Refracted, bottom-reflected ray propagation in a channel with time-dependent linear stratification

Abstract

A model study has been undertaken of refracted, bottom-reflected (RBR) ray propagation in a time-dependent, linearly stratified acoustic channel. The analysis is relevant to the acoustic propagation effects of large-scale variations in the channel and pertains to the influence of these variations upon sound-speed stratification. Such ocean phenomena as surface tides, low-frequency internal waves, and advected intermediate and large-scale inhomogeneities—all of which may have scales comparable to or larger than representative acoustic path lengths of a few kilometers to a few hundred kilometers—are included. For numerical work, a measure of the influence of large-scale variations upon sound-speed stratification is provided by a thermistor string installation in the Straits of Florida. The analysis shows that any refracted, bottom-reflected ray contributes a signal that is very nearly constant in spreading loss, but phase modulated at the frequency of the environmental variation in the channel. Thus, for a simple harmonically time-dependent channel the signal at the receiver is a sum of constant amplitude, but sinusoidally phase modulated signals. Comparisons with data from a 7-NM propagation multipath in the Straits of Florida demonstrate qualitative agreement between theory and experiment.