Broadband parametric imaging of breaking ocean waves

Abstract

An acoustic array was deployed in the near-surface layer of a fetch-limited coastal inlet to image breaking waves using only the sound radiated in the band (400 Hz to 2000 Hz) from the breaking region. The breakers were assumed to possess predominantly spiller characteristics. For this frequency band, the wavelength of sound in bubble-free water is much larger than the surface wave height and the depth of the breaker bubble plume, so both were considered insignificant. The 15-element array was configured as a sparse horizontal cross with an 8 m aperture, bottom moored, and positioned nominally 3 m beneath the surface. Propagation from the source to the array elements assumed dipole sources, an acoustically flat surface, and an acoustically thin bubble plume. The radiating region was parameterized by a broadband two-dimensional Gaussian profile: information from up to six independent frequencies was combined to yield a maximum-likelihood image. Analysis shows that the images align closely with the wind and can be observed moving downwind with a speed roughly equal to 70% of the phase speed of the dominant wind waves. A model of acoustic source strength which is linear in log frequency is found fit the data reasonably well, and model parameters are provided for a single wind speed. Unlike other imaging experiments, this technique provides measurements of the size and shape of the bubble-creation region at or near the peak of the radiated autospectrum.