Recent results from basin-scale ocean acoustic wave propagation: Implications for the general theory of the approach to saturation

Abstract

Long-range ocean acoustics as a problem in wave propagation through random media is unique in several ways. First the propagation takes place in the ocean sound channel (a waveguide), and second, a primary source of sound speed fluctuations are the anisotropic and inhomogeneous ocean internal waves (not turbulence). Recent observations of 75 Hz, broadband transmissions over several thousand kilometers have revealed fascinating results on the approach to saturation. Two distinct propagation regimes are observed in the same pulse: a fully saturated regime characterized by Rayleigh statistics, and an unsaturated regime in which the probability density function for intensity is nearly log-normal. The transition between these two regimes is extremely rapid. The saturated region of the pulse is composed of acoustic energy traveling near the waveguide axis, while the unsaturated region is composed of acoustic energy strongly refracted by the waveguide. This fact suggests a fundamental stabilizing influence by the waveguide. Application of path-integral theory, which was successful in describing shorter-range ocean experiments, cannot describe these two regimes. Recent efforts using semiclassical and ray chaos theory have led to new insights into the observational results and to the failure of the path-integral theory, yet a complete theoretical description remains elusive.