Exploring for groundwater in sub-Saharan Africa: Insights from integrated geophysical characterization of a weathered basement aquifer system, central Malawi

Abstract

Study Region: Mkonga village, ∼35 km southwest of the capital city of Lilongwe on the central plains in Malawi, Africa, located at the crest of a local topographic-high characterized by crystalline basement. Study Focus: Groundwater aquifer systems in the shallow buried crystalline basement are geologically complex reservoir targets with historically low drilling success rates. Understanding the basement weathering profiles and their implications for aquifer architecture is crucial for drilling productive wells. This is particularly critical in developing countries where water demands are rising. This study employs an integrated approach of 2-D seismic reflection and 2-D electrical resistivity imaging, constrained by borehole data to characterize the local basement aquifers in the crystalline-dominated terranes of central Malawi. New Hydrological Insights for the Region: The integrated geophysical and borehole data reveal a layered weathering profile: shallow thin laterite and sub-lateritic layers, the saprolite, the saprock, and the fractured/fresh basement. We identify four potential water-bearing zones consisting of the base of the sub-laterite layer, the saprolite, the saprock and the fractured basement. The most productive boreholes are those that intersect low resistivity anomalies (< 20 ohm-m) that are collocated with strong but discontinuous reflectors. These targets are interpreted to represent water-bearing zones that connect the saprolite and saprock to recharge pathways at the base of the sub-laterite.

Keywords

• Basement aquifers
• Electrical resistivity imaging (ERI)
• Seismic reflection
• Saprolite
• Saprock
• Fractured basement