On bistatic sea surface scattering: Field measurements and modeling

Abstract

The bistatic scattering cross section of the sea surface, σ, is studied, along with a model for σ and its comparison with field data. The data are horizontal spatial coherence and ensemble-averaged intensity, which represent integral measures of sea surface bistatic scattering, and the model for σ is used to generate these same properties for comparison with the field data. The data are from an experiment conducted in shallow waters off southern Florida, using a sound frequency of 30 kHz. Directional wave measurements were made with a wave buoy positioned within 100 m of the acoustic measurements, with the environment characterized by rms wave heights of O(10) cm and wind speeds of 1–4 m/s. In the model σ is divided into two components: σr associated with scattering from the rough, air/sea interface, and σb associated with scattering from near-surface bubbles. The second-order small slope approximation is used to compute σr, which is a much improved approach over the traditionally used composite roughness model. The primary advantage in the small slope approximation was the resulting smooth behavior in σr over a broad range of scattering angles. Directional wave data obtained by the wave buoy were converted to an estimate of the 2-D spatial correlation function of sea surface roughness, C(ξ,final_sigma), for use in the scattering calculations. An analysis of the effective correlation properties of C(ξ,final_sigma) suggested that an isotropic correlation function C(ρ), based on the directionally averaged wave-number spectrum, would be equally effective in the scattering calculations. Model-data agreement was quite satisfactory, regardless of whether C(ξ,final_sigma) or C(ρ) was used in the scattering calculations.