True 2-D Resistivity Imaging from Vertical Electrical Soundings to Support More Sustainable Rural Water Supply Borehole Siting in Malawi

To improve borehole siting for rural water supply, an advanced resistivity method was adapted for developing country use and demonstrated in Malawi. The method was designed to be low cost, developing-country accessible, efficient. It allows single or multiple operators to acquire the multiple vertical electrical soundings (VESs) required that are inverted together in 2-D, to give a true cross-section of subsurface resistivity. Application at four sites generated true cross-sections of subsurface resistivity to around 100 m depth relevant to groundwater-resource investigation. A wide range of (hydro)geological features was identified, including fractured/weathered basement, gneiss domes, well-developed fault zones and several types of deltaic deposits. Imaging performance appears comparable to that of 2-D surface ERT (electrical resistivity tomography) that uses more expensive equipment, often unaffordable in developing countries. Based on the subsurface configurations determined and hydrogeological conceptualisation subsequently undertaken, the local aquifer potential could be evaluated, thereby providing a decision-making basis for future borehole siting at the sites surveyed. The technology is far superior to conventional 1-D VES, electromagnetic profiling or magnetic profiling currently used for borehole siting in Malawi. Technology adoption currently under consideration nationally would make use of existing VES capacity and permit much improved targeting of aquifer resource, more sustainable siting of boreholes and greater future resilience of Malawi’s rural water-supply infrastructure.