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Waves, Bubbles, Noise and Underwater Communications

Abstract : The long-term goal is to develop physical models for the role of surface waves and bubbles on the performance of high-frequency, coherent underwater communications systems. These models will be used as the basis of improved signal processing algorithms for underwater acoustic modems, and to develop predictive algorithms for the performance of modems operated beneath wind and waves. Modem performance is impacted by signal-to-noise ratio at the receiver and reverberation levels determined by scattering from the sea surface and seafloor. Waves impact both of these processes by radiating noise when they break and by focusing and scattering surface-reflected sound. They also influence surface reverberation by injecting micro-bubbles into the upper ocean boundary layer that scatter and attenuate sound. Because of these wave-drive processes, one long-term goal is to measure, model and exploit the surface-focused acoustic arrivals occurring on short length scales beneath sea swell and study their impact on underwater communications systems in the littoral and near-shore regions. The final long-term goal of this grant, to understand the physical mechanisms responsible for the Knudsen spectrum for wind-driven underwater ambient noise, has been achieved. Work over the past 12 months has focused on 2 main objectives, which have been to: (1) analyze and model the Surface Processes and Communications Experiment (SPACE08) data collected as part of the field expedition at the WHOI Coastal Observatory during fall of 2008, and (2) incorporate the effects of attenuation and scattering of bubbles into a propagation model that also accounts for surface scattering in the presence of surface gravity waves.