Forward scattering from the sea surface: Observations of both subtle and profound effects of bubbles in single-interaction measurements

Abstract

For frequencies of O(10) kHz and above, field data show that near-surface bubbles impact forward scattering from the sea surface in three phases. The first occurs under mild conditions (wind speed less than 5–7 m/s); here a pulse forward scattered from the sea surface is extended in time, but only at levels ∼30 dB below the peak level, which itself is not attenuated. The second occurs under more vigorous conditions (wind speed 7–12 m/s); here a significant energy loss is observed, but time and angle spreading (dominated by rough surface scattering) remain relatively unchanged. The third occurs under still more vigorous conditions (wind speed greater than ∼12 m/s). Here, there is near total occlusion of the sea surface, time and angle spreading are manifestly altered, and bubble-mediated energy loss becomes bounded by scattering from bubbles. Examples from ASIAEX East China Sea and other archival data sets will be discussed along with a model for bubble-mediated energy loss in forward scattering from the sea surface. In the case of near total occlusion, an interesting example of the knock-down of horizontal coherence will be discussed along with a model that utilizes the van Cittert-Zernike Theorem. [Research supported by ONR Ocean Acoustics.]