Mid-ocean acoustic current profiling from submerged buoys

Summary form only given, as follows. An intercomparison experiment with two buoy-mounted upward-looking Acoustic Doppler current profilers (ADCP´s) was carried out during 7-11 January 1986 in the Florida Current at a water depth of 670 m. A 75 kHz profiler with 30° beam angle was moored at 610 m depth in one mooring and a second profiler, working at 150 kHz and a beam angle of 20° and located at 470 m depth was moored about 800 m upstream. Directly underneath both systems, Aanderaa current-meters (ACM´s) were moored for comparison. Acoustic profiling was in time intervals of 10 minutes, with 400 pings per ensemble using 4.3 m bin width at the 150 kHz system, and 200 pings and 8.0 m bin width at the 75 kHz system. The 75 kHz system gave significant echo amplitudes over the full depth range of 600 m, the 150 kHz system to about 330 m above the transducer. Current speeds measured by both systems in the depth range 470-300 m differed by only 1.4 - 2.4cm/s; comparison with ACM currents was also within a few cm/s. Near the surface, Doppler current speeds were biased toward low values by side-lobe returns traveling straight up to the surface and back. This biassing was extended to greater depth in periods of strong mooring inclination by the method of bin number reassignment which is used to assure that Doppler currents are calculated from approximately the same depth level regardless of the inclination of individual beams. The overall result was that ADCP´s mounted on top of subsurface moorings can be used for current profiling over vertical ranges of hundreds of meters, but that special precautions have to be taken to get good data from near the surface.