The SOFAR channel (short for sound fixing and ranging channel), or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...

is at its minimum. The SOFAR channel acts as a

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

for sound, and low frequency

sound waves In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

within the channel may travel thousands of miles before dissipating. An example was reception of coded signals generated by the Navy chartered ocean surveillance vessel ''Cory Chouest'' off

Heard Island The Territory of Heard Island and McDonald Islands (HIMI) is an Australian external territory comprising a volcanic group of mostly barren Antarctic islands, about two-thirds of the way from Madagascar to Antarctica. The group's overall size ...

, located in the southern Indian Ocean (between Africa, Australia and Antarctica), by hydrophones in portions of all five major ocean basins and as distant as the

North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...

and

North Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

.Figure 1 of the referenc
"The Heard Island Feasibility Test"
(Munk) shows ray paths to receiving locations. Table 1 lists the sites with one being a Canadian research vessel with a towed array off

Cape Cod Cape Cod is a peninsula extending into the Atlantic Ocean from the southeastern corner of mainland Massachusetts, in the northeastern United States. Its historic, maritime character and ample beaches attract heavy tourism during the summer mont ...

. This phenomenon is an important factor in ocean surveillance. The deep sound channel was discovered and described independently by

Maurice Ewing William Maurice "Doc" Ewing (May 12, 1906 – May 4, 1974) was an American geophysicist and oceanographer. Ewing has been described as a pioneering geophysicist who worked on the research of seismic reflection and refraction in ocean basin ...

and

J. Lamar Worzel J. Lamar Worzel (February 21, 1919 – December 26, 2008) was an American geophysicist known for his important contributions to underwater acoustics, underwater photography, and gravity measurements at sea. Life Worzel was born on February ...

Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...

and Leonid Brekhovskikh at the Lebedev Physics Institute in the 1940s. In testing the concept in 1944 Ewing and Worzel hung a hydrophone from ''Saluda'', a sailing vessel assigned to the

Underwater Sound Laboratory The Naval Undersea Warfare Center (NUWC) is the United States Navy's full-spectrum research, development, test and evaluation, engineering and fleet support center for submarines, autonomous underwater systems, and offensive and defensive weapons ...

, with a second ship setting off explosive charges up to away.

Principle

Temperature is the dominant factor in determining the speed of sound in the ocean. In areas of higher temperatures (e.g. near the ocean surface), there is higher sound speed. Temperature decreases with depth, with sound speed decreasing accordingly until temperature becomes stable and pressure becomes the dominant factor. The axis of the SOFAR channel lies at the point of minimum sound speed at a depth where pressure begins dominating temperature and sound speed increases. This point is at the bottom of the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

and the top of the deep isothermal layer and thus has some seasonal variance. Other acoustic ducts exist, particularly in the upper

mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...

, but the ray paths lose energy with either surface or bottom reflections. In the SOFAR channel, low frequencies, in particular, are refracted back into the duct so that energy loss is small and the sound travels thousands of miles. Analysis of Heard Island Feasibility Test data received by the

Ascension Island Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overseas Territory o ...

Missile Impact Locating System The Missile Impact Location System or Missile Impact Locating System (MILS)Both full names are found in references. is an ocean acoustic system designed to locate the impact position of test missile nose cones at the ocean's surface and then the pos ...

hydrophones at an intermediate range of from the source found ''surprisingly high''

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...

s, ranging from 19 to 30 dB, with unexpected phase stability and amplitude variability after a travel time of about 1 hour, 44 minutes and 17 seconds. Within the duct sound waves trace a path that oscillates across the SOFAR channel axis so that a single signal will have multiple arrival times with a signature of multiple pulses climaxing in a sharply defined end.The "History of the SOFAR Channel" reference has a recording and sonogram of the effect. That sharply defined end representing a near axial arrival path is sometimes termed the SOFAR finale and the earlier ones the SOFAR symphony. Those effects are due to the larger sound channel in which ray paths are contained between the surface and critical depth.The term also has a biological oceanography application. Critical depth is the point below the sound speed minimum axis where sound speed increases to equal the maximum speed above the axis. Where bottom lies above critical depth the sound is attenuated, as is any ray path intersecting surface or bottom.Figure 2 on page three of the Williams/Stephen/Smith reference is helpful in understanding critical depth, the SOFAR channel, the entire channel and the ray paths involved. The channel axis varies most with its location reaching the surface and disappearing at high latitudes (above about 60°N or below 60°S) but with sound then traveling in a surface duct. A 1980 report by Naval Ocean Systems Center gives examples in a study of a great circle acoustic path between

Perth, Australia Perth is the capital and largest city of the Australian state of Western Australia. It is the fourth most populous city in Australia and Oceania, with a population of 2.1 million (80% of the state) living in Greater Perth in 2020. Perth is ...

and

Bermuda ) , anthem = "God Save the King" , song_type = National song , song = " Hail to Bermuda" , image_map = , map_caption = , image_map2 = , mapsize2 = , map_caption2 = , subdivision_type = Sovereign state , subdivision_name = , e ...

with data at eight locations along the path. At both Perth and Bermuda the sound channel axis occurs at a depth of around . Where the path meets the

Antarctic convergence The Antarctic Convergence or Antarctic Polar Front is a marine belt encircling Antarctica, varying in latitude seasonally, where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the sub-Antarctic. Antarctic waters pr ...

at 52º south there is no deep sound channel but a in depth surface duct and a shallow sound channel at . As the path turns northward, a station at 43º south, 16º east showed the profile reverting to the SOFAR type at .

Applications

The first practical application began development during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

when the

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

began experimenting and implementing the capability to locate the explosion of a SOFAR bomb used as a distress signal by downed pilots. The difference in arrival times of the source at an unknown location at known locations allowed computation of the source's general location. The arrival times form hyperbolic lines of position similar to

LORAN LORAN, short for long range navigation, was a hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee system but operated at lower frequencies in order to provide an improved range u ...

. The reverse, detection of timed signals from known shore positions at an unknown point, allowed calculation of the position at that point. That technique was given the name of SOFAR backwards: RAFOS. RAFOS is defined in the 1962 edition of

The American Practical Navigator ''The American Practical Navigator'' (colloquially often referred to as ''Bowditch''), originally written by Nathaniel Bowditch, is an encyclopedia of navigation. It serves as a valuable handbook on oceanography and meteorology, and contains use ...

among the hyperbolic navigation systems. The early applications relied on fixed shore stations, often termed SOFAR stations. Several became acoustic research facilities as did the Bermuda SOFAR Station which was involved in the Perth to Bermuda experiment. The records of the Bermuda station are maintained by the

Woods Hole Oceanographic Institute The Woods Hole Oceanographic Institution (WHOI, acronym pronounced ) is a private, nonprofit research and higher education facility dedicated to the study of marine science and engineering. Established in 1930 in Woods Hole, Massachusetts, it ...

(WHOI). In the recent past SOFAR sources were deployed for special purposes in the RAFOS application. One such system deployed bottom moored sources off

Cape Hatteras Cape Hatteras is a cape located at a pronounced bend in Hatteras Island, one of the barrier islands of North Carolina. Long stretches of beach, sand dunes, marshes, and maritime forests create a unique environment where wind and waves shape ...

, off Bermuda and one on a seamount to send three precisely timed signals a day to provide approximately accuracy. The first application quickly became of intense interest to the Navy for reasons other than locating downed air crews. A Navy decision in 1949 led to studies by 1950 recommending the passive sonar potential of the SOFAR channel be exploited for the Navy's Anti-Submarine Warfare (ASW) effort. The recommendation included one that $10 million a year be spent on research and development of the system. By 1951 a test array had proven the concept and by 1952 additional stations were ordered for the Atlantic. The first major exploitation of the SOFAR channel was to ocean surveillance in a classified program that led to the Sound Surveillance System (SOSUS). That system remained classified from inception until the fixed systems were augmented by mobile arrays to become the Integrated Undersea Surveillance System with the mission and nature of the system declassified in 1991.It is not entirely coincidental that some of the SOSUS shore facilities, termed Naval Facilities (NAVFAC), were located in the vicinity of older SOFAR stations. For example

Naval Facility Bermuda Naval Facility Bermuda, or NAVFAC Bermuda, was the operational shore terminus for one of the Atlantic Sound Surveillance System (SOSUS) array systems installed during the first phase of system installation and in commission from 1955 until 1992. ...

and

Naval Facility Point Sur Naval Facility Point Sur was one of 30 secret sites worldwide that were built during the Cold War to detect Soviet submarines. In 1958, the U.S. Navy built a Naval Facility (NAVFAC) ½ mile south of Point Sur on the Big Sur coast to provide subma ...

. The local acoustics were already well known. Earthquake monitoring through the use of SOSUS after limited civilian access was granted to the

Pacific Marine Environmental Laboratory The Pacific Marine Environmental Laboratory (PMEL) is a federal laboratory in the National Oceanic and Atmospheric Administration (NOAA) Office of Oceanic and Atmospheric Research (OAR). It is one of seven NOAA Research Laboratories (RLs). The PM ...

(PMEL) of the

National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

in 1991 revealed ten times the number of offshore earthquakes with better localization than with land-based sensors. The SOSUS detection could sense earthquakes at about magnitude two rather than magnitude four. The system detected seafloor spreading and magma events in the

Juan de Fuca Ridge The Juan de Fuca Ridge is a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the east ...

in time for research vessels to investigate. As a result of that success, PMEL developed its own hydrophones for deployment worldwide to be suspended in the SOFAR channel by a float and anchor system.

Other applications

Comprehensive Nuclear-Test-Ban Treaty Organization The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is an international organization that will be established upon the entry into force of the Comprehensive Nuclear-Test-Ban Treaty, a Convention that outlaws nuclear test explosions. It ...

(CTBTO) - International Monitoring System (IMS) *

Missile Impact Location System The Missile Impact Location System or Missile Impact Locating System (MILS)Both full names are found in references. is an ocean acoustic system designed to locate the impact position of test missile nose cones at the ocean's surface and then the pos ...

(MILS): System to localize impact and location of test missile nose cones * Ocean acoustic tomography: A technique to measure ocean temperatures and currents by the time delay of sounds between two distant instruments *

Project Mogul Project Mogul (sometimes referred to as Operation Mogul) was a top secret project by the US Army Air Forces involving microphones flown on high-altitude balloons, whose primary purpose was long-distance detection of sound waves generated by Soviet ...

, based on the conjectured existence of a similar channel in the

upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

Search for Malaysia Airlines Flight 370 Searching or search may refer to: Computing technology * Search algorithm, including keyword search ** :Search algorithms * Search and optimization for problem solving in artificial intelligence * Search engine technology, software for findi ...

: Sounds carried by the SOFAR channel were analyzed to determine if they detected a possible ocean impact of a passenger jet that disappeared in the Southern Indian Ocean

In nature

Mysterious low-frequency

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

s, attributed to

fin whale The fin whale (''Balaenoptera physalus''), also known as finback whale or common rorqual and formerly known as herring whale or razorback whale, is a cetacean belonging to the parvorder of baleen whales. It is the second-longest species of cet ...

s (''Balaenoptera physalus''), 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.Orientation by Means of Long Range Acoustic Signaling in Baleen Whales
R. Payne, D. Webb, in Annals NY Acad. Sci., 188: 110–41 (1971)

Popular culture

The novel ''

The Hunt for Red October ''The Hunt for Red October'' is the debut novel by American author Tom Clancy, first published on October 1, 1984, by the Naval Institute Press. It depicts Soviet submarine captain Marko Ramius as he seemingly goes rogue with his country's cutt ...

'' describes the use of the SOFAR channel in submarine detection.

Footnotes

References

External links

The SOFAR or deep sound channel
from

NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

How is sound used to study undersea earthquakes?
(Recording of the March 11, 2011 Honshu, Japan earthquake recorded at a hydrophone located near the Aleutian Islands)
A sound pipeline
from the

National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the Nati ...

SOSUS, the "Secret Weapon" of Underwater Surveillance
by Edward C. Whitman. ''Undersea Warfare''
Richard Muller, UC Berkeley – lecture on waves, SOFAR and the Roswell UFO Incident
{{physical oceanography, expanded=none Acoustics Anti-submarine warfare Navigation Oceanography Sonar