Near-Transducer Errors in Acoustic Doppler Current Profiler Measurements

Previous studies have shown that velocities measured near the Acoustic Doppler Current Profiler (ADCP) transducers are biased low. The error was associated with two causes: acoustic ringing and the flow disturbance created by the submerged part of the ADCP in its vicinity. Acoustic ringing defines the resonance of the transducer and the associated electronics occurring after the acoustic pulse is generated until the vibrations die out. The local flow disturbance is induced by the flow blockage and the drag acting on the ADCP and it is commensurate with the ADCP submergence and its geometry. Given that these errors contaminate the measured velocities in the bins close to the transducers, they are collectively addressed in this paper as the near-transducer error. The paper presents results of a study conducted in a laboratory flume with an ADCP specifically designed for reducing the ringing effect (ZedHed produced by Teledyne RD Instruments). The measurements were conducted in shallow, low-velocity flows (i.e., bulk flow velocities between 0.5 ft/s and 3 ft/s and flow depths between 1 ft and 3 ft) where the near-transducer error becomes increasingly significant. The results show that by operating the ADCP with minimum blanking distance the error in the velocity estimates near the transducers can be up to 8.23% from the mean velocity in the undisturbed channel (for a flow depth of 2 ft and velocity of about 3 ft/s), and up to 27.5% when the flow is very shallow (for a flow depth of 1 ft and bulk velocity of about 1 ft/s). The results of the study also indicate that the major fraction of the near-transducer bias error is due to the ADCP blockage and the drag acting on the submerged part of the ADCP, with ringing playing a secondary role.