Doppler measurement of velocity structure beneath surface gravity waves

Abstract

The acoustic Doppler method has proved particularly useful for the investigation of small scale velocity structure associated with wave motions, shear instability, and turbulence. The incoherent bistatic approach permits measurement of the velocity field at meter scales within the crests of large and steep waves, allowing direct comparison with predictions in the region where theory is most likely to break down. At the other end of the scale, coherent Doppler measurement at megahertz frequencies can be used to identify and estimate the magnitude of turbulence beneath breaking waves. We contrast these different approaches using observations with a bistatic incoherent Doppler sonar placed on the floor of the North Sea and an array of monostatic coherent sonars deployed on the ocean surface off the California coast. The observations illustrate the power of Doppler techniques in resolving various aspects of wave dynamics which have been inaccessible using traditional methods.