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Where When How N/D 2017 - J/F 2018 : Page 92

Figure 3: The relationship between depth and the speed of sound. ators monitoring the SOSUS arrays in the early days detected many sounds whose sources were at first puzzling, but definitely weren’t submarines (one particular un-known sound operator dubbed the “Jezebel Monster”). Later the low-fre-quency sounds were identified as whale vocalization. Unfortunately, for many years all SOSUS data was highly classified. But with the fall of the Soviet Union, the SOSUS data was declassified in 1991 and made available to marine mammalogists, opening up a whole new area of research and understanding of the Leviathans of the sea. By the 1960s the U.S. Navy was so en-amored by the prospects of sound that it even experimented with the use of these long-range sound transmissions as a life-saving tool. The concept was that ship-wrecked sailors or downed pilots could drop a small explosive charge set to ex-plode in the SOFAR channel. The arrival times of the signal at a number of widely spaced listening stations ashore would then be used to triangulate the survivor’s position. However, the idea was ultimately rejected in favor of radio beacons. Oceanographers were also interested in the SOFAR channel for more than mili-tary applications. They realized that the speed and direction of deep ocean cur-rents could be measured using floats de-signed to drift with the current within the channel and transmit low-frequency acoustic signals (SOSUS hydrophone ar-rays were initially used as receiving sta-tions). Due to the high expense of the drifting transmitters, the project turned to lower-cost receivers, which recorded transmissions from moored stations. At a predetermined time the floats surfaced and uplinked their data to satellites. With time, aside from ocean currents, oceanog-raphers found that precise measurements of the travel times between widely spaced sources and receivers could be used to measure large-scale ocean temperature variability. Slowing Down Sound Figure 4: Although speed of sound in the SOFAR channel is relatively slow, the transmission of sound within it is very efficient because refraction tends to con-fine sound energy to within this zone. The outer edges of sound waves escaping from this zone will enter water in which the speed of sound is higher. This causes the wave to speed up but then pivot back into the minimum-velocity layer. 92 • • • • • NOV/DEC/JAN/FEB 2017/2018 “Where When How -Turks & Caicos Islands” While it’s easy to see why the SOFAR chan-nel became an important phenomenon of great military and scientific interest, the question remains: What causes it? Basically, it occurs because sound does not travel at

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