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A little focus on bathymetry

Continue bathymetric mapping of the Southwest Indian Ridge is one of the spots that the campaign has set Smoothseafloor. As with the TOBI, the goal is to refine our knowledge of the structure of the seafloor and by the same tectonic and operation of opening a large scale as in the detail.

N / O Marion Dufresne is equipped with a multibeam sonar Sea Falcon 11 (TSM 5265B) which specifically allows the acquisition of bathymetry fine. It integrates real-time operation information from the central position of the vessel (vertical, roll, pitch, heading, speed boat) and inertial navigation (latitude, longitude). Given the study area, the sonar is used in deep mode, in the ranges 4000 m and 5000 m. The sonar emits beams simultaneously on five adjacent sub-parallel, perpendicular to the ship's route. Each of these 5 bundles corresponds to a specific transmission frequency of about 12 kHz, the center beam is the most powerful. Receiving acoustic sonar signals is ensured by five identical panels, arranged transverse to the boat. The receiving antenna is composed of 3 sub-antennas, a power plant, comprising a single panel, and two to port and starboard respectively, with 2 panels each.

The transceiver of 5 beams simultaneously multiplies by 5 the number of data in the direction of advance of the vessel and thus have a sample, and thus a resolution equivalent X and Y, for vessel speed of about 13 to 14 kts. The resolution on the merits of the order hundredth of the height of water. The depths of minimum and maximum use of 100 m and 11,000 m respectively. Cross coverage is 12 km per 3000 m of water. During

Smoothseafloor, originality of work lies in the fact that we use the sonar in an unusual mode, namely at a very low speed of the vessel, simultaneously with the TOBI survey. This can significantly increase the density of probes and consequently the resolution of the mapping on the merits. It is thus possible to achieve resolution of 20m, 10m and even after data cleaning and processing artifacts, which is remarkable given the average depth of about 4500m, the area in which we work.