Scientists Ewing and Worzel conducted an experiment in 1944 where explosive charges were detonated at varying distances from a research vessel to test the theory that low-frequency sound could travel long distances underwater. Their experiment was successful and discovered the SOFAR channel, where sound waves are ducted horizontally in the ocean. This discovery led the US Navy to develop the SOSUS system during the Cold War to detect Soviet submarines over long ranges using hydrophone arrays. The SOFAR channel allows for underwater sound communication over hundreds of miles and has applications in navigation, climate monitoring, and wildlife observation.

1. SOund Fixing And Ranging

2. History of the SOFAR Channel
• In 1944, ocean scientists, Maurice Ewing and J. Worzel,
aboard the R/V Saluda set sail 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

3. Discovery of the SOFAR Channel
• Shot 43 recorded aboard the R/V Saluda on April 3, 1944.
• Charges were exploded at a depth of 4000 feet and a range of 320 nautical miles.
Times are labeled for 370, 371, ... , 374 seconds following the explosion. Channel 1
shows time markers. The remaining channels show the received signal after different
types of signal processing. (Adapted from Ewing and Worzel, 1948. Ewing and Worzel
recording from Fig. A.1 of Munk et al., 1995)

4. • The U.S. Navy used the ability of low-frequency sound to
travel long distances in the deep ocean 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. Later 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.
• Sailors 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.

5. • In the early 1960's 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 (See
How is sound used to navigate underwater?). The
project was called SOFAR (for SOund Fixing and
Ranging), giving the SOFAR channel its name.

6. • 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 (See SOFAR Floats).
Because the drifting sound sources were expensive, the approach was soon
turned around to use drifting receivers. 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 (See RAFOS
Floats). 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 (See How is sound used
to measure global climate change?).

7. References
• http://www.dosits.org/science/sndmoves/4
a.htm
• http://oceanexplorer.noaa.gov/explorations
/sound01/background/acoustics/media/sof
ar.html
• Activities:
http://www.coexploration.org/bbsr/classroo
mbats/html/body_sofar.html