The use of phased-array Doppler sonars near shore

Phased-array Doppler sonars (PADS) have been used to probe an area several hundred meters on a side with 8-m spatial resolution, sampling every second or less with under 2 cm s(-1) rms velocity error per sample. Estimates from two systems were combined to produce horizontal velocity vectors. Here, concerns specific to use of PADS in shallow water are addressed. In particular, the shallower the water is, the larger the fraction of bottom backscatter, so the stronger the bias is toward zero Doppler shift in the estimates. First, direct comparisons are made with other current measurements made during the multi-investigator field experiment "SandyDuck,'' sponsored by the Office of Naval Research, which took place in the autumn of 1997 off the coast of Duck, North Carolina. The coherences between PADS and in situ current measurements are high, but the amplitude of the sonar response is generally low. To explore this further, a simplified model of wave shoaling is developed, permitting estimates of wave-frequency velocity variances from point measurements to be extrapolated over the whole field of view of PADS for comparison. The resulting time-space movies of sonar response are consistent with quasi-steady acoustic backscatter intensity from the bottom competing with a variable backscatter level from the water volume. The latter may arise, for example, from intermittent injection of bubbles by breaking waves, producing patches of high or low acoustic response that advect with the mean flow. Once this competition is calibrated via the surface wave variance comparison, instantaneous measured total backscatter intensities can be compared with an estimated bottom backscatter level (which is updated on a longer timescale, appropriate to evolution of the water depth or bottom roughness) to provide corrected sonar estimates over the region.