C. S. Hayek I. W. Schurman C. A. Boyles J. H. Sweeney
The Johns Hopkins Univ., Appl. Phys. Lab., Johns Hopkins Rd., Laurel, MD 20723-6099
Low-frequency sound, backscattered from an annulus of ocean approximately 9 km wide at a nominal 43-km range, was measured on a 128-hydrophone horizontal line array (HLA) during an experiment in the Gulf of Alaska. Fourier analysis of the backscattered signal permitted separation of the Bragg-shifted returns from the 250-Hz carrier frequency. The Bragg-reflected energy was resolved in azimuth by beamforming the HLA data. Azimuthally varying sea surface backscattering strengths, corresponding to measured wind speeds, were obtained using an exact solution for the backscattered field applied to a two-dimensional Donelan water wave spectrum. Normal mode model predictions using the calculated scattering strengths agreed with the up- and down-shifted Bragg energy measurements in both level and azimuthal dependence. [Work supported by SPAWAR PMW 192.]