Use of the magnetotelluric method in the study of the deep Maestrichtian aquifer in Senegal

Abstract

The magnetotelluric (MT) sounding method has been employed to study the deep aquifer of Senegal. Results from nine soundings measured along a 350 km long east-west profile demonstrate that the method provides valuable information about the geometry on the bottom of the Maestrichtian aquifer and that it can be used as a tool for estimating the effective porosity of the aquifer. One-dimensional inversion yields layered models clearly delimiting the bottom of the fresh water aquifer. On the other hand, due to weak electrical resistivity contrasts atop the Maestrichtian sands, the upper limit of the aquifer is not well resolved by MT. However, this limit is generally known from water well data. The effective porosity can be estimated from MT data using Archie's formula if well log data about the pore fluid salt content is provided. Two-dimensional images of the Senegal basin obtained from the nine soundings show geoelectrical features in good agreement with the known geology and hydrogeology. Distortion analysis of the MT impedance tensors and 2D resistivity imaging allow the definition of four main geological features controlling the deep aquifer: the subsidence of the basin, the Linguère fault, the Ndiass horst and the Gassane mafic intrusion.