Effects of horizontally varying internal wavefields on multipath interference for propagation through the deep sound channel

Abstract

Fluctuations of CW signals transmitted through real ocean paths are characterized by smooth slow variations in phase and rapid noise-like variations in transmission loss. The variations are thought to be the results of internal waves, which perturb the sound-speed field and thereby produce time-varying multipath interference. This study is concerned with modeling the effects of large-scale horizontal and vertical perturbation of the sound-speed field on multipath interference of CW signals, for the particular case of long-range propagation through the deep sound channel. The vertical profile is approximated by two layers of linear sound-speed gradient and arbitrary horizontal variations are introduced by numerical methods. The influence of horizontal wavelength of idealized internal wavefields is studied by using model simulations of time-varying transmission. The travel times along the individual ray paths are shown to be influenced by fluctuations of the sound-speed field with horizontal scales which are (1) approximately equal to the cycle distance of the ray; or (2) greater than several cycle distances of the ray. Both scales of disturbance induce multipath interference but the latter also induces temporally correlated phase shift for all ray paths. The models are used to interpret results of fixed system measurements of CW transmission fluctuations over a 700-NM range. Model computations show that horizontally invariant internal waves cannot cause both the phase and transmission loss fluctuations which are consistent with experiments. When horizontal fluctuations are introduced, statistics of CW transmission fluctuations match experimental results.