Characterization and location of the laminated layer within hard rock weathering profiles from electrical resistivity tomography: implications for water well siting

This research aims to quantify the geophysical signature of the laminated layer, one of the main layers constituting the weathering profile of hard rocks. This laminated layer acts as a marker for locating the underlying groundwater productive stratiform fractured layer (SFL). The study is based at two sites on the interpretation of 50 km of electrical resistivity tomography (ERT) profiles, compared with outcrops and boreholes by geophysical modelling. For the first time, the geophysical signature of the laminated layer, located at the base of the saprolite, is characterized within granite formations. Where the stratiform weathered layer is detected by pole-dipole ERT profiles, the laminated layer is identified as a resistant layer on 90% of the SFL length using an appropriate inversion method. In addition, this layer is also revealed for the first time in certain types of metamorphic formations; here it is revealed in micaschists (62% of the SFL length). The location of the laminated layer in the weathering profile is important (1) for water well siting by determining if an underlying SFL exists in the weathering profile and (2) for assessing the residual thickness of the saprolite, and then evaluating water storage and the protection of the SFL aquifer.