Evaluation and preliminary results from an autonomous 3-axis coherent acoustic Doppler velocity profiler

Laboratory based Acoustic Doppler Velocity Profilers (ADVP) have found recent application to sediment transport and turbulence studies, due to their ability to provide vertical profiles with sub-centimetric spatial resolution at turbulent and inter-wave timescales. Adaptation of these instruments to autonomous units capable of prolonged deployment in marine environments is not straight forward due largely to the necessary reductions in power consumption, and the resulting trade off between operating frequency and signal to noise ratio, impacting upon the systems ability to de-alias ambiguous velocities over the whole profile. In the present paper, the system design, laboratory evaluation, and preliminary field trial results for an autonomous, 3-axis coherent ADVP will be presented. The system described operates at 1 MHz utilising a dual pulse repetition frequency to combat phase wrap velocity aliasing, and provides profiles over a 1.5 m range recording at 10 Hz, with a deployment duration of up to 5 weeks. In the laboratory, the instrument has been evaluated in a sediment tower, by comparing measured settling velocities obtained from spheres to theoretical estimates. The field trial was carried out in a shallow estuary, where the ADVP was evaluated relative to independent measurements of the three velocity components, and an assessment of the instruments dynamic range was conducted. The implications of the results to using autonomous ADVPs to probe sediment transport processes in marine bottom boundary layers will be discussed