Autonomous underwater vehicle D. Allan B. maps the deep seafloor
In the spring of 2005, MBARI launched a specialized autonomous underwater vehicle (AUV) designed to map the seafloor with higher resolution than possible with traditional ship-based systems. Within months the mapping AUV—later christened the D. Allan B. after MBARI’s late, long-time board member, D. Allan Bromley—was enabling new science discoveries in the deep sea.
Even though 70 percent of our planet is covered by ocean, less than five percent of the deep sea has been studied. MBARI's AUV, the D. Allan B. extends our ability to explore the deep sea, much in the same way that planetary rovers have given us a fresh understanding of Mars. Unlike those vehicles, the D. Allan B. can fly into the unknown and return all in a day's work. In many ways, ocean science is more challenging than space exploration because light and sound are attenuated within meters in the ocean, forcing researchers to rely on high-precision acoustics to map seafloor features.
Seafloor mapping using an AUV has some distinct advantages over mapping using traditional shipboard sonars. The resolution of a sonar system depends on, among other things, how close the sonar is to the bottom. Because MBARI's AUV can fly as low as 30 meters (about 90 feet) above the seafloor, the on-board sonars can distinguish fine-scale details that would be invisible from shipboard sonars. In addition, an AUV can map an area three times faster than a ship towing sonar equipment.
The D. Allan B. is a torpedo-shaped vehicle, about five meters long, equipped with four mapping sonars that operate simultaneously during a mission. Each of the sonar devices on the AUV measures a distinctive aspect of the seafloor, all by bouncing sound waves off the bottom and measuring the strength of the reflected signals. A multibeam sonarcollects bathymetry information for creating detailed relief maps, much like the topographic maps used by hikers. Two sidescansonars image the character of the seafloor,producing photograph-like views of the bottom. These signals show subtle topographic features only a few centimeters high, and also indicate whether the seafloor is soft or hard, rocky, muddy, or silty. Finally, a sub-bottom profiling sonar images the structure beneath the seafloor,detecting boundaries between layers of mud and rock below the seabed. Together, these data reveal the nature of the seafloor today and help us understand how it formed.
To map an area as completely as possible, the D. Allan B. is programmed to fly in a “lawnmower” pattern, making a series of side-by-side passes, or swaths, up and back across the zone to be mapped. The swaths overlap by varying amounts, so the AUV also makes a few passes at 90 degrees to the first passes in order to determine how the swaths line up with one another.
In addition to sonars, the AUV also carries a navigation system, an electric propulsion motor, an on-board computer, and an emergency drop weight. The drop weight can be jettisoned should the vehicle become stranded, allowing the AUV to float—eventually—to the surface. The sonar instruments are packed into the AUV's half-meter (21-inch) diameter shell. Each instrument is contained in a sealed pressure housing that can withstand repeated dives to 6,000 meters (roughly 18,000 feet). Operating 50 meters above the seafloor, D. Allan B. maps about a square kilometer for every hour of surveying.
The huge quantities of data gathered by the vehicle are processed using “MB-System”, specialized software developed by MBARI’s Dave Caress and a colleague at the Lamont-Doherty Earth Observatory. MB-System is an open-source software package for the processing and display of bathymetry and backscatter imagery data derived from multibeam, interferometry, and sidescan sonars. “We process the data at sea to verify that the vehicle is functioning properly,” explains Hans Thomas, AUV group leader, “And in many of our missions, we are passing the data on to scientists to aid in their dives with ROVs or other research tools. It can be quite a challenge.”
The D. Allan B. has been deployed from the MBARI research vessels Point Lobos, Zephyr and Western Flyer, as well as the R/V Thomas Thompson operated by the University of Washington, andR/V Atlantis operated by the Woods Hole Oceanographic Institution. With a new portable crane proposed for 2008, the AUV group will be able to deploy the D. Allan B. from many more ships of opportunity. This will enable MBARI scientists to study areas beyond Monterey Bay and will foster future collaborations with researchers around the globe.
“It's very gratifying to now be achieving true one meter lateral resolution at greater operational efficiency than anyone else in the world”, says project lead Dave Caress, “We're also having great fun. I'm now happily overworked dealing with surveys of unprecedented resolution of submarine canyons, methane hydrate deposits, fault structures, seamounts, and spreading centers, and we've really just gotten started.”
MBARI contributors: Knute Brekke, Dave Caress, Doug Conlin, T. Craig Dawe, Andy Hamilton, Rich Henthorn, Scott Jensen, Bill Kirkwood, Rob McEwen, Paul McGill, D.J. Osbourne, Karen Salamy, Farley Shane, Mark Sibenac, Mark Talkovic, Hans Thomas, Duane Thompson