WIND-SPEED DEPENDENCE OF ACOUSTIC BACKSCATTERING

The results from acoustic scattering measurements conducted in the Nor th Sea during the Synthetic Aperture Radar and X Band Ocean Nonlineari ties (SAXON) - Forschungsplattform Nordsee (FPN) experiment are presen ted. Scattering strength values were obtained for selected frequencies (5-70 kHz) and for wind speeds up to 20 mis. The grazing angle depend ence was measured for 50 kHz at three different sea state conditions. Two grazing angle regimes were identified (above and below 60 degrees) where the wind speed dependence of acoustic backscattering shows a di fferent behavior. One grazing angle from each regime (normal incidence and 45 degrees) was investigated to explain the physical processes th at dominate the backscattering in the different wind speed regimes. In the low grazing angle regime the backscattering strength is caused by the small-scale surface roughness at low wind speeds and by subsurfac e bubbles at high wind speeds. There is only weak dependence of backsc attering in the intermediate wind speed regime. Scattering and absorpt ion of a random swarm of interacting bubbles inside the propagation ra nge lead to undefined acoustic intensities in front of the surface. Th us scattering from a boundary layer instead of sea surface backscatter ing has to be considered. The presented acoustic data set is a base fo r further correlation with radar measurements that were conducted simu ltaneously. Good correlation between electromagnetic and acoustic back scattering is expected for the wind speed regime in which the scatteri ng mechanisms are the same.