1-cm Resolution Seafloor Surveys Combining Wide Swath Lidar, Multibeam Sonar, Stereo Cameras, and INS on an Articulating ROV Toolsled

The Monterey Bay Aquarium Research Institute has developed a low altitude, ROV-based seafloor mapping system that combines lidar laser ranging, multibeam sonar, and stereo photographic imagery to survey steep and complex terrain. When operated at a 3 m standoff, this system maps seafloor topography with a 1 cm lateral resolution and simultaneously collects 2.5 mm resolution color photography. The highest resolution bathymetry data are collected using a newly developed 3D at Depth Wide Swath Subsea Lidar (WiSSL) laser scanner. This lidar scans continuously across a 90° field of view, achieving a 1 cm sounding spacing at a 3 m altitude for speeds <= 1.0 m/s. Each sounding has a laser footprint of 0.8 cm. A 400-kHz Reson 7125 multibeam sonar provides 5 cm lateral resolution bathymetry across a 135° swath. Dual Prosilica GX1920 2.4 Mpixel color cameras provide color stereo photography of an 80° field of view illuminated by dual xenon strobes with a 2.5 mm raw image pixel resolution. Position and attitude data are provided by a Kearfott SeaDevil Inertial Navigation System (INS) integrated with a 300 kHz Teledyne RD Instruments Doppler velocity log (DVL) and a Paroscientific pressure sensor. The sensor package is mounted into an articulating frame within an ROV Ventana toolsled that rotates from pointing the sensors vertically down to horizontally forward, allowing surveys of flat to vertical seafloor. The articulation is automated by closing a control loop around the difference between the forward and backward beams of the DVL. The ROV survey paths are controlled autonomously in two modes. On slopes less than 20°, survey lines defined by waypoints are achieved by line-following using the realtime INS data. On steeper slopes, survey lines are defined as constant-heading depth transects. In either case, a 3-m survey line spacing insures overlapping coverage between adjacent lines. Following sea trials in early 2018, the articulating survey system has been used offshore Southern California to map a methane gas seep mound at 800-m depth in Santa Monica Basin and the trace of the active San Diego Trough Fault where it crosses the San Gabriel Channel. Surveys have also been conducted of deep-sea sponge and coral communities in and near Monterey Bay.