MEASUREMENT OF OCEAN WAVE DIRECTIONAL SPECTRA USING DOPPLER SIDE-SCANSONAR ARRAYS

A technique is presented for extraction of ocean wave directional spec tra using Doppler side-scan sonars. Two 103-kHz steerable side-scan be ams from a freely drifting subsurface platform are used to estimate ho rizontal water surface velocity due to waves. The side-scan targets ar e clouds or layers of microscopic air bubbles, confined to within a fe w meters of the ocean surface. Upward-looking sonars are used to estim ate the surface height power spectrum. The side-scans are steered to c ompensate for any platform yaw, and corrections to the measured wave v elocity due to horizontal and vertical platform motion must be applied . Measurements of wave velocity with the side-scans in this geometry a re found to have acceptable spectral signal-to-noise properties at hor izontal ranges up to 150 m in the frequency range 0.05-0.35 Hz, with d egraded performance at greater range. Time series of horizontal wave v elocity at 16 range points along each of two orthogonal beams are used in a maximum-likelihood array processing technique to estimate the di rectional spreading functions versus frequency. Numerical and experime ntal tests suggest that the maximum-likelihood method can reconstruct adequately unimodal and bimodal wave fields. These methods are demonst rated with ocean data from the northeastern Pacific.