Azimuthal dependence of Bragg scattering from the ocean surface

Frequency selective scattering of water-borne acoustic waves by the rough s ea surface (Bragg scattering) has been observed, in particular during the C ritical Sea Test series. The directional nature of the gravity wave spectru m observed by Mitsiyasu, Donelan, Banner and others implies that the interf ace scattering will be directional, that is, depend upon azimuth relative t o the receiver. During the ocean exercise Critical Sea Test 4, Bragg scatte ring was observed at 250 Hz over a wide range of azimuths using a linear hy drophone array with approximately one degree azimuthal resolution. The ampl itude of the Bragg scattering and its dependence on azimuth closely matched model predictions based on the Donelan 2-D wave spectrum, first-order pert urbation theory and a normal mode reverberation model. (C) 1999 Acoustical Society of America. [S0001-4966(99)05503-4].