Marine Acoustics and Acoustic Oceanography
C. S. Murty (OAT Group*, National Institute of Oceanography, Goa-403 004, India.)
e-mail: csmurty@csnio.ren.nic.in
ABSTRACT
Acoustic propagation characteristics of the waters of the Arabian Sea and the Bay of Bengal including the equatorial Indian Ocean have been examined using the climatic data and recent hydrographic measurements in these regions. These studies enabled identification of the regions of significant variations in sound speed and sound channel axis depths. Numerical simulations of propagation using ray and PE methods enabled prediction of acoustic field (trajectories, signal travel times and the energy losses associated with spreading and absorption of energy) while the generalized and stochastic methods of inversion provided estimates of the sound speed / thermal variations from travel time measurements. For a propagation range of 300 km, computed acoustic losses due to spreading, showed variations between 90 dB and 120 dB for different eigen rays while the frequency dependent chemical absorption loss for a source at 400 Hz (200 Hz) central frequency accounts for about 15.25 dB (4.25 dB). At 400 Hz, absorption takes place at the rate of 1.76x 10-3 dB/km over the ambient value.
A reciprocal acoustic transmission experiment, using acoustic transceivers of Webb's design with central frequency 400 Hz, was conducted in the eastern Arabian Sea during May 1993, covering a range of 〜270 km for 10 days to study propagation along 12.5ﾟN. Measured travel time perturbations of the stable eigen rays enabled reconstruction of sound speed anomaly which was further transformed to temperature anomaly using a linear relation. The six-hourly thermal anomaly maps obtained for the four days of observation reproduced known regional physical features such as the general warming of the upper layers and the influence of the Red Sea waters.
Numerical simulations of long range (〜7000 km) propagation carried out as part of the Heard Island Feasibility Test revealed a gradual increase in the vertical extent of the ray paths scanning the water column from the southern ocean towards tropics.A hydrographic survey was undertaken in the eastern Indian Ocean for the collection of baseline data as part of ATOC program of the Indian Ocean which aims at monitoring the ocean warming signal. Numerical simulation of acoustic propagation along the geodesic paths from a hypothetical source near Cape Leewin to two receiver locations one near Madras (India) and the other in the southeastern Arabian Sea was carried to enable identify suitable locations for the deployment of acoustic source(s) and receivers forfuture ATOC. The methodology developed in conjunction with remote sensing and TOPEX/POSEIDON data sets on Sea Surface Height (SSH) anomaly would address the variability and in understanding some of the oceanic processes in addition to the inputs to monsoon forecasts.
INTRODUCTION
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 this institute, towards developing tomography techniques independently during mid eighties.
*G.s. Navelkar, A.K. Saran, A.M. Almeida, S.Prasanna Kumar, T.V. Ramana Murty, Y. K. somayajulu, Vijayan Fernando.
