The site specific behavior pertains to the fluctuations of the surface parameters like the temperature, wind, rain, bottom type like muddy, sandy, rocky, salinity based on closeness to river mouth, the biological sound based on the habitat.6 Thus, the site-specific impact, relates to both the underwater propagation characteristics and the ambient noise at the receiver location.7,8 The shallow water sonar operations have to deal with higher ambient noise as well as random fluctuations in the propagation properties.
The initial deep water studies undertaken by the NATO and the WARSAW countries covered the temperate or polar regions due to the conflicts being limited in those geographic locations. Temperate/polar locations are characterised by refraction of sound around the sound axis while propagating underwater from one location to the other.9 The depth of the sound axis varies from 50 metres in case of the polar region to 2,000 metres in tropical waters closer to the Equator. This variation of the depth of the sound axis from the polar region to the tropical region translates to hypsometrically deep waters in the tropical waters behaving as shallow waters acoustically, resulting in multiple interaction of the sonar signal with the surface and the bottom.10
The vast coastline and the massive study requirements cannot be addressed by the R&D organisations alone…
The tropical waters are further characterised by diurnal and seasonal fluctuations in the surface parameters and the propagation characteristics present much more complexity in the tropical waters rather than the temperate/polar regions. Thus detailed understanding of the propagation characteristics and their inter-dependence on the medium characteristics needs to be enhanced to ensure optimum sonar performance.
The ambient noise sources in the coastal waters apart from having higher intensity also present site-specific behavior.11 Furthermore, the abundance of marine life in the tropical littoral waters increases the dominance of biological components of the ambient noise as a result of the use of acoustic signals by these marine creatures for communication, foraging and breeding.12 One marine creature that stands out in terms of its dominant contribution to the ambient noise levels in the spectral band of sonar operations is the Snapping Shrimp that is known to generate very high intensity sound to intimidate its prey.13 Thus, it is imperative that we develop a detailed understanding of the characteristics of the Snapping Shrimp acoustic signals with respect to the sonar spectrum and generate an underwater map of the Snapping Shrimp beds to facilitate enhanced sonar performance for effective ASW deployment.
The characterisation has to account for the propagation properties in the IOR and then evaluate the impact on sonar performance.
Snapping Shrimp Noise
The two principal genera of noise producing Snapping Shrimps are Crangon (Alpheus) and Synalpheus. These species (about three centimetres in length) are far different from commercial shrimps that are noiseless. They possess enlarged claws which produce a vigorous snap when closed. They are found in large colonies and there is typically a continuous succession of snaps which causes an intense crackling noise resembling the burning of dry twigs. The crackle merges into sizzle or a hiss as we move away from the shrimp bed.
As the third largest body of water on the Earth, the Indian Ocean is an important resource…
The habitat of the Snapping Shrimps can be characterised as enumerated below:
(a) Temperature: The distribution is certainly governed by water temperature more significantly in terms of duration of temperature at the bottom of the sea. Certain critical periods of life cycle such as spawning and larval development appear to require a considerable period of warmer temperatures, near or above 11º C. Typically 15ºC at the bottom of the sea is considered suitable.
(b) Geographical Distribution: The species are reported to be prevalent in the tropical and sub-tropical regions. Available literature on sound measurements from latitude 9º S to 33º N indicate about the same levels over shrimp beds irrespective of the latitude.
(c) Depth: While most species of Snapping Shrimp genre have been reported to be found within 0-55 m water depth, density crackling has been reported to be higher in shallower waters. These conditions are satisfied in some 70 per cent of the world’s near shore, within a band of some 400 of the Equator.
(d) Bottom: They are known to be bottom living species that seek concealment in holes provided by coral, stones, shell, calcareous algae and other solid objects. However, they have been found in other bottom types as well.
The geo-strategic scenario compels India to play a very critical role in the Indian Ocean Region (IOR)…
The Shrimps emit sharp pulses caused by the rapid closing of an asymmetrically-large claw, simultaneously ejecting a jet of water. The narrow pulse (3-8 ms) ensures a broadband frequency content while the peak amplitude (typically 150 dB re 1mPa2 @ 1m) is substantial to mask any sonar receiver. The collective cacophony of a colony of such Shrimps can easily exceed wind induced ambient noise sources by 10-15 dB, even at 15 m/s wind speed.14
The average spectral band of Snapping Shrimp noise is presented in Fig. 4. At low frequencies, below 2 kHz, the noise produced by Snapping Shrimps is negligible except at very low sea state (close to zero). The shrimp crackle is predominant and nearly constant in the frequency band 2-15 kHz, measured by a non-directional hydrophone. The noise produced by Snapping Shrimps is continuous and independent of hydrophone depth. There appears to be no pronounced seasonal variation and little diurnal variation, the level during the night hours being only a few dB higher than during the day time.
The Snapping Shrimp noise is predictable as
(a) The noise producing shrimps are widely distributed within their geographic range.
(b) They are confined predominantly to specific water depths and bottom types.
(c) The population is stable.
(d) The noise produced is continuous and of uniform characteristics.
While the importance of Snapping Shrimp in ambient noise production has long been realised [9] and there have been several recent descriptions of individual shrimp acoustics15 there is still very little information about their collective behaviour. If Snapping Shrimp are as loud and ubiquitous as reports indicate, acoustical oceanographers need to know much more about the ensemble statistical characteristics in order to better reject the noise or even to exploit it in active and passive system processing. The peculiarities of the IOR need to be understood with extensive study efforts including biological sound and the propagation characteristics that can impact signal behaviour and way ahead formulated that can ensure consistent and reliable sonar performance in the tropical littoral waters of the IOR.
The push for a ‘blue water’ navy entails that the two categories of platforms – aircraft carriers and submarines, are high valued and critical assets…
Way Ahead
The sonar deployment in the tropical littorals of the IOR presents very critical challenges and demands substantial efforts to map the propagation and ambient noise characteristics of the deployment location to mitigate the random fluctuations. In the warm tropical and sub-tropical waters, the Snapping Shrimps manifest as the most dominant source of ambient noise in the typical sonar Frequency Band from 2 to 15 kHz. The unique propagation characteristics of the IOR further complicate the Snapping Shrimp noise being received at the sonar front end to attempt any mitigation. The challenge is to understand and model the properties of shrimp noise along with the propagation properties to reduce its impact on sonar and underwater acoustic telemetry systems.
An important aspect that merits attention is that the exclusive characteristic of the Snapping Shrimp noise at the sonar receiver can be used as an advantage. The fact that the precise knowledge of the Snapping Shrimp noise along with the propagation properties at the location is critical for the optimum performance of the sonar can be strategically used against our adversaries by denying them that advantage. Only our sonar deployed for surveillance or underwater weapons with precise inputs will be able to perform effectively in these waters.
Abundant availability of Snapping Shrimps (Crangon (Alpheus) and Synalpheus) are reported in the warm waters of the tropical regions.16 Authors have reported dominance of Snapping Shrimp noise in warm waters at varied locations across the globe and confirmed their findings with real ambient noise data recording and analysis.17 The three sites, evaluated by the authors are San Diego (California), Haifa (Israel) and Singapore. The data was recorded using portable DAT recorders from a small boat. The analysis presents a comparison of ambient noise spectrum with the Wenz curves for deep waters.18
The above study19 is a classic example of academic involvement in field experiment very critically required to cover distance across the vast Indian coastline spread over 7,500 km. The achievement in this case has been the design and development of the hardware and algorithms for such a study that could be replicated on a large scale. The unique propagation properties in the IOR will have to be factored prior to formalising the hardware and software. A comprehensive study methodology including deployment strategy, data collection protocol and analysis plan has to be formalised on a specific location as a pilot project.
The IOR with its tropical littoral waters presents unique challenges for operational deployment of sonar for tactical as well as strategic purpose…
The US example20 cannot be directly replicated in India as here the academic involvement in field experiments of such nature is minimal. Therefore, strategists have to evolve a way to foster partnerships between the academia and the Indian Navy. The model has to be home grown as international collaboration may not be conducive for such strategic studies as confidentiality of the data and analysis findings may be compromised. Indigenous effort has to be initiated and a sustainable program evolved.
The participation of the Indian Navy is inescapable to retain the confidentiality of the data and the analysis results. A viable partnership model for such real data collection and analysis is proposed. The research consciousness of the armed services has been significant and cannot be understated. A significant number of officers with brilliant academic credentials are being spared from active service to pursue their Master’s programme in academic institutes including IITs, IISc and DIAT. The pool of officers at various academic institutes with sea-going capabilities and with their understanding of the deployment challenges of sonars and underwater systems can be utilised for such efforts.
A national level programme needs to be formulated with modular approach to define multiple research problems in a hierarchical manner. The academic pursuits of the student participants have to be dovetailed with the project deliverables. Project proposals have to be jointly written by naval officers, DRDO scientists and faculties at the academic institutes to achieve the larger goal. The basic deployment strategy formulation can be undertaken by Masters-level projects by naval officers for multiple research problems.
The deployment strategy and data collection protocols have to be well structured to ensure optimum utilisation of the time at sea so that real data collected can be analysed and inferences drawn for wide range of research problems subsequently. Raw data storage under matured filing system will have to be evolved supported by sophisticated retrieval software. The algorithms and hardware design could be undertaken by DRDO scientists and naval officers in the initial stages as part of a Master’s programme and can be subsequently taken up at doctorate level for more evolved investigations. The project investigators have to prioritise the research problems to be able to generate quantifiable results/deliverables at regular intervals to keep the sponsors and participants motivated. The detailed strategy for maintaining confidentiality of the data and the analysis results also need to be evolved.
A detailed project document needs to be worked on with application specific formulation. The organisational structure and involvement of various stakeholders needs to be articulated and reviewed. The deliverables and the resource allocation will have to be deliberated and the project proposal approved based on the detailed project report.
Conclusion
Snapping Shrimp noise has been reported to manifest itself as a dominant ambient noise source in the tropical and sub-tropical littoral waters of the IOR. The unique propagation characteristics further add to the site specific behavior, thus detailed mapping of the Snapping Shrimp beds in the IOR need to be initiated. The mitigation strategy to enhance sonar performance has to be an indigenous effort. The vast coastline and the massive study requirements cannot be addressed by the R&D organisations alone, thus mass involvement is required that can be sustained only with academic participation. This massive study requirement will require vast student resources and high quality research potential available in the academia along with the infrastructure, scientific ship and the expertise available with the various R&D organisation.
The Snapping Shrimp noise characteristics have been discussed in the paper with the propagation characteristics of the tropical littorals of the IOR. The challenges of the sonar operations in the IOR have been enumerated with reference to ASW deployment. The authors have attempted to put the aspect of Snapping Shrimp noise in perspective and present an approach for enhanced sonar performance. A navy-academia partnership has been proposed with participation of a pool of naval officers and DRDO scientists at various levels of R&D (postgraduate or doctorate) requirements for the study.
Notes
1. Ibid 13-14.
2. G. Wenz, “Acoustic Ambient Noise in the Ocean: Spectra and Sources,” J. Acoust. Soc. Am. 51, 1010-1024, (1971).
3. Wagner, Daniel H., W. Charles Mylander and Thomas H. Sanders, Operations Analysis Manual, third edition, Monterey: Naval Institute Press, 1999.
4. Paul C. Etter, Underwater Acoustic Modelling and Simulation (Spon Press, Taylor and Francis Group, London and New York, third edition, 2003).
5. R. O. Nielsen, Sonar Signal Processing (Artech House, Boston, 1991).
6. Ibid 18.
7. F. B. Jenssen, William A. Kuperman, Michael B. Porter and Henrik Schmidt, Computational Ocean Acoustics (Springer-Verlag, 2000).
8. F. B. Jensen and C. M. Ferla, “Numerical Solutions of Range Dependent Benchmark Problems,” J. Acoust. Soc. Am., vol. 87, 1499-1510 (1990).
9. Ibid 13-14.
10. Ibid 18.
11. John A. Hildebrand, “Anthropogenic and Natural Sources of Ambient Noise in the Ocean”, Marine Ecology Progress Series, Vol 395, Pp. 5-20, 2009.
12. Richardson, W. J., Charles R. Greene Jr., Charles I. Malme, Denis H. Thomson, Marine Mammals and Noise. Academic Press, San Diego, California, 1995.
13. Ibid 9.
14. Potter, J. R., Lim, T. W. and Chitre, M., “Ambient Noise Environment In Shallow Tropical Seas And The Implications For Acoustic Sensing”, Proceedings of Oceanology International 97 Pacific Rim, Singapore, 1, pp. 191-199, 1997.
15. Au, W.W.H., “The Acoustics Of Snapping Shrimp In Kaneohe Bay”, J. Acoust. Soc Am., 1996, 99(5) Pt. 2 2533.
16. Potter, J. R. and Chitre, M., “Statistical Models For Ambient Noise Imaging In Temperate And Tropical Waters”, J. Acoustical Society of America, 100 (4), Pt 2, 2738, 1996 b.
17. Ibid 28.
18. Gordon, M. Wenz, “Acoustic Ambient Noise in the Ocean: Spectra and Sources”, J. Acoustical Society of America, 34 (12), Pp. 1936-1956, Dec 1962.
19. Ibid 28, 30, 32.
20. Ibid 9, 11.
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This is the best article I have read in this web site so far. Majority of Naval officers are interested in increasing the number of weapon carrying vehicle rather than giving importance to its safety. Till last year all our warships. and frigates were working without Active Towed Array Sonar (ACTAS) system.