COHERENCE OF ACOUSTIC SCATTERING FROM A DYNAMIC ROUGH-SURFACE

Numerical studies of the angular and temporal coherence of rough-surfa ce acoustic scattering are presented at kh = 12, 20, and 32, where k i s the acoustic wave number and h is the root-mean-square surface heigh t fluctuation. The computations are based on point-source illumination of a one-dimensional pressure-release Pierson-Moskowitz dynamic rough surface, and the Kirchhoff approximation. In the region near specular , for a source-location grazing angle of 20-degrees, the angular varia tion of the scattered field coherence is found to predominantly depend on the product of k and a fixed length scale. The computed results sh ow only a mild dependence on kh. For the same geometry, the temporal v ariation of the scattered field coherence is found to depend predomina ntly on (g DELTAt/U)kh, where g is the acceleration of gravity, DELTAt is the time shift, and U is the wind speed. A general method for scal ing the scattered field coherence in terms of k, DELTAt, U, and the an gular separation of the field points is suggested based on the finding s. The effects of wind direction and surface-wave frequency cutoff on acoustic scattering are also noted.