Temporal fluctuations of the sound speed fiel an how they affect acoustic mode structures and coherence

Abstract

In an idealized shallow water propagation channel with smooth boundaries and range independent sound speed profiles, normal modes can accurate ly describe the entire sound field which can be predicted using normal mode models. We also know that fluctuations in the sound field can be caused by fluctuations in the sound speed profile or by source/receiver motion. These phenomena are deterministic a nd can be simulated by changes in the mode shape or by a combination of the motion of modes past the receiver. If the fluctuations are small then small changes will occur in the mode shape or in the mode positions, hence the phase response will be approxim ately linear and our propagation is “phase coherent” relative to the background noise. Furthermore, spatial and temporal averaging is possible, which enhances the signal -to- noise ratio (SNR). But random fluctuations of the sound speed caused by multipath interactions with the boundaries can totally distort the acoustic modes reducing and sometimes annihilating phase coherence. The research community seeks to understand the effects of internal waves on temporal coherence and a considerable number of experim ents using fixed system to observe both oceanographic and acoustic fluctuations has been conducted. On the other hand, the applied Navy is more concerned with mobile platforms and underwater communications in which spatial coherence is measured instantaneously. Thus, the long term temporal coherence observed by basic research has little interest to mobile platforms. In this work we seek to understand coherence in terms of the normal acoustic mode structure. This structure can be randomized by fluctuati ons of the sound field and fluctuations of the boundaries. The research proposed here emphasizes the temporal fluctuations of the sound speed profile and how they affect acoustic mode structures and coherence. To achieve that, the MMPE ( Monterey -Miami Para bolic Equation) model will be used to predict the mode shapes in a range dependent channel and random fluctuations will be introduced to observe how the modes are distorted in space and time. In turn, we can use these mode structures to estimate the tempor al and spatial coherence of the mode arrivals allowing us to compare predictions of coherence for individual acoustic modes with observations.