On bistatic sea surface scattering: Field measurements and modeling

The bistatic scattering cross section of the sea surface, sigma, is studied , along with a model for sigma and its comparison with field data. The data are horizontal spatial coherence and ensemble-averaged intensity, which re present integral measures of sea surface bistatic scattering, and the model for sigma is used to generate these same properties for comparison with th e field data. The data are from an experiment conducted in shallow waters o ff southern Florida, using a sound frequency of 30 kHz. Directional wave me asurements were made with a wave buoy positioned within 100 m of the acoust ic measurements, with the environment characterized by rms wave heights of O(10) cm and wind speeds of 1-4 m/s. In the model sigma is divided into two components: sigma(r) associated with scattering from the rough, air/sea in terface, and sigma(b) associated with scattering from near-surface bubbles. The second-order small slope approximation is used to compute sigma(r), wh ich is a much improved approach over the traditionally used composite rough ness model. The primary advantage in the small slope approximation was the resulting smooth behavior in sigma(r) over a broad range of scattering angl es. Directional wave data obtained by the wave buoy were converted to an es timate of the 2-D spatial correlation function of sea surface roughness, C( xi,sigma), for use in the scattering calculations. An analysis of the effec tive correlation properties of C(xi,sigma) suggested that an isotropic corr elation function C(rho), based on the directionally averaged wave-number sp ectrum, would be equally effective in the scattering calculations. Model-da ta agreement was quite satisfactory, regardless of whether C(xi,sigma) or C (rho) was used in the scattering calculations. (C) 1999 Acoustical Society of America. [S0001-4966(99)01904-9].