UXO Detection at Jefferson Proving GroundUsing Ground-Penetrating RadarSteven A. Arcone, Kevin OÕNeill, Allan J. Delaney, April 2000and Paul V. Sellmann

Approved for public release; distribution is unlimited.

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US Army Corpsof Engineers ¨

Engineer Research andDevelopment Center

Abstract: Ground-penetrating radar (GPR) was usedto detect UXO and nonordnance on the 40-acre site (lot54) of Jefferson Proving Ground, Indiana. The UXO areburied about 1 m deep in a clayey silt for which the soilwater content ranged from moist near the surface tonear saturation at about 1 m. A 16-bit radar was used toprofile along previously established lines and transectsover emplaced artificial targets. Data were recorded at48Ð64 traces/s with minimal towing speeds during bothdry and rainy weather . Target responses at both 300(time range of 50 ns) and 600 MHz (30 ns) ranged fromdiscrete diffractions to short reflection segments. Soilloss greatly attenuated diffraction hyperbolas. Theoret-ical analyses of these hyperbolas give an average soildielectric constant of 10 at both 300 and 600 MHz. Thephase polarity of many of the reflected and dif fractedwavelets indicate targets with wave impedances higherthan that of the soil. It is therefore assumed that thesetargets are metallic and the responses of some, whoselocations correlate with the position of UXO on burial

maps, are shown in detail. Theoretical modeling of wave-let propagation for this soil confirms the high rate of at-tenuation (47Ð66 dB/m round-trip), the maintenance ofwaveform, a shift in wavelet local frequency , and re-sponse to a typical UXO. It is concluded that GPR iseffective for finding targets in this type of soil to no morethan 2-m depth. It is recommended that future surveysutilize high trace acquisition rates to capture the full tar-get responses, and a prowed, heavy dielectric antennasled to improve antenna-to-ground coupling and to de-flect surface obstacles such as vegetation. Broadband,three-dimensional numerical modeling of scattering fromUXO-shaped targets in soil, with diverse orientations ofantenna and target relative to one another , suggeststhat target length and diameter may be inferred fromresonance patterns in backscattered signals. The sim-ulations also reveal some ways in which oblique target/antenna orientations along a transect may af fectfrequency-dependent response patterns, providing cluesas to target shape as well as positional ambiguities.

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COVER: Commercial antenna transducer units thatproduced the 300- (left) and 600-MHz wavelets. Theplastic tubs improved ground contact.

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UXO Detection at Jefferson Proving GroundUsing Ground-Penetrating Radar

Steven A. Arcone, Kevin OÕNeill, Allan J. Delaney, April 2000and Paul V. Sellmann

US Army Corpsof Engineers ¨Cold Regions Research &Engineering Laboratory

PREFACE

This report was prepared by Dr . Steven A. Arcone, Research Geophysicist, Snow andIce Division; Dr. Kevin OÕNeill, Research Civil Engineer, Applied Research Division; AllanJ. Delaney, Physical Sciences Technician, Snow and Ice Division; and Paul V. Sellman,Geologist, Civil Engineering Research Division, Research and Engineering Directorate,U.S. Army Cold Regions Research and Engineering Laboratory (CRREL), Engineer Re-search and Development Center (ERDC), Hanover, New Hampshire.

This work was jointly sponsored by the U.S. Army Environmental Center (AEC), Aber-deen Proving Ground, Maryland, under MIPR Number 3767, 15 April 1997, Geor geRobitaille, Program Manager, and by the ERDC Environmental Quality Technology (EQT)Program, Work Unit Innovative Geophysical Technologies for Enhanced Buried UXO Dis-crimination (AF25, 6.2), Dr. John Cullinane, Program Manager.

The authors thank Dr. Dwain Butler, Dr. Janet Simms, and Jose Llopis of GeotechnicalLaboratory, ERDC, Vicksburg, Mississippi, for supplying necessary technical informationand for helping to coordinate field activities, and thank the Thayer School of EngineeringNumerical Methods Laboratory, particularly Dr. S.A. Haider, for providing technical assis-tance.

This publication reflects the personal views of the authors and does not suggest or reflectthe policy, practices, programs, or doctrine of the U.S. Army or Government of the UnitedStates. The contents of this report are not to be used for advertising or promotional pur -poses. Citation of brand names does not constitute an of ficial endorsement or approval ofthe use of such commercial products.

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CONTENTS

Preface ............................................................................................................................. iiIntroduction ..................................................................................................................... 1Site description ................................................................................................................ 1Soils ................................................................................................................................. 2Dielectric theory .............................................................................................................. 3Equipment and methods .................................................................................................. 3

Radar system ........................................................................................................... 3Field profiling .......................................................................................................... 4Post-processing and display of data ........................................................................ 5Profile interpretation ................................................................................................ 5

Results and discussion ..................................................................................................... 5Control studies ......................................................................................................... 5Grid survey: 300 MHz ............................................................................................. 8Grid survey: 600 MHz ............................................................................................. 12Distribution of ε« ..................................................................................................... 12

Theoretical discussion ..................................................................................................... 13Modeling.......................................................................................................................... 15Incident field and polarization parameters ...................................................................... 22Modeling results .............................................................................................................. 23Conclusions and recommendations ................................................................................. 28Literature cited ................................................................................................................. 30Abstract ............................................................................................................................ 32

ILLUSTRATIONS

Figure1. Location of the 40-acre site at Jefferson Proving Ground ....................................... 22. Typical form of a radiated GPR wavelet ................................................................. 43. Distributions at the 40-acre site with radar profile transects superimposed ............ 64. 300- and 600-MHz diffraction profiles and waveform responses to a

metal disk buried 58 cm deep .................................................................................. 75. Soil moisture profiles at five locations .................................................................... 76. Sample segment of a 300-MHz profile and the result of applying a

background removal filter ....................................................................................... 107. Selected target responses within segments extracted from the 300-MHz

profiles, and sample traces containing diffracted or reflected wavelets .................. 108. Likely interpretations for two of the responses in Figure 7 ..................................... 139. Selected target responses within segments extracted from the 600-MHz

profiles, and sample traces containing wavelets ...................................................... 14

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10. Distribution of ε«at both 300 and 600 MHz ........................................................ 1511. Dielectric permittivity curves and attenuation rates for the average ε«,

range of σ, and probable values of frel and εoo at JPG ........................................ 1612. Model 400-MHz wavelets before and after round-trip propagation within

1 m of ground characterized by the soil parameters measured ........................... 1713. Radiation beamwidths for a model of our antennas and the average

ε«= 10.4 found at JPG.......................................................................................... 2114. Basic setup, with target below ground surface, in its reference orientation ........ 2315. Normalized scattered electric field above a projectile surrounded by soil

and oriented in the Y direction, as a function of frequency ................................. 2416. Normalized scattered E field above target for various projectile inclinations

and rotations θd, θx ...................................................................................................................... 2517. Normalized scattered E field when projectile axis is in (x,z) plane, from

different vantage points Za, with different declinations θd .......................................... 2518. Backscattered Ez field magnitudes along transect, at various frequencies,

when the projectile is inclined at 45° with nose up, and the antenna isrotated .................................................................................................................. 26

19. Backscattered Ez field magnitudes along transect, at various frequencies,when the projectile is inclined at 45° with nose down, and the antenna isrotated .................................................................................................................. 27

20. Scattered |Ez| values, when the incidence direction and incident E fieldorientations are in the (x,z) plane, for various incidence angles ......................... 28

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INTRODUCTION

The remote detection of unexploded ordnance(UXO) has generally relied on the electromagnetic in-duction (EMI) and ground-penetrating radar (GPR)methods. EMI, the most common method, finds targetsthat contrast strongly in electrical conductivity with theirhost soil by detecting 10- to 40-kHz magnetic fieldsinduced from the target. EMI systems are mobile anddo not require ground contact, and their performance isnot degraded by minor variations in topography. How-ever, they have little ability to detect nonmetallic tar -gets. GPR, which generally operates between about 50and 1000 MHz, is sensitive to contrasts in both con-ductivity and dielectric permittivity . Thus GPR candetect plastic, glass, concrete, or wood when their per-mittivity contrasts with that of a moist soil. In addition,GPR shows the continuity of soil horizons and thus candetect burials of foreign tar gets by the disturbance ofthose horizons. GPR performance requires good an-tennaÐground contact and a complex soil permittivityand conductivity that does not rapidly attenuateradiowave energy. The 50- to 300-MHz range usuallyprovides optimum penetration for a given soil conduc-tivity and permittivity. In this report we discuss GPRresults from a site within the Jefferson Proving Ground(JPG), where ordnance is buried within a highly attenu-ating soil and for which reports of GPR performance(NAVEOD 1997) are negative.

Our objectives were 1) to assess the ability of GPRto detect UXO-type tar gets at JPG, and 2) to under -stand the nature and causes of target responses to fos-ter better tar get discrimination or explain dif ficultiestherein. We were interested in the effect of the soil elec-

trical properties upon the GPR survey parameters offrequency, towing speed, and data acquisition rate. Wesurveyed a site where the types, locations, depths, andapproximate orientations of both ordnance (metallic)and nonordnance (metallic and nonmetallic) are docu-mented. We used supplied location maps to identify theprobable nature of targets detected. The specified depthsare rarely stated to exceed one meter. We used a com-mercially available GPR system and towed 100-, 300-and 600-MHz- (mid-band values) frequency antennasalong established lines. The higher two frequencies pro-vide high resolution of subsurface objects and are withinmilitary specification for ordnance detection, and theantennas are small and mobile. We used the signal char-acteristics obtained from ordnance, nonordnance, andobjects we emplaced to determine soil permittivity anddiffraction waveforms.

Three-dimensional numerical modeling employed anintegral method that allowed incorporation of a reason-ably realistic subsurface antenna radiation pattern aswell as arbitrary target shape and orientation. Key ide-alizations were the assumption of homogeneous groundand exclusion of ground surface effects. The model wasused to compute fully polarimetric mono- and bistaticscattering over a broad band, for positions directly overa target as well as along a continuous transect passingby the target.

SITE DESCRIPTION

JPG (Fig. 1) is located in southern Indiana. Our in-vestigations were performed within lot 54, known asthe 40-acre site (Fig. 1). The vegetation at this site is

UXO Detection at Jefferson Proving GroundUsing Ground-Penetrating Radar

STEVEN A. ARCONE, KEVIN OÕNEILL, ALLAN J. DELANEY,

AND PAUL V. SELLMANN

predominantly sedge and grass with a few trees. The areahas a nearly level to gently sloping topography with smallgullies providing only a few meters of relief. The site hasbeen specially prepared with approximately 246 unfused,buried ordnance and 234 nonordnance tar gets buriedwithin 1 m of the surface. The ordnance includes small,handheld munitions, 2-ft- (0.6-m-) long projectiles, and5-ft- (1.6-m-) long bombs. Strong radio background noisewas present at this site.

We conducted our surveys from 23 to 25 October1997. The weather was clear on 23 October , and thenlight rain fell late on the 24th and all day on 25 Octo-ber. All of our profile transects were freshly mowed,but clumps of woody vegetation and tractor ruts madefor uneven antenna towing and short periods of poorantennaÐground contact.

SOILS

The soils in this area formed in a thin surface layerof windblown silt (loess) and underlying glacial drift(Nickell 1985). Some residual soils may reside near the

limestone bedrock interface where small rock fragmentsoccur. The bedrock ranges from about 1.5 to 7.6 m deep.The soils are part of either the Cobbsfork-A vonburg(CA) or Cincinnati-Rossmoyne-Hickory (CRH) soilmap units. The CA soils are found on level to gentlysloping surfaces, are generally poorly drained, and havea seasonally high water table. They can also have a panlayer, which can create a perched water table at a depthof 0.3Ð1 m for a significant part of the year. The CRHsoils are found on level to steep surfaces and are classedas well drained to moderately well drained. They mayalso have a pan layer of low permeability and perchedwater tables at depths of 0.5Ð1 m during the winter andspring. In general, both soil units have a 15- to 20-cmsurface soil consisting of silt loam varying from darkbrown to gray brown. The permeability tends todecrease with depth because of an increase in clay con-tent in the pan layers.

X-ray dif fraction studies performed by the U.S.Army Waterways Experiment Station in Vicksburg,Mississippi, show the soil mineralogy to be primarilyquartz and plagioclase feldspar with clay minerals gen-erally absent or in minor amounts. This is consistent

AA

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40-Acre Site

Figure 1. Location of the 40-acre site at Jefferson Proving Ground.

2

with other studies of loess in the southern United States(Rodbell et al. 1997), which show a predominantamount of quartz, and also with studies of loess in inte-rior Alaska (P”w” 1955).

DIELECTRIC THEORY

We present the theory of radiowave propagation insoils in this section in order to provide a basis for mod-eling the GPR propagation at JPG later. The discussionpertains to engineering-grade silt- and clay-size soils,but of little clay mineralogy so that it applies to thesoils at JPG. The radar detectability of a subsurface tar-get strongly depends on the radiowave speed, v, andattenuation rate, β, of the burial soil. The speed deter-mines the shape of the antenna beamwidth. β deter-mines how deep the radiowaves will penetrate andreturn to the surface with enough strength to be detected.These quantities are related to the relative complex per-mittivity of soil, εs*, such that

v = c/Real (εs*1/2) (1)

and

β ω εdB / m ag s( ) =

20 1 2log exp / Im * /i c

(2)

where i = (Ð1)1/2, ω is frequency in radians/s, andc = 3 × 108 m/s is the wavespeed in free space. Thequantity εs* is determined by the Debye relaxationpermittivity (Debye 1929), εrel, and a contributionfrom the very-low-frequency soil conductivity , σ(Siemens/m, or S/m), such that

εs* = εrel Ð iσ/ωε0 (3)

where

εrel = εoo + (εst Ð εoo)/(1 + i f/frel). (4)

ε0 is the dielectric permittivity of free space (a con-stant), εst is the low-frequency, ÒstaticÓ value of the rela-tive soil permittivity , εoo is the very-high-frequencyvalue, f = 2πω is the wave frequency in hertz (Hz), orfrequency component for a radar wavelet, and frel isthe soil relaxation frequency . The quantity εrel deter-mines the dipole moment density induced in a materialby a passing wave. The quantity frel is a characteristicfrequency above which the induced dipoles no longerstay in phase with the incident radiowave. Thesedipoles then generate interference, which ef fectively

slows and attenuates the incident radiation. This processis dispersive, which means that dif ferent frequencieswithin the incident wavelets propagate at different speedsand attenuation rates (Stratton 1941, Brillouin 1960, andFeynman et al. 1964).

Values of εst are directly related to the water contentfor nonmineralogic clays and silts (T opp et al. 1980)and generally equal εs* for f < 600 MHz. Above about600 MHz, laboratory investigations (Hoekstra andDoyle 1971, Hoekstra and Delaney 1974) show thatthe adsorbed water on the particle surfaces of the clayfraction (Tice et al. 1982) strongly influences thedielectric properties. The adsorption process lowers boththe high dielectric constant (81) of the normally freewater and also the free water relaxation frequency (22GHz) to about 1Ð3 GHz. Values of εoo are generallyrelated to the dry soil density, range from about 2.5 to3.5, and equal εs* at frequencies above about 100,000MHz. For soils of JPG and the radar frequencies used,σ was lar ge enough to also influence attenuationbecause it determines how much of the radiowaveenergy will be converted into conduction currents.

The resulting values of εs* for silty and clay-richsoils at frequencies above about 100 MHz provide highvalues of β and wave speeds lower than would beexpected for sandy soils with the same volumetricwater content. The β values (a quantitative discussionis given later) increase as f approaches, and thenexceeds, frel, and are orders of magnitude greater thanattenuation rates caused by the geometric spreading ofthe radiation energy. At frequencies below about 1000MHz, the real part, ε′ = εst, often referred to as thedielectric constant, determines the wave speed v throughthe relation

v = c/ε′1/2 (5)

for a variety of frozen and unfrozen silts and(nonmineralogic) clays (Hoekstra and Delaney 1974,Topp et al. 1980, Delaney and Arcone 1984).

EQUIPMENT AND METHODS

Radar systemWe used the GSSI (Geophysical Survey Systems,

Inc.) SIR system model 2 and models 3207 (100 MHz),5103 (300 MHz), and 101C (600 MHz) antenna trans-ducers. We used the control unit to set the time range(in nanoseconds, ns) for the echo traces, the data ac-quisition rate (48 or 64 traces/s), the sampling density(512 samples per trace), trace sample density (16-bit),and time variable gain (TVG) for the traces. We cali-brated our settings with the antennas set over tar gets

3

that we emplaced. The resulting reflections required alarge amount of gain (up to 65 dB) at time ranges ofonly 50 (300 MHz) and 30 ns (600 MHz). The shorttime ranges were sufficient to capture target responsesthroughout the site but were also necessary to limit back-ground radiowave interference, which beat with theradar returns at the high-gain time ranges. The high gainhad the negative ef fect of amplifying small antennaimpedance mismatches and low-amplitude clutter (un-wanted events), which probably originated from radia-tion that leaked onto the antenna housing and cables.These events usually arrive at constant time delay andtheir interference can be alleviated with a horizontalÒbackground removalÓ filter. However, in our case,where short time ranges were used, electronic jitter anderratic movement of the antenna may have caused theseevents to arrive at variable amplitude and so theirinterference was not consistently reduced by filtering.

All antennas were resistively loaded dipoles. Thesmaller, 300- and 600-MHz antennas are shielded withsemi-cylindrical housings to alleviate above-surfaceclutter. These frequencies are ÒlocalÓ (also known asÒinstantaneousÓ) values, which correspond with thedominant periods and lie approximately at the centerof the received wavelet spectrum. They are consider-ably below the manufacturerÕs specifications for theseantennas (400 and 900 MHz, respectively), whichgenerally apply to operation in air or on the ground withlower values of dielectric permittivity and loss thanencountered here. Their transmitters do not exceed 8W (peak power) in order to protect the nearby receiver.

One-hundred-MHz data were also acquired but are notdiscussed because the direct coupling between theseantennas, which lasts approximately 30Ð50 ns, obliter-ated any near-surface returns. The antenna directivitybecomes increasingly confined beneath the antenna asε′ increases (Arcone 1995). The typical shape of a trans-mitted GPR wavelet for either the 300- or 600-MHzantenna system is shown in Figure 2. The phase polar-ity sequence of the half-cycles defines the wavelet phase(Arcone 1995).

Field profilingWe profiled along established transects and entered

electronic event markers on the profiles at previouslyestablished, 100-ft (30-m) distance marks. The transectsgenerally deviated from a straight line by 1Ð2 m, andsometimes by as much as 5 m to avoid isolated bushes,trees, and severe ruts. Consequently we cannot recon-struct our exact position along the lines, and errors ofas much as 20 ft (6 m) may occur in our interpretationof distance between markers. We dragged all ourantennas by vehicle at less than 1 m/s for long distancesand by hand for the small surveys over emplaced tar -gets. All antennas were polarized perpendicular to thetransect direction. We determined that vehicle reflec-tions were not in the data by comparing profilesrecorded with and without the vehicle. We placed thesmaller antennas in a fiberglass box to alleviate erraticantenna-to-ground coupling. However, the uneven tow-ing speed over the rough ground also degraded theappearance of the profiles.

0 10 20 30 40 50

Time (ns)

Am

plitu

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linea

r sc

ale)

Figure 2. T ypical form of a radiated GPR wavelet (darkened area).The frequency characterization of the wavelet refers to the inverseof the dominant wavelet period (in this case, about 300 MHz).

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Post-processing and display of dataWe first band-pass-filtered (very wide settings, e.g.,

50Ð600 MHz for the Model 5103 antenna) the recordeddata to alleviate high-frequency electronic noise andlow-frequency, above-surface clutter . We normalizedthe number of data traces between event markers overthe targets we emplaced to compensate for changes indragging speed. We did not normalize the longer pro-files with the 100-ft (30-m) marker spacing becausevehicle speed varied between any two markers.

We used both linear and nonlinear gray-scale for -mats to indicate signal strength, and used an amplitudeformat to display the profiles for the targets we buried.In this format, positive phase is indicated by lightertones and negative phase by darker tones. We used anintensity format to display the profiles of the perma-nent targets at JPG. In this format, which is insensitiveto phase, strength is indicated by the intensity of darkertones.

Profile interpretationThe main objectives of the profile analysis are to

determine if ordnance tar gets had been detected andthe range of ε′values for the site soil. The permittivityanalysis used the dif fractions caused by radar scatter -ing from targets. In this method we matched the hyper-bolic shape of the dif fractions with theoretical hyper -bolas for a given value of ε′ (Jezek et al. 1979, Clarkeand Bentley 1994, Arcone et al. 1998). The main dis-advantages of this approach are 1) the hyperbolas canactually be responses to linear soil inclusions, in whichcase the hyperbolas are distorted reflections that resultwhen the transect obliquely intersects the inclusiondirection (Jezek et al. 1979) and are thus artificiallywide; and 2) an erratic towing speed, which would dis-tort the hyperbolic image. Item 1 was not consideredimportant because of the depositional process of thesoil (glacial drift and loess) and because of probablehistorical tilling. Item 2 is a concern and for this reasona statistical study is presented.

Target detection depends on the presence of eitheror both dif fractions and reflections and also on theirphase polarity. Both the strength and phase polarity ofa reflected or diffracted event depend on the reflectivityof a target, which is determined by its Fresnel reflec-tion coefficient, R, such that

R = (εs*1/2 Ð εt*1/2)/( εs*1/2 + εt*1/2) (6)

where εt* is the complex permittivity for the tar getmedium (Wait 1970). Although this formula applies toplane wave incidence upon lar ge flat reflectors, weinvoke its use because of the small in-situ wavelengths(30 cm at 300 MHz) relative to the larger sizes of some

of the targets known to be buried at JPG and because itpredicts the correct phase polarity. For a metal tar get,assumed to be ordnance, εt* is orders of magnitudehigher than εs* and produces a wavelet with a phasestructure opposite to that produced when εt* is lowerthan εs*.

It is unlikely that any geologic or or ganic inhomo-geneity in the JPG soil we profiled had a higher εt*than that of the soil itself. Consistent horizons are vir-tually absent in our data, which means that electricallyimportant changes, such as in moisture content, weregradational. In addition, ε′of limestone is generallybetween 8 and 10 (Parkhomenko 1967), which is nearthat of the soil and precluded strong bedrock reflections.

RESULTS AND DISCUSSION

Control studiesOur objectives for the control studies were to obtain

profile responses and scattered waveforms for buriedmetal reflectors, as well as soil moisture and conduc-tivity profiles. We conducted these studies either out-side or along the perimeter of the 40-acre site (Fig. 3).We buried two 9-inch- (23-cm-) diameter metal disksat depths of 1 1 (28 cm) and 23 (58 cm) inches. Theremoved soil was highly compact and did not appear tohave excess moisture. Therefore, we think that no sig-nificant soil drying took place between removal andreburial. The 300- and 600-MHz diffractions from thedeeper target (Fig. 4) (the response to the more shallowtarget is not sufficiently separated from the direct cou-pling between antennas to facilitate analysis) best fittheoretical diffraction hyperbolas for ε′ = 9.3 and 8.6 at300 and 600 MHz, respectively. The values of ε′, whichwe computed from the wavelet round-trip travel timewhen the antennas were over the center of the targets,are 9.5 and 8.7, respectively . In accordance with themeasurements, dielectric dispersion theory (eq. 4, anddiscussed below) predicts that the 600-MHz valueshould be slightly less than the 300-MHz value.

The accompanying traces in Figure 4, whose posi-tions within the profiles are indicated by arrows, showthe forms of the scattered wavelets within the dif frac-tions. The wavelets have a negativeÐpositiveÐnegativesequence to the phase polarity of the dominant half-cycles. This sequence is typical for the relative polaritywiring of these GSSI antennas and is characteristic oftargets whose wave impedance (eq. 6) is higher thanthat of the surrounding media. Targets characterized byan ε′ value less than that of the soil matrix would pro-duce a similar wavelet but with opposite phase polarityof the individual half-cycles. The local frequency isindicated for the wavelets.

5

Figure 3. Distributions at the 40-acre site with radar profile transects superimposed. Linearrows indicate the profile directions. In Figure 3b, the small arrow along line C marks abomb (cross symbol), and a bracket along line K indicates targets whose responses arediscussed later . Also shown are the locations of the targets we buried and the soil coreswe extracted for moisture profiles.

a. Ordnance and nonordnance distributions.

b. Ordnance-type distributions.

6

60-mm Mortar81-mm Mortar76-mm Projectile90-mm Projectile105-mm P152-mm Projectile155-mm Projectile175-mm ProjectileBombsSoil CoresBuried Dis

N M L K J I H G F E D C B A

4309800

4309700

4309600

4309500

4309400

4309300641200 641300 641400 641500 641600 641700 641800

Nor

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g (m

)

Easting (m)

N

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3

5

4

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N M L K J I H G F E D C B A

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4309300641200 641300 641400 641500 641600 641700 641800

Nor

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NonordnanceOrdnance

The amplitude along the 600-MHz hyperbolic as-ymptotes in Figure 4 rapidly fades with distance fromthe target. This indicates a very high soil attenuationrate per meter; the change in antenna directivity withangle to the target (discussed later) is an insignificantloss factor. Commercial GPR systems at these frequen-cies commonly have a performance figure of about 100Ð120 dB and a dynamic range (the amplitude range vis-ible in any particular trace) of about 60Ð70 dB. This

latter range is consistent with the gain we added beforerecording and with soil attenuation rates discussed later.

The volumetric water content profiles of soil coresamples (obtained before rain occurred) show valuesfrom about 15% at the surface to as high as 40% at 30Ð180 cm deep (Fig. 5). The water contents within theprofile of the core obtained about 24 hours after steadyrain had begun ranged from 23 to 45%, the latter ofwhich is at about saturation for this type of soil. The

Figure 4. 300- (left) and 600- (right) MHz diffraction profiles and waveform responses to ametal disk buried 58 cm deep. The amplitude of the 600-MHz diffraction fades rapidly asdistance increases from the target. The diffracted wavelets (darkened areas) within the sampletraces have a half-cycle polarity sequence that is characteristic of the response of this an-tenna systemÕs wavelets from a metal target.

0.5

0.4

0.3

0.2

0.1

0.00 5 10 15 20 25 30 35

Depth (cm)

Vol

umet

ric M

oist

ure

Con

tent

(cm

3 cm

–3)

Figure 5. Soil moisture profiles at five locations.

a. Profile 1.

7

0

10

20

30

40

50

–4 –3 –2 –1 0 1 2 3 –44 –3 –2 –1 0 1 2 3

10

Tim

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300 MHz376Trace

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600 MHz372Trace

Phase

core obtained above the 58-cm-deep tar get showed amoisture content that varied from 15% at 15 cm deepto 24% at 50Ð70 cm deep. According to Topp et al.(1980), these water contents correspond with ε′ valuesranging from about 7 (15%) to 12 (25%), which agreeswell with our values of 8.6Ð9.5 for this soil column. Atime-of-flight analysis, in which we ascribed theappropriate ε′ to 10-cm soil increments (Fig. 5b) andthen calculated the time delay in each increment, givesan effective ε′ value of 9.5.

Soil conductivity, σ, was measured at several sitesby the Waterways Experiment Station (Llopis in prep).Using four-electrode Schlumberger soundings, concur-rent with our work, they found the soil resistivity struc-ture to be dominated by a near -surface, 2- to4-m layer with approximately 0.025 < σ < 0.04 s/m.These values strongly influence radiowave attenuationbelow about 400 MHz, while the imaginary part of ε*,

0.5

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0.00 10 20 30 40 50 60 70

Depth (cm)

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oist

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6 8 9 10 12 13ε LayerValues

0.5

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0.00 50 100 150 200

Depth (cm)

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umet

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oist

ure

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tent

(cm

3 cm

–3)

Low-VolumeSample

Figure 5 (contÕd). Soil moisture pro-files at five locations.

ε″, influences attenuation above 400 MHz (discussedlater). Although ε″ was not measured, the high amountof gain used for the radar measurements indicates a highattenuation rate for this soil and will be seen to be con-sistent with the above range of σ.

Grid survey: 300 MHzThe transect lines on the 40-acre site are superim-

posed on maps of tar get and ordnance distribution inFigure 3. The lines surveyed are designated as transectsA, B, C, etc., and were 1300 ft (396 m) long. The classof target (ordnance or nonordnance) is indicated on themap. Additional information regarding exact locationand type of target, target depth, and approximate orien-tation is available. All ordnance are metal.

We show a typical 300-MHz profile segment beforeand after horizontal background removal filtering in Fig-ure 6. Intensity is linearly proportional to signal ampli-

b. Profile 2.

c. Profile 3.

8

tude in the profile. The time range of 50 ns, beyondwhich noise became severe, corresponds to about 2.3m of penetration for ε′ = 10.4, the average valueobtained at JPG (discussed later). The direct couplingbetween antennas occupies about 8 ns of the record andmasks part of the responses to some of the targets. Thenoise bands between about 30 and 40 ns are probablycaused by radiation leakage onto the cables that reflectedback to the receiver and internal system mismatchreflections caused by either the high value of ε′ at thesurface or poor system design. In both cases, erraticground contact, caused by uneven topography and jerksin the towing, caused the amplitude of these bands tovary and precluded the efficacy of horizontal filtering.No distinct and extended horizons indicative of soilstratification or a bedrock interface appear within the2Ð3 m of radar penetration along any of our profiles.

We use arrows to identify several targets of anoma-lously high amplitude in the unfiltered profile of Fig-ure 6. These targets are characterized by both hyper -

0.5

0.4

0.3

0.2

0.1

0.00 10 20 30 40 50 60 70

Depth (cm)V

olum

etric

Moi

stur

e C

onte

nt (

cm3

cm–3

)

0.5

0.4

0.3

0.2

0.1

0.00 50 100 150 200

Depth (cm)

Vol

umet

ric M

oist

ure

Con

tent

(cm

3 cm

–3)

Rain Day Prior to Sampling

Figure 5 (contÕd).

bolic diffractions (between 859 and 865 ft) and shortreflection segments. Although the background filter -ing reduces the noise bands and the direct coupling, italso attenuates many target responses, especially thosecharacterized by short reflection segments such asoccur near 926, 915, and 900 ft. The use of more traces(i.e., a wider ÒwindowÓ) in the background filter wouldretain these reflection responses at the expense of lessnoise reduction.

In general, we detected a total of 30 high-amplitudetargets along all 14 lines, of which 14 responses withinsegments of these profiles are shown in Figure 7. Thephase structure of several of the tar get wavelets(transects C, left; G, left; K, left; and I) identifies themas being of higher impedance than the surrounding soilmatrix. They are therefore probably metal. Other targetwavelets have a phase structure opposite that of themetal response (transect F; transect K, right). Wepresume these responses to be from the nonordnancetargets. Some tar gets exhibit waveform resonance,

d. Profile 4.

e. Profile 5.

9

Tim

e (n

s)

Dep

th (

m)

0.0

1.0

2.0

0

50

40

30

20

10

0.0

1.0

2.0

0

50

40

30

20

10

949 940 930 920 910 900 890 880 870 859

Distance Along Line F (ft)

{

Noise Bands

Direct Coupling

Figure 6. Sample segment of a 300-MHz profile (top) and the result of apply-ing a background removal filter (bottom). The arrows indicate strong targets.The short filter window required to alleviate the erratic character of the back-ground noise decreases the prominence of the target responses.

0

1

2

Dep

th (

m)

533 ft 572 ft0

1

2D

epth

(m

)

629 ft 672 ft

251

8 ft(2.4 m)

Transect C Transect C

283

Figure 7. Selected target responses within segments extracted from the 300-MHz profiles, andsample traces containing diffracted or reflected wavelets (darkened areas). The distances alongeach transect are shown at the top of the segments. Each segment is 500 traces long, and tracenumber is indicated on the sample traces. The lower horizontal scale is in 100-trace incre-ments. The depth scale is based on an ε′ of 10.4.

a. Transect C.

10

0

1

2

Dep

th (

m)

1235 ft 1207 ft0

1

2

Dep

th (

m)

884 ft 846 ft

225

Transect F Transect F

285225

0

1

2

Dep

th (

m)

512 ft 553 ft0

1

2

Dep

th (

m)

763 ft 811 ft

180

Transect G Transect G

179 193

0

1

2

Dep

th (

m)

239 ft 289 ft0

1

2

Dep

th (

m)

1204 ft 1245 ft

Transect K Transect K

239 326

347

256

Figure 7 (contÕd).

b. Transect F .

d. Transect K.

c. Transect G.

11

within which the first few half-cycles have the metalresponse waveform (transect C, right), while other tar-gets exhibit resonances within the direct coupling andcannot be similarly analyzed (transect G, right; transectL). These latter resonances occur where tar gets areextremely close to the surface; i.e., where tar get mul-tiple reflections occur before soil attenuation dampensthem significantly. In all cases, diffraction asymptotesare so highly attenuated as to be either barely visiblefor small targets (transect G, left), as was seen in thecontrol studies of Figure 4, or not visible at all wherethey should occur at the ends of short reflection segmentsrecorded above more extended targets (transect C, left).

We give probable interpretations for the tar getresponses seen along transect C (left side) and alongtransect K (left side) in Figure 8. The response alongtransect C is about 7 ft (2.1 m) long, which is close tothe length of a heavy bomb (1.6 m). The location ofthis response (533Ð572 ft) is marked in Figure 3b ashaving such ordnance. The more complex responsealong transect K is about 12 ft (3.7 m) long and occursin a section marked as having several medium-size mor-tars and projectiles. The JPG UXO ordnance brochuresshow many of these projectiles to be about 2 ft (0.6 m)long. We therefore speculate that this transect segmentspanned a series of close projectiles.

Grid survey: 600 MHzWe conducted this survey along four of the 14 grid

lines. We show profile segments containing the sevenstrongest target responses (Fig. 9) in a nonlinear for -mat because the amplitudes of all the returns areextremely weak, hence the appearance of much morenoise (gray speckle patterns) in the profiles thanappeared at 300 MHz.

Small segments of dif fractions are apparent in allthe target responses in Figure 9; the lack of asymptotesindicates the high signal attenuation. Almost all of theresponses have well-defined wavelets that exhibit theproper phase for a high-impedance tar get. Theseresponses originate from depths less than about 0.5 mand would not have been resolved from the direct cou-pling of the 300-MHz system. Although the time rangeis only 30 ns, very few tar get responses occurred atgreater than about 20-ns range. Therefore, the penetra-tion of this frequency in this soil was limited to lessthan 1 m deep.

Distribution of ε′We compared the more prominent diffractions seen

in both the 300- and 600-MHz surveys with modelhyperbolas to produce distributions of ε′ and to com-pute an effective mean permittivity value for the over-

Figure 7 (contÕd). Selected target responses within segments extracted from the 300-MHz pro-files and sample traces containing diffracted or reflected wavelets (darkened areas). The dis-tances along each transect are shown at the top of the segments. Each segment is 500 traceslong, and trace number is indicated on the sample traces. The lower horizontal scale is in 100-trace increments. The depth scale is based on an ε′ of 10.4.

e. Transects I and L.

12

0

1

2

Dep

th (

m)

718 ft 763 ft0

1

2

Dep

th (

m)

35 ft –7 ft

Transect I Transect L

252240

Figure 8. Likely interpretations for two of the responses in Figure 7.

burden above the tar gets. Seventy diffractions wereidentified in the 300-MHz survey and 48 in the 600-MHz survey. We used a statistical approach to averagethe computational inaccuracies caused by the minor dif-ferences in distance scale that resulted from uneventowing speeds. Seventy hyperbolas were clear enoughin the 300-MHz profiles to be used and 48 in the 600-MHz profiles. Both distributions (Fig. 10) exhibit nearlythe same average value and similar standard deviations(s.d.), although their distributions are slightly dif fer-ent. The 300-MHz distribution is more bimodal, whichmay reflect dif ferent orientations of deeper tar gets;extended targets whose axial direction crosses that ofthe transect produce hyperbolic reflections ratherthan diffractions, with values of ε′ reduced by the sineof the intersection angle (Jezek et al. 1979). The 600-MHz distribution may reflect less tar get orientationbecause it was limited to only lines AÐD and to shal-lower depths.

THEORETICAL DISCUSSION

The average values of ε′, the range of σ, the fact thatε′ may be slightly less at 600 MHz than at 300, and the

high radar gain and faded diffraction asymptotes, bothof which indicate high attenuation rates, allow us toestimate the soil dielectric properties at JPG (Fig. 11a)and theorize their effect upon antenna beamwidth andwaveform. We assume frel is about 3 GHz (Hoekstraand Delaney 1974); if it was any lower, then ε′ at 600MHz would be distinctly lower than it is at 300 MHz.We chose a value of 3 for εoo, which is characteristic ofdry soils and applies to frequency values where wateris nondispersive. The JPG values of σ strongly influ-ence β below about 400 MHz, while εrel is the domi-nant factor above about 400 MHz (Fig. 11b). Unfrozenmineralogic clays can exhibit even stronger absorptionand may preclude any radar penetration at our frequencyrange and above (Hoekstra and Doyle 1971).

We computed the effects of the soil properties uponthe round-trip propagation of both a model of our 300-MHz wavelet and an ideal monocycle-type waveform(Fig. 12). A monocycle is the shortest possible pulse anantenna can radiate and may be nearly achieved withunshielded antennas. It therefore has a wider bandwidththan our system model and would provide the highestpossible target resolution. Both wavelets begin propa-gation with a local frequency of 400 MHz (Fig. 12a),which is that of our antenna model 5103 when used on

13

General Purpose Bomb

87 in. (221 cm)

7 ft (2.1 m)

0

1

2Dep

th (

m),

ε =

10

533 ft 572 ft

Transect C

Trace0

Trace500

0

10

20

30

40

50

Tim

e (n

s)

300 MHz

Trace251

Dep

th (

m),

ε =

10

0

1

2

239 ft 289 ft

Transect K

Trace0

Trace5000

10

20

30

40

50T

ime

(ns)

Trace251

Trace256

Trace326

24 in (61 cm)

24 in (61 cm)

26 in. (66 cm)

12 ft (3.7 m)

0

0.5

1.0

Dep

th (

m)

22 ft 45 ft0

0.5

1.0

Dep

th (

m)

617 ft 644 ft

Transect A Transect A

233290

0

0.5

1.0Dep

th (

m)

719 ft 687 ft0

0.5

1.0 Dep

th (

m)

213 ft 180 ft

Transect B Transect B

208195

0

0.5

1.0Dep

th (

m)

508 ft 539 ft0

0.5

1.0 Dep

th (

m)

1155 ft 1119 ft

Transect C Transect D

258173 305

Figure 9. Selected target responses within segments extracted from the 600-MHz profiles, and sample tracescontaining wavelets. The penetration of this frequency is about 1 m.

14

low-permittivity material (e.g., ε′ = 4). Both waveletsthen experience strong attenuation caused by conduc-tivity alone (Fig. 12b) but maintain their original formin this case and show no shift in local frequency*. Fig-ures 12c and d show that shifts in local frequency willoccur when the dielectric relaxation is considered.Despite the shift, the wavelet form is changed little.

The value of ε′ also determines the antenna direc-tivity (Arcone 1995). Horizontal dipoles on the groundsurface show a greater concentration of energy withinthe subsurface vertical plane that includes the antennaaxis than in the plane perpendicular to the axis (Fig.13). As ε′ increases, the radiation will become moreconfined within the plane containing the antenna axis.This means that our profiles were mainly sensitive tothe ground directly beneath the transects. The greaterwidth of the pattern perpendicular to the axis (and alongour transects) shows that the rapid fading we observedwithin diffraction asymptotes was due to soil attenua-tion and not to radiation directivity.

MODELING

In this section we describe some basic modeling thatis intended to do two things: 1) demonstrate how thespecific kinds of scattering effects from elongated UXO-type targets can give systematic clues to the characterof the target; and 2) show how such scattering can causeambiguities in the pattern of measured signal. As anentrance into modeling the kind of phenomena con-fronted in the field, we constructed a simple integralequation model of radar scattering in a uniform loessybackground. Our focus is on the kinds of fields radi-ated from a dipole-type antenna, their interactions withUXO-form targets, and the scattered signals that result.We do not include complications here which may arisefrom the presence of a ground surface near the tar get,but concentrate only on the transmissions and reflec-tions back and forth between the antenna and tar get.Elsewhere, using more sophisticated modeling tech-niques (OÕNeill et al. in prep), we treat tar gets withproximate ground surfaces. These latter investigationsgenerally reveal that basic resonance and polarimetricscattering phenomena that we see here using the inte-gral equation model (infinite homogeneous soil back-ground) usually persist when the tar get is near theground surface.

Within the bounds of our assumptionsÐinfinite soilconstituting a uniform background around the tar getÐwe treat the physics of the antenna radiation and targetscattering (re-radiation) rigorously. All governing rela-tions used here begin with statements tantamount toHuygenÕs Principle (Kong 1990). To simulate bothradiation from the buried target and from the transmit-

Figure 10. Distribution of ε′ at both 300 (black) and 600 MHz (grey).

*The distortion is small in all cases for our system model(Fig. 12b, c, d, bottom) and the attenuation is comparable tothe gain we used during recording to bring reflections to thelevel of the direct coupling. The shift in local frequency forthe ideal wavelet (Fig. 12c, d, top) is severe enough to pre-clude its detection by a receiver antenna identical to the trans-mitter antenna. In contrast, the local frequency of our systemmodel is lowered to only 300 MHz by the 3-GHz relaxationfrequency (Fig. 12c, bottom). A value of 6 GHz has lesseffect (Fig. 12d, bottom).

15

25

20

15

10

5

0

Per

cent

Occ

uren

ces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 >17

Relative Permittivity (ε )

300 MHzε = 10.5s.d. = 4.2

600 MHzε = 10.4s.d. = 3.5

Relative Permittivity (ε′)

300 MHzε′ = 10.5s.d. = 4.2

600 MHzε′ = 10.4s.d. = 3.5

εs = 10.4

εoo = 2.7frel = 3 GHzσ = 0.25Upper – Real PartLower – Imaginary Part

σ = 0.025

σ = 0.04

1110810 1010910Frequency (Hz)

12

10

8

6

4

2

0

Com

plex

Per

mitt

ivity

, ε*

1110810 1010910

110

210

310

410

Rou

nd T

rip A

ttenu

atio

n (d

B m

–1),

β

Frequency (Hz)

σ = 0.025

σ = 0.04

Figure 11. Dielectric permittivity curves and attenuation rates for the averageε′, range of σ, and probable values of f rel and εoo at JPG. A dielectric relax-ation at about 3 GHz strongly increases β above about 300 MHz, yet wavevelocity (determined by the real part of εs*) remains constant to about 600MHz.

a. Dielectric permittivity curves.

b. Attenuation rates.

16

–1.0

1.0

0.8

0.6

0.4

0.2

0.0

Time (ns)

Am

plitu

de

10 20 30 400 50

–0.2

–0.4

–0.6

–0.8

Incident Wavelet = Ideal MonocycleLocal Wavelet Frequency = 400 MHz

–1.0

1.0

0.8

0.6

0.4

0.2

0.0

Time (ns)

Am

plitu

de

10 20 30 400 50

–0.2

–0.4

–0.6

–0.8

Incident Wavelet = GPR Model WaveformLocal Wavelet Frequency = 400 MHz

Figure 12. Model 400-MHz wavelets before and after round-trip propagation within1 m of ground characterized by the soil parameters measured. The wavelets re-flect from an interface for which R = Ð1 (eq. 6), which only reverses the phase ofthe wavelets so that all wavelet distortion and attenuation are due to propagation.

a. Incident waveforms.

17

Figure 12 (contÕd). Model 400-MHz wavelets before and after round-trip propaga-tion within 1 m of ground characterized by the soil parameters measured. Thewavelets reflect from an interface for which R = Ð1 (eq. 6), which only reverses thephase of the wavelets so that all wavelet distortion and attenuation are due topropagation.

–4

3

2

1

Time (ns)

Am

plitu

de

10 20 30 400 50

0

–1

–2

–3

Soil Resistivity = 25 ohm-mNo Soil RelaxationLocal Wavelet Frequency = 400 MHzRound Trip Attenuation = 49 dB/m

x 10–3

–3

3

2

1

Time (ns)

Am

plitu

de

10 20 30 400 50

0

–1

–2

Soil Resistivity = 25 ohm-mNo Soil RelaxationLocal Wavelet Frequency = 400 MHzRound Trip Attenuation = 47 dB

x 10–3

b. Reflections from purely resistive soils.

18

–3

4

3

2

Time (ns)

Am

plitu

de

10 20 30 400 50

1

0

–1

–2

Soil Resistivity = 25 ohm-mSoil Relaxation Frequency = 3 GHzLocal Wavelet Frequency = 300 MHzRound Trip Attenuation = 66 dB

x 10–4

Soil Resistivity = 25 ohm-mSoil Relaxation Frequency = 3 GHzLocal Wavelet Frequency = 170 MHzRound Trip Attenuation = 61 dB/m

–10

5

0

–5

Time (ns)

Am

plitu

de

10 20 30 400 50

x 10–4

Figure 12 (contÕd).

c. Reflections from resistive soil with dielectric relaxation at 3 GHz.

19

Soil Resistivity = 25 ohm-mSoil Relaxation Frequency = 6 GHzLocal Wavelet Frequency = 200 MHzRound Trip Attenuation = 57 dB/m

–1.5

1.0

0.5

0.0

Time (ns)

Am

plitu

de

10 20 30 400 50

–0.5

–1.0

x 10–3

Soil Resistivity = 25 ohm-mSoil Relaxation Frequency = 6 GHzLocal Wavelet Frequency = 330 MHzRound Trip Attenuation = 58 dB

–8

10

8

6

Time (ns)

Am

plitu

de

10 20 30 400

4

–6

2

0

–2

–4

x 10–4

50

Figure 12 (contÕd). Model 400-MHz wavelets before and after round-trip propa-gation within 1 m of ground characterized by the soil parameters measured.The wavelets reflect from an interface for which R = Ð1 (eq. 6), which onlyreverses the phase of the wavelets so that all wavelet distortion and attenu-ation are due to propagation.

d. Reflections from resistive soil with dielectric relaxation at 6 GHz.

20

ting antenna, we consider electric currents over the sur-face of a perfect electric conductor (PEC), where thetangential electric field E is negligible. In terms ofintegrals over the metal surface S,

(7)

where r and r′ are observation and integration fieldpoints, respectively; µ is the magnetic permeability ofthe soil, taken here to be that of free space (4 π × 10Ð7

H/m); k is the electromagnetic wavenumber (mÐ1); andE (V/m) and H (A/m) are the radiated electric and mag-netic fields, respectively. The quantity α is a geometri-

cal factor equal to the solid angle subtended by the sur-face required to exclude the singularity when r equalsr′, divided by 4 π. Thus at a point on a smooth, i.e.,locally flat surface, α is one-half. The surface currentsJ are equal to ̂n × H, where the unit normal vector ̂npoints outward from the surface, while the tangentialelectric field, proportional to ̂n × E, has been consid-ered negligible (perfect conductor). The scalar Greenfunction g is

(8)

Taking the vector cross product of ̂n with equation(7) for H leads to

(9)

ε = 10.5

–15 dB

–10

–5

62°22°

Figure 13. Radiation beamwidths for a model of our antennas and the average ε′ =10.4 found at JPG. The solid curve is the beam pattern within the plane of the transectand is perpendicular to the direction of antenna polarization. The dotted curve is thebeam pattern in the plane of the antenna polarization, which is perpendicular to thetransect direction. Individual pattern 3-dB beamwidths are indicated at the point ofthe arrows. A typical form of the radiated wavelet is also shown. The curves assumethat the transmitter and receiver antennas are coincident. They are actually spacedabout 15 cm apart in the 300-MHz unit and slightly less in the 600-MHz unit.

21

g

eR

R= ≡ ′ikR

4π, – .r r

α ωµ( ) ( )r E r r r J r= ′ ′( ) ′( ) +

∫∫ dS i gS

Ð ,

ωµ ,r r J r E r+ ∇∇ ′( ) ⋅ ′( )

( )i

kg

incÐ

2

α ,r H r r r J r H r( ) ( ) = ′ ∇ ′( ) × ′( ) ( )∫∫ dS gc inc

SÐ

J r n r r r n r H r′( ) ( ) ⋅∇ ′( )( )} = ( ) × ( )g inc– ˆ , ˆ .

α r J r r J r r r( ) ( ) ′ ( ) ⋅ ′( )( )∇ ′( ){∫∫– ˆ –dS g

Sn ,

62

We approximate the tar get geometry using N sur -face elements, each being a flat triangular facet. Toachieve the numerical formulation we assume furtherthat the current J is approximately constant over eachsuch element, associating its value with the location ofthe elementÕs centroid, ri, i.e., Ji = J(ri) for the ith ele-ment. Locating r only at the centroid of each elementmeans that what follows α is always one-half. Alto-gether we obtain

(10)

where Sj is the area of the jth element. Thus over eachelement we need only evaluate the integral

Vp dS

gp

p s t nS

ijij

= ∫∫ ′ =∂∂

, , , (11)

where s and t are locally constructed tangential coordi-nates, and n is the local outward normal coordinate.We used four -point Gaussian quadrature to ensureaccuracy in this integration.

Locating ri at each element centroid in successionprovides N vector equations in the N unknown currentelements Ji. These currents are tangential to the sur -face. Expressing the ultimate governing relations interms of tangential s and t components we obtain

(12)

.

This is the algebraic system to be solved. In effect, themethod consists of applying point matching over a PEC,with pulse basis functions.

This formulation was chosen in part for its simplic-ity and hence programming convenience. Because tan-gential electric field components drop out, the initial

governing equation is reduced at the outset. Associat-ing the degrees of freedom with element centroids andlocating observation points there cause all Òself ele-mentÓ singularities to drop out of computations: in allself element integrations

(13)

Once the currents are obtained from solution of theabove algebraic system (12) they can be substituted inthe equivalent of (7), expressed numerically in a man-ner similar to (12), to obtain scattered E and H fields atany observation point. In line with the assumptionsoutlined above, we calculate the (re)radiated fields,ignoring the influence of the ground surface disconti-nuity, i.e., as if the radiating currents were in an infinitesoil medium. The computer program was tested againstdetailed near-field solutions for cases where analyticalsolutions are available (sphere), and against reasonable-ness checks for signal loss as a function of distancewithin the soil.

INCIDENT FIELD AND POLARIZA TIONPARAMETERS

The basic setup assumed for the antenna-tar getsystem is shown in Figure 14. The antenna is simulatedby a sheet of currents deemed to be a reasonableapproximation of those on a metal surface drivenas a dipole antenna. We adjusted this current distribu-tion so that its subsurface radiation pattern resemblespatterns considered representative of those from a di-pole antenna resting on a ground surface. To accom-plish this, the antenna current distributions are expressedas

(14)

where wy = 0.8 m and w z = 0.4 m. In choosing unit

value maximum current magnitude, we implicitly nor-malize all simulations with respect to that dimensionof input. In their general features, i.e., 3-dB beamwidth,lobe patterns, and angular locations where the fieldsdecline rapidly towards minima, patterns from thesedistributions fit our needs here.

Our antenna model thus consists of a rigorous rep-resentation of a synthetic or idealized structure. Theresulting fields are strictly in accordance with MaxwellÕs

22

J r r n Hji

i i i, ˆgn

inc∂∂

′( )

= ×

1

2J n s s s Jsi i j ij i j ij sj– ˆ ˆ ˆ ˆ –⋅( ) + ⋅( )[ ]∑ Vs Vnj

12t H J n si i ti i j ij– ˆ – ˆ ˆ= ⋅ ⋅( ) +[∑ Vtinc

j

i j ij tj i i+ ⋅( ) ] = ⋅Vn incˆ ˆ ˆt t J s H

J yy z

z zw, ˆ cos

–,( ) = ( )

azπ

y y w z z w, – , –≤ ≤a y a z

12 J n J r r Ji i j i

j

– ˆ , –dS gS

′ ⋅( )∇ ′( )

∫∫∑

ˆ .n Ji ji

⋅( ) = = ∂∂

0g

n

n t s t Ji j ij i j ij tjˆ ˆ ˆ ˆ⋅( ) + ⋅( )[ ]∑ =Vs Vnj

t s J n ti j ij sj i jˆ ˆ – ˆ ˆ+ ⋅( ) ] ⋅Vn (( ) +[∑ Vt

jij

equations, which underlie HuygenÕs principle and formthe basis for describing all electromagnetic wave phe-nomena. Any input to our problem that violates thoseequations to some degree thwarts our ability to checkthe ultimate output for electromagnetic consistency, e.g.,energy conservation. Beyond this, the antenna modelcontains two basic features in common with the actualantennas in frequent use: broader, smoother pattern inthe H plane, which is usually coincident with the sur -vey transect direction; and beamwidth that depends onfrequency in the manner of a current sheet or set ofaperture fields. In practice, numerical computation ofthe incident H values at each element centroid was avery minor burden, in terms of both programming andCPU time.

Because the fields radiated by the antenna are notplanar, we will indicate antenna orientation rather thanspeak of the polarization of the incident field. We al-ways assume that currents run in its long directionwith the same distribution relative to the antenna.Especially because we are surveying at shallow depths,the incident field is nonuniform in both magnitudeand direction over the tar get geometry. In practice,the orientation of the E field over most of thetarget is reliably in the E plane of the antenna. In the(x,z) plane, which contains the transect line, the mostconsistent E field orientation results from the referenceantenna orientation in Figure 14: the transmitted E fieldis always normal to that plane, parallel to the Y axis.

For received signals we assume that a horizontally ori-ented antenna records the computed E y and Ez, with-out applying the antenna pattern to its receiving char -acteristics.

MODELING RESULTS

In what follows, the survey transect is always 0.7 mabove the target, as if along a ground surface. We some-times consider cases in which the antenna is variouslyrotated relative to the tar get axis or relative to thetransect direction. The latter rotation was not enlistedsystematically in the field measurements at JPG dis-cussed here. However , it of fers insight into possibletarget scattering behavior when the target is arbitrarilyoriented relative to the incident field, which was likelyto be the case during a survey. Some of the features inthe field profiles push questions to the fore: how canthe target response from points not above the tar getarrive relatively early in time, then appear to fade some-what, then return to higher amplitude? Rotations of thetarget relative to the antenna are seen below to imposeinteresting spatially and frequency dependent patternsin the scattered signal. Relative rotation of antenna andtarget also suggest ways of discriminating hard cases.

First we examine scattering responses when the pro-jectile is level, beginning with it oriented in the Ydirection (θd = 0 = declination; θx = 90° = horizontal

Figure 14. Basic setup, with target below ground surface, in its reference ori-entation. The fully polarimetric, orientable antenna is above it, at some posi-tion on a survey transect line.

X

Y

Z

antennatransect

line

target

23

and deepening its inclination diminishes the first, low-est mode and brings out the higher modes, ultimatelymore strongly than the first. Given the dielectric con-stant, this kind of pattern allows one to infer the targetlength from the separation of the peaks/troughs in thepattern. Previous investigators (Geng et al. 1999) havedemonstrated the existence of shape-dependent naturalresonant modes in currents in, and associated radiationfrom, metallic bodies of revolution (BORs). These cor-respond to electromagnetic Òfree vibration,Ó after anyforcing effects have died out, at frequencies less than 1MHz, consisting primarily of decay patterns. Here wesee what are in many ways comparable ef fects in theradar frequency range, when responses are forced andthe resonant frequencies we seek are real valued.

As θx => 0 we lose the kind of amplitude patternin the previous two figures, but fortunately we gainanother, which adds basic information as long as θd <90°. The upper line in Figure 17 shows the normalizedscattered E field magnitude when the antenna is directlyabove a level projectile (Za = 0). Both antenna and tar-get are in the reference configuration (Fig. 14). Thelower line shows results for the same case but with theprojectile inclined 45 ° (θd = 45 °, θx = 0) and theantenna is located at Za = 0.72. Both results are shownvs. Re{k}a where a is the radius of the uniformlycylindrical portion of the target. Basically, both of theseshow most of the first cycle in the well-establishedcyclical scattering cross-section pattern for a circularcylinder, in the resonance region, under normal inci-dence with transverse E field (Bowman et al. 1988).

rotation about X axis, relative to reference target orien-tation). In this case, given the reference antenna orien-tation (Fig. 14), the incident E field is aligned with thetarget. Although the same effects described below aregenerally apparent without normalization, for general-ity we scale the magnitude of the received signal by themagnitude of the incident field at the tar get, Eo, as afunction of frequency . Figure 15 shows normalizedbackscatter response across the band. The clear maxi-mum occurs at 100 MHz where, given the dielectricconstant assumed (10.2 + i 2.8 × 10Ð3), the subsurfacewavelength is almost exactly twice the tar get length.That is, the projectile responds most strongly when itcan serve as a half-wavelength dipole antenna. Thechange in the tar getÕs cross-sectional shape along itsaxis does not appear to interfere significantly with thiseffect.

Perhaps more revealing than this horizontal case,similar scattering behavior is apparent when the targetis inclined further , even when it is rotated about thevertical (x) axis. The lower (solid) line in Figure 15shows the effect of adding a 45° inclination (θd) to thetarget, keeping it in (x,y) plane. We see a clear succes-sion of higher modes for higher multiples of L/(λ/2).This pattern persists for a variety of inclinations androtations (Fig. 16), as long as the impinging radiationtravels longitudinally along the surface of the target tosome degree. Here the ÒtotalÓ scattered field magni-

tude Etot is defined as | | | |E Ey z2 2+ . Rotating the

target back towards the reference orientation (θx = 0°)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400 500

|Ey|/E

o

f (MHz)

θd = 0

θd = 45o

Figure 15. Normalized scattered electric field above a projectilesurrounded by soil and oriented in the Y direction, as a functionof frequency .

24

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 1 108 2 108 3 108 4 108 5 108

30, 60

45, 4560, 30

f (Hz)

| Etot|/E o

Figure 16. Normalized scattered E field above target for variousprojectile inclinations and rotations θd, θx.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

| Ey|/E

o

ka

θd = 0, z

a = 0

θd = 45o, z

a = 0.72

Figure 17. Normalized scattered E field when projectile axis isin (x,z) plane, from different vantage points Z a, with differentdeclinations θd.

The peak in the curve occurs at the ka value one wouldexpect for an infinitely long circular cylinder with ra-dius a, when creeping waves around the tar get inter-fere constructively with direct backscatter from the frontof the target. Thus a plot of results for this target orien-tation implicitly reveals the value of a. We note that thetapering down of target cross section into its nose doesnot seem to affect these results, i.e., a refers to the ra-dius of the uniform section of the target cylinder.

The appearance of resonance peaks in the patternscan provide essential information about tar get geom-etry. But they can also lead the surveyor astray . Thepositions where these peaks appear depend both on fre-quency and vantage point. Thus, for example, low fre-quencies coming from an antenna to the side of the tar-get (the low-frequency portion of the incident beam iswider than the higher frequency) could provoke strongpeaks. However, when the antenna has been moved,

25

different frequency content in the beam and dif ferentincidence angles would cause dif ferent responses tobecome relatively more prominent. A key to successfuldiscrimination will be to identify various tar get char-acteristics in stages, starting with things like depth andorientation, and then use these to calibrate subsequenttreatment of the data.

Examination of the broadband response along thetransect can reinforce the above tar get length estima-tion, while adding information about the variation ofits geometry along its axis. Figure 18 showsbackscattered Ez amplitudes along the transect whenthe projectile is inclined at 45° with its nose up (i.e., θd= 225°, θx = 0). This orientation does indeed occur inreality, though it is less common than a nose-down sub-surface rotation. In any case, our purpose here is pri-marily to study the fundamental effects of target orien-tation, whatever it may be. In this and all subsequentfigures, scattered field values are not normalized. Toprovide the most general illumination, the antenna hasbeen rotated horizontally 45 ° so its axis is oblique tothe transect direction, producing an incident E field nei-ther aligned with nor transverse to the tar get axis. Inpractice this can be accomplished by two measurements,

one with parallel and the other with perpendicular an-tenna orientation; these results can then be combinedfor a completely general polarimetric characterization.

Figure 18 shows a progression over increasing fre-quency, from more or less a single event (peak) at 100MHz, through two and three events at 200 MHz and300 MHz, respectively, and four events at 400 MHz.That is, the number of peaks/events corresponds to thenumber of half wavelength multiples in the target length.In this case, the E z backscattered field component isgenerated by the tendency of the tar get to induce cur-rents preferentially in the general direction of its longaxis. In fully polarimetric surveying, this tendency, to-gether with the circumferential creeping wave phenom-ena illustrated in Figure 17, produces characteristic andvery revealing induced ellipticity and depolarization inthe scattered signals (OÕNeill et al. 1999).

The pattern in Figure 18 is altered in certain inter -esting details when the projectile direction is reversed,that is, with the same overall orientation but with itsnose down and tail up ( θd = 45 °, θx = 0). Figure 19shows backscattered Ez magnitudes for this case. It isevident that the upward pointing flattened end of thetarget has a prominent influence in changing the pat-

Figure 18. Backscattered E z field magnitudes along transect, at variousfrequencies, when the projectile is inclined at 45 ° with nose up ( θd = 225°,θx = 0), and the antenna is rotated θa = 45°.

26

terns relative to those in the previous figure. In this nose-down case, increasing the frequency causes the scatter-ing lobes to coalesce into smaller events on one largerbackground lobe, shifted towards Za < 0. This is evenmore evident relative to the nose-up case when the Eycomponent is examined as well. This background lobeoccurs in the range of Z a when the incident radiationimpinges most directly on the flattened tar get end. Atthe same time, we must note that this end orientationeffect is not solely due to localized reactions inducedon the flattened end itself. The dramatic rise of the back-scatter for Za < 0 at frequencies corresponding to inte-gral numbers of L/(λ/2) indicates that this signal mag-nitude enhancement involves the current pattern downthe entire length of the target.

Overall, in this set of examples we note that thereare dramatic shifts in the magnitude of the response,depending on which frequencies come to the fore. Wealso note that the strongest response is sometimesshifted spatially of f to the side of the tar get location,depending on its orientation.

In conclusion, we show three bistatic scattered fieldvalues (|Ez|) over the (x,z) cutting plane, for an inclinedprojectile. They were obtained from 3-D finite element

simulations, at the single frequency of 150 MHz, in-cluding soil properties as specified above but adding aground surface. That is, the material below the X = 0has the aforementioned electrical properties, while thatabove it has the electromagnetic characteristics of freespace.

Figure 20 shows bistatic scattering patterns for inci-dence at subsurface angles of 45°, zero, and Ð45° rela-tive to the vertical axis, where negative incidence anglesare taken to be in the first quadrant (positive x and z).Analysis based on Fresnel coefficients and superposi-tion of multiple reflections explains some apparentlybaffling features in the total scattered field shown: asthe wave initially reflected from the tar get strikes theunderside of the ground surface, it reflects back down-wards. This downwards reflection interferes construc-tively with the upgoing component; that is, it does notchange sign on reflection. This means that we see val-ues of |E z| increase near the ground surface. We alsosee an overall pattern of two minima and two maximaalong certain lines between the target and the surface.This is basically a standing wave interference patternbetween upgoing and downgoing waves, and possibleexamples of this were seen in Figure 7 for transects G

Figure 19. Backscattered E z field magnitudes along transect, at variousfrequencies, when the projectile is inclined at 45 ° with nose down ( θd = 45°,θx = 0), and the antenna is rotated θa = 45°.

27

and L where the target appeared to be near the surface.Most strikingly, the direction along which the stron-gest portions of the scattered field appear does not al-ways run through the (mono-static, backscatter) obser-vation point; or it does so, but not when the antenna isdirectly over the center of the target. Thus these simu-lations illuminate the ways in which interference pat-terns and directional biases in the signal, dependent onsurface location and target inclination, can produce de-ceptive returns on the surface.

CONCLUSIONS AND RECOMMENDATIONS

Soil attenuation at JPG was extremely high, as evi-denced by the high amount of gain required to displaythe signals received from within only the first meter ofdepth. The gain values of 50Ð65 dB that we applied areconsistent with the general values of round-trip attenu-ation for 1 m of depth predicted by our propagationmodels. GPR bandwidths should be centered no higherthan 600 MHz because the round-trip attenuation permeter of depth above this frequency may exceed thedynamic range of the radar. This fact precludes the useof any microwave system for detection of UXO. These

high attenuation rates also greatly reduce off-axis sen-sitivity to targets so that full-diffraction signatures arerarely seen. We therefore recommend that the antennatowing speeds be kept very slow and data acquisitionrates very high to ensure recording of as much of thediffraction responses as possible. We further recom-mend that an all-digital system eventually be developedso that extremely high rates (e.g., 512Ð1024 traces/s)would further allow noise reduction by trace stacking.

The most likely range of soil dielectric constants (5Ð14) is consistent with the measured moisture contents.The average value of about 10 predicts a narrowbeamwidth in the plane parallel with the antenna polar-ization axis. This means that for normal towing opera-tions, wherein the antennas are polarized perpendicu-lar to the transect direction, the system is sensitive toground mainly beneath the antennas. We therefore rec-ommend that coverage be increased by using a multi-channel radar system. Such operation is standard withpresent commercial systems, but its implementationcompromises the data acquisition rate. Therefore, thisis further reason for development of a real-time digi-tizing GPR system above 300 MHz that could acquiredata at 512Ð1024 traces/s.

Horizontal filtering, normally an effective means of

Figure 20. Scattered |E z| values, when the incidence direction and incident Efield orientations are in the (x,z) plane, for various incidence angles.

a. 45° from the left.

28

|Ez|

0.1000.0930.0860.0790.0710.0640.0570.0500.0430.0360.0290.0210.0140.0070.000

Ð1.5 Ð1 Ð0.5 0 0.5 1 1.5

1.5

1

0.5

0

Ð0.5

Ð1

Ð1.5

X

Z

Figure 20 (contÕd).

c. Ð45° (from 45 ° to the right of the vertical).

b. Vertical.

29

|Ez|

0.2000.1860.1710.1570.1430.1290.1140.1000.0860.0710.0570.0430.0290.0140.000

Ð1.5 Ð1 Ð0.5 0 0.5 1 1.5

1.5

1

0.5

0

Ð0.5

Ð1

Ð1.5

X

Z

|Ez|

0.5000.464

0.429

0.3930.3570.321

0.2860.2500.214

0.179

0.1430.107

0.071

0.0360.000

Ð1.5 Ð1 Ð0.5 0 0.5 1 1.5

1.5

1

0.5

0

Ð0.5

Ð1

Ð1.5

X

Z

reducing weak internal and external noise (or clutter)that occurs at constant time delay , was only partiallysuccessful. We have ascribed this result to the ef fectsof uneven towing speeds and erratic contact betweenthe ground surface and antenna faces upon antenna leak-age. We recommend that a simple antenna platform,consisting of a heavy, high-density polyethylene Òboat,Óbe used to contain the antenna. The prow of the boatwould help to deflect vegetation, and the weight wouldimprove ground contact. The dielectric permittivity ofthe material is intermediate between air and ground andmay improve the antenna impedance match with the nearsurface. The use of this platform may make the ampli-tude of the clutter bands more uniform and subject to betterremoval with horizontal filters.

We recommend that improved jitter control be speci-fied in the design of future transmitters and receivers.Jitter is the slight shift in time that occurs in signal po-sition from trace to trace. Although jitter may be only1Ð2 ns, its reduction could significantly reduce noisewhen the trace length is only 30Ð50 ns.

We recommend that antennas for UXO detection re-main on the surface. Airborne profiling, even at mini-mal heights of a few tens of centimeters, introduces anextra dielectric layer (air) into the propagation medium.In airborne profiling, returns from uneven subsurfaceinterfaces will be strongly refracted along the surfaceand not be received because the value of ε′ = 1 for aircontrasts sharply with the ε′ of the soil.

The wavelet shape transmitted by present, commer-cially available systems is probably most practical. Ourtheoretical studies show that shorter pulse length, whichmay be available with unshielded antennas, may shifttoo much in local frequency to be detected after propa-gation.

Future surveys should address the problem of totalsoil saturation. Our surveys were conducted in earlyfall, by which time these soils are usually below satu-ration. However, saturation levels are usually reachedby late winter and can persist into late spring. Satu-rated soils have higher values of εs* and frel and maytherefore offer less attenuation per meter at 300Ð600MHz than would moist soils. However, mechanical op-eration in mud may be a serious mobility problem.

EMI systems have shown themselves to be quite suc-cessful at UXO detection, even in site conditions thatare unfavorable for GPR. At the same time, simula-tions show the distinct possibility that GPR may revealkey target characteristics, such as length and diameter.This suggests that more such simulation should be doneand integrated into signal interpretation, and that themost important offering of GPR may be in the realm ofdiscrimination more than UXO detection. Fully polari-metric GPR is both possible and potentially advantageous.

LITERATURE CITED

Arcone, S.A. (1995) Numerical studies of the radia-tion patterns of resistively loaded dipoles. Journal ofApplied Geophysics, 33: 39Ð52.Arcone, S.A., D.E. Lawson, A.J. Delaney, J.C.Strasser, and J.D. Strasser (in prep) Ground-penetrat-ing radar reflection profiling of groundwater and bed-rock in an area of discontinuous permafrost. Geophys-ics.Bowman, J.J., T.B.A. Senior, and P .L.E. Uslenghi(Ed.) (1988) Electromagnetic and Acoustic Scatteringby Simple Shapes. New York: Elsevier.Brillouin, L. (1960) Wave Propagation and Group Ve-locity. New York: Academic Press.Clarke, T.S., and C.R. Bentley (1994) High-resolu-tion radar on Ice Stream B2, Antarctica: Measurementsof electromagnetic wave speed in firn and strain his-tory from buried crevasses. Annals of Glaciology, 20:153Ð159.Debye, P. (1929) Polar Molecules. Mineola, New York:Dover Publications.Delaney, A.J., and S.A. Arcone (1984) Dielectric mea-surements of frozen silt using time domain reflectom-etry. Cold Regions Science and Technology, 9: 39Ð46.Feynman, R.P., R.B. Leighton, and M. Sands (1964)The Feynman Lectures on Physics, Vol. II. Reading,Massachusetts: Addison Wesley Longman.Geng, N., C.E. Baum, and L. Carin (1999) On thelow-frequency natural response of conductivity andpermeable targets. IEEE Transactions on Geoscienceand Remote Sensing, 37(1): 347Ð359.Hoekstra, P., and A.J. Delaney (1974) Dielectric prop-erties of soils at UHF and microwave frequencies. Jour-nal of Geophysical Research, 79: 1699Ð1708.Hoekstra, P., and W.T. Doyle (1971) Dielectric relax-ation of surface adsorbed water. Journal of Colloid andInterface Science, 36: 513Ð521.Jezek, K.C., C.R. Bentley, and J.W. Clough (1979)Electromagnetic sounding of bottom crevasses on theRoss Ice Shelf, Antarctica. Journal of Glaciology, 24:321Ð330.Kong, J.A. (1990) Electromagnetic Wave Theory. NewYork: John Wiley and Sons.Llopis, J.L. (in prep) Soil resistivity measurements atJPG. Engineer Research and Development Center ,Vicksburg, Mississippi, Technical Report.NAVEOD (1997) Unexploded ordnance (UXO): Anoverview. Naval Explosive Ordnance Disposal Tech-nology Division, UXO Countermeasures Department,Indian Head, Maryland.Nickell, A.K. (1985) Soil Survey of Jefferson County,Indiana. U.S. Department of Agriculture, Soil Conser-vation Service, 169 pp.

30

OÕNeill, K., S.A. Haider, S. Geimer, and K.D. Paulsen(in prep) Effects of a proximate grounds surface on ba-sic broadband polarimetric scattering from buried me-tallic targets. IEEE.Parkhomenko, E.I. (1967) Electrical Properties ofRocks. New York: Plenum Press.P”w”, T.L. (1955) Origin of the upland silt nearFairbanks, Alaska. Geological Society of America Bul-letin, 67: 699Ð724.Rodbell, D.T., S.L. Forman, J. Pierson, and W.C.Lynn (1997) Stratigraphy and chronology of Missis-sippi Valley loess in western Tennessee. GeologicalSociety of America Bulletin, 109: 1134Ð1148.Stratton, J. (1941) Electromagnetic Theory. New York:McGraw-Hill.

Tice, A.R., J.L. Oliphant, Y. Nakano, and T.F.Jenkins (1982) Relationship between the ice and un-frozen water phases in frozen soil as determined bypulsed nuclear magnetic resonance and physical des-orption data. USA Cold Regions Research and Engi-neering Laboratory, CRREL Report 82-15.Topp, G.C., J.L. Davis, and A.P. Annan (1980) Elec-tromagnetic determination of soil water content: Mea-surements in coaxial transmission lines. Water Re-sources Research, 16: 574Ð582.Wait, J.R. (1955) Scattering of a plane wave from acircular dielectric cylinder at oblique incidence. Cana-dian Journal of Physics, 33: 189Ð195.Wait, J.R. (1970) Electromagnetic waves in stratifiedmedia, 2nd Ed. New York: Pergamon Press.

31

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April 2000 Technical Report

UXO Detection at Jefferson Proving Ground Using Ground-Penetrating Radar

Steven A. Arcone, Kevin OÕNeill, Allan J. Delaney,and Paul V. Sellmann

U.S. Army Engineer Research and Development CenterCold Regions Research and Engineering Laboratory72 Lyme Road ERDC/CRREL Technical Report 00-5Hanover, New Hampshire 03755-1290

Ground-penetrating radar (GPR) was used to detect UXO and nonordnance on the 40-acre site (lot 54) of Jefferson Proving Ground, Indiana. The UXO areburied about 1 m deep in a clayey silt for which the soil water content ranged from moist near the surface to near saturation at about 1 m. A 16-bit radar wasused to profile along previously established lines and transects over emplaced artificial targets. Data were recorded at 48Ð64 traces/s with minimal towingspeeds during both dry and rainy weather. Target responses at both 300 (time range of 50 ns) and 600 MHz (30 ns) ranged from discrete diffractions to shortreflection segments. Soil loss greatly attenuated diffraction hyperbolas. Theoretical analyses of these hyperbolas give an average soil dielectric constant of10 at both 300 and 600 MHz. The phase polarity of many of the reflected and diffracted wavelets indicate targets with wave impedances higher than thatof the soil. It is therefore assumed that these targets are metallic and the responses of some, whose locations correlate with the position of UXO on burialmaps, are shown in detail. Theoretical modeling of wavelet propagation for this soil confirms the high rate of attenuation (47Ð 66 dB/m round-trip), themaintenance of waveform, a shift in wavelet local frequency, and response to a typical UXO. It is concluded that GPR is effective for finding targets in thistype of soil to no more than 2-m depth. It is recommended that future surveys utilize high trace acquisition rates to capture the full target responses, and aprowed, heavy dielectric antenna sled to improve antenna-to-ground coupling and to deflect surface obstacles such as vegetation . Broadband, three-dimensional numerical modeling of scattering from UXO-shaped tar gets in soil, with diverse orientations of antenna and tar get relative to one another,suggests that target length and diameter may be inferred from resonance patterns in backscattered signals. The simulations also reveal some ways in whichoblique target/antenna orientations along a transect may af fect frequency-dependent response patterns, providing clues as to ta rget shape as well aspositional ambiguities.

Dielectric constant Unexploded ordnance (UXO)Ground-penetrating radar

Standard Form 298 (Rev. 8-98)]Prescribed by ANSI Std. 239.18

U.S. Army Environmental Center Office of the Chief of EngineersAberdeen Proving Ground Washington, DC 20314-1000Maryland