Measurement of Ocean Wave Directional Spectra Using Doppler Side-Scan Sonar Arrays

Abstract

A technique is presented for extraction of ocean wave directional spectra using Doppler side-scan sonars. Two 103-kHz steerable side-scan beams from a freely drifting subsurface platform are used to estimate horizontal water surface velocity due to waves. The side-scan targets are clouds or layers of microscopic air bubbles, confined to within a few meters of the ocean surface. Upward-looking sonars are used to estimate the surface height power spectrum. The side-scans are steered to compensate for any platform yaw, and corrections to the measured wave velocity due to horizontal and vertical platform motion must be applied. Measurements of wave velocity with the side-scans in this geometry are found to have acceptable spectral signal-to-noise properties at horizontal ranges up to 150 m in the frequency range 0.05–0.35 Hz, with degraded performance at greater range. Time series of horizontal wave velocity at 16 range points along each of two orthogonal beams are used in a maximum-likelihood array processing technique to estimate the directional spreading functions versus frequency. Numerical and experimental tests suggest that the maximum-likelihood method can reconstruct adequately unimodal and bimodal wave fields. These methods are demonstrated with ocean data from the northeastern Pacific.