Forward scattering from fetch-limited and swell-contaminated sea surfaces

By using a Helmholtz–Kirchhoff (HK) approach described in a previous paper [J. Acoust. Soc. Am. 91, 831–822 (1992)], the forward scattering from nonfully developed sea surfaces is examined and the full three-dimensional scattering problem is addressed. Two types of departures from the limiting case of the fully developed sea are considered: fetch-limited seas and the more general case of seas that are time-evolving and swell-contaminated. Estimates of several statistical parameters of the forward scattered field are calculated for these surfaces as ensemble averages and are compared to those obtained for fully developed seas due to the same wind speed. Results are obtained for frequencies from 0.2 to 2 kHz, for moderate angles of incidence and moderate wind speeds. It is found that the fetch strongly affects the coherence and strength of the forward scattered field at and near the specular direction. In this frequency range, discrepancies in excess of 11 dB with respect to the fully developed case are predicted. For swell-contaminated surfaces, the scattering coefficient and the coherence are strongly influenced by the relative contributions of sea and swell to the total roughness and directional characteristics of the surface. Swell-contaminated sea surfaces having the same total roughness but different sea-swell compositions are shown to result in significantly different scattering patterns.