Stochastic effects in ocean acoustics. Advances in theory and experiment

Abstract

As the need to operate in the kilohertz range increased in the 1960s, existing approaches to ocean acoustics, based on sound-speed profiles and ray-tracing, became quite inadequate. The raggedness of acoustic transmissions, previously regarded simply as noise, was so pronounced at the higher frequencies that the stochastic properties themselves became an urgent area of research. Several major ocean transmission trials were carried out to obtain fluctuation data. The trial at Cobb Seamount produced results that could not be explained by existing random-wave propagation theory. The research that followed led to significant advances in several areas. New propagation theory emerged based on the parabolic equations for the acoustic field moments. In particular it was the construction of analytical solutions of the equation for the fourth moment that provided the first satisfactory explanation of the Cobb intensity fluctuation spectra. Advances in numerical simulation methods and capability have allowed pictures of the full stochastic acoustic field in an ocean transmission trial to be produced. These have not only confirmed experimental variances and cross correlations but have also revealed the spatial structure of the field, including interesing features such as the elongated sound ribbons that arise in some cases.