Testing and evaluation of a dual-frequency six-beam acoustic Doppler current profiler

Abstract

Observations of current velocity in near-surface and near-bottom boundary layers are critically important for many scientific, operational, and engineering applications. NOAA's National Data Buoy Center (NDBC) presently maintains approximately 110 weather and oceanographic buoys. Of these, some are tasked to measure full current proflies, including near-surface currents, in an operational real-time system. Nortek developed a dual-frequency, six-beam acoustic Doppler current profiler to meet the needs of observing the complete water column velocity profile, including the near-surface or near-bottom currents (depending on the mounting orientation). Three acoustic beams point upward or downward in the traditional current profiler mode. The other three acoustic beams are directed horizontally and spaced equally around the circumference of the profiler with 120 deg spacing between the beams. These beams measure the two-component horizontal currents at the level of the instrument. This geometry allows the observation of current velocity near the surface for buoy-mounted profilers and near the bottom for bottom-mounted profilers. Near-surface and water column current velocity profile observations from a 6-beam Nortek profiler mounted on an NDBC 3 m discus buoy are compared with current velocity profile measurements from a bottom mounted 1 MHz Nortek AWAC acoustic Doppler wave and current profiler located within 100 m of the buoy location.