Marine Acoustics and Acoustic Oceanography
CS Murty
National Institute of Oceanography
The fact that the oceans are opaque to the electromagnetic radiation, the use of sound waves as effective carriers of transmission over long distances in the ocean lead to investigations on "ocean acoustic tomography". This has been proved to be a viable tool for monitoring ocean variability. Following this, studies were initiated at the National Institute of Oceanography, Goa, towards developing tomography techniques independently during mid eighties. These studies were confined to understanding of the acoustic field of the two major basins - Bay of Bengal and Arabian Sea including the region north of the equator. While the oceanographic features of the north Indian Ocean underline the importance of the dynamics of these two marginal seas, one needs detailed investigations in a time series mode to address (a) the nature of advective field within the ocean interior, (b) the degree of orderliness of the observed equator-ward and pole-ward Western Boundary Currents, (c) the nature of local forcing and remote forcing on the flow field and the associated upwelling and (d) the influences of deep water circulation. The annual mean sound speed profiles in the northern Indian Ocean were obtained from quasi-synthetic environmental data. In the upper 40-m of the Arabian Sea, these profiles show uniform values while they increase with depth in the Bay of Bengal. An interesting feature common to both the marginal seas is the depth limited nature of sound speed profile, where the magnitude of the sound speed at the sea surface exceed the near bottom values. In the Arabian Sea sound speed shows maximum spatial variations between 200 to 1500 m due to a combined effect of salinity (to a large extent) and temperature. Spatial valiations in sound speed in the Bay of Bengal are marginal as compared to the Arabian Sea. The depth of sound channel axis varies between 1450 and 1850 m in the Arabian sea as compared to 1100-1750 m in the Bay. In general, the depth of channel axis increases towards the northern latitudes in the Arabian Sea, while it decreases in the Bay of Bengal. The mean depths of the channel axes are 1750 and 1500 m in the Arabian Sea and the Bay of Bengai respectively. The sound speed at the axis shows variation up to 4 m/s and 2 m/s respectively. The eastern Bay of Bengal and the Andaman Sea are characterized by low axial depths and high axial sound speeds. The sound speed profile in the Arabian Sea shows the presence of large gradients above the axis of the SOFAR channel in contrast to that in the Bay of Bengal and also a relatively higher sound speed values at any given depth indicating a strong wave guide nature. Acoustic model simulation experiments related to the forward and inverse aspects of ocean tomography have been developed. For the forward model, ray theoretical approach and to a less extent the parabolic equation method were followed while for the inverse model deterministic and stochastic inverse methods were used. The deterministic method is based on the generalized inverse approach for the estimation of the model parameter (sound speed perturbation) in a near stable or stratified ocean. Resolution matrices obtained through singular value decomposition helped to examine the correctness of the solution.
