On the determination of breaking surface wave distributions using ambient sound

Abstract

When surface gravity waves break, they generate an acoustic signal which can be used to derive information about their properties and distribution. Observations of sound formed in this way have been obtained at 1, 10, and 40 m below the surface in the fetch limited environment of Georgia Strait, British Columbia. Interpretation of the results is aided by a model of randomly distributed sound sources on the ocean surface. It is shown that the signal variability is governed by the number density, or mean spacing of the breakers, by the tendency for breakers to repeat at half the wave frequency, and by the group structure of the wave field. Calculated wave field properties reveal several features which can be related to changes in the wind, including variations in wave period, mean breaker spacing, and the average acoustic strength of individual breakers. Bubble clouds caused by the breaking waves can also be detected, and there is evidence that they contribute to the trapping of sound in a shallow layer close to the surface.