Extraction of Ocean Wave Directional Spectra Using Steerable Doppler Side-Scan Sonars

Abstract

A new technique is presented for extraction of ocean wave directional spectra using steerable Doppler side-scan sonars. The method is designed for use from a subsurface platform at the 25-m depth. Two 360° steerable side-scan beams at 103 kHz are used to estimate along-beam 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 wavenumber decomposition of radial velocity versus range profiles. A wavenumber-directional grid is then coherently incremented over one 360°, 64-ping sweep of the side-scan beams, which requires deconvolution 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 accurately realistic uni- and bidirectional seas over the frequency range 0.09?0.30 Hz and is insensitive to random noise. The method is demonstrated through comparison with conventional array methods using data collected in the northeast Pacific. The resolution of this method is severely limited by the combined effects of short temporal and spatial coherence scales of natural ocean waves, and geometric consequences of the finite side-scan beam aperture (<200 m).