Aquifer characterization in the Ouarzazate Basin (Morocco): A contribution by TEM and RMT data

Abstract

ABSTRACTIn this case study we present the first application of the radio magnetotelluric (RMT) technique in Africa. We applied the RMT and transient electromagnetic (TEM) methods to study an aquifer of the Ouarzazate Basin between the High Atlas and the Anti Atlas in Morocco. The main objective of the survey was to constrain the geometry of the near surface sediment layer by mapping the aquitard, an underlying clay layer.We collected RMT data in the frequency range of 16 kHz to 207 kHz and TEM data in the time range of 4 μs to 1 ms along an 8 km long profile in a desert area crossing an oasis region. After 2D inversion of the RMT data and 1D inversion of the TEM data, we carried out 1D joint inversion to enhance the resolution of the subsurface resistivity structure.From the inversions we derived the resistivity distribution down to a depth of about 100 m and identified a relatively resistive layer as the aquifer sediments, overlying a more conductive clay layer. Inversion statistics indicate that the depth of the clay layer is well resolved. Compared with the lithological logs of drillings located at distances of 150 m to 3 km from the profile, the depth of the clay layer beneath the aquifer in the south of the geophysical section agrees to ± 3 m. Furthermore, we observed a lateral resistivity contrast close to the surface separating the desert area and the oasis, due to differences in humidity.