A Bistatic Phased-Array Doppler Sonar for Wave–Current Research

Abstract

ave breaking and wave-forced flows are important to air?sea interactions and to the transport and dispersal of materials at sea. But recent measurements have shown a discrepancy in the Eulerian response to wave groups compared to scientists? current theoretical understanding of wave?current interactions. Flow structures on scales of centimeters to meters occur underneath breaking waves, and larger-scale flows are driven by wave?current interactions (e.g., Langmuir circulation, alongshore flows). Such details of the vertically resolved flow are just beginning to be modeled, and observational guidance is needed. Here a new instrument is described that is intended to measure waves and currents over a 2D vertical plane underwater, resolving two components of velocity on this plane. Initial observations were made near the Scripps Pier (La Jolla, California), where steep waves and strong currents can be reliably found, yet logistics are not too burdensome. To get the spatial resolution desired using 200-kHz sound, ping-to-ping ?coherent processing? would have be used for Doppler estimation; however, near shore the reverberations remain too strong for far too long to get any coherence, unlike the previous experience in deep water. In view of this, using much higher frequencies (>1 MHz) with ?incoherent processing? is suggested; the increased attenuation at higher frequencies then would subdue the reverberation problem, but with comparable space?time resolution.