Mapping Groundwater in Regolith and Fractured Bedrock using Ground and Airborne Geophysics: Case studies from Malawi and Brazil

Abstract

Water supply exploration in rural regions of eastern Malawi (Africa) and northeastern Brazil (South America) is complicated by the highly irregular nature of aquifers in weathered regolith and crystalline bedrock. Drilling success can be as low as 30%. Improvements in the success rate can be achieved by using simple geophysical investigations to target the thickest zones of weathering. Sophisticated geophysical surveys that image the full range in aquifer types can further improve the success rate to 70%. In Malawi, a total of 250 hand pumps were sited using a combination of borehole geophysics (natural gamma, conductivity and magnetic susceptibility), ground-based frequency-domain electromagnetics (Geonics EM34 and Apex Max-Min), and two-dimensional electrical resistivity imaging. In a similar terrain in northeastern Brazil, resistivity profiling and Very Low Frequency electromagnetic surveys are the mainstay of water exploration. A recent project introduced ground-based and airborne frequency-domain electromagnetics (Geonics EM34 and Aerodat helicopter electromagnetics) to improve the imaging of complex fracture zones and the detection of saline aquifers. Helicopter-borne surveys are a valuable tool in groundwater exploration because they cover large areas at high resolution. In both case studies, inversion of geophysical data integrated with remote sensing images and hydrogeology has increased the chances of drilling successful wells and finding potable water for rural communities.