Groundwater Exploration in Weathered Crystalline Rocks: Combining Remote Sensing and On-Field Structural Analysis With Ert Measurements.

Summary This study aims at proposing implantations for new groundwater extraction boreholes for the town of Grand-Santi in French Guyana, an area that lies mostly on weathered crystalline rocks. Although surface water is abundant, its poor quality makes it more relevant to target underground water resources. In such a geological context, higher exploitable flow rates are usually obtained by drilling boreholes close to faults where vertically fractured layers tend to enhance higher permeability. The strategy adopted here thus relies on 1) refining the geological and structural map of the target area; 2) performing electrical resistivity tomography across suspected discontinuities to confirm their existence and 3) propose optimized borehole implantation close to identified fractured corridors. As ERT measurements implies a large effort, especially in this tropical forest context, structural preanalysis through remote sensing proved very efficient to optimize ground investigations positioning. Results allowed us to determine the geometry of the various geological formations encountered and to refine the evaluated positions of faults, which is of prime importance to reach highly fractured layers while avoiding local clay accumulations. As a result, we could propose 10 suitable drilling sites over the three main zones of interest, thanks to seven ERT profiles.