Multifrequency electromagnetic method for the hydrogeophysical characterization of hard-rock aquifers: the case of the upstream watershed of White Bandama (northern Ivory Coast)

In West Africa, the drinking water supply relies on the hard-rock aquifers. In Ivory Coast, the population growth along with the climate changes make drinking water resources highly vulnerable. The White Bandama upstream watershed, northern Ivory Coast, is located on a hard-rock aquifer which geometry and potential water resources are not yet well characterized. Indeed, the heterogeneous subsurface in this region shows high variability in the hydraulic conductivity inducing difficulties in the hydrogeological exploration. The determination of the geometry and hydrodynamic properties of the aquifer are required for a sustainable management of this water resource and for a better choice of future well locations. This study presents a hydrogeophysical approach using the multifrequency electromagnetic device PROMIS®, as well as lithology logs and geological information of a 30 x 30 km zone in the north-western part of the White Bandama catchment. Our geophysical data are interpreted with 1D multi-layer models consistent with the discontinuities observed in lithology logs and the geology of the site. Results allow to precise the local thicknesses of the 3 main units of our study area down to 50 m, being from top to down, saprolite, a fissured-rock zone and the rock substratum. Between the saprolite and the fissured zone, the main aquifer unit constitutes the interesting target for productive water wells. Its thickness ranges from 15 to 30 m. A detailed knowledge of the local aquifer geometry constitutes the first and crucial step before going further into a complete hydrogeological study.