Combined ERT and GPR methods for investigating two-stepped lateritic weathering systems

Abstract

Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) surveys have been carried out in eastern Senegal for investigating two-stepped lateritic systems developed upon crystalline basement rocks. The geo-electrical pseudo-sections derived from ERT data provide a broad view of the lateritic weathering mantle, which is characterised by a decrease in resistivity with increasing depth. High resistivities characterise hardened ferruginous materials, in particular, the ferricrete, while the lower resistivities are related to the clayey horizons such as the saprolite. GPR provides high-resolution and continuous subsurface data on the hardened ferruginous horizons. The underlying clay horizons, however, drastically restricted the investigation depth. An anomalous area showing an obvious disruption of the reflections was detected in one of the radar records, suggesting contrasting iron and clay contents of superimposed lateritic weathering materials with differentiated porosity rate and kaolinite amount. Both ERT and GPR allow quite similar investigation depths of the hardened ferruginous surface horizons. Observations from trial boreholes located near the profiles contribute to better interpretation of the ERT and GPR results. The thickness of the weathering horizons depicted from ERT and GPR data roughly corresponds to that measured in the boreholes. The two geophysical methods are affected in different ways by lateritic terrains, but ERT and GPR surveys combined with borehole observation help to reduce ambiguous geological interpretations. ERT and GPR images show the variability of the lateritic weathering facies identified in the boreholes of each landsurface, ‘Middle-’ and ‘High-glacis’, depending on different erosion and weathering-ferruginisation processes that have shaped these landsurfaces and allowed the development of more or less mature underlying lateritic weathering profiles.