Rapport de campagne MaRoLiS Penfeld. Large de l’aéroport de Nice 10-16 Novembre 2020

The offshore operations carried out during the MaRoLis PENFELD cruise aimed at detecting the preparatory signs of submarine landslides and assessing their potentially catastrophic consequences. The fact that in 1979, offshore Nice Airport, this type of geohazard induced a tsunami, the breakage of communication cables and human losses illustrate the societal impact of those operations. Beyond the 1979 events, the detection in 2007 of mechanically weak zones in the upper 30 m of sediments indicate that some sectors of the continental shelf are prone to failure. To gain insight into the characterization and understanding of the mechanisms underlying the development of a catastrophic failure, continuous monitoring of pore fluid pressure in sediments is carried out with piezometers and followed in real time since 2015, thanks to the EMSO Ligure Nice cabled underwater observatory (European Multidisciplinary Seafloor Observatory, https://www.emso-fr.org/EMSO-Ligure-Nice). Along this line, the French-German ANR-DFG MODAL project aims at installing new instrumented rods (TIPS) to record deformation in the upper 50 m of sediments and identify failure planes (https://anr.fr/Project-ANR-17-CE01-0017). To this aim, a geotechnical site characterization cruise followed by a deployment cruise for the TIPS were planned. The MaRoLiS Penfeld cruise addressed the geotechnical site characteristion using piezocone soundings down to 50 m below seafloor with the PENFELD rig. Based on 19 soundings carried out during 5 dives of the Penfeld offshore Nice Airport, conclusions can be drawn regarding the accuracy of the measurements made by two different piezocones, the care which should be taken to interpret piezocone sensor data, the identification of target sites for the future installation of two TIPS instruments, the possibility of installing TIPS by stopping rod penetration every 2 m. In between Penfeld deployments a multi-beam bathymetric survey was made from the “Baie des Anges” to Moncao.