Marine Karst Environment Characterization Using Jointly Seismic Imaging, Marine ERT and Geotechnical Data

Summary Marine calcareous environments during aerial phase may become highly heterogeneous. The dissolution can form karst systems characterized by sinkholes and caves. Marine geophysical explorations of such soils prior to building projects should include several methods, such as seismic imaging and electric resistivity tomography, largely used on terrestrial studies. Our study focused on a marine renewable energy site off Saint-Nazaire (France), where the substratum is mainly formed of fractured calcarenite lying on dolomitic sands. Seismic imaging was used jointly with marine ERT (MERT) and the data were compared to geotechnical measurements performed on boreholes. The seismic data had limited penetration due to the high reflectivity of the substratum, whereas MERT was able to detect possible dissolution areas where conductive saturated sediment filling was identified. The MERT profiles fit quite well with the geotechnical data except between 0 and 10m below seabed. The difference was explained by the connected fractures and dissolution pockets only visible at the MERT scale. Using MERT and geotechnical conductivities in a simple electrical model of the soil, we could estimate the volume of connected fractures in the upper fractured calcarenite layer, which volume exceeds 10-15% of the total volume.