EXTRACTION OF OCEAN WAVE DIRECTIONAL SPECTRA USING STEERABLE DOPPLER SIDE-SCAN SONARS

A new technique is presented for extraction of ocean wave directional spectra using steerable Doppler side-scan sonars. The method is design ed for use from a subsurface platform at the 25-m depth. Two 360 degre es steerable side-scan beams at 103 kHz are used to estimate along-bea m horizontal water motion due to waves at ranges out to 250 m. Upward- looking sonars are used to estimate the surface height power spectrum. This new method, dubbed ''coherent wavenumber summation,'' uses waven umber decomposition of radial velocity versus range profiles. A wavenu mber-directional grid is then coherently incremented over one 360 degr ees, 64-ping sweep of the side-scan beams, which requires deconvolutio n in time and spatial orientation. The resulting directional spreading functions are then incoherently averaged over 30-50 min of side-scan data. Numerical testing shows that this method can reconstruct accurat ely realistic uni- and bidirectional seas over the frequency range 0.0 9-0.30 Hz and is insensitive to random noise. The method is demonstrat ed through comparison with conventional array methods using data colle cted in the northeast Pacific. The resolution of this method is severe ly limited by the combined effects of short temporal and spatial coher ence scales of natural ocean waves, and geometric consequences of the finite side-scan beam aperture (<200 m).