MBARI mapping AUV operations: In the Gulf of California

Abstract

The Monterey Bay Aquarium Institute (MBARI) has conducted high-resolution multibeam bathymetry, sidescan, and subbottom sonar surveys in the Gulf of California using an autonomous underwater vehicle (AUV) as part of a three-month, two-ship oceanographic expedition. The seafloor mapping AUV D. Allan B. was operated from R/V Zephyr, achieving 22 successful 17-hour missions during March and April 2012. The 1-m lateral resolution seafloor maps and subbottom profiles were used to guide subsequent ROV Doc Ricketts dives made from MBARI's R/V Western Flyer, which combined visual surveys, sampling, and in situ experimentation by several different science groups. The vehicle is a modularized Dorado Class 54 cm (21 inch) diameter, 5.18 meter (17 feet) length, 733 kg (1616 pounds) air weight. The interior of the AUV floods with water when launched; electronics are contained in either one-atmosphere hermetic housings or in pressure-compensated enclosures. The AUV is ballasted to be about 3.63 kg (8 pounds) positive in seawater. Flotation is accomplished with 6000 meter rated syntactic foam. The D. Allan B. has three mapping sonar systems onboard: a Reson 200 kHz multibeam echosounder, an Edgetech FS-AU Sonar Package which comprises a 100 kHz chirp sidescan, a 410 kHz chirp sidescan, and a 2-16 kHz chirp subbottom profiler. With the onboard Lithium Polymer battery packs, the Mapping AUV has an endurance of approximately 17 hours at a speed of 1.5 meters per second, providing a range of 55 to 85 kilometers. The mapping AUV is equipped with a Kearfott inertial navigation system (INS) with Doppler velocity log, a Paroscientific pressure sensor, and an ultrashort baseline (USBL) and acoustic modem for communications. It also has an Iridium Satellite Short Burst Data modem, a drop weight, radio direction finder, Homer Pro and visible strobe location aids. Propulsion is accomplished with a duct-protected articulated propeller which provides 52 Newtons (12lbf) of thrust at 300 rpm as well as elevator and rudder functions. Turning diameter is less than 20 meters; climb/dive rate is approximately 30 meters per minute. Launches and recoveries were made from the RV Zephyr, a 24m (79 foot) former pilot boat that operates with four crew and four science personnel. The Zephyr is outfitted with a Brook Ocean Technologies J-Frame crane for AUV launch and recovery. Once the AUV is in the water, the mission is sent from a computer on the Zephyr to the AUV over a wireless Freewave Ethernet link. At the start of the mission, the AUV spirals down to the start-of-line while being position-updated with GPS data from the Zephyr aided by the USBL system. A Bottom Pressure Recorder used to provide in situ tide data for bathymetry correction was deployed prior to all AUV missions, and recovered afterwards, using a Benthos Recoverable Transponder mounted to an MBARI-designed frame. The seafloor mapping data were processed using MB-System, an open source software package developed at MBARI and the Lamont-Doherty Earth Observatory. The first half of the mapping AUV surveys focused largely on four transform faults in the central and northern gulf. The combination of high-resolution bathymetry and subbottom profiles enabled the identification of active fault traces and zones of subsurface gas and hydrate. The second half of the Zephyr expedition focused on the Alarcon Rise, a volcanic spreading center. The new surveys provide the first 1-m scale bathymetry coverage of the neovolcanic zone along an entire ocean spreading center segment. The mapping data also identified the locations of several fields of hydrothermal vent chimneys, two of which were confirmed to be currently active by ROV dives.