Mapping sand and clay-filled depressions on a coastal chalk clifftop using gravity and seismic tomography refraction for landslide hazard assessment, in Normandy, France

Abstract

In this study, we present a complete and successful case study where gravity and seismic refraction surveys detect and map previously poorly known sand and clay-filled depressions within the top chalk layers in a costal context, near Dieppe, Normandy, France. This study was commissioned by local authorities after a coastal chalk cliff collapse exposed a sand and clay-filled depression which turned into a > 100,000 m3 landslide. This resulted in a massive clifftop retreat exceeding 40 m, which threatened infrastructure and amenities. For risk and safety assessment, coastal managers commissioned BRGM to (i) determine the depth and extent of the sand and clay-filled depression, and (ii) map the presence of similar cliff-top depressions in a 2-km-long and 400-m-wide band inland of the coast. Both geophysical methods allow the detection and mapping of the sand and clay-filled depressions, which are characterised by a co-localized deepening of the first seismic horizon and a positive gravity anomaly. Seven auger holes confirm the geophysical interpretation, with depth to the top of the chalk >60 m in some instances. A map of the depth to the top of chalk is inverted using the gravity residuals.The successful mapping of the previously poorly-documented sand and clay-filled depressions on the Dieppe clifftop, using both gravity and seismic refraction tomography, was used in part to generate a Coastal Landslide Hazard Zonation map, which is a useful tool for coastal managers who need to make hazard-mitigating decisions.