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

Abstract

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 soundlevel 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.Corresponding author address: Richard Manasseh, Fluid Dynamics Cluster, CSIRO Manufacturing and Infrastructure Technology,

Semiempirical Dissipation Source Functions for Ocean Waves. Part I: Definition, Calibration, and Validation

“…This tends to overestimate the breaking probability by a factor of 1.5 to 2 (Manasseh et al 2006), compared to the present approach in which we consider that several waves (of different scales) may be present at the same place and time. We shall thus correct for this effect, simply dividing P by 2.…”

Wave modelling – The state of the art

“…Consistency of this feature has been confirmed by further investigations of the Lake George wave breaking data by independent means (Babanin and Young, 2005;Manasseh et al, 2006) where the two-phase behaviour of the spectral dissipation has also been obtained.…”

“…A passive acoustic method of detecting individual bubble-formation events developed by Manasseh et al (2006) was found promising for obtaining both the rate of occurrence of breaking events at different wave scales and the severity of wave breaking. A combination of the two should lead to direct estimates of the spectral distribution of wave dissipation.…”

Wave Climate and Wave Mixing in the Marginal Ice Zones of Arctic Seas, Observations and Modelling

“…The most successful so far were methods based on remote sensing, either by bubble-acoustic means (Manasseh et al, 2006) or whitecap observations (Melville andMatusov, 2002, Gemmrich et al, 2008). Both of these methods, however, appear to be affected in the polar seas.…”