3D electrical resistivity investigation and subsurface imaging of the Kasane warm spring area, northwest Botswana

Abstract

Electrical resistivity imaging survey was carried out at the warm spring site in Kasane, northwest Botswana, to investigate the subsurface distribution of geological structures and how they influence the existence of the warm spring. This investigation was carried out along eight 480-m-long parallel profiles with an electrode spacing of about 10 m and 50 m line spacing. We used a Pole–dipole electrode array to acquire the data. The data inversion employed in this study was based on the smoothness constrained nonlinear least-square inversion algorithm. 2D data inversion for all the eight 2D datasets was performed, and the eight 2D datasets were then collated into a single 3D data file before performing a 3D data inversion. The resultant 2D inversion models display the variation of electrical resistivity beneath the respective profiles; whereas, the resultant 3D models show the distribution of electrical resistivity in both the lateral and vertical directions. The 2D inversion results revealed two distinct anomalies with low resistivity values, e.g., < 46.2 Ω m of the topmost layer indicating the presence of silty clays and relatively high resistivity material with resistivity values exceeding 249.3 Ω m, which was attributed to basalts. Also observed in the 2D inversion results are low-intermediate resistivity zones of about 46.2 Ω m extending to greater depths beyond the boundaries of the models, which are attributable to the existence of fluid-filled fractures/faults. The 3D inversion results resolved a laterally extensive high-resistivity basaltic unit, two NE–SW-trending vertical/near-vertical low-resistivity features which are attributable to fluid-filled fractures or faults, and a broad low-resistivity zone occurring beneath a depth of 40 m on the southeastern side of the study area, which is likely to be indicating part of the geothermal reservoir. The low-resistivity vertical/near-vertical features observed in this study are likely to be enabling the flow of geothermal fluids, and the high resistivity basaltic unit acts as a caprock overlying the reservoir.