Non-Acoustic Detection Of Moving Submerged Bodies In Ocean

Although, acoustic techniques are currently the mainstay for detection and tracking of submerged bodies like submarine and autonomous underwater vehicles (AUV), advent of silent submarines and the ensuing reduction of acoustic signatures and development of anechoic coatings is making detection very difficult especially in shallow water environment. Though there are some promising non-acoustic techniques such as magnetic anomaly detection, LIDAR, bioluminescence detection etc for the same purpose, they have some inherent limitations which restrict their use for detection over a wide area and from a large distance. Against this backdrop detection of physical manifestations such as turbulent wake and internal waves generated due to platform movement by optoelectronic techniques increasingly becoming popular for detection and tracking of moving underwater objects. We present here the general features of turbulent wake and internal waves generated by moving submerged bodies and discuss the optoelectronic techniques for their detection. Key word: Sonar, MAD, SAR, Kelvin wake, turbulent wake, internal wave, shadowgraph ISSN: 2278 0211 (Online) www.ijird.com December, 2012 Vol1 Issue 10 (Special Issue) INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT Page 362 1.Introduction Though electromagnetic (EM) waves have their presence in almost all areas of human life starting from military to civilian, certain limitations like high attenuation in water medium precluded their use for underwater applications, thus leaving this domain to sound waves which have obvious advantage over EM waves regarding range in underwater environment. Hence in present scenario acoustic technique such as SONAR (Sound Navigation and Ranging) is the mainstay for detection and tracking of underwater moving objects. Sonars are of two types, active and passive. Active sonar transmits a pulse of sound and then receives the echo produced after reflection by target. But by sending a powerful burst of sound it reveals its position making it an easier target for attack or helping the enemy to take evasive action. Passive sonar uses underwater microphones called hydrophones arranged in the form of an array to detect sound coming from distant targets. By comparing sound received by each hydrophone it is possible to determine the direction of incoming sound. In addition the received sound after comparing with pre recorded sound, the nature of source generating sound can be determined. Hence passive sonars detect, localize and identify enemy submarines if the vessel generates enough noise. But with the development of quieter submarines with many noise reduction techniques such as precisely balancing rotating parts to minimize vibration, mounting machinery on sound absorbing platforms, and covering submarine with anechoic materials which absorb sound, the noise signatures of submarines reduce significantly and are likely to be reduced further. Other concerns in the acoustic methods are range of acoustic transducers which is limited and the effect of ambient noise in the oceanic environment. Hence to detect targets over a wide area in ocean, large numbers of acoustic sensors are to be deployed. Also acoustic sensors may be confused by the decoys fired form enemy platforms which generates similar sounds of submarine. Hence there have been attempts to find alternative methods for detection over a wide area of ocean. One of the techniques is magnetic anomaly detection. The body of submarine is made up of anomaly detector (MAD), a device sensitive to changes in local geomagnetic field can be used to detect submerged submarines. But the strength of magnetic anomaly signal reduces as cube of the distance and hence range is limited upto some thousand feet. Also noises like naturally occurring concentrations and random fluctuations in the geomagnetic field due to solar activity sometimes mask the actual submarine. Moreover, unlike sound and light, magnetic field is a non-propagating phenomenon. Hence it is difficult to determine the www.ijird.com December, 2012 Vol1 Issue 10 (Special Issue) INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT Page 363 directionality of source of disturbance in case of long range magnetic surveillance. Also an actual submarine. Other promising method is LIDAR (Light detection and ranging). Although sea water is opaque to most electromagnetic radiation; blue green light can penetrate a considerable distance through sea water. Similarly to radar an intense pulse of blue-green laser can be transmitted and the reflected light from submarine can be detected by a suitable photo detector. However, surface of submarine hull may tend to absorb more light than the surrounding water, in which case it would be detected as a hole in the naturally occurring level of oceanic backscattering. There are some other concerns too. Many false targets are likely to be detected by such mechanism, since there are plenty of submerged objects in ocean other than submarines including marine animals such as blue whale. The intensity of blue-green light is attenuated by a maximum factor of approximately two for every seven meters it travels through water. Hence this may not be useful for long range detection. There have been attempts to detect heat emitted by submarines. Conventional and especially nuclear submarines draw in substantial quantities of seawater specifically for the purpose of cooling as it relies on recycling of water through a boiler, a steam turbine and a cooling system (whose heat sink is sea water) to convert heat energy produced in the reactor to mechanical energy. About 20% of the thermal energy produced by the reactor goes into propulsion. The rest 80% of the energy goes to surrounding sea water through cooling of boiler and through the heat generated by electrical systems. While this appears to be massive, heat transfer calculations reveal that at a speed of about five knots, the temperature immediately behind the submarine only rises by about 0.2 degrees Celsius. This temperature differential will diminish rapidly as the submarine moves further away at a rate of (x/D) where x is the distance downstream of submarine and D is the diameter of submarine. The warmer water being less denser rises to the surface and eventually encounter water of the same temperature at which point it will not rise further and therefore not detectable. Other interesting phenomenon called bioluminescence seems to be another detection technique. The oceans are populated with organisms like dinoflagellates that emit light when they are disturbed. The moving submarine will naturally cause a local disturbance of the surrounding bioluminescent organism population inducing them to emit light. The intensity of such emission is a function of population density of species, environmental conditions and the speed of subsurface vessel that disturbs the species. The peak of this www.ijird.com December, 2012 Vol1 Issue 10 (Special Issue) INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT Page 364 emission spectrum is in the blue to blue green region (.48 μ) which is the wavelength that can be transmitted in sea water with less attenuation. Hence such effects may be detectable above the ocean surface and may be used to detect the location of subsurface vessels. However, the emission of light by excited organisms at one depth may induce other organisms closer to the surface to emit light. But the presence and population density of such organisms vary with season, geographical location and depth. Also at a particular location their position changes with time. Other limitation for the detection of bioluminescence is the overpowering background noise contributed by the sun and the moon which would render a detection system useless during the day-time and possibly also under certain night-time conditions. Other possible detection methods are detection of different characteristic emanations from modern submarines. particles of paint that slough off the outer surface of the submarine, minute quantities of radioactive substances that escape into sea water through nuclear reactor cooling substance (in case of nuclear submarine) or other effluents that leave a distinctive chemical trail indicating the presence of submarine in that area. The detection of such trail requires direct measurement of very minute quantities of chemical substances at different areas and at different depths of ocean which is very difficult and time consuming. Apart from these techniques the physical surface effects caused by a submerged vessel may be detectable either by accurate measurement of the ocean surface height or by imaging the when it is mobile. The characteristics of the wake will be a function of the speed, depth and size of the vessel. Three separate hydrodynamic phenomena are either directly or indirectly caused by the wake the Bernoulli hump, Kelvin waves, and the surface effect of underwater turbulence and internal waves. If a submarine travels at high speed near the surface of the ocean it produces a characteristic hump of water which is sometimes referred to as the Bernoulli hump. The shape of this disturbance is independent of speed and depth but the size is proportional to DU/h, where D, U, and h are the diameter, speed and operating depth of submarine respectively. Hence it decreases rapidly with decreasing speed and increasing depth. For example, the height of the hump reduces from about six centimeters to one millimeter when a given submarine reduces speed and increases depth from 20 knots and 50 meters to five knots and 100 meters, respectively. Kelvin waves are shaped wake that can be seen to linger behind a moving vessel as shown in Fig.1. They have an angle of approximately 39o which is independent of the size of the vessel or the www.ijird.com December, 2012 Vol1 Issue 10 (Special Issue) INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT Page 365 speed at which it is travelling. The height of wave reduces exponentially with increasing depth and decreasing speed. Also it decays behind the submarine with square root of the distance. These waves are dominant for submarines moving at low depth and high speed. Using the above example, the wave size reduces from about two centimeters to immeasurably small. Figure1. Kelvin waves formed by a moving boat Most of the above mentioned techniques have two basic limitations. First, they belong to the class of problems involving low signalto noise ratio that is inherent and difficult to overcome. Second, they can be defeated simply by operating submarine at higher depths. In this backdrop the turbulent wake and internal wave generated by submerged bodies seem to be the most promising phenomena for detection. 2. Turbulent Wake And Internal Waves The ocean is a stratified medium as density, salinity and temperature changes continuously, especially with depth in the upper 1000 meters as shown in the Fig.2. Since this is the operating zone of the submarine, when a submarine moves it will disturb these profiles due to its propeller motion and displacement of water by it. There will be mixing of layers of water having different density and temperature. This creates a turbulent wake downstream of submarine. www.ijird.com December, 2012 Vol1 Issue 10 (Special Issue) INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT Page 366 Figure 2: Ocean stratification A wake is defined as the non-propagating disturbance produced by a moving body. The cooler (denser) water below the submarine and warmer (less dense) water from top of submarine will be drawn into the wake and gets mixed up. Hence the turbulent wake will have a different density profile with respect to the background. Immediately downstream of the vehicle, the wake, which may contain fluid of nearly constant density, will grow at the same rate in all directions as (x/D) and maintains a circular cross section. However, as the turbulent energy of the wake decays with increasing distance from the body, the restoring action of buoyancy begins to inhibit the vertical expansion of the wake and at the same time enhances the horizontal growth. After reaching a critical time and distance the uniformly mixed wake can no longer maintain itself in the ambient density gradient. At some point behind the body, the wake reaches a maximum vertical size followed by a collapse and spreads horizontally as the fluid returns under the action of gravity to the level at which its density is the same as the environment. Its cross-sectional shape first becomes elliptical and then finally approaches that of a flat rectangle. The critical time for the onset of wake collapse has been found to be dependent on T, where T is the Brunt-Vaisala period, which is defined as 2 1