Bedrock Interface Detection for Sand and Gravel Mineral Reserve Assessment Using 3D ERT

A study describing the use of 3D ERT to determine depth to bedrock, and hence mineral thickness, at a sand and gravel extraction site within river terrace deposits is presented. Two approaches are considered which automatically extract interface depths from 3D ERT models. The first assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The second uses an intrusive sample point to calibrate the model by identifying the resistivity iso-surface associated with the interface, and is referred to here as the ‘known interface method’ (KIM). The results of these two approaches have been tested against direct GPS observations of the interface position that were made after the bedrock had been exposed by quarrying of the river terrace deposits. In this case, the use of intrusive data for model calibration (i.e. the KIM) was essential for recovering accurate depth information from the 3D ERT model. Reliance upon using the steepest resistivity gradient as an indicator of a geological boundary (i.e. the SGM) produced a severe overestimate of interface elevation.