Effect of wind-generated bubbles on fixed range acoustic attenuation in shallow water at 1–4kHz

Abstract

Long-range acoustic propagation in isothermal conditions is considered, involving multiple reflections from the sea surface. If the sea is calm there is almost perfect reflection and hence little loss of acoustic energy or coherence. The effect of wind is to increase propagation loss due to rough surface scattering and the interaction with near-surface bubble clouds. Previously published measurements of wind-related attenuation in shallow water, at a fixed range of 23km, are converted to surface reflection loss by dividing the total attenuation by the expected number of surface interactions. Theoretical predictions of coherent reflection loss are compared with these measurements in the frequency range 0.9–4.0kHz and wind speeds up to 13m∕s. Apart from an unexplained seasonal dependence, it is shown that the magnitude of the predicted rough surface scattering loss is sufficient to explain the measurements if the effect of bubbles is included, and not otherwise. The bubbles are found to play an important catalytic role, not by scattering or absorbing sound, but by refracting it up towards the sea surface and thus enhancing the scattering loss associated with the rough air–sea boundary. Possible explanations for the apparent seasonal variations in the measurements are explored.