In the spring of 1944, ocean scientists, Maurice Ewing and J. Worzel, departed Woods Hole, Massachusetts, aboard the research vessel R/V Saluda to test a theory that predicted that low-frequency sound should be able to travel long distances in the deep ocean. A deep receiving hydrophone was hung from R/V Saluda. A second ship dropped 4-pound explosive charges set to explode deep in the ocean at distances up to 900 miles from the R/V Saluda's hydrophone. Ewing and Worzel heard, for the first time, the characteristic sound of a SOFAR (SOund Fixing And Ranging) transmission, consisting of a series of pulses building up to its climax:
bump bump bump bump bump bump
A single explosive source is heard as a number of separate arrivals, leading to the characteristic signature of a SOFAR transmission building up to its climax. In the words of Ewing and Worzel, "the end of the sound channel transmission was so sharp that it was impossible for the most unskilled observer to miss it." In 1946 Leonid Brekhovskikh and his colleagues in the former Soviet Union independently made a similar discovery while conducting experiments in the Sea of Japan.
Even before WWII ended, the U.S. Navy experimented with the use of these long-range transmissions as a lifesaving tool. The notion was that survivors of a downed aircraft or sinking ship could drop a small explosive charge set to explode in the ocean sound channel. The arrival times of the signal at a number of widely spaced listening stations ashore would then be used to compute the position of the life raft (Lean More). The project was called SOFAR (for SOund Fixing and Ranging), giving the SOFAR channel its name.
The U.S. Navy soon realized that the ability of low-frequency sound to travel long distances in the deep ocean could be used to increase the range at which submarines could be detected. In great secrecy during the 1950's, at the height of the cold war with the former Soviet Union, the U.S. Navy launched a project with the code name Jezebel. It would later become known as the SOund SUrveillance System (SOSUS). Arrays of hydrophones were placed on the ocean bottom and connected by underwater cables to processing centers located on shore. The SOSUS system was very successful in detecting and tracking the noisy Soviet submarines of that era. The sailors operating the early SOSUS arrays also detected some sounds whose sources were at first unknown. One particular unknown sound was attributed to the "Jezebel Monster." The sound was later found to be low-frequency blue and fin whale vocalizations.
Oceanographers subsequently realized that the speed and direction of deep ocean currents could be measured using floats designed to drift with the current at mid-depth and transmit low-frequency acoustic signals at regular intervals. The acoustic signals were originally received on the SOSUS hydrophone arrays and the arrival times were used to compute the float positions. The floats were called SOFAR floats. Because the drifting sound sources were expensive, the approach was soon turned around to use a drifting receiver. Low-cost receivers were designed to drift at mid-depth and record the transmissions from moored sound sources. The floats, called RAFOS floats (SOFAR spelled backwards), surfaced at the end of their lives and radioed the arrival times that they had recorded back to shore via satellite, from which the positions of the floats as they drifted with the ocean currents could be computed. Oceanographers later realized that precise measurements of the travel times between widely space sources and receivers could be used to measure large-scale ocean temperature variability.
- Ewing, M. and Worzel, J.L. 1948, "Propagation of Sound in the Ocean." Geological Society of America Memoir 27.
- Munk, W., Worcester, P. and Wunsch, C. 1995, "Ocean Acoustic Tomography." Cambridge University Press.