Showing papers by "Manhar R. Dhanak published in 2001"

Coastal Oceanography Using a Small AUV

Abstract: Bathymetry, current, temperature, and depth (CTD) measurements using a small, mobile, autonomous underwater vehicle (AUV) platform are described. Autonomous surveys of a shallow water column off the east coast of Florida during December 1997 were carried out using a 2.13-m long, 0.53-m maximum diameter Ocean Explorer series AUV, equipped with a 1200-kHz acoustic Doppler current profiler (ADCP) and a CDT package. At a speed of 1‐2 m s21, this AUV can perform preprogrammed missions over a period of several hours, collecting in situ oceanographic data and storing it on an onboard datalogger. The vehicle may also carry sidescan sonar or a custom small-scale turbulence measurement package or other instruments for subsidiary measurements. The versatility of the AUV allows measurement of oceanographic data over a substantial region, the motion of the platform being largely decoupled from that of any surface mother ship. In the missions of 5 and 11 December 1997, ‘‘lawn mower pattern’’ AUV surveys were conducted over 1 km2 regions on the east coast of Florida, north of Fort Lauderdale, at depths of 7 and 3 m, respectively, in a water column where the depth ranged from 10 to 32 m. During 5 December, the region was subjected to a cold front from the northwest. Local wind measurements show presence of up to 10 m s21 winds at temperatures of up to 108‐158C below normal for the time of the year. The fixed ADCP indicates occurrence of significant internal wave activity in the region. The data collected using the mobile AUV are utilized to develop a map of the bottom topography and examine current, temperature, and density variations in the context of the background information from a fixed bottom‐mounted ADCP and Coastal-Marine Automated Network buoys. The work described here is a significant step in the development of an autonomous oceanographic sampling network, illustrating the versatility of an AUV platform. The data collected during the missions described will form part of a bank for information on the impact of a cold front on shallow subtropical waters. The authors expect to repeat the missions during other such fronts.

An AUV survey in the littoral zone: small-scale subsurface variability accompanying synoptic observations of surface currents

Abstract: A survey of small-scale subsurface variability within the synoptic observational field of an ocean surface current radar (OSCR) using an autonomous underwater vehicle (AUV) is described. The survey involved observation of a developing upper mixed layer in a littoral zone off southeast Florida, on the edge of a strong Florida current during the summer of 1999. Complimentary in situ observations from a bottom-mounted acoustic Doppler current profiler (ADCP), conductivity-temperature (CT) chain arrays, atmospheric measurements from a surface buoy, and CTD and ADCP observations from a surface ship provided the background to the survey. The AUV, the Ocean Explorer, equipped with a CTD, downward and upward looking ADCPs, and a small-scale turbulence package, was used to conduct a continuous 12-h survey of small-to-fine-scale variability within a few grid cells of the surface current radar field. The vehicle repeatedly sampled the same grid in a set pattern at a fixed mid-water depth. Maps of developing spatial distribution of current, salinity, temperature, and rate of dissipation have been developed using the AUV-based observations. The observed features in the current field compare well with the OSCR and the bottom-mounted ADCP measurements.

Active Turbulence Following Using an AUV

Abstract: : The long-term goal is to be able to follow any turbulence structure in the ocean environment with an autonomous underwater Vehicle (AUV) while quantifying, in real time, the turbulence field.