LAUSR

Abstract This study assesses the efficiency of a key geophysical groundwater exploration technique, Electrical Resistivity Tomography (ERT) in part of the Voltaian Supergroup in Northern Ghana. In the Northern Ghana, groundwater has been the main source of water for domestic and other uses. Much of the terrain is underlain by Neoproterozoic sedimentary rocks which have lost their primary porosity to partial metamorphic episodes during the Pan-African Orogeny. As a result, groundwater acquisition in this terrain is challenging compared to crystalline basement aquifers. Borehole success rates (yields ≥ 13 L per minutes) have generally been low (50%) as the terrain has been difficult to characterize with standard geophysical techniques. Previously, when the electromagnetic and vertical electrical sounding (VES) techniques were used to explore for groundwater resources, the success rate was less than 50%. In order to have an improved yield, the 2D Electrical Resistivity Tomography (ERT) approach was used for the investigation to provide information about the lateral and vertical extent of weathering and fracturing in the sub-surface. A total of 30 ERT profiles were run across strike of the underlying lineaments at a maximum profile length of 800 m. The Schlumberger, Dipole-dipole and the Wenner array protocols were used concurrently for the data collection and the GEOTOMO RES2D Inversion software was used for the inversion. Analysis of data on 40 boreholes in 22 selected communities in different Voltaian rock units of drill depths ranging between 92 and 192 m indicates a 80% drilling success rate when drilling is guided by ERT surveys, providing a significant improvement over previous approaches recording a less than 50% success rate. Comparison between the drilling logs and the ERT revealed a strong correlation between the inferred resistivity boundary and lithology changes. On average, the resistivity patterns that indicate a promising zone for drilling a wet well ranges between 10 and 300 Ω-m. This investigation shows that, in hydrogeologically difficult terrains such as the Voltaian Supergroup, a careful application of the ERT with clear understanding of the underlying geological conditions would significantly improve success.