Acoustic Scattering Cross Sections for Truly Composite Wind-Wave Surfaces. Scattering without Bubbles

摘要

Underwater acoustic scattering cross sections for truly composite windwave surfaces are obtained analytically, where the composite surface consists of the usual single gravity-capillary wave surface and an independent ripple structure which rides upon the former. The later consists of an ensemble of solitons, as hydraulic jumps, moving nondispersively in a very thin (moving) surface layer of the gravity-capillary waves. These soliton-ripples are postulated to provide a scattering mechanisms of sufficient strength potentially to account for the often large observed discrepancies 0(10-20) db) in the (back)-scattering cross sections between conventional theory and experiment when high frequencies 0(> or = 10 kHZ) and small grazing angles 0(5-20 deg) are employed, without the assumption of near-surface bubble mechanisms. This is particularly important for the frequent situations when bubbles are demonstrably lacking, even at mean wind speeds up to 0(10 m/sec = 20 knots) and possibly higher. Here, the 'capillary' component of the gravity-capillary wave continuum is not large enough by itself to provide sufficient Bragg scatter, while the soliton-ripples appear capable of so doing. Of course, if near-surface bubbles are verified to be present in sufficient numbers, they by themselves, or with the solition-ripples, can account for the noticeably larger (back-)scatter cross sections obtained experimentally.