Autonomous Underwater Vehicle–Based Hydrographic Sampling

Abstract

An autonomous underwater vehicle (AUV), the U.S. Navy?s Large Diameter Unmanned Underwater Vehicle (LDUUV), was used as a stable platform for rapid, repeated, near-synoptic CTD measurements of estuarine variability in Narragansett Bay. Surveys were made in lawnmower-like patterns at middepth to obtain horizontal profiles and maps, and in vertical yo-yo patterns to obtain vertical profiles. These observations were ground-truthed using standard CTDs on the fixed position of the launch cage and on ship-based surveys around the perimeter of the study area before and after the runs. For the horizontal surveys, a comparison of temperature and salinity time series from the LDUUV and the launch cage CTDs shows that differences are within the range of lateral variability 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 shows indications of hysteresis in the vehicle-obtained data, which can be minimized with improved sampling techniques. Horizontal profiles and maps were obtained for a 2000 m ? 300 m area, which was repeatedly sampled during 1994. An example of a time series from the vehicle shows three crossings of a salinity front with no significant temperature variability. Accompanying platform data show the effects of turns and speed 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?salinity relationships in the AUV-derived data show unique differences related to seasonal changes in Narragansett Bay, including the winter?spring and the subsequent summer?fall transition of 1994. These measurements show the viability of the AUV as a unique tool for hydrographic characterization, under vehicle, environmental, and oceanographic constraints.