Groundwater electromagnetic imaging in complex geological and topographical regions: A case study of a tectonic boundary in the French Alps

Very-low-frequency (VLF), audiomagnetotelluric (AMT), and water geochemistry surveys were performed on the Sur-Fretes Ridge in the French Alps to evaluate the groundwater circulation system within the ridge. At this site, temporal variations of the electric field have been observed in association with water-level variations of neighboring artificial lakes. The Sur-Fretes Ridge is 1 km wide and trends east-west. Water samples were collected at 52 points distributed throughout the area. VLF soundings were carried out along three parallel east-west profiles, and 7 AMT soundings were carried out along an east-west profile on the top of the ridge. This site is characterized by a rugged topography of the ridge where geological and topographic trends are almost perpendicular, making the structure fully three dimensional. We constructed a 2-D resistivity model of the ridge from 2-D and 3-D analyses of the VLF and AMT data, associating geology and topography models. When combined with the water chemistry data, a model is proposed for groundwater percolation below the ridge across the geological contacts. This study demonstrates that electromagnetic imaging in a highly heterogeneous context can be combined with water chemistry to map groundwater circulation at the kilometer scale. The approach is relevant for hydrogeological and environmental applications.