Continuous Time–Space Sampling of Near-Surface Velocities Using Sound

Abstract

A phased-array Doppler sonar (PADS) system is described that uses sound at frequencies near 200 kHz to probe an area several hundred meters on a side with 7?20-m spatial resolution. The area can be sampled every second or less with under 2 cm s?1 rms velocity error per sample. Radial velocity estimates from two or more systems can be combined to produce time series of horizontal velocity vector maps over the irregularly shaped overlapping region. Such extensive and continuous sampling in time and space permits analysis via direct 3D Fourier transformation, for example, producing complete wavenumber?frequency spectra. Free waves, bound harmonics, finite-amplitude effects, Doppler shifting by currents, etc., can be studied. Extended temporal sampling permits investigations into lower-frequency vortical and internal wave modes as well as surface waves, and of the modulation of these by tides. A pair of PADS was deployed as part of SandyDuck, a large collaborative field experiment held in 1997 near Duck, North Carolina. An example drawn from SandyDuck data illustrates use of the technique, demonstrating that both mean flow and oscillatory (wave) motions can be detected.