The selection of mapped areas was based on the results of a statistical model for calculating probable cold-water coral (Madrepora oculata) habitat (Fabri et al., 2017). Thus, data from the AUV-mounted multibeam echosounder were used to map in high resolution different areas of the Cassidaigne canyon identified as habitats for cold-water corals. These areas have been the subject of subsequent dives using a vehicle capable of acquiring optical imagery (photos, videos) to validate the distribution calculated by the statistical model (VIDEOCOR1 oceanographic cruise). The mapped areas are listed below:

• A series of small knolls located at 200 m depth (area named Cassis-200) was mapped accurately (Figure 2).
• A rocky ridge (named Cassis-500) was located around 500 m depth (Figure 2).
• A cliff (at the top of which is an overhang, named The Wall, 300-600 m deep) was mapped thanks to an innovative navigation of the AUV: its trajectory was planned to follow a ridge, in servo-control with respect to the water surface (and not with respect to the bottom as it is usually the case). This allowed us to map the cliff edge, shown in Figure 3.
• A site named SW Flank was mapped because it had been predicted as a suitable habitats for cold-water corals.

Figure 2: Three-dimensional view of high-resolution bathymetry (2 m) over the canyon head coral area. Cold water corals (CWC) are located at 500 m depth (Cassis-500) under a rocky ridge and, at 200 m depth (Cassis-200) on a series of small rocky mounts (knolls).

Figure 3: Three-dimensional view of high-resolution bathymetry (2m) over the coral area nammed The Wall between 300 and 600 m depth, at the exit of the canyon (from Fabri et al., 2019).

The bathymetric data produced after the two BATHYCOR campaigns were processed by the Data Processing team of Geoscience Marine (Gaillot A. and Pierre D., 2018). The bathymetric map produced (2m resolution), gathering all the data, was used during several campaigns listed below:

• To secure the navigation of the submarine vehicles during different trial campaigns in the area (ESSTECHROV_2017; ESSTECHROVGEN17, ESSHROV_AN_2019), in particular of the H-ROV Ariane, one of whose characteristics is to be able to navigate in the rough areas of the canyons,
• To secure the navigation of the first scientific campaign of the Ariane H-ROV (VIDEOCOR1) in 2017.