Multimodal Geophysical Characterization of Subsurface Structures - Insights from Independence Hall, KNUST, Ghana

A borehole drilling attempt at Independence Hall, KNUST, was unsuccessful due to an unexpected subsurface composition, a highly weathered and fractured layer beneath what was initially presumed to be the bedrock. To better understand the geological structure and recommend an optimal drilling approach, an integrated geophysical investigation was conducted. Seismic refraction, seismic reflection, and geo-electrical resistivity methods, including single-channel resistivity profiling and Electrical Resistivity Tomography (ERT), were utilized. A 200-meter survey line was established across the failed borehole site, with seismic refraction covering 100 meters, seismic reflection spanning 64 meters, and both ERT and single-channel resistivity surveys extending the full 200 meters. The results revealed a complex subsurface comprising three distinct layers including, a sandy topsoil, a sand and gravel formation, and a jointed-weathered granite layer. Seismic refraction data further indicated alternating competent and incompetent layers, with evidence of faulting. Despite the challenging subsurface conditions, the study suggests that the anomalous structure is not laterally extensive, and viable locations for drilling remain within the area. This investigation highlights the significance of geophysical surveys in delineating subsurface structures, optimizing borehole placement, and reducing the risk of drilling failures in geologically complex terrains.