INVESTIGATING GROUNDWATER POTENTIAL IN BASEMENT AQUIFERS USING RESISTIVITY THRESHOLD, CENTRAL MALAWI

Basement aquifers are conventionally manifested as fractures within the basement or the weathered zone (saprolite/saprock) above the bedrock and often regarded as low-productivity aquifers. Geophysical surveys such as seismic and electrical resistivity are often used to delineate aquifers and identify optimum well locations. However, drilling through geophysical anomalies alone sometimes lead to low-yielding wells or even dry wells. Improving the success rate of drilling requires a better understanding of the basement weathering profile and the aquifer structure within basement profile. Here we use seismic reflection, magnetic, and electrical resistivity imaging (ERI) techniques constrained by well data to investigate local basement aquifers in central Malawi. Previously, wells drilled at the site had a success rate of 30% and struggled to provide a sustainable water source. ERI data was collected along nine profiles in dipole-dipole and Wenner-Schlumberger arrays. Seven seismic reflection profiles were collected with five lines being co-located with ERI. Source parameter imaging (SPI) of the magnetic data was used to estimate the top to magnetic basement in the area. Depth to basement estimates were made correlating ERI, seismic, magnetic derived SPI, and well data. The inverted ERI profiles reveal three electrical layers: thin (5-20 m) laterally continuous high-resistivity zone (layer-1), an intermediate (20-60 m), discontinuous and electrically heterogeneous layer (layer-2), and a deeper high-resistivity layer with discrete vertical/sub-vertical low-resistivity zones. Wells drilled along the profiles show that layer-1 is a lateritic topsoil, layer-2 is composed of deeply weathered portions of the underlying metamorphic bedrock (saprolite), and layer-3 is characterized by a combination of regolith (saprock) and fractured bedrock. Using the concept of resistivity threshold values constrained by both dry and producing wells, we find that there are two potential water bearing zones. The first zone occurs within layer 2 (lower/upper saprolite, 50–300 ohm-m) and aquifers (1–10 ohm-m) within this zone are discontinuous. The second zone represents deep weathered fractures within the basement rocks (layer 3). Although, the resistivity threshold values proposed for this study may be unique to the locality, we suggest that our approach is critical for optimizing the success rate and total volume of water exploration in areas of shallow basement aquifers.