Vertical coherence of ambient noise in shallow waters of Bay of Bengal

Ambient noise field in shallow waters is very complicated due to wave guide nature of the environment and multiple reflections between the sea surface and the sea bottom. Time series measurements of ambient noise from an automated noise measurement system deployed in shallow waters of Bay of Bengal is analysed for vertical coherence. Vertical coherence for different sea states is estimated for almost isovelocity sound velocity profile and the effect of wind forcings on coherence pattern is addressed here. Time averaged coherence for wind generated noise is a relatively stable feature and is independent of the temporal variations in the ocean environment. This has been demonstrated using estimates spread over a period of time for wind forcings of beau fort 2 to 5. In such an environment, stability in coherence pattern is seen for wind speeds above 4 beaufort supporting the viewpoint that spatial coherence of shallow water, wind generated noise is stable and predictable, making it a good candidate for inversion to obtain the geo-acoustic parameters of the bottom