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

摘要

Two-dimensional electrical resistivity tomography has been employed to investigate the subsurface structure of a thick lateritic weathering mantle overlying a granitic bedrock in southeastern Senegal. The resistivities were measured along two kilometric profiles insuring continuous coverage. Exploration pits exposed the different weathering layers, i.e., a saprolite, a mottled zone, a soft ferricrete and a ferricrete, whose respective thicknesses were used to constrain the measured apparent resistivity, despite their spatial variations. Colour-modulated pseudo-sections of apparent resistivity versus pseudo-depth including the groundsurface topography clearly show spatial variations in electrical properties of the weathering layers since their 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 more useful results about the real resistivity ranges of the weathering layers. The resulting geo-electrical images document the geometric relations between the different layer boundaries, in particular those of the aquifers with the bedrock and groundsurface topographies. The spatial relationships between the granitic bedrock and groundsurface topographies suggest that a large part of the actual lateritic weathering mantle is allochthonous. This also implies that the actual topography of the bedrock surface was mainly shaped by weathering processes while the hillslope geomorphic patterns result from erosion processes or lateritic weathering of reworked materials leading to ferricrete development according to the different landforms observed. It is suggested that climatic changes were implied in the landscape evolution of our study area.