Acoustic Doppler Profiler

dopplergraphicAcoustic Doppler Profiler: The Wind Energy Center is operating an Acoustic Doppler Profiler to measure wave heights at prospective offshore wind turbine locations off the coast of Hull, Massachusetts.

The on-site ocean wave and current data for the Hull Offshore Wind Project are taken by the Wind Energy Center using an acoustic Doppler profiler (ADP). The ADP used in the Hull Offshore study is an acoustic device designed to measure volume-averaged, three-dimensional fluid velocities in water, at several depths simultaneously. The three transducers of the ADP each send out four narrow-band, 9 kHz signals per second, which are reflected off of particles in the water, and the surface. The return signals are analyzed to recover the range to the reflecting object, and the component of that objects velocity parallel to the beam. These range and velocity data from the three beams are combined and resolved into x,y and z components of velocity averaged over each of 25 range bins or cells. The blanking region of the water column is the space close to the ADP transducers for which the signal returns too quickly to be measured, since the transducers are still ringing from sending the signal.

ADP Volume-Averaging

menonboatThe ADP being used at Harding ledge has a high-precision pressure transducer, that is used to measure the depth of the water column above the ADP. These data are also logged at 4 Hz. The water depth time series is analyzed to garner the significant wave height, and peak spectral period which characterize the sea state for each hour. The Directional characteristics of the waves are determined from the acoustic velocity data.

The Data collected by the ADP of relevance to the Hull Offshore Project are: current speed and direction, and water depth, reported half-hourly, and first-order wave characteristics: significant wave height, peak spectral period, dominant direction, and directional spreading all reported hourly.