Bedrock Detection and Mineral Thickness Assessment Using 3D Electrical Resistivity Tomography (ERT)

In this study, we demonstrate the use of 3D ERT for detecting depth to bedrock below river terrace sand and gravel, and quantify its performance against borehole control data. The approach to ERT bedrock detection considered here 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 study site was located within a geological setting that has proven to be impossible to adequately characterise using conventional discrete sampling approaches (i.e. boreholes and trial pits) due to the considerable heterogeneity of the deposit. Comparisons between borehole and the SGM derived bedrock surface elevations indicated a reasonable agreement between the two, thereby establishing a basis, in this case, on which to estimate mineral volumes using 3D ERT and the SGM. The bedrock surface calculated from the resistivity data provided a means of estimating mineral volumes at a site for which a meaningful reserve calculation could not be made using conventional approaches to site investigation.