1 Woods Hole Oceanographic Institution, Dept. of Applied Ocean Physics
and Engineering, Woods Hole, MA 02543, United States
2 University of Tennessee, Mechanical and Aerospace Engineering and
Engineering Science, Knoxville, TN 37996, United States
3 University of Washington, Box 357940, Seattle, WA Box 357940, United
States
4 Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Iba 305-8567, Japan
In August 2000, we used the Autonomous Benthic Explorer (ABE) to measure the heat flux from the Main Endeavour Field on the Juan de Fuca Ridge. The survey strategy utilized many of the unique strengths of a fully autonomous, untethered robotic vehicle to obtain high quality mapping results of both the seafloor and the hydrothermal plumes. ABE made 10 dives ranging in length from 5 hours on the bottom to over 30 hours on the bottom. One of the dives was dedicated to mapping the bathymetry using a mechanically scanned sonar, all other dives were dedicated to plume mapping. Total time on the bottom for the cruise was 114 hours covering over 225 km of bottom tracks. The survey technique took full advantage of ABE's ability to determine its location using returns from a conventional long-baseline transponder net, execute preprogrammed patterns of tracklines, hold depth, and follow the bottom. On most dives, we programmed ABE to cover the area above the main field (700mx300m) at constant depth in a grid pattern of 20 meter-spaced tracks. On other dives, ABE covered the sides of survey area with vertical spacing of 10 meters. When necessary to avoid the seafloor, ABE's bottom following took precedence over holding depth, allowing both side and top tracklines to be tightly connected to the bottom and the valley wall to the west of the Main Field. Newly developed high-capacity lithium ion battery packs enabled ABE to run surveys nearly 5 times as long as on previous expeditions. The assembled battery uses 378 lithium ion D cells, combined into 9 element clusters and 126 cell packs. Using a WHOI-developed system for charging, real-time monitoring, and recharge, these batteries proved to be reliable, safe, and simple to use. This extended endurance allowed a single, continuous survey to last for more than 30 hours, covering 59 kilometers over 2 full tidal cycles. Instruments on ABE included a Sea Bird pumped and ducted CTD, a MAVS acoustic current meter, a turbidity sensor, and a redox potential (Eh) measuring instrument. Using these instruments, we identified the boundaries of the plumes and determined the heat flux. The effect of the plumes on the vehicle was much larger than anticipated, often driving the vehicle 10 meters or more above its assigned depth despite the application of full counteracting thrust. We obtained an estimate of the plume velocity by combining water velocity from the MAVS current meter with a vehicle vertical velocity estimate derived from the vehicle pressure depth. We further strengthened the vertical water velocity estimate by invoking a dynamic model of the vehicle vertical response. We determined the model structure and coefficients from an open-loop system identification run performed at depth at the end of one dive.