Three-Dimensional Electrical Resistivity Imaging of a Karstic Limestone Aquifer in the Bima Sandstone Contact Zone, Gombe State

The contact zone between the Cretaceous Bima Sandstone and carbonate units exposed in Gombe State presents a complex hydrogeological setting where dissolution features in the carbonate sequence create karstic aquifers of high productivity but uncertain geometry and extent. This study applies three-dimensional electrical resistivity tomography to characterise the karstic limestone aquifer system in the Gombe-Bajoga corridor using a fully 3D data acquisition and inversion protocol. A 100 by 100 metre electrode grid comprising 100 electrodes was deployed at each of three sites, acquiring measurements in multiple directions to achieve full 3D spatial coverage. Data were inverted using a regularised 3D inversion algorithm to produce volumetric resistivity models at each site. The 3D resistivity volumes reveal low-resistivity anomalies at depths of 10 to 45 metres interpreted as water-filled karst cavities and fractures, with a spatial density of approximately 3 to 5 karst features per 10,000 square metres of surveyed area. The largest individual karst feature has estimated dimensions of 40 by 25 by 15 metres based on the resistivity model volume below the cavity threshold resistivity. Pump tests at boreholes sited within the 3D-imaged karst zones confirm specific capacities 4 to 8 times higher than boreholes drilled outside the karst zone, validating the geophysical characterisation. The study provides a 3D karst aquifer model that directly improves borehole siting success rates in this geologically complex water supply setting. Keywords: 3D ERT, karst aquifer, Bima Sandstone, Gombe State, limestone