Stability of vertical coherence of ambient noise in shallow waters off the Indian coast

140-143<span style="font-size:9.0pt;font-family: " times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-US">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. Vertical coherence for high sea state is estimated for different coastal sites with dissimilar sound speed profiles, 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 above 4 beau fort. Coherence for sandy site (off Cuddalore) with reflective seabed is mostly positive, due to noise field that is fairly symmetric around the horizontal whereas for silty site (off Cochin), the absorbing seabed gives rise to an asymmetric field. Off Vizag, the upward refracting sound speed profile that leads to a surface duct makes coherence more symmetric than the other sites.