AUTONOMOUS UNDERWATER VEHICLE-BASED HYDROGRAPHIC SAMPLING

An autonomous underwater vehicle (AUV), the U.S. Navy's Large Diameter Unmanned Underwater Vehicle (LDUUV), was used as a stable platform fo r rapid, repeated, near-synoptic CTD measurements of estuarine variabi lity in Narragansett Bay. Surveys were made in lawnmower-like patterns at middepth to obtain horizontal profiles and maps, and in vertical y o-yo patterns to obtain vertical profiles. These observations were gro und-truthed using standard CTDs on the fixed position of the launch ca ge and on ship-based surveys around the perimeter of the study area be fore and after the runs. For the horizontal surveys, a comparison of t emperature and salinity time series from the LDUUV and the launch cage CTDs shows that differences are within the range of lateral variabili ty around the study area observed at run depth from the ship. For the yo-yo surveys, a comparison of LDUUV CTD and standard CTD profiles sho ws indications of hysteresis in the vehicle-obtained data, which can b e minimized with improved sampling techniques. Horizontal profiles and maps were obtained for a 2000 m x 300 m area, which was repeatedly sa mpled during 1994. An example of a time series from the vehicle shows three crossings of a salinity front with no significant temperature va riability. Accompanying platform data show the effects of turns and sp eed changes on the data acquisition process. In the maps, temperature and salinity variability were observed with horizontal scales of order 100 m, finestructure advected by the tidal current. Temperature-salin ity relationships in the AUV-derived data show unique differences rela ted to seasonal changes in Narragansett Bay, including the winter-spri ng and the subsequent summer-fall transition of 1994. These measuremen ts show the viability of the AUV as a unique tool for hydrographic cha racterization, under vehicle, environmental, and oceanographic constra ints.