Ambient sea noise as the indicator of dynamic physical processes at the sea surface

Abstract

It is well known that an ambient sea sound is generated by wind, mostly throughout the processes of wave breaking and accompanying injection of acoustically active air bubbles. The resulting sound level is highly correlated with wind speed and has consequently been propounded that the ambient sea noise can be used to estimate wind speeds. It has been proved that despite of significant scattering the accuracy is similar to other marine wind measurement techniques. Tentatively the acoustic ambient sea noise could be used also as an alternative technique for estimating a set of diverse dynamical processes like the wind wave energy dissipation, rain rate and rain type, and also as the indicator of bubbles presence. The simplicity of the passive acoustic buoys technology and sensors endurance to withstand the hard conditions in highly dynamic and corrosive environment make it very suitable technique. The measurements of the ambient sea noise in the Baltic Sea jointly with other parameters as wind speed, air bubbles entrainment, rain rate and wave field statistics were performed using autonomic hydroacoustic buoys. The measurements of ambient noise in a mid-frequency range (350–12500Hz) were carried out with the acoustic imaging of bubble plumes structure performed at 130 kHz to find associations between noise and parameters of bubble population. Relationships between bubble clouds parameters, the wind speed and the ambient sea noise were derived, providing a better way to predict a bubble population presence on the basis of the noise level. A good correlation quality between the wind speed and the ambient sound level is achieved by considering a sound propagation conditions in the area. The hydrophones should be deployed outside of the Baltic Seasonal Acoustic Waveguide — a subsurface one — during the winter/spring season and the deep water waveguide in the summer/autumn period. Using multisensory autonomic hydroacoustic buoy of the next generation, the noise and wind wave statistics were measured simultaneously including the registration of the noise from single breaking event to estimate breaking probability and intensity of the wave processes.