The SOFAR channel is centred on the depth where the cumulative effect of temperature and water pressure combine to create the region of minimum sound speed in the water column. Pressure in the ocean increases linearly with depth, but temperature is more variable, generally falling rapidly in the main thermocline from the surface to around a thousand metres deep, then remaining almost unchanged from there to the ocean floor in the deep sea. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is called the sound channel axis. Near Bermuda, the sound channel axis occurs at a depth of around 1000 metres. In temperate waters, the axis is shallower, and at high latitudes it reaches the surface. Sound propagation is constrained to the channel by refraction, which makes sound travel near the depth of slowest speed. If a sound wave propagates away from this horizontal channel, the part of the compression front furthest from the channel axis travels faster, so the wave front curves back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fibre. Mysterious low-frequency sounds, attributed to fin whales, are a common occurrence in the channel. Scientists believe fin whales may dive down to this channel and "sing" to communicate with other fin whales many kilometers away. During World War II, Dr. Maurice Ewing suggested that dropping into the ocean a small metal sphere, specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots. The novel The Hunt for Red October describes the use of the SOFAR channel in submarine detection. The conjectured existence of a similar channel in the upper atmosphere, theorized by Dr. Ewing, led to Project Mogul, carried out from 1947 until late 1948.
Applications
Ocean acoustic tomography: A technique to measure ocean temperatures and currents by the time delay of sounds between two distant instruments
Search for Malaysia Airlines Flight 370: Sounds carried by the SOFAR channel were analyzed to determine if they detected a possible ocean impact of a passenger jet which disappeared in the Southern Indian Ocean
SOSUS: Hydrophone system to detect submarine movements during Cold War
