Eliminating the Blanking Distance for Buoy-Mounted
Acoustic Doppler Current Profilers
222 Severn Avenue
Annapolis, MD 21403
NOAA National Data Buoy Center
Stennis Space Center, MS 39529
NOAA Chesapeake Bay Office
Annapolis, MD 21403
Observations of near-surface current velocity from buoy-mounted ADCP’s are critically important for
many scientific, operational, and engineering applications. Accurate measurements of near-surface currents are required for studying the dynamics of surface features, such as freshwater plumes, harmful algal blooms, and surface contaminants; as well as useful for search and rescue operations and to corroborate HF radar maps of current velocity. However, near-surface current velocity has always been intrinsically difficult to measure with acoustic Doppler current profilers mounted on surface buoys due to the necessary deployment depth below the surface and the blanking distance to the first measurement cell.
Nortek developed Aquadopp Z-Cell Profiler, 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. For buoy-mounted applications, three acoustic beams 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. The horizontal beams operate at a frequency of 2 MHz and may be configured to sample at a distance of up to 2.5 m away from the buoy (blanking distance). The Z-Cell offers the profiling beams in a 1 MHz or 600 kHz acoustic frequency, for a nominal profile range of 20 and 50 m, respectively.
New observations of near-surface currents from Nortek Aquadopp Z-Cell Profilers deployed on various NOAA buoy platforms are discussed in detail. Near-surface and water column current velocity profile observations from a Z-Cell Profiler mounted on an NDBC 3 m discus buoy (northern Gulf of Mexico) are compared with current velocity profile measurements from a bottom mounted Nortek AWAC acoustic Doppler wave and current profiler located within 50 m of the buoy location. A tidal analysis suggests that velocity data from the horizontal beams (Cell 0) are of good quality and consistent in direction and magnitude with the velocity measurements in cells below, with the AWAC velocity and with theory. Cross-spectral analysis indicate that synoptic band velocity measured at Cell 0 is highly correlated (90%) with velocity at Cell 2 and may be up to 25% higher than velocity measured 2.5 m lower in the water column. During periods of increased wind and wave activity, vertical shear in the upper 3 m of the water column are observed to be on the order of 20 cm/s for over 48 hours before the water column becomes well mixed. Current speed and direction differences between Cell 0 and lower cells project a spatial separation of water parcels as much as 20 km/day, with a mean separation of 8.5 km/day.
Fig. 1 Drawing of Nortek Aquadopp Z-Cell Profiler and example bottom mount and buoy mount applications. The blanking distance is eliminated and velocity is measured at Cell 0 using horizontally directed acoustic beams.
Fig. 2 Three Z-Cell velocity profiles showing high velocity shear between Cell 0 and Cell 1 near the surface [top]. Compass plots of wind direction and current direction for each current profile [bottom]. Wind speed in given in the lower legend. Wind vector has arbitrary but constant magnitude (only plotted to indicate wind direction). Current vector has correct magnitude and direction.