Any disturbance of the physical environment caused by a submarine suggests the possibility of remote detection. The disturbance must be measurable at a distance and must be discriminated from the background of similar naturally occurring disturbances. Such an anomaly is frequently called an observable. To be useful, a detection system must perform two functions — detection and discrimination — to some degree of confidence.
The following is a brief description of some of the more frequently discussed means of nonacoustic detection:
- Local Chanees in the Earth’s Maenetic Field
As a large piece of ferrous metal, the steel-hulled submarine causes a local disturbance in the earth’s magnetic field. This disturbance or anomaly can be detected with a device (MAD) that measures the local magnetic field. If a nonmagnetic hull material is used, the magnetic signature decreases but is not eliminated, since the submarine contains some ferrous parts, and the nonmagnetic shell does not shield the magnetic effects of this internal material.
- Bioluminescent Detection
The sea contains bioluminescent organisms of many kinds, the most relevant to detection being dinoflagellates. These organisms can generate light when they are physically stimulated in the boundary layer of a submarine or in its wake. This phenomenon has been studied as a method for detecting submarines from the air or space.
- Submarine-generated Waves on the Surface of the Ocean
Moving submarines, at high speeds and shallow depths, generate surface waves behind them. At reasonable depths and speeds, however, wind-generated surface waves mask the minute submarine waves.
- Submarine-generated Internal Waves
Internal waves are oscillations of the thermocline that can be caused by a solid body moving in the ocean. Internal waves in turn cause water motion at the surface that is not directly observable but that can innuence preexisting wind-generated ripples and waves on the surface. These changes in the surface can in principle be detected by radar. The ocean always contains internal waves that are generated by storms, currents, tides, whales, surface ships, and submarines.
- Submarine-related Changes in the Sea Surface Temperature
Submarine nuclear reactors generate an enormous amount of heat, which ultimately must be rejected into the surrounding seawater. Water has a very high capacity to absorb heat with a small change in temperature, however, and a moving submarine raises the water temperature by a very small amount. A moving submarine may also change the temperature of the sea surface by mixing lower cooler water with upper water, thereby leaving a trail of cool surface water that could be detected with infrared (heat) sensors.
- Laser Detection
The sea is relatively transparent to blue-green light. A burst of blue-green laser light could penetrate the sea, reflect off an object, and return to the sensor. The round-trip travel time of the laser-~urst indicates the depth of the object, but cannot discriminate, for example, between a large whale and a submarine.
Magnetic Anomaly Detection
Magnetic anomaly detection (MAD) devices are used to detect changes in the background magnetic induction that are associated with submarines. Terrestrial magnetism usually varies slowly over distance, but when a submarine is present, the field changes rapidly and may be detected by a low flying aircraft carrying MAD equipment.
Submarines contain a large amount of metal that becomes magnetized in the course of normal operations. The permanent magnetic field associated with the submarine remains until active measures are used to demagnetize it. The earth’s magnetism induces a transient magnetic field that depends on the spatial orientation of the submarine. The total magnetic field of the submarine is the vector sum of the permanent and induced magnetic fields.
The strength of the magnetic field at a distance from the submarine is inversely proportional to the third power of distance. The shape of the earth’s magnetic field lines are distorted by the submarine according to how far away it is. The earth has a strong and very complex magnetic field that varies with time and location. On a small scale the earth’s magnetic field is very irregular, and small natural magnetic anomalies associated with ore deposits may be indistinguishable from submarines by MAD equipment. When searching areas in which there is a high level of geologic noise, MAD operators must set their receivers at a low sensitivity. According to a Navy study, “At these settings it will be difficult, if not impossible, to see a small submarine anomaly. Parts of the Norwegian Sea and the seas around Iceland are areas where geologic noise may interfere with MAD operations.”
The U.S. currently deploys two types of MAD equipment on its ASW aircraft. These systems can detect the submarine magnetic field at no more than a few thousand feet. Area magnetic surveillance is technically feasible with a distributed system of many MAD systems. But even if some highly sensitive MAD system were widely distributed in the ocean, simple countermeasures could render it virtually useless. Small dummy submarines could carry coils that reproduce a magnetic signature of a much larger submarine. Military submarines themselves could carry coils that neutralize their own magnetic field by imposing an equal and opposite magnetic field from the coil.
Detection of Submarine-induced Bioluminescence
The primary sources of ocean bioluminescence are certain species of the plankton dinoflagellates. The mechanical stimulus of a moving submarine hull and its turbulent wake will elicit luminescence from organisms disturbed or killed. The power and persistence of this light is a function of the organisms’ population density and species, environmental conditions and submarine speed. Luminescence is expected to be strongest in the turbulent regions associated with a submarine — that is, hull boundary layer and the wake. The radiant flux of an individual organism varies widely among species. The most common may radiate .002 x 10·9 watts, while other organisms may radiate 20 x 10″9 watts or more.
The population density of bioluminescent organisms varies with location and depth. According to one study, “Under natural conditions, bioluminescence is maximum around midnight and minimum around midday. This diurnal periodicity is attributed in part to downward migration of the organisms during the day and return migration to surface waters at night. Most luminescence is found between 50 and 150 meters and is associated with dense dinoflagellate populations in continental shelf areas up to 60 degree north latitude. Maximum luminescence frequently occurs at the thermocline. The amount of light generated by a submarine wake can be estimated by multiplying the volume of water disturbed by the wake, the number of organisms per unit volume, and the light power emitted per organism. Measurements of ocean bioluminescence suggest typical values for the North Atlantic and North Pacific as 10″6 to 10·5 watt/m/micron11 The reason for analysis overestimates, as to the light outputs of a wake, is that it is generally assumed that all organisms glow constantly, when in fact dinoflagellates flash only intermittently for a duration of about 100 milli-seconds.
In order to reveal the presence of a submarine, the light energy must pass through some depth of water and atmosphere and still be sufficiently strong relative to reflected and scattered sunlight or moonlight to be detected. Exponential transmission loss is assumed between the source and the surface. It is clear that during the daytime the bioluminescence is lost within the surface reflection. At night, disturbances on the sea surface may be detectable. However, for submarines below 50 meters the signal to noise ratios may be too unfavorable and submarine wakes generated below 50 meters are unlikely to reach the surface. In essence, it is the depth of the submarine-generated lig~t that precludes its detection from above the surface.
Detection of Surface Waves Generated by Submarines
The physical effects and problems associated with detection of submarine surface waves are related to the near-field and far-field waves which are generated by a moving body — a large submarine. Comparing submarine generated waves with typical wind-generated surface waves, it is noted that the submarine wave is negligibly small relative to wind waves.
The near-field disturbance of the surface appears as a hump of water (sometimes called a Bernoulli hump) over the moving submarine which dies rapidly with distance from the submarine. The general shape of the disturbance is not very sensitive to changes in depth, but the height of the disturbance increases as the square of the speed and decreases as the square of the depth. The surface disturbance is limited in extent to a few ship-lengths. The amplitude of the wave is very small but, under certain circumstances, measurable. An OHIO-class submarine running at 20 knots and at 30 meters depth would generate a wave at most 15 centimeters high. Under more realistic patrol conditions (5 knots at 100 meters), the wave is on the order of a millimeter.
The far-field disturbance shows up as a wedge-shaped Kelvin wave pattern behind a moving source-sink pair. In general, both transverse and divergent waves may be present, and these are contained within an angle of 19.5 degrees to both sides of the line of motion. For typical speeds and depths, transverse waves dominate. Wave height varies with the submarine diameter, speed, depth, distance and length. Speed and depth are the most important factors, since wave height decays exponentially with increasing depth and decreasing speed. The waves decay slowly behind the submarine, with the square root of the distance. Even for very shallow depths and speeds up to 12 knots, the surface wave is only of the order of millimeters, and for depths greater than 100 meters, no wave is generated at reasonable speeds.
The near-field wave, or Bernoulli hump, is a single, localized perturbation, a few hundred yards in extent, and is three orders of magnitude below the peak of a typical wave spectrum. The prospects of detecting such a disturbance are extremely dim, irrespective of the sensitivity of a space-based system. The far-field Kelvin wave pattern covers a greater area but it can only be produced at high speeds and shallow depths. With the mildest of precautions, these waves are virtually nonexistent.
Submarine-generated Turbulent Wakes and Internal Waves
As the submarine moves through the water, some of the energy of propulsion goes into generating a turbulent wake behind the hull. Typical wake lengths associated with submarines below 125 feet are on the order of 100 yards at 6 knots and 30 yards at 2 or 3 knots. It can be assumed that the submarine wake will disturb the temperature structure of the seawater. Cooler water from below the submarine will be drawn up into the wake, and warmer water from above will be drawn downward into the wake. The mixed wake will therefore be slightly cooler than the water just above it and slightly warmer than the water just below it. Studies suggest that the wakes may be detectable a few kilometers downstream before the turbulence decays to an undetectable level. When the turbulent wake collapses, it can drive an internal wave in the density-stratified layers of the ocean. Submarines also generate internal waves by the movement of the hull alone. These internal wave patterns associated with the hull are sharply concentrated along the line of motion. Internal waves cannot be seen directly as undulations of the surface, unlike a submarine wake which attains a maximum height of 8-25 meters above the hull at a distance of 300-3000 meters behind a submarine traveling at 5 knots. Once this wake ceases to grow or collapse vertically, it usually continues to spread horizontally. This wake may be detectable a few kilometers downstream before the turbulence decays to an undetectable level.
Internal waves cannot be seen directly as undulations on the surface. The internal wave generates horizontal currents near the surface that modulate existing surface ripples whose wavelengths are on the order of centimeters. The modulation takes the form of changes in the ripple wavelength and steepness, which in turn alters the radar scattering properties of the rippled surface. The modulation of surface waves can in principle reveal the pattern of underlying internal waves. Synthetic aperture radar can be tuned so that the radar backscatter depends on the wavelength of the short surface waves. It is known that the submarine wake will collapse fairly rapidly so that the potential energy in the wake can be transferred to inner waves. A recent review of all the subsurface hydrodynamic mechanisms that could modulate the surface ripple field concluded that although the large surface gravity waves have a dominant effect on the surface ripples, the surface wave modulations by the internal wave can still be shown to be observable.
The surface manifestations of internal waves, or the vortices of an internal wake, may be linked to the presence of a thin film of natural organic material and oil that is commonly found on the ocean’s surface. Movement by the submarine can sweep the film into regular patterns which might be detectable by a sensor with sufficient spatial resolution. The slicks reduce the surface tension, which can affect the wave characteristics and energy dissipation in capillary waves. The variation in surface roughness may then be detected using the synthetic aperture radar.
Detection of Submarine-generated Temperature Changes
Submarines change the temperature of the water in two ways: by mixing the thermocline, and by direct heating through the reactor cooling system. These two processes may tend to cancel each other, since upwelling of cool, deep water is offset by rector heating. If either a cool or warm temperature anomaly is present at the surface, it may be detected by ASW forces.
The temperature of the ocean surface can be measured by measuring the infrared or microwave radiation emitted by the surface. Since only surface temperature is detectable, only the submarine-induced temperature anomalies that reach the surface can be detected.
Assuming that a 5-knot submarine’s heat is mixed into a wake 11 meters behind the propeller, then at a distance of 1,100 meters downstream, the temperature of the wake is only 0.020 C higher than the surrounding water. At 20 knots it is 0.005° c.
Detection of Submarines by Laser
Lasers can be an active nonacoustic detection device because of the depths to which blue-green light penetrates seawater. Such a detection system would consist of an airborne laser/detector which would send short pulses into the ocean and from the return energy determine if a pulse had been either reflected off or been absorbed by a subsurface object. The laser must have sufficient power to compensate for round-trip attenuation and the large reflection Joss off the submarine. The greatest Joss by far occurs in the few hundred meters of seawater through which the beam must pass. Since moderate fog strongly attenuates blue-green light due to scattering and clouds have much the same effect, and since clouds and fog cover 60 percent of the ocean’s surface, both laser detections and detections of surface temperature anomalies involve relatively poor risk systems.
Most of the technologies discussed can be defeated simply by operating the submarine deeper. The signal-to-noise ratios decrease dramatically, usually by several orders of magnitude, with an increase in depth on the order of 100 meters. Operating submarines below 100 meters should foil most foreseeable nonacoustic detection systems. This may not apply to the detection of internal wave effects. Not enough is understood about this phenomena to properly evaluate its detection possibilities, but at the same time no breakthrough in this direction seems to be in sight. For most systems, it is likely that relatively short-range sensors on aircraft are more feasible than long-range sensors on satellites.