Abstract:

Measurements of the spatial coherence of high-frequency (30-kHz) sound forward scattered from the sea surface are discussed, along with a simple interpretive model based on the Kirchhoff approximation. The data are from an experiment conducted off south Florida in waters 25-m deep. For the model, the bistatic cross section is first computed via the Kirchhoff integral, and a marginal density function for intensity versus horizontal arrival angle generated; this function is then Fourier transformed to give a model for the horizontal spatial coherence. The Kirchhoff integral requires a representation of the two-dimensional surface wave correlation function, which is itself derived from estimates of the directional wave spectrum. The latter were measured with a directional wave buoy that operated in the immediate vicinity of the acoustic measurements. A model [Donelan et al., Philos. Trans. R. Soc. London, Ser. A 315, 509--562 (1985)] is used to fill in for the necessary directional spreading information. Sensitivities of horizontal spatial coherence to array acquisition geometry, wind speed, surface wave directional properties and mean-square slope, are discussed. [Work supported by ONR Code 321OA.]