Sofar channel
Encyclopedia
The SOFAR channel or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the speed of sound
is minimal. The SOFAR channel acts as a waveguide
for sound, and low frequency sound waves
within the channel may travel thousands of miles before dissipating. This phenomenon is an important factor in submarine warfare
. The deep sound channel was discovered and described independently by Dr. Maurice Ewing
, and Leonid Brekhovskikh
in the 1940s.
The SOFAR channel is centered on 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 increases linearly with depth, but temperature is more variable, generally falling rapidly in the main thermocline
from the surface to around a thousand meters deep, then remaining almost unchanged from there to the ocean floor in the deep sea
. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient
. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is called the sound channel axis.
Near Bermuda
, the sound channel axis occurs at a depth of around 1000 meters. In temperate waters, the axis is shallower, and at high latitudes (above about 60°N or below 60°S) it reaches the surface.
Sound propagates in the channel by refraction
of sound, which makes sound travel near the depth of slowest speed. If a sound wave propagates away from this horizontal channel, the part of the wave furthest from the channel axis travels faster, so the wave turns back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fiber
.
Mysterious low-frequency sound
s, attributed to humpback whales
and other baleen whales, are a common occurrence in the channel. Scientists believe 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 into the ocean a small metal sphere (called a SOFAR bomb
or
SOFAR disc
), specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots.
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.
bottom limited: A situation that exists when the ocean bottom occurs at a depth less than the critical depth. Convergence zone propagation is prevented from occurring. Deep sound channel propagation is restricted to a deep source.
conjugate depth: For a source below the sonic layer depth, that depth below the deep sound channel axis where the sound speed equals the speed at the source depth.
convergence zone: That region in the deep ocean where sound rays, refracted from the depths, return to the surface. They are focused at or near the surface in successive intervals. A convergence zone is a sound-transmission channel in the deep ocean [2,500 to 15,000 feet (850 to 4,500 meters)] produced by the combination of pressure and temperature changes.
cutoff frequency: That frequency below which an acoustic signal will tend not to be trapped in a layer or duct and is a function of the depth of the layer.
critical depth: The depth below the deep sound channel (DSC) axis at which the sound speed is the same as it is at the sonic layer depth. The critical depth is the bottom of the DSC.
deep layer: In underwater acoustics, the layer of water between the lower edge of the main thermocline and the ocean bottom. It is characterized by a nearly constant temperature and a positive sound-speed gradient caused by pressure.
depth excess (DE): In underwater acoustics, 1. The difference between the bottom depth and the depth at which the sound velocity is equal to either the surface velocity when there is no layer depth, or the maximum velocity in the surface layer. 2. The difference between the depth at which the maximum near-surface sound velocity recurs and the bottom depth. It is applied to convergence zone propagation.
depth required: In underwater acoustics, the minimum depth required for a reliable convergence zone to exist. It is 200 to 300 fathom
s below the critical depth.
diffraction loss: The loss of sound energy that occurs when sound rays traveling in a sound channel experience leakage out of the channel and thus out of the main wave front.
downslope enhancement: Acoustic energy from a source in shallow water changes from a bottom bounce path to a convergence zone or sound channel path as it travels to deeper water and is concentrated down the slope to a receiver in deep water. Also called the megaphone effect. See upslope enhancement.
half channel: In underwater acoustics, an upward-refracting condition where the sound-speed gradient is positive from the surface all the way to the bottom. In a half channel, sound waves behave as if in a very thick surface duct.
limiting depth: In underwater acoustics, the depth below the deep sound channel axis where the sound speed equals the maximum near-surface sound speed.
limiting ray: In underwater acoustics, the sound ray that becomes tangent at the depth where the sound speed is at maximum; it delimits the outer boundary of direct (before reflection) sound rays.
surface duct (SD): In underwater acoustics, a zone below the sea surface where sound rays are refracted toward the surface and then reflected. The rays alternately are refracted and reflected along the duct out to relatively long distances from the sound source.
shadow zone: In underwater acoustics, a region in which very little sound energy penetrates, depending upon the strength of the lower boundary of the surface duct. It is usually bounded by the lower boundary of the surface duct and the limiting ray. There are two shadow zones: the sea surface, beneath which a shadow is cast by the surface in the sound field of a shallow source, and the deep-sea bottom, which produces a shadow zone in the upward-refracting water above it.
sonic layer depth (SLD): The depth of maximum near-surface sound speed above the deep sound channel.
sound channel: In underwater acoustics, that region in the water column where the sound speed first decreases to a minimum value with depth and then increases in value, due to pressure. Above the depth of minimum value, sound rays are bent downward; below the depth of minimum value, rays are bent upward, resulting in the rays being trapped in this channel and permitting their detection at great ranges from the sound source.
sound channel axis: The depth of minimum sound speed within a sound channel. Abbreviated as DSCA for the deep sound channel axis or SSCA for the secondary sound channel axis.
topographic shading: The disruption of convergence zone (CZ) or deep sound channel propagation by ocean bottom features such as seamounts, guyots, ridges, or islands. This disruption causes large shadow zones. Depth excess is destroyed for CZ propagation when a source is one-half the CZ range from such a bottom feature.
upslope enhancement: Acoustic energy from a source in deep water changes from a convergence zone or sound channel path to a bottom bounce path as the bottom shoals and is concentrated up the slope to a receiver in shallow water. Also known as the inverse megaphone effect.
vertex depth: In underwater acoustics, the depth in the water at which a refracted sound ray becomes horizontal.
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
is minimal. The SOFAR channel acts as a waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
for sound, and low frequency sound waves
Underwater acoustics
Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and...
within the channel may travel thousands of miles before dissipating. This phenomenon is an important factor in submarine warfare
Submarine warfare
Naval warfare is divided into three operational areas: surface warfare, air warfare and underwater warfare. The latter may be subdivided into submarine warfare and anti-submarine warfare as well as mine warfare and mine countermeasures...
. The deep sound channel was discovered and described independently by Dr. Maurice Ewing
Maurice Ewing
William Maurice "Doc" Ewing 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 basins, ocean bottom photography, submarine sound transmission , deep sea coring of the ocean...
, and Leonid Brekhovskikh
Leonid Brekhovskikh
Leonid Maksimovich Brekhovskikh was a Russian/Soviet scientist known for his work in acoustical and physical oceanography.-Life:...
in the 1940s.
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and water pressure (and, to a smaller extent, salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
) combine to create the region of minimum sound speed in the water column
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
. Pressure in the ocean increases linearly with depth, but temperature is more variable, generally falling rapidly in the main thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
from the surface to around a thousand meters deep, then remaining almost unchanged from there to the ocean floor in the deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...
. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient
Sound speed gradient
In acoustics, the sound speed gradient is the rate of change of the speed of sound with distance, for example with depth in the ocean,or height in the Earth's atmosphere. A sound speed gradient leads to refraction of sound wavefronts in the direction of lower sound speed, causing the sound rays to...
. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is called the sound channel axis.
Near Bermuda
Bermuda
Bermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, its nearest landmass is Cape Hatteras, North Carolina, about to the west-northwest. It is about south of Halifax, Nova Scotia, Canada, and northeast of Miami, Florida...
, the sound channel axis occurs at a depth of around 1000 meters. In temperate waters, the axis is shallower, and at high latitudes (above about 60°N or below 60°S) it reaches the surface.
Sound propagates in the channel by refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
of sound, which makes sound travel near the depth of slowest speed. If a sound wave propagates away from this horizontal channel, the part of the wave furthest from the channel axis travels faster, so the wave turns back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
.
Mysterious low-frequency sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
s, attributed to humpback whales
Humpback Whale
The humpback whale is a species of baleen whale. One of the larger rorqual species, adults range in length from and weigh approximately . The humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the...
and other baleen whales, are a common occurrence in the channel. Scientists believe humpback whales may dive
Diving
Diving is the sport of jumping or falling into water from a platform or springboard, sometimes while performing acrobatics. Diving is an internationally-recognized sport that is part of the Olympic Games. In addition, unstructured and non-competitive diving is a recreational pastime.Diving is one...
down to this channel and "sing" to communicate with other humpback whales many kilometers away.
During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, Dr. Maurice Ewing
Maurice Ewing
William Maurice "Doc" Ewing 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 basins, ocean bottom photography, submarine sound transmission , deep sea coring of the ocean...
suggested that dropping into the ocean a small metal sphere (called a SOFAR bomb
Sofar bomb
In oceanography, a sofar bomb , occasionally also referred to as a sofar disc, is a long-range position-fixing system that uses explosive sounds in the deep sound channel of the ocean to pinpoint the location of ships or crashed planes...
or
SOFAR disc
Sofar bomb
In oceanography, a sofar bomb , occasionally also referred to as a sofar disc, is a long-range position-fixing system that uses explosive sounds in the deep sound channel of the ocean to pinpoint the location of ships or crashed planes...
), specifically designed to implode at the SOFAR channel, could be used as a secret distress signal by downed pilots.
The novel The Hunt for Red October
The Hunt for Red October
The Hunt for Red October is a 1984 novel by Tom Clancy. The story follows the intertwined adventures of Soviet submarine captain Marko Aleksandrovich Ramius and CIA analyst Jack Ryan.The novel was originally published by the U.S...
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
Project Mogul
Project 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 atomic bomb tests. The project was carried out from 1947 until early 1949...
, carried out from 1947 until late 1948.
Related terms
Terminology related to The SOFAR or deep sound channel from the United States' Navy Supplement to the DOD Dictionary of Military and Associated Terms:bottom limited: A situation that exists when the ocean bottom occurs at a depth less than the critical depth. Convergence zone propagation is prevented from occurring. Deep sound channel propagation is restricted to a deep source.
conjugate depth: For a source below the sonic layer depth, that depth below the deep sound channel axis where the sound speed equals the speed at the source depth.
convergence zone: That region in the deep ocean where sound rays, refracted from the depths, return to the surface. They are focused at or near the surface in successive intervals. A convergence zone is a sound-transmission channel in the deep ocean [2,500 to 15,000 feet (850 to 4,500 meters)] produced by the combination of pressure and temperature changes.
cutoff frequency: That frequency below which an acoustic signal will tend not to be trapped in a layer or duct and is a function of the depth of the layer.
critical depth: The depth below the deep sound channel (DSC) axis at which the sound speed is the same as it is at the sonic layer depth. The critical depth is the bottom of the DSC.
deep layer: In underwater acoustics, the layer of water between the lower edge of the main thermocline and the ocean bottom. It is characterized by a nearly constant temperature and a positive sound-speed gradient caused by pressure.
depth excess (DE): In underwater acoustics, 1. The difference between the bottom depth and the depth at which the sound velocity is equal to either the surface velocity when there is no layer depth, or the maximum velocity in the surface layer. 2. The difference between the depth at which the maximum near-surface sound velocity recurs and the bottom depth. It is applied to convergence zone propagation.
depth required: In underwater acoustics, the minimum depth required for a reliable convergence zone to exist. It is 200 to 300 fathom
Fathom
A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...
s below the critical depth.
diffraction loss: The loss of sound energy that occurs when sound rays traveling in a sound channel experience leakage out of the channel and thus out of the main wave front.
downslope enhancement: Acoustic energy from a source in shallow water changes from a bottom bounce path to a convergence zone or sound channel path as it travels to deeper water and is concentrated down the slope to a receiver in deep water. Also called the megaphone effect. See upslope enhancement.
half channel: In underwater acoustics, an upward-refracting condition where the sound-speed gradient is positive from the surface all the way to the bottom. In a half channel, sound waves behave as if in a very thick surface duct.
limiting depth: In underwater acoustics, the depth below the deep sound channel axis where the sound speed equals the maximum near-surface sound speed.
limiting ray: In underwater acoustics, the sound ray that becomes tangent at the depth where the sound speed is at maximum; it delimits the outer boundary of direct (before reflection) sound rays.
surface duct (SD): In underwater acoustics, a zone below the sea surface where sound rays are refracted toward the surface and then reflected. The rays alternately are refracted and reflected along the duct out to relatively long distances from the sound source.
shadow zone: In underwater acoustics, a region in which very little sound energy penetrates, depending upon the strength of the lower boundary of the surface duct. It is usually bounded by the lower boundary of the surface duct and the limiting ray. There are two shadow zones: the sea surface, beneath which a shadow is cast by the surface in the sound field of a shallow source, and the deep-sea bottom, which produces a shadow zone in the upward-refracting water above it.
sonic layer depth (SLD): The depth of maximum near-surface sound speed above the deep sound channel.
sound channel: In underwater acoustics, that region in the water column where the sound speed first decreases to a minimum value with depth and then increases in value, due to pressure. Above the depth of minimum value, sound rays are bent downward; below the depth of minimum value, rays are bent upward, resulting in the rays being trapped in this channel and permitting their detection at great ranges from the sound source.
sound channel axis: The depth of minimum sound speed within a sound channel. Abbreviated as DSCA for the deep sound channel axis or SSCA for the secondary sound channel axis.
topographic shading: The disruption of convergence zone (CZ) or deep sound channel propagation by ocean bottom features such as seamounts, guyots, ridges, or islands. This disruption causes large shadow zones. Depth excess is destroyed for CZ propagation when a source is one-half the CZ range from such a bottom feature.
upslope enhancement: Acoustic energy from a source in deep water changes from a convergence zone or sound channel path to a bottom bounce path as the bottom shoals and is concentrated up the slope to a receiver in shallow water. Also known as the inverse megaphone effect.
vertex depth: In underwater acoustics, the depth in the water at which a refracted sound ray becomes horizontal.
See also
- Bathythermograph (BT)BathythermographThe bathythermograph, or BT, is a small torpedo-shaped device that holds a temperature sensor and a transducer to detect changes in water temperature versus depth. Lowered into the water from an underway ship, the BT records pressure and temperature changes as it is dropped through the water...
- Ocean acoustic tomographyOcean acoustic tomographyOcean Acoustic Tomography is a technique used to measure temperatures and currents over large regions of the ocean. On ocean basin scales, this technique is also known as acoustic thermometry. The technique relies on precisely measuring the time it takes sound signals to travel between two...
- Project MogulProject MogulProject 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 atomic bomb tests. The project was carried out from 1947 until early 1949...
- SOSUSSOSUSSOSUS, an acronym for Sound Surveillance System, is a chain of underwater listening posts across the northern Atlantic Ocean near Greenland, Iceland and the United Kingdom — the GIUK gap. It was originally operated by the United States Navy for tracking Soviet submarines, which had to pass...
- ThermoclineThermoclineA thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
- Underwater acousticsUnderwater acousticsUnderwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and...
External links
- The SOFAR or deep sound channel, from NOAA
- A sound pipeline, from the National Academy of SciencesUnited States National Academy of SciencesThe National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
- 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