Multibeam Sonar Mapping of the Explorer Ridge with an Autonomous Underwater Vehicle

In June of 2002, we demonstrated the SM2000 multibeam sonar on the Autonomous Benthic Explorer (ABE) as part of a NOAA-funded Ocean Exploration cruise to the Explorer Ridge (49 46'N, 130 16'W). The same sonar was used on several earlier studies from the JASON remotely operated vehicle, but in this effort the vehicle operated with full autonomy from the surface vessel, freeing the ship up for other tasks such as vessel-mounted multibeam survey and CTD casts. The AUV survey focussed in the area of the Magic Mountain vent field. The SM2000 operates at 200 khz and has a narrow-beam transmitting transducer and 80 receive elements, which are typically processed to form 128 beams. As the commercially available SM2000 topside computing hardware is incompatible with ABE, we recorded raw element data from the SM2000 head using a WHOI-built logger, and performed all beamforming and bottom detection after recovery. The configuration in which we used the SM2000 resulted in a 30 times improvement in the density of the sonar soundings over the mechanically scanned sonar previously used on ABE. ABE ran tracklines at the height of 40 meters off-bottom with 60 meter trackline spacing. While the SM2000 has considerably higher maximum range, we chose these parameters based on compatibility with other sensors (magnetometer, plume sensors, and other sonars). The multibeam sonar provides a view of the seafloor that is unprecedented in detail. In particular, small faults and fissures that are not resolvable with the mechanically-scanned sonar can be seen clearly in the multibeam data. This information was used to guide the ROV ROPOS on later dives in the Magic Mountain vent field. Many active vents were located along subtle structural features that were evident in the SM2000 bathymetry. For example, some chimney clusters are located along small faults with variable offset, whereas others are located on discrete but localized sulfide mounds. In each case, the ROV was able to easily locate and follow the trend of these small-scale features based on the SM2000 bathymetry.