Using Ground-Penetrating Radar to Detect Layersof Discontinuous Dielectric Constant

K.-J. S. Kung* and Z.-B. Lu

ABSTRACTMany sandy soil materials consist of layers with different textures.

Water in an unsaturated sandy soil can be tunneled and start to flowexpeditiously and preferentially as it encounters inclined coarse sandlenses. Along these funnel-type preferential-flow pathways, soil mois-ture is close to saturation, while the coarse sand lenses underneaththe pathways are very dry. To nondestructively detect the existenceof a coarse sand lenses is important in order to predict and estimatethe influence of funnel-type preferential flow. The objective of thisstudy was to demonstrate that ground-penetrating radar (GPR) candetect layers with a sharp discontinuity in dielectric constant fromtheir surroundings. A GPR with a 450-MHz antenna was towed onthe surface of a diving pool to detect a 2.4-m-long polyvinyl chloride(PVC) plate submerged in the pool at different depths and inclina-tions. The scenario would be similar to detecting a dry coarse layerembedded in a uniform saturated sandy soil. Results showed that GPRcould accurately detect the depth, dimensions, and inclination of theplate from the 25- to ISO-cm depth. Because sandy soils generallyhave an electrical conductivity of 10 to 1000 times less than the waterin the pool, we expect that a 450-MHz GPR has the potential to ac-curately detect depth, dimensions,- and inclination of an embeddedcoarse sand layer up to the 3-m depth.

MANY sandy soil materials deposited by rivers,lakes, glaciers, oceans, or wind consist of strata

laid down at different times and under different cir-cumstances. The different strata have different poros-ities and hydraulic conductivities. The way soil islayered markedly affects how water and contaminantsmove to groundwater. Recently, funnel flow was ob-served in field experiments conducted in Plainfieldsands in central Wisconsin (Kung, 1990) and laterconfirmed in laboratory experiments (Kung, 1993),which demonstrated that water in an unsaturated sandysoil profile can be funneled and start to flow expedi-tiously and preferentially as it encounters inclined coarsesand lenses with abrupt textural discontinuity. The in-clined coarse sand lens functions as an impermeablelayer to unsaturated flow. To accurately predict thefate of a contaminant and determine the susceptibilityof groundwater to contamination under a given land-use strategy in sandy soils, it is essential to developtechniques to nondestructively ascertain the mor-phometry of soil horizon and geologic strata in orderto disclose the existence and locations of the prefer-ential paths. A field experiment by Kung and Dono-hue (1991) showed the potential of GPR to map thelocation and inclination of soil layers in a sandy va-dose zone.

The GPR technique is similar in principle to reflec-tion seismic and sonar techniques. The GPR is a broadband width impulse radar with transmitting antennathat sends a pulse of electromagnetic wave (with a

K.-J.S. Kung, Dep. of Soil Science, and Z.-B. Lu, Dep. of Ag-ricultural Engineering, Univ. of Wisconsin, Madison, WI 53706.Received 2 Oct. 1991. * Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:335-340 (1993).

frequency ranging from 10 to 1000 MHz) into the soil.Abrupt changes in electrical properties cause part ofthe transmitted signal to be reflected. The reflectedsignal is detected by the receiving antenna and stored(Morey, 1974). A detailed description of fundamentalGPR principles can be found in Ulriksen (1982). Thepenetration depth of the radar signal depends on thedielectric properties of the ground, controlled largelyby the soil electrical conductivity and soil water con-tent.

The GPR technique has been initially used in geo-physics and engineering. Conventionally, lower fre-quency GPR antenna (i.e., 80 to 200 MHz) were usedto detect deep and large geological boundaries (i.e.,boundaries and fractures in bedrock or glacier,groundwater tables, sedimentation strata) (Morey, 1974;Ulriksen, 1982). More recently, GPR has been usedto map soil horizons (Shih and Doolittle, 1984; Collinet al., 1989), or embedded man-made structures suchas barrels (Davis and Annan, 1989), and in detectinghydrocarbon and organic compounds in sandy soils(Olhoeft, 1986). It offers a way of viewing a medianondestructively.

Along funnel-type preferential-flow pathways, soilmoisture is close to saturation, while the coarse sandlenses underneath the pathways are very dry (Kung,1993). This contrast in soil moisture content, and hencedielectric constant, makes GPR an ideal instrument todetect the embedded coarse sand lenses that could trig-ger preferential paths in sandy soil profiles. Never-theless, the accuracy of the GPR technique fordelineating depth, dimensions, and inclination of soillenses has not been rigorously tested. The objectiveof this study was to test how accurately a GPR withhigher frequency antenna (450 MHz) can detect flatand inclined layers with sharp discontinuity in dielec-tric constant from their surroundings. Ideally, labo-ratory experiments should have been conducted in alarge sand tank where a coarse sand layer was packedin uniform fine sandy soil at a specific depth and in-clination. To detect a 2.4 by 2.4 m layer embedded1.5 m deep requires a 5-m long by 5-m wide by 2-mdeep sand tank to minimize noise caused by the walleffect. Our preliminary tests showed that it is difficultto achieve a uniform and repeatable packing schemein a tank of such size. Microlayers caused by non-uniform packing made the GPR image very noisy. Inaddition, it would be very time consuming and costlyto repack a sand tank of such size. Therefore, an al-ternative approach was tested using the diving pool atthe University of Wisconsin-Madison as a surrogatefor the sand tank.

MATERIALS AND METHODSLaboratory experiments were conducted to test the accuracy

of GPR in detecting a submerged PVC plate at different depths

Abbreviations: GPR, ground-penetrating radar; PVC, polyvinylchloride.

335

336 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

Table 1. Dielectric constant and electrical conductivity of thepool water, polyvinyl chloride (PVC) plate, and dry andsaturated sand.

Material

Pool waterPVC plateDry, coarse sandSaturated fine sand

Dielectricconstant

8133

30

Electricalconductivity

mS m-1

45<io-5io-4-o.i0.1-10

and inclinations. A 2.4-m long by 1.2-m wide by 1-cm thickPVC plate was lowered into a 15-m long by 12-m wide by3.8-m deep diving pool to different depths and inclinations.The plate was hung by four nylon ropes so that its depth andinclination could be easily and exactly controlled. After eachdesired plate configuration was achieved, a 450-MHz GPRantenna, contained in a styrofoam floating device, was towedacross the center of the plate.

A GPR with a 450-MHz antenna (pulse EKKO IV, Sensorsand Software, Mississauga, ON, Canada)1 was tested. Thetransmitting and receiving antenna were separated by 12.7 cm.This GPR unit was powered by a 12-V battery and controlledby a computer. The raw, digital signals reflected from the platewere stored directly onto a hard disk. This GPR unit offers astacking option (i.e., collecting 2" traces at the same locationwhere n is a positive integer) to average out background noiseand operates under a step mode (i.e., collecting data only atdesignated locations) to ensure a constant distance betweentraces. In this study, the stacking was set at 128 and the dis-tance between two adjacent traces was 10 cm.

The dielectric constant and the electrical conductivity of thewater in the diving pool and the PVC plate are given in Table1. In a medium with relatively low electrical conductivity, thereflection coefficient, r, of a one-dimensional normal incidentis (Ramo et al., 1984):

r = Ve~2) [1]

as the wave travels from the first medium, with a dielectricconstant, of e,, into the second medium with e2. Using thisformulation as an approximation (the wave may still be spher-

1 Mention of product or trade names does not constitute endorse-ment by the Univ. of Wisconsin-Madison to the exclusion of oth-

ical before it reaches the far field), the reflection coefficientfrom a submerged PVC plate is 67.7% and that of an embeddedcoarse sand lens in 51.9%. In other words, to detect a sub-merged PVC plate in the pool experiment was similar to map-ping a dry, coarse layer embedded in a saturated, fine sand.

The GPR was first tested to determine if it could accuratelymeasure the depth and boundary of a flat plate. The PVC platewas lowered into the pool to a 30-cm depth and the antennawas towed over the plate center along the long side. The two-way travel time of a reflected signal was recorded in nanose-conds, as was the number of the trace when the antenna crossedthe edge of the plate. The antenna was towed across the platethree times. The experiment was repeated with the plate low-ered at 10-cm intervals to 100 cm.

In the second experiment, the accuracy of GPR to measurethe depth, dimension, and inclination of an inclined PVC platewas evaluated. Two ends on the long side of the plate werelowered to different depths. The exact depths at 11 configu-rations with inclination ranging from 5 to 18° were measured(Table 1). The same procedure mentioned above were re-peated. Based on the wave propagation velocity calculated bylinear regression analysis in the first experiment, the two-waytravel times of the second experiment were converted to depthsand compared with the measured depths.

In the third experiment, the effect of electrical conductivityof the water on the performance of the GPR was evaluated.The first experiments were repeated while the electrical con-ductivity of water in the pool was increased to 0.134 S m-1.In this case, the flat plate was lowered only to 80 cm becausethe GPR images were too noisy to detect the plate at greaterdepth. The depths and angles of the three configurations of theinclined plates are shown in Table 2.

RESULTS AND DISCUSSIONFigure 1 shows three GPR wiggle traces of the flat

plate at different depths. The x axis on top was the tracenumber, and the y axis represented the two-way traveltime of the reflected signal in nanoseconds. These rawimages were taken when the antenna was towed fromthe center toward an edge of the plate. The only en-hancement made to these images was the gain. The gainwas increased to enhance the clarity of the reflected sig-nals. The gain factor used to enhance each image isshown in Fig. 1.

The same software package also offered a program toconvert the raw digital GPR image into an ASCII data

Table 2. Measured and calculated depth and angle of the inclined plate at 11 different configurations collected in water.

Test

Measuredplate depth

Shallow DeepMeasured

plateangle

Two-waytravel time

Calculatedplate depth

Shallow Deep Shallow DeepCalculatedangle ofthe plate

-cmElectrical conductivity of 0.045 S m~'

123456789

1011

83.039.5

121.035.065.557.033.529.540.026.026.0

105.565.0

152.069.5

100.599.581.083.0

101.091.5

102.0

5.36.07.38.18.3

10.011.212.614.515.618.2

42.522.760.022.035.030.021.019.524.017.717.7

Electrical conductivity123

39.530.035.0

70.570.085.0

7.39.4

11.8

23.520.522.5

52.034.572.537.050.547.742.042.550.046.050.5

of 0.134 Sm-1

37.536.5

—

83.136.8

124.135.165.853.932.829.339.825.125.1

39.131.836.7

105.464.4

153.470.3

101.995.381.983.1

100.791.3

101.9

72.970.5nat

5.26.56.98.38.59.8

11.612.814.515.718.3

7.99.1na

t Reflected signal was too weak to measure.

RUNG & LU: DETECTING LAYERS OF DISCONTINUOUS DIELECTRIC CONSTANT 337

DEPTH = 30 CMGAIN = 3

DEPTH = 60 CMGAIN = 3

DEPTH = 90 CMGAIN = 3

45 ^Fig. 1. Typical ground-penetrating radar (GPR) images of flat

plates at different depths.

11015 20 25 30 35 40

100

o 90

•£ 80a(Ua 70

60;cu

50

<aI 30

45 50

•-•-•-• Data from the 3rd exp.Y = 2.4159 X - 17.6931R = 0.991EC = 134 mS m

•iii-Data from the 1st exp.Y = 2.3414 X - 16.3636R2 = 0.998EC = 45 mS m

110

100

90

80

} 70

!• 60

50

40

I 30

to2020 25 30 35 40 45

Two—way Travel Time (ns)Fig. 2. Linear regression of the two-way travel time vs. measured

depths of the flat plate.

file. The two-way travel time of the reflected signal ineach trace could be determined exactly through the AS-CII file. The two-way travel times could be determinedexactly through the ASCII file. The two-way travel timesof the flat plates were found to be linearly related to themeasured depths with R2 = 0.998 (solid lines in Fig.2), suggesting that the GPR could accurately detect thedepth of a layer after its was calibrated. It was also foundthat the GPR could not detect the plate as soon as it leftthe edge of the plate. The trace at the edge of the platewas marked with an arrow in each image, indicating thatthe GPR had a very small view angle in water and couldmeasure the dimension of the 2.4-m horizontal plate ac-curately.

It should be noted that, in the pool, the media betweenthe GPR and the plate was homogenous. In soils, therecould often be several horizons or features between thesoil surface and the first coarse sand layer. The speed ofan electromagnetic wave in a medium is inversely pro-portional to the square root of the dielectric constant ofthe media. Soil horizons of different textures generallyhave different moisture contents and dielectric constants.Thus, the wave travel velocity in field soil is not a con-stant and the depths to reflected layers could not directlyconvert from the two-way travel time. Therefore, it isimportant to first calibrate the GPR at several depths ina field.

The thickness of the plate could not be determinedfrom the GPR image. The true thickness of the PVCplate was only 1 cm, while the GPR image showed atriple-band with a two-way travel time of about 10 ns,indicating that images from two adjacent soil layers couldbe superimposed and indistinguishable. Multiple reflec-tions appeared on GPR images when the plate was 30,40, and 50 cm deep (no similar problem at greater depths).In the pool, the amplitude of the second reflection fromthe plate at 40 cm was approximately 40% of that fromthe plate at 80 cm. This indicates that layers near thesoil surface (in the radiation near or intermediate fields)could generate a "ghost" image of soil layers in farfield. In seismic research, software packages have beendeveloped to reduce this type of noise.

Figure 3 shows four raw GPR images of the inclined

PVC plates at different depths and inclinations from thesecond experiment. Because more energy was reflectedaway, the images of inclined plates appeared weakerthan those from flat plates and required more gain. How-ever, larger gain amplified unwanted background noisesand, hence, images in Fig. 3 are much noisier than thosein Fig. 1. Moreover, the background noises could befiltered out (Fig. 4). To reduce noises in these four cases,differentiating across adjacent traces and averaging downseven sample points along each trace were used.

All four images showed that inclined plates could beclearly detected by the GPR. The traces at the edges ofeach inclined plate were again marked with arrows. Asshown in Table 1, the depth of an inclined plate couldbe accurately calculated from the linear regression, Y =2.3414X — 16.3636, obtained from the first experiment.However, the edges of the inclined plates could not beas accurately delineated in the GPR images as those ofhorizontal plates. This is because more energy from aside lobe could be reflected by an inclined plate.

In the third experiment, where the water had a higherelectrical conductivity (i.e., 134 mS m-1), the relation-ship between measured depths and the two-way reflec-tion times was linear for flat plates with an R2 = 0.991(dashed line in Fig. 2). Once again, the depth and in-clination of an inclined plate could be calculated fromthe two-way travel time (Table 2) and the boundary onthe shallower end of the inclined layer could be detectedby GPR. However, the reflected signal from the deeperend of the inclined plate at 80 cm had similar amplitudeto ambient noises. This effect occurred because the skindepth 8 (or penetrating depth) of a wave in a conductoris inversely proportional to the square root of its electri-cal conductivity. A medium with larger electrical con-ductivity has a smaller skin depth. As an electromagneticwave penetrates into a conductor to a depth, z, its mag-nitude is exponentially damped out by z/S (Ramo et al.,1984). Therefore, an electromagnetic wave is quicklydamped out as it penetrates into a silty or clayey soil oflarge electrical conductivity.

In dry, coarse sand, electrical conductivity typicallyranges from 10~4 to 0.1 mS m-1 (Morey, 1974). There-fore, it was expected that the GPR could detect a layer

338 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993

DEPTH = 57/99.5 CMGAIN = 8

ca LO

DEPTH = 40/101 CMGAIN = 15

DEPTH = 26/102 CMGAIN = 15

DEPTH = 121/152 CMGAIN = 40

ground-penetrating radar (GPR) images of inclined plates at different depths and angles.

deeper than 150 cm because the skin depth in sandy soilsis more than twice that of the water in the first experi-ments. The GPR unit used in these experiments clearlydetected inclined soil layers at a depth of 3.2 m in asatiated sandy profile at the Bordan Experimental Site inOntario, Canada (Kung and Annan, 1991, unpublisheddata). Conversely, as wet, clayey soils generally haveelectrical conductivities ranging from 10-1 to 1000 mSm"1 (Morey, 1974), images from the 450-MHz antennaare restricted to depth < 80 cm. For soils of high elec-trical conductivity, a lower frequency antenna may beuseful because the skin depth is inversely proportionalto the square root of the frequency.

The fact that the GPR could delineate the edge of thehorizontal submerged PVC plate is because the GPR'sbeam angle in water is very small (beam angle can bethought of as how wide a GPR can see). Assuming SnelPslaw could be applied to estimate reflection in the nearfield of radiation for the first-order approximation, thebeam angle would change from Bl to 02 as the wave

travels from a medium with a dielectric constant el intoanother medium with e2, as follows:

sin(e1)/sm(e2) = [2]

In air, the half beam angle and dielectric constant are90° and 1, respectively. The dielectric constant of wateris 81; therefore, the half beam angle in water is only6.4°. Because water has the largest dielectric constant,the GPR images collected in the diving pool representedthe best-case scenario in testing whether the resolutionof GPR is enough to delineate the boundary of a layer.In sandy soils in the field where the dielectric constantranges from 3 to 30, the beam angle will be wider. Witha larger beam angle, a wider area is scanned and hencethe edges could not be sharply delineated.

We tested the worst-case scenario by examining howwell the GPR would see plates in air. Two 2.4-m-wideplywood plates were hung at 3 and 4 m from the surface,where the suspended antenna was towed laterally in air

DEPTH = 57/99.5 CMGAIN = 8

DEPTH = 40/101 CMGAIN = 15

„. is in s in

DEPTH = 26/102 CMGAIN = 15

DEPTH = 121/152 CMGAIN = 40

05

101520

3540

45

50Fig. 4. Filtered ground-penetrating radar (GPR) images of inclined plates at different depths and angles.

in si in s inCD CD K K

KUNG & LU: DETECTING LAYERS OF DISCONTINUOUS DIELECTRIC CONSTANT 339

Fig. 5. Ground-penetrating radar (GPR) images of two plywoodplates in air at 3 and 4 m from the GPRs towing surface:(A) both plates parallel to the towing surface; and (B) thefirst plate on a 15 ° inclination.

at 1.6 m above the ground. In this case, the SIR-8 GPRsystem (Geophysical Survey System, Hudson, NH) wasused because the EKKO IV system emits less power andcould not clearly detect the plywood layers, whose di-electric constant is 2 and reflection coefficient only 17.2%in air. Data in Fig. 5A clearly indicate that the GPRdetected the plates before reaching and after passing theedges of the plates. The plate center, its two edges, and1 m from either edge were marked with vertical lines.The GPR image showed that the 2.4-m plate hung at 4m had a width of approximately 4 m and when hung at3 m had a width >3 m. Moreover, when the first platewas inclined to 15° while its center was maintained at 3m from the towing surface, the GPR image of the firstplate was significantly distorted (Fig. 5B). Not only thewidth of the first plate increased to approximately 3.3m, but the whole first plate was shifted to the left byabout 0.5 m. In seismic literature, this phenomenon iscalled migration.

This worst-case scenario probably would never occurin soils because the beam angle in sandy soils is muchsmaller than that in air. In very wet, sandy soil, thedielectric constant ranges from 20 to 30 and the halfbeam angle is 10.5 to 13°. In very dry, sandy soil, the

dielectric constant ranges from 3 to 6 and the half beamangle increases to 24 to 35°, but still significantly smallerthan that in air.

When water is applied with a certain critical rate in asandy soil profile with embedded coarse sand lenses, theoverlying finer sand above the coarse sand lens becomesessentially saturated while the coarse sand lenses remainvery dry (Kung, 1993; Ju and Kung, 1993). This sug-gests that, by appropriately wetting the soil profile beforescanning, errors in the GPR image because of large beamangle could be minimized. This moisture manipulationcould also increase the reflection coefficient, r, becausethe €i in Eq. [1] could be increased while the e2 is main-tained small. This would help to enhance the clarity ofthe coarse sand lenses in the GPR image.

CONCLUSIONSResults from laboratory experiments conducted in a

diving pool showed that a GPR with 450-MHz antennacould detect the depth, dimensions, and inclination of a2.4-m-wide PVC plate at depths of 30 to 150 cm. Be-cause electromagnetic waves would be quickly dampedout as they penetrated into a medium of high electricalconductivity, the 450-MHz antenna is only suited for usein sandy soils. In order to minimize the error in the GPRimages, a sandy soil profile of low ionic content shouldfirst be carefully wetted to minimize the GPR beam angleand maximize the reflection coefficient from the coarsesand lens.

ACKNOWLEDGMENTSThe authors thank Mr. John Paine, director of the recrea-

tional facilities at the Univ. of Wisconsin-Madison, for per-mitting us to use the U.W. diving pool. We also thank Dr. F.Madison of the Wisconsin Geological and Natural History Sur-vey for providing the GSSI SIR-8 GPR equipment. Researchwas supported by USDA/CSRS Grant no. 9000452, USGSGrant no. 14-08-0001-G1921, and Univ. of Wisconsin WaterResources Center Grant no. DINH. The views and conclu-sions contained in this document are those of the authors andshould not be interpreted as necessarily representing the offi-cial policies, either expressed or implied, of the U.S. govern-ment.

340 SOIL SCI. SOC. AM. J., VOL. 57, MARCH-APRIL 1993