Correcting for beam spread in acoustic Doppler current profiler measurements

Spatial homogeneity assumptions inherent in the conversion of directly measured acoustic Doppler current profiler (ADCP) beam to Cartesian coordinates for the Janus configuration are investigated. These assumptions may be adequate for large-scale flows, such as tidal currents and wind-forced upwelling. However, for high-frequency features, such as internal solitons and turbulence, the velocity fields may vary over scales comparable to the divergence of the acoustic beams. Equations are derived for beam spreading, and it is shown that a first-order correction can be applied to improve velocity measurement accuracy. Two cases are examined. First, the effects of the spatial and temporal convolution inherent in beam spreading from the Janus configuration ADCP are applied to a model internal solitary wave. It is shown that the corrected vertical velocities have deviations of less than 2 mm s(-1) for distances up to 30 m from the transducer face and are approximately 3 times more accurate than the uncorrected velocities for distance up to 20 m from the transducer face. Next, under a "frozen turbulence'' hypothesis, the method is applied to processing turbulence data. It is demonstrated that the horizontal longitudinal velocity can be markedly improved.