Signal Coherence Modeling for Shallow Water with Rough Boundaries.

A normal mode model is used to predict signal coherence for a sound source moving horizontally in shallow water. In the model, modal amplitude or modal phase are assumed to fluctuate. The fluctuations are ascribed to the effect of the surface or bottom roughness on sound propagation. For frequencies at which only a few modes are important, namely water depths of the order of 1.5 wavelengths, model coherence was calculated for sensor configurations in the plane containing source and receivers. For source motion towards the receivers, it was found that for phase and amplitude fluctuations the coherence obtained are identical and that the coherence, which can drop to zero over a few wavelengths, is independent of the fluctuation distribution width. For source motion towards the receivers, coherence is controlled by mode strengths and mode propagation constants. In contrast, if the source is in motion in a circle about a vertical array, coherence depends upon the distribution width, and for the same distribution width coherences for phase fluctuations are proportionately lower than those for amplitude fluctuations. Author