Estimating ocean wind wave spectra by means of underwater sound

Abstract

Measurements of the spectrum of ocean surface reradiated acoustic signals generated by an omnidirectional, sinusoidal source are used to estimate the ocean surface wave-height spectrum, and the results are compared with wave-staff measurements made simultaneously in the scattering area. Plane-wave physical optics theory is applied to a long-crested sea surface model to verify Marsh's hypothesis that for low sea states the spectrum of acoustic waves reradiated by the rough, moving sea surface consists of a specular component at the transmitted frequency and a scattered component, which is the weighted surface wave-height spectrum. The approach employs theories by Parkins, Clay and Medwin, and Pierson. The weighting function applied to the surface spectrum depends on signal frequency, experimental geometry, and mean-square wave height. Continuous-wave signals at 127 and 1702 Hz were transmitted from fixed, omnidirectional sources to receivers 4.5 and 31 km downrange. Additionally, wave height was monitored continuously by resistance wave staff. The time series records of sound-pressure level and ocean wave height were analyzed for spectral content. Agreement between theory and experiment is good when the acoustic sideband levels are not more than 25 dB below the level of the specular component. The experimental results are also interpreted in light of Parkins' spherical wave theory, and the differences between the plane and spherical wave theories are discussed.