DIVISION S-10- WETLAND SOILS

Using Ground-Penetrating Radar to Delineate Subsurface Featuresalong a Wetland Catena

D. R. Lapen,* B. J. Moorman, and J. S. Price

ABSTRACTContinuous descriptions of the spatial variability of subsurface

materials are desirable in many hydrogeological studies. However,traditional point-measurement techniques, such as soil coring and pitexcavation, are destructive and provide an incomplete characterizationof the subsurface. In this study, ground-penetrating radar (GPR) wasused to continuously and nondestructively map shallow subsurfacefeatures along a small wetland catena in southeastern Newfoundland.Detailed profiles of soil dielectric constant and common midpointvelocity surveys were used to determine radar pulse velocities throughsubsurface features. Major reflectors identified in the study included:(i) organic soil-mineral soil contact, (ii) placic horizons (saturatedmineral soil-unsaturated mineral soil contacts), (Hi) water tables (un-saturated mineral soil-saturated mineral soil contact), and (iv) mineralsoil-bedrock contact. Thicknesses of major soil features were estimatedfrom radar profiles and compared with thicknesses determined fromsoil core-auger data. The relationship between estimated and observedthicknesses is strong (r = 0.99). Spatial relationships between placichorizons and wetland community types were also identified.

BLANKET BOGS are peat deposits that occur on nearlylevel to gently sloping areas in cool, moist climates

where yearly evapotranspiration is less than precipitation(Tansley, 1949). However, blanket bogs (Sphag-nofibrists) on the southeastern tip of Newfoundland donot fully occupy the land surface. Well-drained heathlands, isolated patches of tuckamoor (thickets of balsamfir [Abies balsamea (L.) Mill.] generally <1 m high),and small lakes and ponds occupy the areas not coveredby bog. Unlike the other major community types in theregion, blanket bogs require surface saturation to develop(Wells and Pollett, 1983). This hydrological conditionpromotes the slow decay of organic matter, which helps tokeep accumulation rates higher than decay rates (Crum,1988). In areas with suitably moist climates, surfacesaturation is largely governed by slope morphology,relative topographic position, and the hydraulic proper-ties of subsurface materials (Freeze and Cherry, 1979).In Newfoundland, surface water conditions requisite forpeat development are normally found in wet depressionsand above soils with restricting layers, such as placichorizons, that impede the vertical flow of water (Wells,1981; Wells and Pollett, 1983; Davis, 1984). Hydrogeo-logical controls have been found to play a predominantrole in the development and distribution of peat lands

D.R. Lapen, Dep. of Geography, Queen's Univ., Kingston, ON, CanadaK7L 3N6; B.J. Moorman, Ottawa-Carleton Geosciences Centre, Dep. ofEarth Sciences, Carleton Univ., Ottawa, ON, Canada, K1S 5B6; J.S.Price, Dep. of Geography, Univ. of Waterloo, Waterloo, ON, CanadaN2L 3G1. Received 13 Mar. 1995. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 60:923-931 (1996).

in areas where atmospheric water inputs are not limited(Ugolini and Mann, 1979; Taylor and Smith, 1972).

In many hydrogeological investigations it is desirableto characterize the spatial variability of subsurface fea-tures. However, traditional point-measurement tech-niques, such as soil coring and pit excavation, usuallyprovide an incomplete description of the subsurface,especially where access is limited. In addition, thesemethods can be time consuming, highly destructive, andexpensive.

Ground-penetrating radar is an alternative method formapping shallow subsurface features. The GPR is rela-tively inexpensive to use, nondestructive, fast, and pro-vides spatially continuous data (Davis and Annan, 1989).A transmitter on the GPR emits short pulses of high-frequency (10-1000 MHz) electromagnetic energy intothe ground. A receiver picks up energy reflected off ofsubsurface features with electrical properties differentfrom those of the overlying or surrounding medium.Knowing the time it takes for a radar pulse to travelfrom the transmitter to the reflector and back to thereceiver, and the propagation velocity of the pulsethrough the material, it is possible to determine thedepth of a subsurface reflector (Davis and Annan, 1989).Ground-penetrating radar has been used successfully tomap peat deposits (Bjelm, 1980;Chernetsovetal., 1988;Hanninen, 1992; Theimer etal., 1994), soil stratigraphy(Olson and Doolittle, 1985; Hubbard et al., 1990;Mokmaetal., 1990; Moorman, 1990), bedrock topogra-phy (Collins et al., 1989; Davis and Annan, 1989), andthe water table (Sellman et al., 1983; Shih et al., 1986).

The primary objectives of this investigation were to:(i) use GPR to continuously delineate, in two-dimensionalcross section, subsurface features along a wetland catena,and (ii) identify spatial associations between subsurfacefeatures and community type along the catena.

MATERIALS AND METHODSStudy Area

The study site is located at Cape Race, Newfoundland (Fig.1). The climate is characterized by cool summers, frequent fog,

Abbreviations: CMP, common midpoint; C, propagation velocity in freespace; D, reflection depth and/or thickness; A>, bulk density; EC, electricalconductivity; GPR, ground-penetrating radar; LCTZ, lowland communitytransition zone; k, regression constant; K, apparent dielectric constant;K\, K of overlying material; KI, K of underlying material; Kw, K of water;ATP, AT of dry peat; K^i, saturated hydraulic conductivity; n, total porosity;PC, peat core; PVC, polyvinyl chloride; R, reflection coefficient; S,relative saturation; SC, soil core; SP, soil pit; /, pulse travel time; 9,volumetric soil water content; UCTZ, upland community transition zone;V, propagation velocity.

923

924 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

GPR Transect

* * | Blanket Bog

Heath

Tuckamoor

L'.'.'l Community Transition

-"I Pools/Ponds

Fig. 1. Study site. Location is approximately 1 km north of the Atlantic Ocean.

and high precipitation (Banfield, 1983). The average annualprecipitation at Cape Race is 1379 mm, of which 12% fallsas snow (Environment Canada, 1982). Topography is gentlyrolling, with most slope gradients below 7 %. Geological forma-tions are Hadrynian in age and consist mainly of slate, grey-wacke, argillite, and siltstone (Heringa, 1981). A sandy tosilty-sand glacial till veneer (generally <1.5 m thick) drapes thebedrock surface. Much of the till has undergone pedogenesis.Sphagnofibrists, Placocryods, Cryaquods, and Humicryods arecommon in the area (Heringa, 1981).

The two major vegetation community types in the regionare blanket bog and heath (Wells and Pollett, 1983). Typically,

mature blanket bogs are dominated by a surface cover ofsphagnum moss [Sphagnum fuscum (Schimp.) Klinggr.] andare generally <2.0 m deep (Wells, 1981). Bog peat is composedprimarily of decomposed vegetation that originated from:mosses (Sphagnum spp.), sedges (Carex spp.), deer grass(Scirpus cespitosus L.), cotton grass (Eriophorum spp.), sweetgale (Myrica gale L.), Labrador tea (Ledum groenlandicumOeder), laurel (Kalmia spp.), leatherleaf (Chamaedaphne caly-culata [L.] Moench.), and black crowberry (Empetrum nigrumL.). Heath land vegetation, which includes sweet gale, Labra-dor tea, laurel, lowbush blueberry (Vaccinium angustifoliumAit.), black crowberry, and lichens (Cladonia and Cladina

45 _

9 4 0 -d

u 35 _

Figure 3

}:•:•:•:] ORGANIC SOIL

Figure 5

UPLAND BDG UCTZ HEATH LCTZ LOWLAND BOG

T~40

—I"80 120 160 200 240

—1——T280 320 360 400 440 480

Distance (n)Fig. 2. Profile of ground-penetrating radar transect.

LAPEN ET AL.: DELINEATING SUBSURFACE FEATURES ALONG A WETLAND CATENA 925

spp.), covers mainly well-drained Humicryods (Meades,1983).

GPR TransectA 500-m GPR profiling transect (Fig. 2) was run through

the upland bog, upland community transition zone (UCTZ),heath land, lowland community transition zone (LCTZ), andthe lowland bog (Lapen, 1994). Water tables were approxi-mately 0.1 to 0.15 m below the bog and community transitionzone surfaces at the time of the survey (June 1993). The soilsat the upland and lowland bog sites can be characterized asSphagnofibrists overlying Cryaquods with placic horizons. TheUCTZ consists of about 0.1 to 0.4 m of peat and muckoverlying a Placocryod. The heath land soils are primarilyHumicryods that were unsaturated during the GPR survey.The LCTZ is composed of about 0.1 to 0.2 m of peat andmuck overlying Cryaquods without placic horizons.

Physical PropertiesCores of the mineral soil underlying the bogs were acquired

by a 7.6-cm-diam. soil corer and split-spoon sampler installedon a tractor-mounted drill rig. Gravel and cobbles encounteredduring sampling, and problems associated with the tractorsinking into the saturated organic soils, restricted the numberand location of sampling sites. Dry-weight bulk density (A,),volumetric soil water content (6), total porosity (n), and relativesaturation (S = Q/ri) were determined from the cores(McKeague, 1978). A Micromeritics 5100 Sedigraph particle-size analyzer (Micromeritics, Norcross, GA) was used to deter-mine the silt (<50.0 and >2.0 um) and clay fraction (<2.0urn) of the mineral soil samples. A mini-McCauley samplerwas used to acquire peat cores from the bogs. Dry-weightbulk density, 0, n, and S were determined on 5-cm sectionsof the peat cores. The electrical conductivity (EC) of the bogpore water was estimated in the field using an Orion Model140 conductivity/salinity meter (Orion Research Inc., Boston,MA). Water table elevations along the transect were determinedfrom groundwater wells made of PVC pipe. Hydraulic heads,in the mineral soils underlying the bogs, were ascertained frompiezometers (1.5-cm i.d. PVC pipe) with 10-cm screenedintakes. The saturated hydraulic conductivity (Kxl) at selecteddepths in the peat profile were determined from "bail" tests(Freeze and Cherry, 1979) on piezometers (1.5-cm i.d.~PVCpipe) with 6-cm screened intakes.

Radar SystemThe PulseEKKO IV GPR (Sensors and Software Inc., Mis-

sissauga, ON) was used for this study. It consisted of two 100MHz antennas, a 400-V transmitter, a receiver with built-insignal digitizer, and a control unit that produced the timingsignals and processed the data. A laptop computer linked tothe control unit was used to enter profile parameters andstore data. The system was portable and easily operated by atwo-person team. Radar traces were made every 1.0 m alongthe transect (0-500 m) using an antenna spacing of 1.0 m.

Common midpoint velocity surveys were carried out todetermine bulk propagation velocities (V) through subsurfacematerials. These surveys involved the collection of traces withincreasing antenna spacing while keeping the midpoint positionbetween the antennas constant. Annan and Davis (1976) andArcone (1984) provided detailed discussions of the CMP surveytechnique and limitations in layered ground. Three CMP sur-veys were conducted along the transect: upland bog (0-m

mark), heath land (200-m mark), and the lowland bog (400-mmark) (Fig. 2). Reflection depths and thicknesses betweenreflections (D) were estimated by

[1]

where / = pulse travel time.The radar profiles had gain and topographic correction ap-

plied to them. Gain was used to enhance the strength andspatial pattern of major reflections. Surface elevations, acquiredby stadia measurements, were used to topographically correctthe radar profiles.

Dielectric ConstantThe dielectric constant (K) and EC of a medium govern

radar propagation velocities through that medium (Davis andAnnan, 1989). When EC is small, however, it can be ignoredbecause K overwhelmingly controls V (Shih and Doolittle,1984). The simplified relationship between K and V can beexpressed as:

V = -£• [2]l-wf L J

where C = propagation velocity in free space (3 x 108 m

Knowledge of variation in the dielectric properties of soilscan also be used to help confirm interpretations of GPR reflec-tions. The larger the dielectric gradient between two adjacentsubsurface materials, the stronger will be the reflected radarsignal (Sellman et al., 1983). The reflection coefficient (K)expresses this gradient and has the form

D _ [3]

where KI = dielectric constant of the overlying material, andK2 = dielectric constant of the underlying material.

For the split-spoon mineral soil cores, K was calculatedfrom the following empirical relationship (Topp et al., 1980):

K = 3.03 + 9.30 + 146.062 - 76.763 [4]The K of organic soil was determined using an empirical

equation developed by D. Halliwell (1992, personal communi-cation) for moss (Sphagnum spp.) and sedge (Carex spp.) peatfrom the Hudson Bay area; it is expressed as

K = [Kp + Q(Kkw - 1)]"* [5]

where Kv = dielectric constant of water, Kp = dielectricconstant of dry peat, k = regression constant.

RESULTS AND DISCUSSIONRadar Image Interpretation

Two sections of the GPR transect were interpreted:the upland bog-UCTZ and the lowland bog-LCTZ. Theinterpretation process involved: (i) identifying major sub-surface features from ground truth observations, (ii)identifying major reflectors from the radar images, (iii)using detailed vertical profiles of K, at selected locationsalong the transect, to estimate V through subsurfacefeatures (Eq. [2]) and to assess velocities measured by

926 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

Distance (m)0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

b) 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

•I—•

Q

0 .

0.5

1.0

'1.5.

2.5

3.0

3.5

HEATH

"j'-yK^-^y-^f^i^0^ Horizon1^;

: _ - _ - _ - _ : : _ - _ - _ : : _ - _ : : _ BEDROCK:

Fig. 3. (a) Radar profile of upland bog and upland community transition zone (UCTZ) and (b) interpretation of radar profile with adjusteddepth scale. Reflections: A = soil surface, B and C = possible water content interfaces in peat profile, D = organic soil-mineral soil contact,E = placic horizon (saturated mineral soil-unsaturated mineral soil contact), F = water table (unsaturated mineral soil-saturated mineralsoil contact), G = mineral soil-bedrock contact.

CMP surveys, (iv) using Eq. [1] to estimate reflectiondepths and thicknesses between them; (v) comparingestimated and observed depths/thicknesses of subsurfacefeatures.

Time scales are shown on the radar profiles. Sincesingle depth scales could not be applied to the entireprofile, the interpreted sections were constructed so thatdielectrically distinctive subsurface features could beillustrated with a single and linear depth scale. Radarprofiles of the heath land soils are not presented in thisstudy because many of the reflections of interest aretypically <40 cm below the surface and were producedby soil horizons. Quantitative assessments of reflectionsat these very shallow depths could not be confidentlymade given the GPR system configuration used in thisstudy (Annan, 1992).

Upland Bog and Upland CommunityTransition Zone

Peat cores were acquired at the 1- and 75-m markson the GPR transect and mineral soil cores underlying

the bog were obtained at the 75-m mark (Fig. 3). Physicaland electrical properties of the soil cores are presentedin Fig. 4. The EC of the peat pore water at the 1-mmark was 83 nS cm"1 near the peat surface and 128 nScm"1 at the base of the peat profile. Given these EC valuesand a 100 MHz antenna frequency, Eq. [2] estimates ofV are valid (Davis and Annan, 1989).

The surface of the upland bog is convex in profileand slopes toward the 110-m mark. The UCTZ andunsaturated heath land extend from 110 m to the 144-mmark and from 144 m to the 322-m mark, respectively(Fig. 2). The average propagation velocity through thepeat at the 1-m mark was estimated, using Eq. [2], tobe 0.04 m ns"1. This relatively slow propagation velocityis due to the high water content of the peat; averagevolumetric water content of the peat at the 1-m markwas 0.80 m3 m"3.

At the time of the GPR survey, the water table inthe peat was approximately 0.1 m below the surface.Reflections from the water table could not be interpretedat this shallow depth. The sources of the linear reflections

0.3

LAPEN ET AL.: DELINEATING SUBSURFACE FEATURES ALONG A WETLAND CATENA

K

927

Db (g cm"3) S (cm3 cm 3 )1 0

R V (m ns )80 -0.4 0.0 0.4 0.00 0.08

1.13

Db (g cm ) Grain size (%) S (cm cm" )2 0 100 0

K R V (m ns )1 0 50 -0.4 0.0 0.4 0.0 0.2

Sand

\r: Silt

Clay

Fig. 4. Profiles of physical and electrical properties: (a) peat profile at 1-m mark and (b) mineral soil profile under bog at 75-m mark. Letterson depth scale denote reflections (see Fig. 3). Clear = peat; hatching = bioturbated and amorphous sands, gravels, and cobbles; diagonalstripes = amorphous to weakly platy sands and gravels; dark horizontal stripe = placic horizon; stipple = weakly platy sands and silty sands;dashes = bedrock.

in the peat (B and C) are unclear. They may have beendue to abrupt changes in peat water content not detectedby peat core analysis (cores taken a few days aftersurvey), or multiples of the surface reflection. ReflectionB was estimated to be approximately 0.6 m below thesurface at the 1-m mark. Rainwater a few days prior tothe survey may have caused the permeable surface peatand mosses to expand with water. This would increasethe dielectric contrast between the surface peat (<0.60m below the surface) and the less permeable peat belowit. This reasoning is supported by the fact that the K^iof the peat at 0.70 to 0.65 m (Table 1) is generally smallerthan that of the peat above it. Hydraulic characteristics ofpeat are largely governed by the degree of peat decompo-sition and its botanical constituent (Crum, 1988). Theseproperties can be spatially variable in the profile of many

Table 1. Ranges of saturated hydraulic conductivity (K^t) for peatin the upland and lowland bog.

Hydraulic conductivityDepthm0.25-0.300.45-0.500.65-0.70

Upland bog

10-MO-'

Lowland bog

io-2-io-J

blanket bogs in southeast Newfoundland (Northland As-sociates Ltd., 1989).

Observed and estimated depths of the organic soil-mineral soil contact (D) are essentially the same (Table2). The saturated water content of the mineral soil imme-diately underlying the peat at the 75-m mark was foundto be rather high for sandy soils (0.54 m3 m~3). Fieldobservations indicated that plant root bioturbation in-creased porosity by loosening the soil. As a result, the

Table 2. Comparison of observed and estimated thicknesses ofsoil zones for the upland and lowland bog sites. Estimateddepths were determined using Eq. [2] (see Fig. 3 and 5 forreflection label identification). Pearson's correlation betweenobserved and estimated thicknesses is 0.99.

Site

Upland bog

Lowland bog

Reflectionlocation on

transect

A to D (PCl-m mark)A to D (PC 75-m mark)D to E (SC 75-m mark)E to F (SC 75-m mark)F to G (SC 75-m mark)A to D (PC 395-m mark)A to D (PC 351-m mark)D to E (SC 351-m mark)E to F (SC 351-m mark)F to G (SC 351-m mark)

ThicknessObserved

1.591.000.320.580.301.661.150.320.460.60

Estimated

1.601.040.350.650.251.701.140.380.540.49

928 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

dielectric gradient between the peat and mineral soil isrelatively small.

A strong reflection in the radar profile occurs at E(Fig. 3). Ground truth data suggest that this reflectioncoincides with the location of a thin placic horizon (Fig.4b). Placic horizons, which are common in many of themineral soils underlying bogs in Newfoundland, are thin(generally <3 cm thick), strongly cemented soil horizonsin which Fe is a major constituent of the cementingmaterial (Agriculture Canada, 1978). The placic horizonreflection abruptly stops at the 138-m mark on the radarprofile. No placic horizon was found in the excavatedpit located at the 155-m mark or in pits beyond it in theadjacent heath land soils. Relative saturation profiles atthe 75-m mark (Fig. 4b) and large differences in thehydraulic head above and below the placic horizon (Fig.3b, Table 3) clearly show that the placic horizon perchesthe groundwater. In addition, the "middle" piezometerat the 75-m mark remained dry during 4 mo of observa-tion. The sharp change in relative saturation at the placichorizon (Fig. 4b) coincides with an abrupt decrease inK; hence the relatively high reflection coefficient (0.36).Cobbles and gravels as well as the "wavy" placic horizonprofile (observed in excavated pits) probably contributedto the jagged nature of reflection E.

Ground truth information coupled with the strengthand lateral continuity of reflection E indicate that thegroundwater is perched by the placic horizon along itsentire extent. Moreover, the end of the placic horizon(138-m mark) coincides with the end of the UCTZ,suggesting that the placic horizon has augmented peatdevelopment and the growth of Sphagnum spp. in theUCTZ by restricting vertical drainage.

Observed and estimated thicknesses of the mineral soilbetween D and E at the 75-m mark are very similar(Table 2). Much like the placic horizon at the 75-mmark, the placic horizons observed in the soil pits at the104- and 130-m marks are approximately 33 cm belowthe surface of the mineral soil. Using the average K ofthe soil between the basal peat and placic horizon at the75-m mark, V between D and E was estimated to be0.05 m ns"1. Propagation velocities through the mineralsoils overlying the placic horizon could not be determinedwith confidence using the CMP technique; however, Vthrough the soils between the peat surface and the placic

Table 3. Hydraulic head data for piezometers installed in themineral soil underlying the upland and lowland bogs. Piezome-ter locations are shown in Fig. 3b and 5b. "Upper" refers tothe piezometer nearest to the peat surface, "middle" refersto the middle piezometer, and "lower" refers to the deepestpiezometer in the piezometer cluster. Head data were referencedto a common datum.

Site Piezometer Hydraulic head

Upland bog

Lowland bog

UpperMiddleLowerUpperMiddleLower

41.6empty40.336.5not working35.5

horizon was estimated by CMP survey to be 0.04 mns"1.

The propagation velocity of the radar pulse throughthe unsaturated mineral soil between reflections E andF at the 75-m mark was estimated to be 0.10 m ns"1.The thickness of this soil zone was interpreted from theradar profile to be 0.65 m. This thickness agrees wellwith that determined from auger and soil core data atthe 75-m mark (Table 2). The reflection at F is faintand is believed to delineate the surface of another watertable. The indistinct reflection is probably a result ofthere being a small dielectric gradient at the unsaturated-saturated soil contact (Fig. 4b) or possible signal interfer-ence and cancellation. The observed and interpretedthickness of the soil between F and the top of the bedrock(G) were determined to be 0.30 and 0.25 m, respectively(Table 2). The velocity of propagation through the satu-rated soil (F-G) was estimated, using Eq. [2], to be 0.07m ns"1; a velocity fairly consistent with that for saturatedsands (Davis and Annan, 1989). Common midpoint sur-vey estimates of V through this soil zone were difficultto resolve. However, V through the soil between theplacic horizon and bedrock was found by CMP surveyto be 0.08 m ns"1.

The reflection from the bedrock surface is strong,which is due to an abrupt decrease in K (Fig. 4b). Thepropagation velocity of the bedrock was estimated byCMP surveys to be 0.09 m ns"1. This velocity is similarto that for shale (Davis and Annan, 1989). Core/augerdata, and the lack of strong reflections below G, supportour contention that G is the mineral soil-bedrock contact.Interestingly, the upland blanket bog stops where thebedrock and mineral soil profiles crest (approximatelyat the 110-m mark). This and manual peat depth measure-ments southeast of the 0-m mark indicate that the uplandbog initiated in a concavity where soil water conditionswere probably favorable for pedogenesis and peat accu-mulation. The bedrock topography in the heath land wasfound by soil auguring to be approximately 1.7m belowthe surface.

Lowland Bog and Lowland CommunityTransition Zone

Peat cores were taken at the 351- and 395-m marks,while soil cores of the mineral soil underlying the bogwere acquired at the 351-m mark (Fig. 5). The LCTZextends from 322 m to the 332-m mark. The convexlowland bog begins at the 332-m mark, gently slopes to470 m, and then abruptly drops to a stream at the 500-mmark. The very strong reflection at B was estimated tobe approximately 0.6 m below the bog surface at the395-m mark and is probably a multiple reflection fromthe surface or an interface separating layers havingdifferences in degree of humification and water content(Fig. 6a). Table 1 shows that there is a substantialdecrease in K at the 0.65-m level supporting the lattercontention. The reflection at C was estimated to beapproximately 0.95 m below the surface at the 395-mmark and coincides with a zone of peat with low bulk

LAPEN ET AL.: DELINEATING SUBSURFACE FEATURES ALONG A WETLAND CATENA 929

. Distance (m)3} 303 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490

b) 303 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490

0 .

0.5.

f 1"£ 1.5J

g-2-OJQ

2.5.

3.0.

3.5.

Fig. 5. (a) Radar profile of lowland bog and lowland community transition zone (LCTZ) and (b) interpretation of radar profile with adjusteddepth scale. Reflections: A = soil surface, B and C = possible water interfaces in peat profile, D = organic soil-mineral soil contact, E =placic horizon (saturated mineral soil-unsaturated mineral soil contact), F = water table (unsaturated mineral soil-saturated mineral soilcontact), G = mineral soil-bedrock contact.

density (Fig. 6a) and high volumetric water content (0.87m3 m~3); however, it too may be a reflection multiple.

The propagation velocity through the lowland bog peatat the 395-m mark was estimated to be 0.04 m ns~'.The average volumetric water content of the peat at the395-m mark was 0.81 m3 nr3. The EC of the lowlandbog peat pore water was 80 p,S cm"' near the peat surfaceand 101.6 uS cm"1 at the base of the peat at the 395-mmark. The estimated depths of the peat at the 351- and395-m marks agree with those measured manually (Table2). The weak reflection at the organic-mineral soil con-tact (D) is probably due to the fairly high volumetricwater content of the mineral soils (0.52 m3 irr3) as aresult of bioturbation by roots increasing soil porosity(Fig. 6b). Unlike the peat surface profile, the organicsoil-mineral soil contact is concave.

The very strong reflection at E coincides with thelocation of a thin placic horizon. Soil core (Fig. 6b) andpiezometer data (Table 3) at the 351-m mark suggestthat the placic horizon perches the groundwater. Thestrength of the reflection at the 351-m mark (R = 0.44)is due to a very large dielectric gradient caused by the

placic horizon keeping the soils immediately below itunsaturated (Fig. 6b). The reflection begins at about the332-m mark and laterally extends under the lowland bogto approximately the 490-m mark. The reflection is fairlycontinuous; however, there is a break or dip in thereflection at the 400-m mark. It is noteworthy that unlikethe upland site, the placic horizon in the lowland areadoes not extend into the community transition zone. Theobserved thickness of the soil material between D andE at the 351-m mark is similar to that measured fromthe radar profile (Table 2). The propagation velocitythrough this soil zone was estimated to be 0.05 m ns"1.Again, CMP surveys determined V through the soilbetween the bog surface and placic horizon to be 0.04m ns""1.

The reflection at F corresponds with an unsaturatedsoil-saturated soil contact (Fig. 6b). The reflection iswavy, somewhat discontinuous, and is like portions ofwater table reflections mapped by Shih et al. (1986) andDavis and Annan (1989). The reflections adjacent to Fare so closely spaced that signal interference or cancella-tion has probably occurred. This interference may have

930

a)SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

0.0

0.5

i i .o0)Q

1.5

Db (g c m 3 ) S (cm3 cm 3 )0.0 0.3

R V (m ns )80 -0.4 0.0 0.4 0.00 0.08

IG

• 1 . 1 5 '

-3 3 -3Db (g cm ) Grain size (%) S (cm cm )1 2 0 1 0 0 0 1

R V (m ns 1 )50 -0.4 0.0 0.4 0.0 0.2

.0.25Sandi

'Silt

Clay

0.44

Fig. 6. Profiles cf physical and electrical properties: (a) peat profile at 395-m mark and (b) mineral soil profile under bog at 351-m mark. Letterson depth scale denote reflections (see Fig. 5). Clear = peat; hatching = bioturbated and amorphous sands, gravels, and cobbles; diagonalstripes = bioturbated and amorphous sands and gravels; dark horizontal stripe = placic horizon; stipple = weakly platy sands and siltysands; dashes = bedrock.

contributed to the discontinuous nature of the reflection.Moreover, F has a small amplitude and is not diffuse inappearance. Soil core observations at the 351-m mark andradar profiles indicate that the thickness of the saturatedmineral soil overlying the bedrock (G) was generallygreater than that of its upland bog counterpart duringthe time of the survey. At the 351-m mark, the measuredthickness of this soil zone differs from the observedthickness by 0.11 m (Table 2). The average V betweenF and G at the 351-m mark was estimated by Eq. [2]to be 0.07 m ns"1. The CMP surveys determined V tobe 0.07 m ns"1 through the soil materials between theplacic horizon and bedrock.

The interpretation of the bedrock reflection (G) wassubstantiated by soil core data at the 351-m mark andthe radar stratigraphic appearance below the reflection.The bedrock topography is clearly concave hi profile.The propagation velocity through the bedrock was esti-mated to be 0.09 m ns"1.

CONCLUSIONSSeveral noteworthy conclusions can be drawn from

the results of this study:1. GPR is an effective, fast, and nondestructive tool for

delineating subsurface features. However, without

appropriate ground truth information, quantitative in-terpretations of GPR images can be dubious.

2. The use of the 100 MHz antennas, the low claycontent of the mineral soils, and the relatively lowEC of the soil pore water resulted hi nondispersivesignal propagation and allowed velocity profiles ofthe organic and mineral soil to be estimated by Eq.[2] (Davis and Annan, 1989).

3. Common midpoint surveys determined the bulk Vof(i) soils between the bog surface and the placic horizon(0.04 m ns"1), (ii) soils between the placic horizonand bedrock (0.08 and 0.07 m ns"1), and (iii) bedrock(0.09 m ns"1). However, vertical profiles of K, de-rived from volumetric water content data (Topp et al.,1980; D. Halliwell, 1992, personal communication),provided a more detailed velocity structure of thelayered medium. For example, V was estimated for(i) peat soils (0.04 m ns"1), (ii) soils between peatand the placic horizon (0.05 m ns"1), (iii) unsaturatedmineral soil zone underlying the placic horizon (0.10and 0.09 m ns"1), and (iv) saturated soil overlyingbedrock (0.07 m ns"1). These velocity approximationsprovided a strong agreement between estimated andobserved thicknesses of subsurface features. ThePearson's correlation between estimated and observedsoil layer thicknesses is 0.99.

LAPEN ET AL.: DELINEATING SUBSURFACE FEATURES ALONG A WETLAND CATENA 931

4. GPR made it possible to determine the lateral continu-ity of hydrologically and ecologically important sub-surface features in an area where extensive groundtruthing was restricted by saturated soils. The resultsgenerally showed that placic horizons continuouslyunderlie the blanket bogs and UCTZ, and perch thegroundwater along their entire extent. Placic horizonsrestrict vertical drainage, keep water levels high,and therefore are important in sustaining wet surfaceconditions that are requisite for bog development.

5. The study suggests that when the mineral soil underly-ing peat has a relatively high volumetric water con-tent, the dielectric gradient and resulting reflectionat the peat-mineral soil contact can be fairly small.

ACKNOWLEDGMENTSWe wish to thank Dr. A. Judge and the Geological Survey

of Canada for the use of the GPR system. The technical helpof R. Hoag is also appreciated. Funding for this project wasprovided by Natural Science and Engineering Research Councilgrants to Dr. J.S. Price and Dr. R. Gilbert (Queen's University),and a Horton Research Grant awarded to D.R. Lapen bythe American Geophysical Union. Helpful comments wereprovided by Dr. C. Wang, Dr. G.C. Topp, and Dr. W.D.Reynolds, Centre for Land and Biological Resources Research,Agriculture Canada, Ottawa.