Archaeological ProspectionArchaeol. Prospect. 13, 147–150 (2006)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.281

Ground-penetratingRadarMappinginClay:SuccessfromSouthCarolina,USA

WENDYWEAVER*

Brockington and Associates, Atlanta, Georgia, 6611 Bay Circle, Suite 220, Norcross, GA30071, USA

ABSTRACT Aground-penetrating radar (GPR) survey conducted in a clay-rich floodplain of the Savannah River,SouthCarolina, producedwhatwereverydifficult tointerpret reflectionprofilescausedbyenergycou-pling changes.These changes were caused by deep hardened plow furrows at the ground surfacethatcausedantennamovement tobeirregular, andenergy tobetransmittedinto thegroundat variousangles, andwith different properties. A dense clay layeralso attenuatedmuch of the radarenergyatabout 50 cm, further obscuring the profiles. Despite these ambiguous data, amplitude mapping atslicesbelowtheclaylayeryieldedimagesofdistinctcircularandlinear features,whichprovedtobepa-lisadewallsdatingfromabout thelateMississippiantoEarlyContactperiod (AD1400-1740).Thissurveyillustrates that it ispossible to obtaingoodGPR resultsevenwhenenergypasses throughdense clayand when reflection profiles are obscured by attenuation and coupling changes. Copyright � 2006JohnWiley&Sons,Ltd.

Keywords: ground-penetratingradar; clay; attenuation; amplitudeanalysis

Introduction

It has long been assumed that ground with anabundance of clay yields poor ground-penetrat-ing radar (GPR) results, as most radar energy isattenuated very close to the surface due to clay’shigh electrical conductivity (Jol and Bristow,2003; Leckebusch, 2003). There have, however,been some interesting GPR successes in clay,where radar energy has penetrated deep enoughto be reflected from targets of interest, and stillreceived back at the ground surface (Conyers,2004a). Preliminary work suggests that themineralogy of the clay in these areas producesa somewhat electrically resistive media. Theseclays appear to be either rock clasts of clay size,or mineral clays, such as kaolonite, which havea low cation exchange capacity (Conyers, 2004b).

A successful GPR survey in very clayeyground was conducted at Riverfront Village,South Carolina, which contains late Mississip-pian to Early Contact Period (AD 1400–1740)remains preserved under floodplain sediments.Previous test excavations indicated the presenceof possible structures and palisade walls as wellas a dense trash midden, and possible refuse andstorage pits.

The survey

A 100� 94 m grid covering the prospectivearea was surveyed using a GSSI SIR-3000system and 400 MHz antennae. Reflection datawere collected in transects 1 m apart over agrassy field that retained furrows from recentploughing. The residual plough furrows mademovement of the antennae along the groundquite difficult as they produced ridges about10–15 cm tall, spaced about 30–40 cm apart.

Copyright # 2006 John Wiley & Sons, Ltd. Received 30 November 2005Accepted 24 March 2006

* Correspondence W. Weaver, Brockington and Associates,Atlanta, Georgia, 6611 Bay Circle, Suite 220, Norcross, GA30071, USA. E-mail: wendyweaver@brockington.org

Preliminary stratigraphical investigations in thesurvey area exposed a clay layer about 40–50 cmbelow the present ground surface that varied inthickness from 5 to 20 cm. This stratigraphicallayer was observed overlying a historic plough

zone in a buried soil horizon containing nine-teenth century artefacts. The clay layer is thoughtto represent deposits from the flood of 1888 thatinundated this floodplain area. Beneath the bur-ied historic plough zone prehistoric features and

Figure1. Reflection profile showing coupling changesdue to surface furrowsand indistinct reflectionsat about 20 ns.

Figure 2. Amplitude slice-map from 21 to 24 ns showing linear and circular features, corresponding to palisade walls and otherhistoric featuresbuilt on thepre-1888 flood living surface.

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Copyright # 2006 John Wiley & Sons, Ltd. Archaeol. Prospect. 13, 147–150 (2006)

artefacts were also found from the late Missis-sippian occupation of the area. Little was knownabout this prehistoric component of the site.

Reflection profiles showed a plethora of veryerratic reflections caused by many near-surface

energy coupling changes as the antennae passedover the hardened furrows (Figure 1). In additionthe dense clay layer appeared to have attenuatedmuch of the radar energy around 20 ns, whichvelocity tests showed to be about 40–50 cm in

Figure 3. Linear post mouldsofa prehistoric palisadewall that were imaged on the GPR slicemapsin Figure 2.

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Copyright # 2006 John Wiley & Sons, Ltd. Archaeol. Prospect. 13, 147–150 (2006)

the ground. All profiles in the grids were slicedin 3-ns slices and reflection amplitudes interpo-lated between profiles using a 1 m search radius.The slice from 21 to 24 ns (about 40–45 cm in theground) yielded an amplitude map that showeddistinct linear and circular features at a depthjust below the buried nineteenth century soilhorizon (Figure 2). Most important, the featuresthat might have caused these distinct patternswere not visually discernable in any of the indi-vidual reflection profiles (Figure 1). The patternsin the southern part of the survey were hypothe-sized to be the remains of post moulds fromvertical wooden logs that made up a palisadewall (Figure 2).

A large open excavation cleared all the over-burden to within the nineteenth century soilhorizon at about 45–50 cm depth (Figure 3). Alinear palisade wall composed of very large postmoulds was uncovered within that layer about10 cm beneath the clay horizon deposited duringthe flood of 1888. These visually distinct featuresin the excavations were not visible in the reflec-tion profiles, probably because they are com-posed of material that is not distinct chemicallyor physically from the surrounding matrix andtherefore do not produce readily visible radarreflections. There are still subtle differences thatcan be measured in the digital reflection data,however, which become apparent in the ampli-tude slice-maps (Figure 2).

Conclusion

Reflection profiles in this area were so ambigu-ous that no usable interpretations could be madefrom them. Surface furrows created many cou-pling changes in the reflection profiles, andenergy attenuation below about 20 ns accentu-ated mostly background noise and not the veryweak reflections generated from the features ofinterest. Even with the seemingly ambiguous

reflection data the amplitude maps constructedfrom all reflection profiles yielded useful mapsthat showed distinct linear and circular patterns.These features proved to be palisade post foun-dations of a pre-contact Mississippian age vil-lage. In this difficult area for GPR data mapping,amplitude analysis of all reflections with a 1 msearch radius proved that usable reflection infor-mation still exists within profiles that exhibitedno readily interpretable reflections to the humaneye. Although the composition of the overlyingclay is not known, it appears to be less electricallyconductive than many clay units found else-where, and still allowed the penetration andreflection of radar energy to at least 1 m in theground. This survey shows that if targets are nottoo deeply buried, good results can be obtainedeven in the densest of clay environments with agood deal of ground surface disturbance.

Acknowledgements

Thanks to Tom Whitley for site excavation gra-phics and Inna Bums and Tamara Bruinik forGPR survey assistance.

References

Conyers LB. 2004a. Ground-penetrating Radar forArchaeology. AltaMira Press: Walnut Creek, CA.

Conyers LB. 2004b. Moisture and soil differences asrelated to the spatial accuracy of amplitude mapsat two archaeological tests sites. Proceedings ofthe Tenth International Conference on Ground-pene-trating Radar, Delft, The Netherlands; 435–438.

Jol HM, Bristow CS. 2003. GPR in sediments: adviseon data collection, basic processing and interpre-tation, a good practice guide. In Ground Penetrat-ing Radar in Sediments, Bristow CS, Jol HM (eds).Special Publication 211, Geological SocietyPublishing House: Bath; 9–27.

Leckebusch J. 2003. Ground-penetrating radar: amodern three-dimensional prospection method.Archaeological Prospection 10: 213–240.

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