Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography

Two-dimensional electrical resistivity tomography has been employed to inve stigate the subsurface structure of a thick lateritic weathering mantle ove rlying a granitic bedrock in southeastern Senegal. The resistivities were m easured along two kilometric profiles insuring continuous coverage. Explora tion pits exposed the different weathering layers, i.e., a saprolite, a mot tled zone, a soft ferricrete and a ferricrete, whose respective thicknesses were used to constrain the measured apparent resistivity, despite their sp atial variations. Colour-modulated pseudo-sections of apparent resistivity versus pseudo-depth including the groundsurface topography clearly show spa tial variations in electrical properties of the weathering layers since the ir apparent resistivity changes faster than their respective thickness. The data from a cross-borehole survey along with estimates of resistivity for aquifers and granite were integrated into the pseudo-sections to provide mo re useful results about the real resistivity ranges of the weathering layer s. The resulting gee-electrical images document the geometric relations bet ween the different layer boundaries, in particular those of the aquifers wi th the bedrock and groundsurface topographies. The spatial relationships be tween the granitic bedrock and groundsurface topographies suggest that a la rge part of the actual lateritic weathering mantle is allochthonous. This a lso implies that the actual topography of the bedrock surface was mainly sh aped by weathering processes while the hillslope geomorphic patterns result from erosion processes or lateritic weathering of reworked materials leadi ng to ferricrete development according to the different landforms observed. It is suggested that climatic changes were implied in the landscape evolut ion of our study area. (C) 1999 Elsevier Science B.V. All rights reserved.