Mid-frequency acoustic tracking of breaking waves

Abstract

Large surface wave breaking events in deep water are acoustically detectable by beamforming at 5-6 kHz with a mid-frequency planar array located 130 m below the surface. Due to the array's modest 1 m horizontal aperture, wave breaking events cannot be conveniently tracked by beamforming alone. However, their trajectories can be estimated by splitting the array into left-right and upper-lower sub-array pairs, beamforming each sub-array toward the source, and computing the left-right and upper-lower beam cross-correlations. The cross-correlations can be used to estimate the relative time delay between sub-arrays of noise generated by breaking waves. The time delays map to a source location on the surface that can be tracked over time. Source tracks estimated from the sub-array cross-correlations match the trajectories of breaking waves that are visible in aerial images of the ocean surface above the array. [Work supported by the Office of Naval Research.]