Passive Acoustic Determination of Wave-Breaking Events and Their Severity across the Spectrum

A passive acoustic method of detecting breaking waves of different scales has been developed. The method also showed promise for measuring breaking severity. Sounds were measured by a subsurface hydrophone in various wind and wave states. A video record of the surface was made simultaneously. Individual sound pulses corresponding to the many individual bubble formations during wave-breaking events typically last only a few tens of milliseconds. Each time a sound-level threshold was exceeded, the acoustic signal was captured over a brief window typical of a bubble formation pulse, registering one count. Each pulse was also analyzed to determine the likely bubble size generating the pulse. Using the time series of counts and visual observations of the video record, the sound-level threshold that detected bubble formations at a rate optimally discriminating between breaking and nonbreaking waves was determined by a classification-accuracy analysis. This diagnosis of breaking waves was found to be approximately 70%–75% accurate once the optimum threshold had been determined. The method was then used for detailed analysis of wave-breaking properties across the spectrum. When applied to real field data, a breaking probability distribution could be obtained. This is the rate of occurrence of wave-breaking events at different wave scales. With support from a separate, laboratory experiment, the estimated bubble size is argued to be dependent on the severity of wave breaking and thus to provide information on the energy loss due to the breaking at the measured spectral frequencies. A combination of the breaking probability distribution and the bubble size could lead to direct estimates of spectral distribution of wave dissipation.