Acoustic propagation and oceanic fine structure.

Abstract

Internal waves distort the sound-speed field in the sea. This time variable distortion causes fluctuations in acoustic intensity and phase in forward transmission experiments. Initial efforts to predict the spectrum of internal wave-induced phase fluctuations were successful. Corresponding success was not achieved in the prediction of acoustic intensity, which is sensitive to smaller vertical scales than phase. Are the oceanic environmental models inaccurate or are the relevant scattering models in error? Here, a statistical picture of the fine-scale wave field is presented. The data consist of 10 000 profiles of density and sound speed, from the sea surface to 560 m, collected over a 20-day period. Vertical wave-number-frequency spectra of sound-speed anomaly are estimated from these profiles. Of relevance is the frequency dependence of the spectrum at the vertical wave number corresponding to the Fresnel scale of a given acoustic experiment. In an Eulerian frame, where vertical advection influences the observations, the frequency dependence varies significantly with vertical wave number. The form, as well as level, of the power spectrum of log intensity is dependent on the Fresnel scale of the experiment. Since estimation of the Fresnel scale is sometimes associated with significant uncertainty, the shape of the log amplitude spectrum might prove a valuable guide to the scattering physics.