The acoustics of ocean waves

When a wave breaks, it creates a sub-surface population of bubbles, each of which ''rings'' for several milliseconds after the instant of formation. These wind-generated bubble sources are important contributors to the ambient noise field in the ocean. Although the noise is a stochastic phenomenon, it possesses a number of stable properties, as a result of which it can be inverted to provide useful information on a variety of ocean parameters and processes. Several ocean-noise inversions schemes are currently being developed, not only in connection with surface phenomena, including bubble production and diffusion at the sea surface, but also with the aim of determining the gee-acoustic parameters of the seabed: the bubble-generated noise contains information about both boundaries, the surface and the bottom. Moreover, the noise also acts as a form of natural acoustic illumination, analogous in some ways to daylight in the atmosphere, offering the potential for imaging objects in the ocean. An ambient noise imaging scheme is described, based on the ADONIS acoustic lens, and an example of an image obtained using only ambient noise illumination is presented. The possibility of obtaining images of the evolving source structure within a breaking wave is also explored.