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

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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 4. m 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 0.2. m (average) and 0.4. m (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. © 2012 Natural Environment Research Council.

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Chambers, J. E., Wilkinson, P. B., Wardrop, D., Hameed, A., Hill, I., Jeffrey, C., … Gunn, D. A. (2012). Bedrock detection beneath river terrace deposits using three-dimensional electrical resistivity tomography. Geomorphology, 177178, 17–25. https://doi.org/10.1016/j.geomorph.2012.03.034

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