Estimation of thickness of weathered granite by inversion of airborne frequency EM data on the basis of a two-layer 1D model

In the framework of preliminary studies for the selection of a nuclear waste repository, a helicopter Electromagnetic (EM) survey was carried out by the Canadian company Aerodat for the French nuclear waste disposal agency (ANDRA) on the granitic site of Neuvy-Bouin (Deux-Sevres, Trance), the French BRGM being the technical advisor for the operation, Interpretation of the apparent resistivity maps at the various frequenties indicated two types of area on the granite, conductive areas of thick weathered overburden and resistive ereas of shallow unweathered granite. An inversion treatment was then applied to the data, on the basis of a two-layer one-dimensional model, for the purpose of determining the quality and thickness of the weathered rocks. The problems encountered in fitting the model and the very good final results obtained are described here.