Combined S-wave Reflection Seismics and ERT to Characterize Bedrock and DNAPL Presence in the Shallow Subsurface

The contamination of soil by DNAPL (solvent, dying residuals etc.) represent a serious threat for the environment. Being heavier than water, DNAPLs can reach deep depth as their migration is controlled by gravity and partially by groundwater flows. As results, those contaminants tend to migrate and accumulate in morphological lows of impermeable layer. Near surface geophysics is often employed for environmental issues in the shallow subsurface. In an area contaminated by DNAPL in Switzerland, Deltares has used a combination of S-wave reflection seismics and Electrical Resistivity Tomography (ERT) to characterize the morphology of the clayey bedrock (observed at around 16 m in existing boreholes in the area) and locate the contaminated portion of the subsurface. By three-dimensional interpretation of seismic data, the clayey bedrock has been extrapolated for the whole area. A channel structure is recognized along the SW-NE direction. These observations are confirmed with the boreholes a priori information. The geoelectrical data confirmed the presence of stiffer, less porous bedrock at around 16 m due to the increase in resistivity in the saturated zone. In a few lines, a shallow high resistive anomaly is observed that can related to a DNAPL plume.