INVERSION FOR THE OCEAN SURFACE WAVE DIRECTIONAL SPECTRUM USING THE DOPPLER-SHIFTED, BRAGG-SCATTERED SIDEBANDS OF LOW-FREQUENCY NARROWBAND ACOUSTIC TONES

The frequency and directional content of ocean surface wave fields can be estimated from high-resolution beamformer measurements in the vertical plane between an acoustic source and a 2-dimensional, horizontal receiving array in a shallow ocean waveguide. The acoustic tones transmitted in the experiment are low frequency - 280, 370, 535 and 695 Hz. From first-order perturbation theory, the frequency shift of a narrowband acoustic tone scattered from the ocean surface is dependent on the surface wave frequency, while the arrival angle deviation depends on the acoustic and ocean surface wavenumber vectors through the Bragg condition. All ocean surface waves are assumed to be linear. The inversion results are compared to the directional ocean surface wave spectrum measured independently using an ocean wave vector sensor (i.e., one that measures ambient pressure and the two horizontal components of flow) near the acoustic source. Complications arising from, and additional information contained within, multi-path acoustic propagation, higher- order scattering from the surface waves, and multiple interactions of the acoustic signal with the surface are addressed. (Work supported by ONR, Code 321(US)).