Underground cavities are ubiquitous across northern Europe's cities and countryside due to a long history of mining and building-stone extraction. Management of cavity-collapse risks requires detailed knowledge of a cavity's geometry, depth and rock-mass characterisation. Current mapping practices are neither sufficiently accurate, detailed nor cost-effective in underground settings. Here, a GeoSLAM Zeb-Revo handheld mobile laser scanner was tested. Its point clouds reproduced planes faithfully (RMS < 10 mm) over typical gallery dimensions (<10 m) and any survey horizontality defect was not measurable. In wellstructured corridor networks, reference distance inaccuracy arising after a 115 m loop and 5Á25 minutes of instrumental drift did not exceed 3 mm over 30 m (1:10 000) and the difference from the reference length was insignificant. Applied to mapping 11 ha of a disused underground stone quarry in the Paris Basin, Zeb-Revo surveys produced accurate (<1 m) base maps adequate for regulatory cavity hazard maps at 1/5000 scale. Geometric knowledge of accessible cavities is therefore no longer a challenge for collapse hazard mapping.

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 m 3 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.

Abstract. Perchlorate (ClO

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