown in figures 14 (borehole JBW-7816) and 15 (borehole 

JBW-7817A). The vertical axis represents the length of 

the borehole, and the horizontal axis represents the twoway traveltime of the radar waves. The profiles are shown 

with the same time-gain function. The radar profiles in 

figures 14 and 15 were processed to improve the data quality 

and facilitate detection of reflectors. A background-removal 

filter was applied to suppress ringing and improve the signalto-noise ratio. This processing also attenuates reflections 

from reflectors parallel to the borehole, such as vertical 

fractures or nearby boreholes within the radar reflection time 

window. The uppermost 7 m of both boreholes were cased 

in steel, precluding penetration of the EM radar wave. The 

data collected near the bottom of the steel casings were more 

reflective because the antennas were above the water table.

In the background data from August 2002, the reflections 

show greater amplitude (greater reflection energy) because 

the transmitter output power was higher at that time. The 

penetration is lowest for the last set of measurements from 

November 2002 collected in borehole JBW-7817A; this profile 

is highly attenuated (fig. 15). No substantial change was 

seen in the penetration on the data sets collected in borehole 

JBW-7816, probably because of the low levels of thermal 

change in that borehole.

The omni-directional and directional data sets were 

examined both individually and as “differenced” data sets. 

Difference data were calculated by subtracting a trace in a 

given data set from the corresponding trace in the background 

data (August). The differencing procedure was performed to 

reveal changes in EM reflectivity that may be related to steam 

movement or heat transport.

Analysis of the Radar Profiles

The peak-to-peak amplitudes of the direct EM waves 

were measured for each data set collected in each of the 

two boreholes for the purpose of checking antenna stability 

and for monitoring changes to the formation (fig. 16). The 

output power of the transmitter antenna was much greater 

in the background data set (August) than in the other data 

sets. A small power decrease between the September and 

the November data sets also was observed. These changes 

are thought to be the result of a change in instrumentation, 

resulting from an antenna repair and possible instability of the 

antenna, resulting in small variations in the transmitter output 

or receiver effectiveness; these changes are not necessarily 

related to the movement of steam or transport of heat. A 

meaningful comparison and differencing of data sets requires 

application of a correction to trace amplitudes for the variable 

transmitter output power.

Most of the reflectors observed in the boreholes surveyed 

using the single-hole radar reflection method in August, 

September, and November 2002 do not appear to change 

with time (figs. 14 and 15); however, several important 

changes in reflector amplitude, lateral extent, and continuity 

were observed in the reflection data. For borehole JBW7816, the amplitude and continuity of two parallel reflectors 

increased in the September data after the start of steam 

injection (Zone 1 indicated by a yellow box on fig. 14). 

In borehole JBW-7817A, the amplitude and continuity of 

several pre-existing reflectors increased between the August 

and September data sets. Two of the reflectors (Zone 2 and 

Zone 3) are indicated with yellow boxes in figure 15. In the 

November data for borehole JBW-7817A, the continuity and 

amplitude of reflectors in the deepest portion of the borehole 

are noticeably reduced or disappeared entirely.

In borehole JBW-7817A, a strong reflector appears in 

the September data at depths below 20 m (Zone 3 on fig. 15) 

that was barely noticeable in the August data; however, this 

reflector is no longer visible in the November data. Several 

different explanations for the increase and decrease in the 

amplitude and lateral continuity of the reflector are possible. 

The appearance of the reflector does not indicate steam 

Collection, Processing, and Interpreted Results of Geophysical Data 17
R1 R1

R2 R2

R1

Zone 1

R1

Zone 1

EXPLANATION

Reflectors selected for analysis

Reflectionzones

TIME, IN NANOSECONDS TIME, IN NANOSECONDS TIME, IN NANOSECONDS

DEPTH, IN METERS BELOW TOP OF CASING

AUGUST 2002 SEPTEMBER 2002 NOVEMBER 2002

+1500

-1500

0

Amplitude, in 

volts per meter

Zone 1 Zone 1

152025303540

50 100 150 50 100 150 50 100 150

152025303540

152025303540

Figure 14. The 100-megahertz omni-directional borehole-radar reflection data collected in borehole JBW-7816 during August, September, and November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. The gain function is the same for all profiles.

18 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
R3 R3

R2 R2

R3

Zone 3

TIME, IN NANOSECONDS TIME, IN NANOSECONDS TIME, IN NANOSECONDS

DEPTH, IN METERS BELOW TOP OF CASING

AUGUST 2002 SEPTEMBER 2002 NOVEMBER 2002

R1 R1

R1

Zone 2

R1

Zone 2

EXPLANATION

Reflectors selected for analysis

Reflectionzones

+1500

-1500

0

Amplitude, in 

volts per meter

Zone 3

Zone 2 Zone 2

Zone 3

50 100 150 50 100 150 50 100 150

14161820222426

14161820222426

14161820222426

Figure 15. The 100-megahertz omni-directional borehole-radar reflection data collected in borehole JBW-7817A during August, September, and November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. The gain function is the same for all profiles.

Collection, Processing, and Interpreted Results of Geophysical Data 19
replacing water in a fracture because (a) it also would have 

been imaged in the November data set, (b) modeling predicted 

the reflectivity of a fracture filled with steam would be a 

factor of 10 less than a fracture filled with water, and (c) a 

phase change in the reflection data would have been observed 

between the background and later data sets if steam had 

replaced water in a fracture. Although theoretically possible, it 

is unlikely that the reflector is caused by a temporary increase 

in the conductivity of the fluid in the fracture. It is improbable 

that variations in the antenna power caused Zone 3, because 

all the reflections would be stronger. It also is unlikely that 

the reflection is off of a borehole, or equipment in a borehole, 

because there were no open boreholes within the radius of 

penetration. All boreholes surrounding the logged well were 

sealed, and the slope of the reflector indicates that, if the 

reflector were caused by a borehole, the borehole would have 

to be strongly deviated.

The most likely explanation is that the pressure of the 

injection increased the aperture of the fracture when it was 

imaged in the September data set. This could possibly happen 

by either forcing debris or weathered rock from the fracture, or 

by compressing the weathered rock along the fracture surface. 

The fracture may have filled with steam by the time that the 

November data set was collected. Steam injection borehole I-4 

is less than 4 m away from borehole JBW-7817A, and within 

the area imaged by the reflection data. If steam had filled the 

fracture, the reflectivity should have decreased by a factor of 

10. Because the reflector was not identified in the November 

data set, it can not be determined if a phase change occurred.

Power Correction

In order to compare the amplitudes of radar reflections 

from different data sets, a correction was applied to account 

for variation of output power between the three acquisition 

DEPTH, IN METERS BELOW TOP OF CASING

DEPTH, IN METERS BELOW TOP OF CASING

JBW-7817A

JBW-7816

AMPLITUDE OF THE DIRECT WAVE, 

IN MICROVOLTS

20,000

40,000

60,000

80,000

0

AMPLITUDE OF THE DIRECT WAVE, 

IN MICROVOLTS

20,000

40,000

60,000

80,000

0

0 5 10 15 20 25 30

0 5 10 15 20 25 30 35 40 45

Figure 16. Peak-to-peak amplitude of the direct radar wave of single-hole radar reflection data collected during August, 

September, and November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

20 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
periods. The correction procedure is based on the assumptions 

that the amplitude of the direct wave should be the same in all 

data sets, and that amplitude should decrease exponentially 

with distance. For each trace, a plot of log Ai

2

and i was 

constructed, where Ai is the signal amplitude for sample i. The 

curves were smoothed and normalized. Examples of these 

curves for two different depths in each borehole are shown in 

figure 17. For each depth, the curves are shown for August, 

September, and November.

Log Ai

2

 varies linearly with i and can be approximated 

by c(i),

where

c(i)  logAi

2

 = -0.0068 i + 1 . (12)

This equation is unique and was derived specifically for 

this site.

To account for the power difference between any two data 

sets, 1 and 2, the correction for sample i in trace j is

 , (13)

where

A1 is the peak-to-peak amplitude of direct wave 

in data set 1 for trace j,

A2 is the peak-to-peak amplitude of direct wave 

in data set 2 for trace j, 

and

A2 is greater than A1.

Correction factors were calculated to account for the 

power difference between the August and September data 

sets and were applied to each sample in all of the traces of 

the September data set. Likewise, correction factors were 

calculated to account for the power difference between the 

August and November data sets and applied to the November 

data set.

correction i( , j A ) e = − xp(2 2 log ( j c) (i A )) exp( log ( j c) (i)) 2 1

Figure 17. Power term calculated for selected single-hole radar reflection data sets from boreholes JBW-7816 and 

JBW-7817A during August, September, and November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

The curves are normalized and smoothed. 

SAMPLES

SAMPLES SAMPLES

SAMPLES

NORMALIZED LOG (AMPLITUDE2) NORMALIZED LOG (AMPLITUDE2)

NORMALIZED LOG (AMPLITUDE2) NORMALIZED LOG (AMPLITUDE2)

JBW-7816 

10.54 METERS BELOW TOP OF CASING

JBW-7816 

42.54 METERS BELOW TOP OF CASING

JBW-7817A 

12.54 METERS BELOW TOP OF CASING

JBW-7817A 

25.04 METERS BELOW TOP OF CASING

50 100 150 200

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0.8

1.0

50 100 150 200 50 100 150 200

50 100 150 200

August

September

November

Collection, Processing, and Interpreted Results of Geophysical Data 21
Spectra Analysis

The mean frequency spectra calculated for each of the six 

100-MHz omni-directional data sets (which is two boreholes, 

JBW-7816 and JBW-7817A, collected at three times, August, 

September, and November 2002) are shown in figure 18. 

The spectra are shown for both the unprocessed data and the 

processed data after removal of the background. Removal of 

the background consists of subtracting an average trace from 

the radar profile, which removes consistent noise, the direct 

wave, and reflectors parallel to the borehole. The mean central 

frequency is about 60 MHz for all data sets. The decrease of 

the magnitude of the central frequency between the three data 

sets for a given borehole can result from either a variation 

of radar-system power between the different acquisitions or 

a change in the medium properties. In this case, most of the 

decrease is assumed to be the result of the power difference 

in the radar system used for the collection of each data set, 

although a small portion may be the result of a change in 

conductivity of the matrix or ground water. For the November 

data, the magnitude corresponding to the central frequency 

is slightly lower for the JBW-7817A data set compared to 

the JBW-7816 data set. This discrepancy could be a result 

of greater heating near JBW-7817A relative to JBW-7816 

(fig. 13). The radar profile from borehole JBW-7817A also 

is more attenuated than the profile from borehole JBW-7816 

(figs. 14 and 15), which is consistent with the hypothesis 

that temperature variation accounts for the difference in the 

magnitude of the central frequency. NORMALIZED MAGNITUDE NORMALIZED MAGNITUDE

JBW-7816 Unprocessed data JBW-7817A Unprocessed data

FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ

NORMALIZED MAGNITUDE NORMALIZED MAGNITUDE

JBW-7816 Processed data JBW-7817A Processed data

0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

0.6

0.8

1.0

1.2

20 40 60 80 100 120

20 40 60 80 100 120

20 40 60 80 100 120

20 40 60 80 100 120

August

September

November

Figure 18. Mean frequency spectra calculated for each single-hole radar reflection data set from boreholes JBW-7816 and 

JBW-7817A during August, September, and November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

Processed data (after removal of the background) and unprocessed data are shown.

22 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
Reflector Analysis

The high density of reflectors in the limestone at the 

quarry resulted in interference between reflections in the 

radar profiles, thus complicating identification of individual 

reflectors. Further characterization of the reflectors required a 

more detailed analysis. A method based on spectral analysis of 

the radar reflection has been developed to characterize fracture 

properties such as the type of filling material and the fracture 

opening (Grégoire, 2001; Grégoire and Hollender, 2004). This 

method has previously been applied to evaluate open fractures 

in a salt mine (Grégoire and others, 2003).

To develop a reliable interpretation of the presence or 

absence of steam in fractures around the logged boreholes, the 

analysis focused on several well-defined reflections identified 

with red lines in figures 14 and 15. Two reflectors were 

identified in the JBW-7816 profile (fig. 14; R1 and R2) and 

three reflectors were identified in JBW-7817A profile (fig. 15; 

R1, R2, and R3). These reflectors were chosen because of 

their high amplitude and the comparably large number of 

traces in which these reflections were observed. The reflector 

analysis consisted of several steps, some of which are shown 

in figure 19:

1. Selection of a section of the radar profile containing 

the reflectors of interest (several traces are selected) 

(fig. 19A).

2. Alignment of the traces based on the maximum 

peak amplitude.

3. Calculation of the mean trace signal.

4. Extraction of the wavelet corresponding to the mean 

reflection in the time domain (fig. 19B).

5. Calculation of the Fourier spectrum of the wavelet in 

the frequency domain (fig 19C).

The selected reflections and their characteristics are 

summarized in table 1.

Comparison of the five reflectors in the August, 

September, and November data sets indicates there was no 

change of polarity with time after the steam injection. Also, 

no major differences in the traveltimes were observed. The 

corrected amplitude spectra for each reflector in each data set 

are shown in figure 20. Comparison of a particular reflector’s 

amplitude spectra in different data sets requires the power 

correction discussed above. The changes in the spectra of the 

reflectors in borehole JBW-7816 were minimal. The reflectors 

in borehole JBW-7817A showed a decrease in amplitude with 

time (and heating) and a slight decrease in central frequency.

In order to compare the amplitude of a particular reflector 

at different times, amplitude ratios are calculated in the Fourier 

domain for each reflector. The amplitude ratio is the ratio 

between a reflector’s maximum amplitude at two different 

times, with the August data set used as the reference. For 

example, the amplitude ratio of a reflector for the November 

data set would be calculated as ANov/AAug, where ANov is the 

maximum amplitude observed in the corrected November 

data set, and AAug is the maximum amplitude observed for the 

same reflector in the August background data set. For several 

reflectors, August data are not available; in these cases, the 

September data set is used as reference. The results are given 

in table 2 and figure 21.

Based on the analysis of the reflection data, it does not 

appear that steam replaced water in the selected fractures 

surrounding the logged boreholes. The reflector analyses 

reveal no changes in polarity and no substantial changes in 

traveltime, and the decrease in amplitude is less than an order 

of magnitude. The largest decrease in amplitude ratio was 0.4, 

whereas the results of numerical modeling indicate that the 

presence of steam in a fracture would result in a much larger 

decrease in amplitude such that the amplitude ratio would 

be much less than 0.1. The observed decreases in fracture 

reflectivity (after power correction) are too small to result from 

steam replacement, but are consistent with increased electrical 

conductivity resulting from increased fluid temperature.

Figure 19. Example of reflector analysis of single-hole radar reflection data: (A) reflector in section of radar profile, (B) mean signal in 

the time domain, and (C) amplitude spectrum in the frequency domain. 

TRACE NUMBER

TIME, IN SAMPLES

TIME, IN SAMPLES FREQUENCY, IN MEGAHERTZ AMPLITUDE, IN VOLTS PER METER

 A. B. C.

60

40

20

5 10 15 0 20 40 60 20 40 60 80 100

20

-20

40

-40

0

MEAN AMPLITUDE,

IN VOLTS PER METER

10

5

0

15

Collection, Processing, and Interpreted Results of Geophysical Data 23
Reflection Results

The analysis of the radar reflection data indicates 

that steam did not replace water in most of the fractures 

surrounding boreholes JBW-7816 and JBW-7817A during 

the steam injection from September to November 2002. This 

conclusion is based on three observations: (1) no change of 

reflection polarity was observed, (2) no substantial variation 

in traveltimes was observed, and (3) the observed decrease 

in reflectivity was too small to result from the replacement 

of water by steam. One possible exception is the reflector 

identified as Zone 3 in the data collected in borehole 

JBW-7817A. The reflector is present in the August data 

and obvious in the September data, but not visible in the 

November data.

The decrease in radar reflection amplitude is larger 

for data collected from JBW-7817A than for data collected 

from JBW-7816, which might be a result of greater heating 

around borehole JBW-7817A. The decrease in amplitude 

could be explained by increased attenuation in the limestone 

because of heating or by the power correction. Because steam 

has not replaced the water in most of the fractures, it is not 

possible to estimate fracture aperture using the reflection data. 

Moreover, for openings that are small relative to the radar 

wavelength (approximately 1 m in limestone at 100 MHz), the 

ratio of reflection coefficients corresponding to two different 

temperatures seems to be independent of the fracture aperture.

Crosshole Radar Tomography

Crosshole radar tomography surveys were run between 

boreholes JBW-7816 and JBW-7817A to identify changes 

in radar velocity and attenuation in the interborehole region. 

Data were collected August 13–16, 2002, as background data 

before the steam injection; September 11–16, 2002, about 

2 weeks after the steam injection began; and November 4–8, 

2002, about 9 weeks after the injection began and 10 days 

before the end of the injection. The results of the theoretical 

modeling were used to interpret changes in the crosshole radar 

traveltime and amplitude data as a result of heating from the 

steam injection.

Data Acquisition

Borehole-radar tomography data were collected with 

Malå GeoScience 100-MHz borehole antennas using a 

sampling frequency of 2,000 MHz and 1,024 samples per 

trace. The transmitting antenna was fixed at different depths 

in borehole JBW-7817A while the receiving antenna was 

lowered from the top of casing to a depth of 32 m in borehole 

JBW-7816. For each antenna, measurements were collected 

every 0.20 m; however, because of size limitations of the 

processing software, the measurement interval was later 

increased to 1.0 m.

In the levelrun configuration, the transmitting and 

receiving antennas are always at the same depth (fig. 3A). 

Levelruns were collected every 0.20 m from the top of casing 

to the bottom of JBW-7816. Levelrun data can be interpreted 

in the field to quickly identify areas of change. Levelruns 

also are used to quantify change in the antenna power and 

to correct tomography data for deviations in traveltime or 

amplitude between different data sets. Levelrun surveys were 

conducted before and after acquisition of each tomography 

data set. Also, measurements in air were collected at the 

start and finish of each tomography survey to provide quality 

control for the stability of the radar equipment.

The temperature of the water in the borehole may affect 

the response of the radar antennas. To test the response of 

Table 1. Power correction for selected reflectors seen in single-hole radar reflection data collected during 2002 in boreholes 

JBW-7816 and JBW-7817A at the former Loring Air Force base study area, Limestone, Maine.

[Distance from the borehole was calculated using a velocity of 101 meters per microsecond]

Reflector

Power 

correction 

September/ 

August

Power

correction

November/ 

August

Power

correction

November/ 

September

Distance from 

the borehole,

in meters

Depth from

surface,

in meters

Change in 

polarity 

September/ 

August

Change in 

polarity

November/ 

August

JBW-7816-R1 1.36 1.42 1.04 1.55 28.54 No No

JBW-7816-R2 1.23 1.26 1.02 3.35 36.54 No No

JBW-7817A-R1 1.33 1.46 1.09 3.14 14.54 No No

JBW-7817A-R2 1.52 2.16 1.42 1.50 25.54 No No

JBW-7817A-R3 1.44 1.66 1.15 3.00 17.54 No No

24 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

FREQUENCY, IN MEGAHERTZ FREQUENCY, IN MEGAHERTZ

AMPLITUDE, IN VOLTS PER METER

AMPLITUDE, IN VOLTS PER METER

AMPLITUDE, IN VOLTS PER METER

AMPLITUDE, IN VOLTS PER METER

JBW-7816 - R1 JBW-7816 - R2

FREQUENCY, IN MEGAHERTZ

AMPLITUDE, IN VOLTS PER METER

JBW-7817A - R3

JBW-7817A - R1 JBW-7817A - R2

August

September

November

0 50 100 150

0 50 100 150 0 50 100 150

0 50 100 150 0 50 100 150

0

100

200

300

400

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

50

100

150

Figure 20. Amplitude spectra calculated 

for the selected reflectors from single-hole 

radar reflection data collected during August, 

September, and November 2002 at the former 

Loring Air Force Base quarry, Limestone, 

Maine. The spectra are corrected using the 

power correction.

Collection, Processing, and Interpreted Results of Geophysical Data 25
the antennas at different temperatures, two PVC pipes were 

placed vertically 8.07 m apart and filled with water of different 

temperatures. The 100-MHz transmitter and receiver used 

for the tomography data collection were placed in the pipes 

and allowed to equilibrate for a minimum of 20 minutes. 

Traveltime measurements collected when both pipes were 

filled with hot water (approximately 50°C) and both with cold 

water (approximately 22°C) were within one sample (0.50 ns) 

of each other. Traveltime measurements collected with the 

transmitter in the hot water and the receiver in the cold water 

were five samples faster (2.5 ns) than when the transmitter 

and receiver were both in water of the same temperature. 

Measurements collected with the transmitter in cold water 

and the receiver in hot water arrived four samples slower 

(2.0 ns) than when the transmitter and receiver were both in 

water of the same temperature. It is possible that the thermal 

instability of the antennas, combined with the difference in 

temperature between the boreholes, may have affected the 

results of the tomograms.

Data Processing and Interpretation

Tomographic inversion is the process by which radar 

wave velocity and attenuation are calculated from field data. 

This process is sensitive to data having a low signal-to-noise 

ratio; thus, the first step in data processing is to eliminate 

traces that are highly attenuated because of large transmitterreceiver raypath distances. Data for raypaths below the bottom 

of borehole JBW-7817A also were eliminated to improve the 

symmetry and reduce distortions in the tomograms. Some 

shallow rays located close to the water table were eliminated; 

amplitudes for these rays were higher in the August data 

set than in the September and November data sets, perhaps 

because of water-table fluctuation. After a preliminary 

analysis of the data, it was decided to further reduce the 

data set because of its size (more than 8,000 measurements). 

Although data were collected every 0.20 m, only data for 

1-m measurement intervals were retained for tomographic 

inversion. Borehole deviation data were included in the array 

geometry to accurately calculate the antenna locations in three 

dimensions. Traveltime and maximum peak-to-peak amplitude 

data for the edited data set were picked using the Malå 

GeoScience RAMAC TOMOCG software package.

Table 2. Amplitude ratios calculated at central frequency 

for the selected reflectors identified in single-hole radar 

reflection data collected during 2002 in boreholes JBW-7816 

and JBW-7817A at the former Loring Air Force Base study area, 

Limestone, Maine.

[-, no data available]

Reflector September/ 

August

November/ 

August

November/ 

September

7816-R1 0.8 0.85 1.1

7816-R2 - - .75

7817A-R1 .5 .4 .7

7817A-R2 - - .8

7817A-R3 .8 .5 .6

REFLECTOR

RATIO OF THE MAXIMUM AMPLITUDES

JBW-7817A

R1

JBW-7817A

R2

JBW-7817A

R3

JBW-7816

R1

JBW-7816

R2

November/

August

November/

August

November/

August

September/

August

September/

August

September/

August

November/

September

November/

September

November/

September

November/

September

November/

September

0

0.2

0.4

0.6

0.8

1.0

1.2

Figure 21. Ratio of the maximum amplitudes in the frequency domain of selected reflectors in single-hole 

radar reflection data from boreholes JBW-7816 and JBW-7817A collected during August, September, and 

November 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

26 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
Traveltime Data

For each trace, the time that the transmitter actually fires 

(called “zero time”) must be determined prior to tomographic 

inversion or interpretation of levelrun data. Based on a 

measurement taken in air at the surface between the two 

boreholes, the zero time was calculated based on the sampling 

frequency. After identifying the zero time, levelrun data sets 

were corrected. For each tomography survey, a mean levelrun 

data set is calculated from data collected before and after the 

tomography. The differential traveltimes from the calibrated 

levelruns are shown in figure 22. These data are used to 

calibrate the tomography data because it is assumed that there 

would be minimal changes in the subsurface properties during 

this short acquisition time.

The calibrated traveltimes for the tomography data are 

shown in figure 23A. An increase in the mean traveltimes for 

deeper raypaths occurs in November 2002 near the end of 

the steam injection. The mean and standard deviation of the 

traveltimes are 0.0818 ± 0.0117, 0.0817 ± 0.0119, and 0.0835 

± 0.0132 microsecond (μs), for August, September, and 

November, respectively.

Velocities were calculated from the traveltime data for the 

known geometry, assuming straight-line raypaths (fig. 23B). 

The mean and standard deviation of the velocities for August, 

September, and November are 105.07 ± 3.98, 105.2 ± 3.93 and 

103.10 ± 2.51 m/μs, respectively. Differences in traveltimes 

between the three data sets are shown in figure 24. When the 

differences were less than or equal to the sampling period, 

they were assumed to be zero.

Because of the small changes in traveltimes between the 

data sets, it was decided to invert the traveltime differences 

instead of inverting the traveltime data. The traveltimedifference values were calculated trace-by-trace using the 

August background data as the reference data set. The 

tomographic inversion was performed in several steps:

1. Levelrun data were inverted in order to generate an 

initial model.

2. This model was input to an algorithm that uses the 

simultaneous iterative reconstruction technique (SIRT) 

(Dines and Lytle, 1979; Censor, 1983; McMechan and 

others, 1987; Stewart, 1992; Lane and others, 2003) to 

generate the tomogram.

3. A smoothing filter was applied to the resulting tomogram 

to suppress artifacts caused by data errors or low raypath 

density in regions of the tomogram.

4. The smoothed tomogram was used as the starting model 

for a second application of the SIRT algorithm to the 

levelrun data, which refines the smoothed tomogram and 

improves the match to the data.

The traveltime-difference tomograms are shown in 

figure 25 in terms of slowness. The September-August 

traveltime-difference tomogram reveals no substantial changes 

in traveltime. For the September data set (corresponding to 

the beginning of the steam injection), changes of medium 

properties appear to be small. The second difference 

tomogram (November-August) shows a small increase in 

slowness (increase in traveltime) at depths below about 20 m. 

The decrease in velocity could be explained by increases in 

the conductivity of the limestone matrix and decreases in the 

permittivity of the water.

Amplitude Data

Examination of the peak-to-peak amplitude of the 

levelrun data shows a change in amplitudes between the three 

acquisition periods (fig. 26). The amplitudes were higher in 

the September acquisition than in the August acquisition, 

which is inconsistent with the expected heating of the rock 

RAY NUMBER

DIFFERENCE IN TRAVELTIME

(SEPTEMBER-AUGUST),

IN NANOSECONDS

DIFFERENCE IN TRAVELTIME

(NOVEMBER-AUGUST),

IN NANOSECONDS

 A. B.

1.5

-1.5

0

-2.0

-1.0

-0.5

0.5

1.0

RAY NUMBER

1.5

-1.5

0

-2.0

-1.0

-0.5

0.5

1.0

0 20 40 60 80 100 120 0 20 40 60 80 100 120

Figure 22. Calibrated crosshole radar levelruns. Differences in traveltimes: (A) September-August 2002 and 

(B) November-August 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

Collection, Processing, and Interpreted Results of Geophysical Data 27
medium during the intervening period; thus, it is assumed that 

the change is a result of variable transmitter output power. 

To account for the variation, the levelrun and tomography 

amplitude data were calibrated. The calibration procedure is 

based on two assumptions: (1) there is no power variation 

during a particular levelrun acquisition, and (2) the radar 

equipment is not affected by changes in temperature during 

a particular levelrun acquisition. The assumed stability is 

because of the short acquisition time needed for the levelrun 

survey. The background data (August, before injection) are 

used as reference and are not modified by the calibration.

The amplitudes of the data sets acquired in September 

and November were calibrated using the function:

exp [a ln (A) + b] , (14)

where 

A is the peak-to-peak amplitude. 

The values of the coefficients a and b were chosen 

by visually fitting the peak-to-peak amplitude data to the 

background data at depths of about 13 to 17 m, where the 

traveltime tomography indicates no substantial changes 

between data sets (fig. 25).

The mean attenuation of the medium was calculated 

from the calibrated peak-to-peak amplitudes. The means 

and standard deviations of the attenuation determined 

from the levelrun data were 3.26 ± 0.324, 3.39 ± 0.413, 

and 3.43 ± 0.424 dB/m, for August, September, and 

November, respectively.

The attenuations of the tomography data also were 

calculated from the peak-to-peak amplitudes for each data 

DISTANCE, IN METERS

TIME, IN MICROSECONDS

RAY NUMBER

VELOCITY, IN METERS

 PER MICROSECOND

 A. B.

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

6 8 10 12 14 0 50 100 150 200 250

90

95

100

105

110

115

August

September

November

DIFFERENCE IN TRAVELTIME

(NOVEMBER-AUGUST 2002), 

IN MICROSECONDS

DISTANCE, IN METERS DISTANCE, IN METERS

DIFFERENCE IN TRAVELTIME

(SEPTEMBER-AUGUST 2002), 

IN MICROSECONDS

 A. B.

-0.005

-0.010

-0.015

0.000

0.005

0.010

0.015

-0.005

-0.010

-0.015

0.000

0.005

0.010

0.015

7 8 9 10 11 12 13 7 8 9 10 11 12 13

Figure 23. (A) Traveltimes of crosshole radar tomography data collected during August, September, 

and November 2002 and (B) radar-wave velocity in August, September, and November 2002 at the former 

Loring Air Force Base quarry, Limestone, Maine. 

Figure 24. Differences in crosshole radar tomography traveltimes: (A) September-August 2002 and (B) NovemberAugust 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

28 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
set. Because the output power of the transmitter and the 

capabilities of the receiver change as a function of battery life, 

antenna temperature, and time, calibration of the tomography 

data was necessary. The tomographic data sets were calibrated 

using the levelrun data collected at the same time. Levelrun 

data are more stable than tomography data because the 

levelrun surveys are completed in about 10 minutes as 

opposed to 6 hours for a full tomography survey. For each 

tomography data set, the same correction was applied for data 

with the same transmitter position. The calibrated attenuations 

are shown in figure 27 for each data set.

The means and standard deviations of the attenuation 

determined from the tomography data were 3.68 ± 1.04, 3.65 

± 0.97, and 3.80 ± 1.09 dB/m for August, September, and 

November, respectively. The increase in the mean value of the 

attenuation over the course of the steam injection is small.

Amplitude differences (fig. 28) were calculated in 

a procedure analogous to the calculation of traveltime 

differences discussed previously. Nearly all the attenuation 

differences are lower than 1 dB/m, and many values are 

negative, which cannot be explained by the presence of 

steam or heating. Data for raypaths traversing the deepest 

part of the interborehole regions, however, show mostly 

positive differences in attenuation; this is consistent with the 

conclusion from the theoretical modeling that heating, or the 

presence of steam, will attenuate the radar signal.

The tomographic inversion of attenuation data followed 

the same procedure used for traveltime data. Inversions of 

levelrun attenuation data were used as starting models for 

inversion of the tomography data. The results of the inversions 

are presented as attenuation differences in figure 29. For 

the data set collected near the end of the steam injection in 

November, a small increase in attenuation can be seen at 

depths below about 20 m, just as an increase in slowness 

(decrease of velocity) was observed.

Tomography Results

The traveltime- and attenuation-difference tomograms 

show no substantial changes in velocity and attenuation 

between the background data collected before the steam 

injection (August) and the data collected about a week after 

the steam injection began (September). For data collected 

about 2 months after the injection began (November), the 

difference tomograms show small increases in slowness 

(indicating a decrease in velocity) and increases in attenuation 

at depths greater than about 20 m. These are the same depths 

where the greatest increase in heating was observed in both 

boreholes based on the fluid temperature logs (fig. 13). 

Although these changes are small, they are consistent with 

an interpretation that (1) increases in the conductivity and 

permittivity of the limestone matrix are caused by heating 

of the rock between boreholes JBW-7816 and JBW-7817A, 

and (2) steam from the injection was not detected near these 

boreholes by the crosshole radar tomography surveys.

10

30

28

26

24

22

20

18

16

14

12

DEPTH, IN METERS

10

30

28

26

24

22

20

18

16

14

12

DEPTH, IN METERS

SLOWNESS

DIFFERENCE,

IN 

NANOSECONDS

PER METER

-0.10

-0.05

0.00

0.05

 0.10

 0.15

 0.20

JBW-7817A JBW-7816 JBW-7817A JBW-7816

 A. B.

DISTANCE, IN METERS DISTANCE, IN METERS

0 2 4 6 8 0 2 4 6 8

Figure 25. Results of the inversion of the traveltime-difference tomography data: (A) September-August 2002 

and (B) November-August 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

Collection, Processing, and Interpreted Results of Geophysical Data 29
PEAK-TO-PEAK AMPLITUDE, 

IN VOLTS PER METER

PEAK-TO-PEAK AMPLITUDE, 

IN VOLTS PER METER

DEPTH, IN METERS

DEPTH, IN METERS

 A.

 B.

AUGUST 2002

SEPTEMBER 2002

NOVEMBER 2002

AUGUST 2002

SEPTEMBER 2002

NOVEMBER 2002

0

500

1,000

1,500

2,000

0

500

1,000

1,500

2,000

5 10 15 20 25 30

DISTANCE, IN METERS

ATTENUATION, IN 

DECIBELS PER METER

6 8 10 12 14

0

2

4

6

8

AUGUST

SEPTEMBER

NOVEMBER

Figure 26. Peak-to-peak amplitude of the crosshole radar levelrun data: (A) unprocessed data and 

(B) calibrated data collected during August, September, and November 2002 at the former Loring Air 

Force Base quarry, Limestone, Maine. 

Figure 27. Attenuation measurements from 

calibrated crosshole radar tomography data 

collected during August, September, and 

November 2002 at the former Loring Air Force 

Base quarry, Limestone, Maine. 

30 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
ATTENUATION DIFFERENCE 

(NOVEMBER-AUGUST 2002),

IN DECIBELS PER METER

DEPTH OF TRANSMITTER, IN METERS DEPTH OF TRANSMITTER, IN METERS

A. B.

0

-0.5

0.5

1.0

1.5

10 15 20 25 30

-1.0

10 15 20 25 30

0

-0.5

0.5

1.0

1.5

-1.0

ATTENUATION DIFFERENCE 

(SEPTEMBER-AUGUST 2002),

IN DECIBELS PER METER

Figure 28. Attenuation differences in crosshole radar tomography data: (A) September-August 2002 and 

(B) November-August 2002 at the former Loring Air Force Base quarry, Limestone, Maine. 

 

A. B.

DISTANCE, IN METERS DISTANCE, IN METERS

DEPTH, IN METERS

DEPTH, IN METERS

JBW-7817A JBW-7816 JBW-7817A JBW-7816

10

12

14

16

18

20

22

24

26

28

30

10

12

14

16

18

20

22

24

26

28

30

0 2 4 6 8 0 2 4 6 8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 ATTENUATION

DIFFERENCE,

IN DECIBELS

PER METER

Figure 29. Results of the inversion of the attenuation-differences in crosshole radar tomography data: 

(A) September-August 2002 and (B) November-August 2002 at the former Loring Air Force Base quarry, 

Limestone, Maine. 

Summary and Conclusions 31
Summary and Conclusions

The U.S. Geological Survey conducted a geophysical 

investigation in an abandoned limestone quarry at the former 

Loring Air Force Base, Limestone, Maine, to evaluate the 

effectiveness of borehole radar methods to monitor the 

movement of steam and transport of heat in the subsurface. 

The investigation was undertaken in cooperation with the 

U.S. Environmental Protection Agency during a steamenhanced remediation pilot study being conducted at the 

fractured-rock site.

Numerical modeling performed as part of this study 

examined the effect of heating on radar-wave propagation. 

Changes in subsurface temperature cause changes in radarwave velocity, attenuation, and reflectivity of fractures. 

Radar-wave velocity was predicted to increase slightly in 

heated water, but to decrease in heated limestone. If steam 

replaced water in the fractures, the velocity of the radar wave 

is higher in steam than in water. Radar-wave attenuation was 

predicted to increase slightly in heated water in the fractures 

and to increase more substantially in heated limestone matrix. 

The attenuation of the radar wave in steam in the fractures 

was slightly less than in water. The radar-wave reflection 

coefficient (reflectivity) of a thin, water-filled fracture 

decreased with heating, and decreased by a factor of 10 if 

steam replaced water in the fracture. The reflectivity increased 

if the conductivity of the water in the fractures increased.

In theory, if steam were present in a fracture, it should 

be detectable by radar methods; however, the 100-MHz 

(megahertz) radar system used for this study is not likely 

to detect steam in the small-aperture fractures (<0.1 mm 

(millimeter)) identified at the site because of the wavelength 

of the radar wave. These results indicate that borehole radar 

may be an effective tool for detecting the change in fracture 

filling from water to steam at sites where the fracture apertures 

are greater than 1 mm. The results also indicate that borehole 

radar has potential to detect substantial changes in temperature 

in the rock matrix near a borehole.

Single-hole radar reflection and crosshole radar 

tomography data were collected before the steam injection, 

in August 2002; about a week after the start of the steam 

injection, in September 2002; and near the end of the 

steam injection, in November 2002. Conventional borehole 

geophysical logs, including fluid temperature, electromagnetic 

(EM) conductivity, and borehole deviation, were collected 

to aid in interpreting and processing the radar data. The 

radar data were interpreted based on the results of the 

numerical modeling.

Temperature in borehole JBW-7816 increased about 

10°C (degrees Celsius) above the August background 

measurements. A maximum temperature of about 17°C was 

recorded at a depth of about 20 to 22 meters (m). In borehole 

JBW-7817A, temperature increased as much as 40°C above 

the background measurements. Temperature increased with 

depth, with the maximum temperature of about 47°C recorded 

near the bottom of the borehole at a depth of about 29 m. 

Larger temperature changes were expected in JBW-7817A 

than in JBW-7816, because JBW-7817A is closer to steaminjection well I-4. Larger temperature changes were expected 

at depth because the steam was injected in the interval 

between about 20 and 40 m below land surface in most of the 

injection boreholes.

The EM-conductivity data for each borehole are quite 

similar all three times that data were collected. Because of 

the numerous metallic objects at the site, the EM tool was 

not calibrated, and the data were collected using a fixed 

scale so only relative comparisons could be made. The tool 

is temperature sensitive, and the measured increases and 

decreases in conductivity were in response to changes in 

temperature as well as formation conductivity. Therefore, 

although the conductivity measured in November appears to 

decrease with depth, there is little confidence in this result, 

because theoretical and laboratory results show that the 

conductivity of rocks increases with temperature.

Profiles of the single-hole radar reflection data show 

substantial attenuation of the radar data in borehole JBW7817A in November. No attenuation is obvious in the data 

from borehole JBW-7816. From the two boreholes, a total 

of five reflectors were selected for detailed analyses across 

the three temporal data sets. Amplitude data were corrected 

for antenna power variations. The spectral content of the 

reflectors showed a small decrease with time. Minimal 

changes in the reflector amplitudes with time were observed 

in borehole JBW-7816, and decreases in reflector amplitudes 

were observed in borehole JBW-7817A. The analysis of the 

radar reflections indicates that steam did not replace water in 

most of the fractures surrounding boreholes JBW-7816 and 

JBW-7817A. This conclusion is based on three observations: 

(1) no change of reflection polarity was observed, (2) no 

substantial variation in traveltimes was observed, and (3) the 

observed decrease in reflectivity was too small to have been 

a result of replacement of water by steam in the fractures. 

There is one fracture, identified as Zone 3 in borehole JBW7817A, where steam may have replaced water in a fracture. 

The fracture is identifiable in the August data, obvious in the 

September data, but could not be detected in the November 

data set. The strong decrease in the amplitude, and thus the 

reflectivity, of the feature prevented the analysis of the polarity 

or the possible change in traveltime.

The decrease in radar reflection amplitude is larger for 

data collected from JBW-7817A than JBW-7816, which is 

consistent with the greater heating around that borehole. 

The decrease in amplitude could be owing to heating of the 

limestone or use of a power correction that was not high 

enough. Because steam did not replace the water in the 

fractures, it was not possible to estimate fracture aperture 

using the reflection data.

Crosshole radar tomography data were examined 

for changes in traveltime and amplitude. Small changes 

in traveltime show about a 2-percent decrease in velocity 

between August and November. Traveltime difference 

32 Use of Borehole-Radar Methods to Monitor a Steam-Enhanced Remediation Pilot Study
tomograms show no change in velocity between August and 

September and show a small decrease in velocity between 

August and November for depths below about 20 m. The 

attenuation data required corrections for power differences 

between the data sets. Attenuation difference tomograms show 

no change between August and September, but show a small 

increase in attenuation between August and November for 

depths below about 20 m. This is the same depth where the 

greatest increase in heating was observed in the temperature 

logs of both boreholes. Although these changes are small, they 

are consistent with the interpretation of the single-hole radar 

reflection data: the conductivity of the limestone increased as 

a result of heating of the limestone matrix between boreholes 

JBW-7816 and JBW-7817A, and steam was not found near 

the boreholes.

References Cited

Birchak, J.R., Gardner, C.G., Hipp, J.E., and Victor, J.M., 

1974, High dielectric constant microwave probes for 

sensing soil moisture: Proceedings of the Institute of 

Electrical and Electronics Engineers, v. 62, p. 93–98.

Bregman, N.D., Bailey, R.C., and Chapman, C.H., 1989a, 

Crosshole seismic tomography: Geophysics, v. 54, no. 2, 

p. 200–215.

Bregman, N.D., Bailey, R.C., and Chapman, C.H., 1989b, 

Ghosts in tomography—The effects of poor angular 

coverage in 2-D seismic traveltime inversion: Canadian 

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—Scientific Investigations Report 2006-5191