SOSUS (Sound Surveillance System)
█ K. LEE LERNER
Utilizing the unique properties of sound transmission in water, during the 1950s, the United States Navy developed the Sound Surveillance System (SOSUS). Code named "Jezebel" the SOSUS system provided critical monitoring of Soviet submarine and ship movements, especially through the critical ocean gaps between Greenland, Iceland, and the United Kingdom (the GI-UK gap). SOSUS systems were so sensitive that trained observers could determine ship type—and in some cases, identify specific ships.
SOSUS used arrays of hydrophones (underwater microphones) strategically placed along the ocean bottom. The hydrophones were connected by cables to onshore monitoring stations.
In addition to localized sound readings (i.e., sounds detected within the expected range of the hydrophones), SOSUS also picked up sounds channeled through specific conditions of state (i.e., pressure, temperature) or salinity that create channels though which sound waves propagate over long distances with minimal resistance and minimal loss of strength. This sound fixing and ranging channel (SOFAR channel) was discovered independently by American and Soviet scientists in 1943 during World War II.
SOFAR channels are capable of transmitting the low frequency, long wavelength sound waves produced by an explosion. Sound waves can be trapped effectively in SOFAR channels and propagate with little loss of energy over distances in excess of 15,500 miles (25,0000 km).
Naval communication systems utilize low frequency, long wavelength signals to enhance communications with submerged submarines. Prior to the widespread use of Global Positioning System (GPS) equipment, the SOFAR channel was also used for navigation and the location of marine craft. Evidence gathered by marine biologists indicates that certain species of whales utilize the SOFAR channel to communicate mating calls over long distances.
In general, the speed of sound depends upon the medium through which the sound waves propagate and the properties of the medium (e.g., state, temperature, pressure, salinity, etc.) Accordingly, the speed of sound differs in air, fresh water, and oceanic saltwater.
Within the ocean, the speed of sound varies with changes in temperature and pressure. When the near-surface layer is well mixed by currents and surface action, the resulting isothermal layer provides uniform propagation of sound. When a temperature gradient exists (e.g., a temperature decrease with increasing depth), the resulting thermocline shows a characteristic decrease in the speed of sound with decreasing temperature. At some depth (approximately 420 fathoms or 750 meters), the variations in temperature become so slight that the water becomes isothermal. As depth increases, so does the pressure. Because pressure is directly proportional to sound wave transmission speeds, as the pressure increases with depth so does the speed of sound.
Specific combinations of temperature, pressure, and salinity may act to create "shadow zones" that are resistant to the propagation of sound waves or that act as reflectors of sound waves. Soviet submarine captains attempted to use these zone or layer to conceal their ships from detection by surface SONAR arrays. The layers could also to "bend" signals detected by the SOSUS array in order to attempt to conceal ship movements. In practice, staying within such layers proved impossible to maintain for extended periods, and intermittent SOSUS plots could be used to track ship movements or provide a probable position to explore with the use of sonar buoys dropped by airplane.
Surface sonar buoys were also used to fill gaps in the SOSUS listening network.
█ FURTHER READING:
Munk W., Worcester P., and C. Wunsch. Ocean Acoustic Tomography. Cambridge: Cambridge University Press, 1995.