Preliminary conclusions

• A complete swath bathymetric survey has been carried out over Ampére , Unicorn and Seine seamounts in the NE Atlantic, the first two belonging to the Horseshoe submarine chain and the third, off Madeira archipelago.

• Grid lines spacing have ensured a sea-floor coverage in the order of 50-75% (for Ampére  and Unicorn smts) up to more than 100% for Seine smt. The investigated depth range varied from few tens metres (top of Ampére ) to 2500-3000m; the spatial resolution provided has been medium to high with an average grid cell size ranging from 20 m (at smts summits) to 50-100 m at depths > 1500 m.

• Ampére  smt is an asymmetric volcanic edifice with southern flank affected by massive erosion in the form of some collapse scars that suggest sudden landslides. A flat topped summit is located over the eastern half of the seamount.

• Unicorn smt is a flat topped edifice with the summit surface located at around -400 m; it is crossed by small volcano alignements running N-S some raising to -330 m. These small volcanoes seem to be relatively recent basaltic edifices.

• Seine smt is flat topped volcanic edifice too, with the terraced surface located at -160 m. The shelf break is interested by incipient erosion along several small canyon heads.

• Poor or null acoustic penetration was achieved through sub-bottom chirp confirming the nature of rocky outcrops and of bioclastic loose or indurated sediments.

• On Seine smt a better penetration was obtained with sparker source seismic profiles. They have shown a relatively transparent seismic unit 40-50 m thick overlying an acoustic basement interpretable as the volcanic basement. The upper unit very probably consists of loose to lithified bioclastic sediments and possibly pyroclastic deposits.

• The bioclastic sand production seems inversely proportional to water depth and is higher on top of Ampére  smt than on top of Seine smt. In this latter smt bio erosion and reworking prevail over the shelf area at -160-170 m; here a bio-calcarenite recalling an "hardground"litho- facies has been extensively recovered.

• The minimum depths detected for Ampére  smt are around 60 m and confirm almost all previous investigations. On the contrary Unicorn smt has the summit at around 330 m depths. In previous bathymetric charts (Bathymetry of NE Atlantic, 1983; Admiralty Chart n 3132, 1995) 256 m and 20 m minimum depths were reported. The density of our soundings rules out the case of having missed sea-bed information.

• Also Seine smt. has a minimum depth around -165 m contrary to what reported on these charts (-89 m).