Broadband parametric imaging of breaking ocean waves

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 assum ed to possess predominantly spiller characteristics, For this frequency ban d, the wavelength of sound in bubble-free water is much larger than the sur face wave height and the depth of the breaker bubble plume, so both were co nsidered insignificant. The 15-element array was configured as a sparse hor izontal 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 tw o-dimensional Gaussian profile: information from up to six independent freq uencies was combined to yield a maximum-likelihood image. Analysis shows th at the images align closely with the wind and can be observed moving downwi nd with a speed roughly equal to 70% of the phase speed of the dominant win d waves. A model of acoustic source strength which is linear in log frequen cy is: found Lit the data reasonably well, and model parameters are provide d 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 (C) 2001 Acoustic al Socie ty of America.