Investigation of the geological and hydrogeological structure of chalk cliffs with visible, thermal infrared and electrical resistivity imaging

This study is focused on assessing the surface and subsurface properties of a coastal cliff undergoing erosion. The investigation takes place along the chalk cliffs located in Sainte-Marguerite-sur-Mer, within the region of Normandy, France. To achieve this, a combination of Electrical Resistivity Imaging (ERI), visible and Thermal Infrared (TIR) imagery, and geotechnical methods (specifically, piezometers) were employed. ERI was chosen due to its high sensitivity to changes in water saturation and salinity within subsurface formations. The research involved two transverse ERI profiles originating from the plateau, crossing the cliff, and extending to the shore platform. Additionally, two longitudinal profiles were conducted on the plateau, running parallel to the coastline, all using Wenner configurations. The primary findings unveiled the distribution of subsurface resistivity, revealing insights into cracks, potential karst formations, and weathered chalky areas. A significant discovery was the identification of a conductive zone at depth, which was associated with saltwater intrusion. The integration of piezometers, intersecting the ERI profiles, allowed for the correlation of resistivity values with the measured salinity data. Areas characterised by brackish water exhibited an apparent resistivity ranging between 4 and 75 ohm.m, commencing at the front of the saline wedge.