*********** +++++++++++++++++++++ 032595B.OAC + Source: ONR Asia + *********** +++++++++++++++++++++ Contributory Categories: BIO,CHM,ENG Country: India From: Workshop on Marine Bio-Acoustics Techniques and Their Applications 11-15 March 1996 National Institute of Oceanography Goa, India KEYWORDS: India: Bioacoustics +++++ Part I/IV Item 1 SOME TOPICS FOR BIOACOUSTIC RESEARCH Commodore R.P. Pruthi Director of Naval Oceanology & Meteorology, Naval Headquarters, New Delhi - 110 011, India The ocean acoustical environment is replete with noise and clutter, and has some similarities to our atmospheric environment. Neither active nor passive sonar is as easy to deploy as it sounds to detect a submerged target because the ocean is full of noises that may mask the real signal at one moment and cause false alarms in the next. The continental shelf is biologically very productive. Some of the species viz., snapping shrimp, croakers, etc. present in the area are important noise producers that result in a significant contribution to the overall ambient noise. Further, dense schools of fish can also act as effective sound reflectors, thus, producing submarine-fike echoes and interference on active sonar and other echo ranging equipment. Marine mammals such as whales, dolphins, etc. could contribute significantly to the local ambient noise and have a strong impact on ASW operations. Their noise can cover a very wide range of frequencies (from 10 Hz to more than 150 kHZ) and in some cases can simulate the propeller counts of a distant contact or resemble the sound of underwater detonations or machinery. Further, all marine man-Lmals have the potential of being a sound reflector. Large whales in particular can produce submarine-like echoes on active sonar. When encountered in pairs or trios, they can even form a larger, more significant sonar target The most likely source of confusion for a Doppler sonar is a whale,which typically swims at normal submarine patrol speeds. Whales have been tracked for days by Naval Task Forces much to their embarrassment. Sensors such as radar can also be decoyed by the presence of mammals. When the mammals come to the surface to breathe, they may produce radar returns that can be mistaken for a brief return from a periscope or snorkel breaching the surface, most fishes and marine mammals produce sound in the water either as a bypro- duct of their feeding activities and motion through the water or for communication. Since several species are known to produce'sound in the frequency bands that are covered by passive acoustic sensors, their impact on acoustic sensors can be significant. Presence of marine life in our waters, therefore, has to be taken into account while preparing doctrines for Anti-Submarine Warfare (ASW) at sea due to the following reasons (a) When present in large numbers, the noise output of several species could become a very significant portion of the ambient noise against which the target must be recognized. Some of the noise produced could also be mistaken as the signature of a target. (b) Large fish and schools of fish can produce submarine like echoes on active sonars. (c) Schools of fish can produce enhanced reverberation thus limiting sonar performance. (d) Biological organisms have an impact on ASW due the alteration of water transparency to sound or light through a phenomenon such as bio-luminescence. The knowledge of the behaviour of various marine species and mammals, including their migration patterns, preferred habitats, feeding grounds, and average density distribution on a seasonal basis in the Arabian Sea and the Bay of Bengal would be useful in ASW. Further, the details of scattering characteristics, frequency' and target strength of the marine life will go a long way in the Navy's endeavour of detecting a submerged target, with the help of underwater acoustics, in an environment dominated by several biological entities. +++++ End Item 1 +++++ Item 2 SALIENT FEATURES OF THE WATERS OF NORTH INDIAN OCEAN C.S. Murty National Institute of Oceanography, Dona Paula, Goa - 403 004, India The ocean medium is transparent to sound waves. While this would enable viewing and documenting the oceanic features, one begins to realize that the oceans are quite noisy. A knowledge of the spectral energy distribution - in space and in time - would, therefore, help in understanding the acoustic propagation. At this Institute the studies on marine acoustics initiated during mid eighties were mostly confined to simulation aspects. Knowledge of the acoustic field is a pre-requisite for any simulation, using acoustic propagation models. In view of this, the sound speeds were derived from the annual mean values of temperature and salinity (Levitus 1982) at 33 standard depths. The annual mean sound speed profiles were computed following Chen and MiUero (1977). In the Bay of Bengal, spatial variations of sound speed are marginal (Fig. 1). However, between l@00 m and 2800 m, the observed variations in the Bay of Bengal are due to the presence of warmer waters in the Andaman Sea.,In general, at any depth sound speeds in the Arabian Sea are higher than the those for the Bay of Bengal. Considering the importance- of the sound channel - the speed and depth of the minimum - on acoustic propagation, their spatial distributions were analysed. The depth of sound channel axis varies between 1450 and 1950 m in the Arabian Sea as compared to 1100-1750 m in the Bay of Bengal (Fig. 2a). The depth of the axis increases towards the north in the Arabian Sea, while it decreases in the Bay of Bengal. The sound speed at the axis shows variations upto 4 m/s and 2 m/s in the two water bodies respectively (Fig. 2b). The eastern Bay and the Andaman Sea, are characterized by low axial depth and high axial sound speed. The acoustic intensity losses due to spreading computed for the Arabian Sea and the Bay of Behgal for different rays Figs. 3a & 3b show variations of about 80dB to 110 dB over 300 km range. The steep angle rays suffer less loss compared to flat angle rays. Acoustic absorption losses at source frequencies of 10.0 and 0.4 kHz indicated that the contributions due to boric acid and magnesium sulphate respectively predominate at these frequencies. The vertical distribution of the total losses obtained by adding contributions of all these components at these frequencies (Figs. 4a & 4b) are useful for SONAR operations and OAT studies. Having analysed the sound speed field, the annual mean sound speed profile (reference profile) for the Arabian Sea and the Bay of Bengal were used for the computation of ray parameters. The ray arrival patterns (Fig. 5) obtained from ray tracing reveal that in the Arabian Sea, purely refracted, steep as well as flat angle rays are adequately resolvable in time. This feature makes ray identification for studies such as tomography easier. In the Bay of Bengal, the flat angle rays arrive almost simultaneously (<10 Ms spread in time) making the ray identification difficult. For validation of the forward model, the travel times of eigen rays for the reference Arabian sound speed profile and the ones obtained for the assumed (winter mean) profile have been used to generate travel time perturbations which were operated by the generalized inverse operator to get the model parameter perturbations in different layers. Having carried out a number of simulation experiments and inversions, the next step was to test the models developed and to see how close one could reconstruct the sound speed anomaly from the measured data. In view of this an acoustic transmission experiment was conducted in the eastern Arabian Sea (Fig. 6) during summer for a short range (270 km) and short duration of ten days. The transceivers TRI and TR2 were placed at depths of 1535 m and 1685 m with reciprocal hourly transmissions and receptions. The received signals were preamplified, complex demodulated, low- pass filtered, sampled and coherently averaged. These signals were cross-correlated with the replica of the source signal during the post processing, to generate amplitude peaks corresponding to the multipath arrivals (Fig. 7). Using the climatological mean sound speed profile for the Arabian Sea along 12.50 (67 - 69.50 E) constructed from Levitus data (1982), acoustic ray tracing was carried out to predict the ray arrival structure at the receiver which was matched (Fig. 8) with the measured travel times. From the travel time perturbations, sound speed perturbations (model parameter) were computed based on generalized inverse method. Six hourly averages of transmission data were used for inversion to obtain time evolving temperature anomaly over the duration of the experiment. For brevity, the thermal anomaly for 2nd May is presented in Fig. 9. From the 2-D temperature anomaly maps, signatures of diurnal variability, a gradual warming of the top layers and intrusion of Red Sea waters could be inferred. In the absence of a high density synoptic coverage of the area and/or adequate current metei moorings, the only w@y to check the correctness of the tomographic measurements is to see whether these measurements reproduce the signatures of the known ocean features. The results from the above inversions are underdetermined, (in the form of a single value for each grid in the vertical domain) causing non-uniqueness in the solution thus leading to poor resolvability of the structure of the ocean. On the otherhand, the result obtained through stochastic method permits the continuous representation of the unknown field. The thermal anomaly field obtained using stochastic inverse revealed the presence of positive (warm) and negative (cold) anomalies (Fig. 10) which are depth independent (barotropic). The integrated heat content values derived (Fig. 11) showed significant spatial deviations of about 8% with reference to the annual mean heat content estimates while the diurnal variations of this parameter are found to be about 2% of the total. These studies highlight the capability of acoustic techniques for accurately monitoring the oceanic temperature changes that are essential for studies on air-sea interaction processes, general ocean circulation and also in the identification of high energetic oceanic regions that support cyclogenesis. Acknowledgements The author is thankful to Dr. E Desa, Director for his keen interest and also acknowledges the deep involvement of the Marine Acoustic Group of NIO in pursuing this study. Financial support from the Department of Ocean Development, Govt. of India, is gratefully Acknowledged. Figure Captions Fig. 1. Annual mean ( ) sound speed profiles in the Arabian Sea and the Bay of Bengal along with the minimum ( - - - ) and the maximum values. Fig. 2. Spatial distribution of SOFAR Channel axis depth (m) and sound speed (m/s) at the SOFAR axis in the northem Indian Ocean. Fig. 3. Intensity loss over 300 km raiige (a) Arabian sea, (b) Bay of Bengal. Fig. 4. Acoustic energy losses at (a) 10 kHz and (b) 0.4 kHz due to chemical absorption. Fig. 5. Launch angle versus travel time of acoustic ray over 300 km range for the Arabian Sea and the Bay of Bengal. g measured signal arrivals at the Fig. 6 Location map showing transceiver (TRl & TR2).moorings and CTD stations. Fig. 7. Waterfall plot showingreceiver. Fig. 8. Identification of eigen rays predicted with measured arrivals. Fig. 9. 2-D temperature anomaly for 2000 hrs of 2nd May to 2000 hrs of 3rd May 1993 reconstructed through tomography. Fig. 10. Temperature anomaly for DAY 1 derived using stochastic inverse method. Fig. 11. fieat content estimated six hourly on DAY I from tomography measurements. +++++ End Item 2 +++++ CMR Disclaimer================================================== This document could contain information all or part of which is or may be copyrighted in a number of countries. Therefore, commercial copying and/or further dissemination of this text is expressly prohibited without obtaining the permission of the copyright owner(s) except in the United States and other countries for certain personal and educational uses as prescribed by the "fair copy" provisions of that countries Copyright Statues. ================================================================ ************** END Msg. B.OAC **************