Acoustic intensity levels varied from -20 to -35 dB in the upper 500 m (case 1), In the presence of the cold core eddy, iso-lines exhibited deviations between 250 and 500 km range extending from 100 m to the abyssal region. Maximum insonified region is present in the vicinity of the channel axis depth that showed variations from 800 to 1500 m (case 2). Having carried out a number of simulation experiments and inversions, for estimating the model parameter with sufficient accuracy, as a next step the validation of the theoretical predictions through field experiment has been carried. A short duration test experiment was conducted in the eastern Arabian Sea by deploying two transceivers, near the sound channel axis, on deep sea moorings at water depths of about 4250 m and 4175 m respectively. Acoustic transmissions were conducted 12.0 N latitude for a duration of ten days (2-12 May 1993), with two transceiver systems separated by a range of 270.92 km. Hourly reciprocal transmissions were carried with a time lag of 30 minutes between each direction. From the multiple arrival patterns, significant peaks corresponding to the predicted ray arrivals of the most stable rays were identified. The travel time perturbations enabled reconstruction of temperature anomaly from the sound speed perturbation. A Iinear relation was used to transform sound speed perturbations in the vertical plane to temperature perturbations. The 2-D anomaly derived from the six hourly mean travel-time data showed a gradual warning of the top layers - signatures of diurnal variability and possibly, intrusion of waters of Red Sea origin.
A field program involving deployment of acoustic sources and receivers to acquire environmental data in the form of travel times of acoustic signals as apart of the Acoustic Thermometry of Ocean Climate (ATOC) for the Indian Ocean study. ATOC program is now well established in the Pacific Ocean. In view of this, N.I.O scientists have taken up the acoustic environmental study of the Indian Ocean-Southern Ocean combine. (This follows the success of the Heard Island Feasibility Test (HIFT) in early 1990s). The results of the analysis of this survey are briefly detailed below. Water mass structure and circulation: Circulation of the eastern Indian Ocean has been examined from the vertical distributions of temperature, salinity and sigma-theta. Large variability in the water mass structure was noticed as one moves from the northern to southern latitudes. The salinity of the surface waters at northern latitudes (5oN) is low in January at the eastern transect and high during March at the westem transect. Below the sea surface the high salinity waters of the Arabian Sea origin influence the upper 100m water column. At the equator, a thick (50-75 m) low salinity layer is noticed. This layer decreases in its thickness away from the equator. The low saline surface waters, between 3o S and 8o S, encountered have been traced to the Indonesian Through flow directed towards west into the Indian Ocean. Warm waters with temperature exceeding 28.50C are present in the vicinity of the equator with layer thickness (100 m) towards north of equator. Towards south, the surface temperature decreased to about 220C at the southern most latitude (250S). The presence of a meso-scale cold core eddy in the Bay of Bengal is evident from the upper layer (400 m) XBT thermal structure north of 50N. Its salinity structure, however, is not known. While vertical gradients of temperature are strong in the thermocline (centered around 100 m depth) in the region north of 50S, they are weak with diffused thermocline south of 80S.
