Bedrock detection beneath river terrace deposits using three-dimensional electrical resistivity tomography

J.E Chambers,P.B Wilkinson,D Wardrop,A Hameed,I Hill,C Jeffrey,M.H Loke,P.I Meldrum,O Kuras,M Cave,D.A Gunn

doi:10.1016/j.geomorph.2012.03.034

Abstract

We describe the use of a fully volumetric geophysical imaging approach, three-dimensional electrical resistivity (3D ERT), for bedrock detection below mixed sand and gravel deposits typical of fluvial valley-fill terraces. We illustrate the method through an analysis of terrace deposits of the Great Ouse River (UK), where up to 4m of sand and gravel have filled the valley bottom during the latest Pleistocene. We use an edge detector to identify the steepest gradient in first-derivative resistivity profiles, which yields an estimate of bedrock depth (verified by drilling) to a precision better than 0.2m (average) and 0.4m (standard deviation). Comparison of a range of drilling techniques at the site has revealed that borehole derived interface depths suffered from levels of uncertainty similar to those associated with the 3D ERT — indicating that the reliability of bedrock interface depths determined using these two approaches is comparable in this case. The 3D ERT method provides a high spatial resolution that enabled a previously unknown erosional bedrock structure, associated with the change from deeper first terrace to second terrace deposits, to be identified in the Great Ouse valley. The method provides a relatively quick method to quantify terrace fill volume over large sites to a greater degree of precision than currently available.

Highlights

River terrace deposits are a focus of considerable scientific, archaeological, and economic interest
A total of 11 locations were drilled within the 3D Electrical resistivity tomography (ERT) imaging area; five of the locations were drilled using only the flight auger; whilst the remaining six locations were drilled with a combination of two or more techniques
The drilling at the site was undertaken as a component of a separate project concerned with optimising sand and gravel deposit sampling strategies, which involved the geostatistical analysis of grading data and the comparison of different drilling technologies (Hill et al, 2011; Jeffrey et al, 2011)

Summary

Introduction

River terrace deposits are a focus of considerable scientific, archaeological, and economic interest. Terrace architecture can provide important information regarding uplift, incision, and landscape evolution (e.g., Boreham et al, 2010; Bridgland, 2010), with the formation of aggradational terraces in some settings correlating closely with climatic cycles (e.g., Bridgland, 2006). These deposits are a rich source of archaeological artefacts preserving a record of Palaeolithic human activity (e.g., Wymer, 1988) and are a major economic resource of groundwater (Gomme and Buss, 2006) and sand and gravel aggregates for construction (Smith and Collis, 2001). Because of the complexity of some deposits, even drilling using densely spaced boreholes can fail to adequately reveal the three-dimensional (3D) structure of a deposit in terms of thickness and composition (Wardrop, 1999)

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Conclusion