Electrical imaging of lateritic weathering mantles over granitic and metamorphic basement of eastern Senegal, West Africa

The electrical properties of several tens of metres of lateritic weathering mantle were investigated by using electrical resistivity tomography (ERT) in two basement areas of eastern Senegal. The field survey was conducted al ong two profiles providing continuous coverage. Colour-modulated pseudosect ions of apparent resistivity vs. pseudo-depth were plotted for all survey L ines, giving an approximate image of the subsurface structure. In the area underlain by granitic basement, the pseudosection suggests a very inhomogen eous weathered layer in which the apparent resistivity changes more rapidly than thickness. In the second area, underlain by schists, the lateral chan ges in electrical properties are less pronounced than those of the granitic area. Interpretation of 2D Wenner resistivity data yielded considerable de tail about the regolith, even without pit information. In both areas, the n ear-surface topsoil comprising undersaturated lateritic material is highly resistive. The intermediate layer with low resistivities (e.g., 20-100 Ohm m) contains clays including small quantities of water. The third, highly re sistive layer reflects the granitic basement. Comparison of ERT survey resu lts with pit information shows general agreement and suggests that ERT can be used as a fast and efficient exploration tool to map the thick lateritic weathering mantle in tropical basement areas with hard rock geology. (C) 1 999 Elsevier Science B.V. All rights reserved.