A. Beauvais et al., Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography, EARTH PLAN, 173(4), 1999, pp. 413-424
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.