Reference sound speed profile and related ray acoustics of Bay of Bengal for tomographic studies

Abstract

Using the archived hydrographic data, a climatological mean sound speed profile has been developed for the Bay of Bengal which will serve as an input (reference profile) to acoustic models. This profile is depth limited with the effective acoustic wave guide situated much below the surface as seen by the variation in the conjugate depth. Ray-tracing carried out using range-independent software revealed that acoustic rays with a flat angle of emergence from the source reach the receiver faster than the steep angle ones. This arrival pattern of the rays is attributed to the weak acoustic waveguide nature of the Bay of Bengal. From the ray arrival structure it is seen that the near axial rays present poor resolution (< 10 milliseconds) though they are the first arrivals, while purely refracted steep angle rays have a better resolution and an organised arrival pattern. For refracted rays, the travel time span-first arrival to last arrival-for different receiver depths, varied from 93.3 ms to 354.4 ms over a range of 300 km. When the source and the receiver are located near the channel axis, a maximum number of eigenrays reach the receiver, which reduces considerably when the receiver location is shiffed (in the vertical) from the channel axis. The intensity of the eigenrays at the receiver showed variation from about-81 dB to-112 dB. The loss associated with the steep angle rays is less compared to the flat angle rays because the steep angle rays sample the upper layer of the ocean (having strong sound speed gradients) more often than other rays. These simulated ray arrival times and their energy level provide a bsis for the identification on the measured ray arrival structure during acoustic field experiments