The electrical image method compared with resistivity sounding and electromagnetic profiling for investigation in areas of complex geology: A case study from groundwater investigation in a weathered crystalline rock environment

Abstract

Small but significant supplies of groundwater are found in weathered crystalline rocks. In areas where weathering is actively occurring as a result of tectonic movement, the weathering zones show major lateral variation. Electrical techniques can be used to detect zones of deep weathering because of the significant resistivity contrast between the different weathering zones. Bulk resistivity decreases as weathering intensity increases. Groundwater reserves are located in the intermediate weathering grades between fresh rock (high resistivity) and clay dominated (low resistivity) weathered material. The determination of accurate thicknesses and resistivities for intermediate zones is complicated by the problems of equivalence and suppression. A review of the use of constant separation traverses (CST), vertical electrical sounding (VES), electromagnetic profiling and electrical imaging is presented from a small research catchment close to Bauchi in Northern Nigeria. Results from each of these methods are compared with field observations and drilling. The use of VES at anomalies located by CST is shown to be unsatisfactory. The most appropriate combination of geophysical techniques is electromagnetic profiling to produce a 2-dimensional map of apparent resistivity and electrical imaging to determine the depth of weathering at the regions of low apparent resistivity detected by the EM measurements. Recent advances in electrical image equipment and interpretation methods make the acquisition and interpretation of these data as rapid as that for VES for an equivalent depth of investigation.