SOFAR channel

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The SOFAR channel ("SOund Fixing And Ranging channel"), or deep sound channel, is a layer of water deep in the ocean where the speed of sound is at a minimum. Within the SOFAR channel, low frequency sound waves may travel hundreds, if not thousands, of miles before dissipating. This phenomenon is an important factor in submarine warfare. The deep sound channel was discovered and described by Dr. Maurice Ewing, in the 1940s.

This channel exists at the depth where the cumulative effect of temperature and water pressure (and, to a smaller extent, salinity) combine to create the region of minimum sound speed in the water column. Pressure in the ocean rises linearly with depth, but temperature varies greatly, generally falling rapidly from the surface to around a thousand meters deep, then rising very slowly from there to the ocean floor. Near the surface, the rapidly falling temperature causes a decrease in sound speed; below the SOFAR channel, the increasing pressure causes an increase in sound speed. Near Bermuda, the lowest density water occurs at a depth of around 1000 metres. In temperate waters, this minimum sound depth is shallower, and it reaches the surface between latitudes of about 60 degrees N or 60 degrees S.

The sound propagation in the channel is due to the refractionary tendencies of sound that makes it travel at the depth of its slowest speed (if a sound wave is drifting away from this horizontal channel, the part of the wave that is closer to the center of the SOFAR channel slows down and causes the wave to "turn" back to the slower depth).

Strange and mysterious low-frequency sounds, attributed to humpbacks and other baleen whales, are a common occurrence here. Scientists think that humpback whales may dive down to this channel and "sing" to communicate with other humpback whales many kilometers away.

During World War II, Dr. Maurice Ewing suggested that dropping a small metal sphere into the ocean, specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots. ^[1]

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.