SOFAR channel

The SOFAR channel (short for Sound Fixing and Ranging channel), or deep sound channel (DSC),[1] is a horizontal layer of water in the ocean at which depth the speed of sound is at its minimum. The SOFAR channel acts as a waveguide for sound, and low frequency sound waves within the channel may travel thousands of miles before dissipating.[2] This phenomenon is an important factor in submarine warfare. The deep sound channel was discovered and described independently by Maurice Ewing, Stanley Wong and Leonid Brekhovskikh in the 1940s.[3]

Underwater speed of sound
Sound speed as a function of depth at a position north of Hawaii in the Pacific Ocean derived from the 2005 World Ocean Atlas. The SOFAR channel axis is at ca. 750-m depth

Details

Rays test
Acoustic pulses travel great distances in the ocean because they are trapped in an acoustic "wave guide". This means that as acoustic pulses approach the surface they are turned back towards the bottom, and as they approach the ocean bottom they are turned back towards the surface. The ocean conducts sound very efficiently, particularly sound at low frequencies, i.e., less than a few hundred Hz

The SOFAR channel is centred on the depth where the cumulative effect of temperature and water pressure (and, to a lesser 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 metres 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 metres. 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 fibre.

Mysterious low-frequency sounds, attributed to fin whales (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.[4]

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.[5]

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.

Applications

  • Ocean acoustic tomography: A technique to measure ocean temperatures and currents by the time delay of sounds between two distant instruments
  • Search for Malaysia Airlines Flight 370: Sounds carried by the SOFAR channel were analyzed to determine if they detected a possible ocean impact of a passenger jet which disappeared in the Southern Indian Ocean
  • SOSUS: Hydrophone system to detect submarine movements during Cold War

See also

References

  1. ^ Navy Supplement to the DOD Dictionary of Military and Associated Terms (PDF). Department Of The Navy. August 2006. NTRP 1-02.
  2. ^ "The Heard Island Feasibility Test" (PDF). Acoustical Society of America. 1994.
  3. ^ Citation for Leonid Maximovich Brekhovskikh ...for pioneering contributions to wave propagation and scattering Archived 2009-10-23 at the Wayback Machine
  4. ^ Orientation by Means of Long Range Acoustic Signaling in Baleen Whales, R. Payne, D. Webb, in Annals NY Acad. Sci., 188: 110–41 (1971)
  5. ^ "Sound Channel, SOFAR, and SOSUS". Robert A. Muller. Archived from the original on 2007-05-16. Retrieved 2007-04-14.

External links

Acoustical oceanography

Acoustical oceanography is the use of underwater sound to study the sea, its boundaries and its contents.

Echo sounding

Echo sounding is a type of sonar used to determine the depth of water by transmitting sound waves into water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes. Echo sounding can also refer to hydroacoustic "echo sounders" defined as active sound in water (sonar) used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish.

The word sounding is used for all types of depth measurements, including those that don't use sound, and is unrelated in origin to the word sound in the sense of noise or tones. Echo sounding is a more rapid method of measuring depth than the previous technique of lowering a sounding line until it touched bottom.

Hydroacoustics

Hydroacoustics is the study and application of sound in water. Hydroacoustics, using sonar technology, is most commonly used for monitoring of underwater physical and biological characteristics.

Hydroacoustics can be used to detect the depth of a water body (bathymetry), as well as the presence or absence, abundance, distribution, size, and behavior of underwater plants and animals. Hydroacoustic sensing involves "passive acoustics" (listening for sounds) or active acoustics making a sound and listening for the echo, hence the common name for the device, echo sounder or echosounder.

There are a number of different causes of noise from shipping. These can be subdivided into those caused by the propeller, those caused by machinery, and those caused by the movement of the hull through the water. The relative importance of these three different categories will depend, amongst other things, on the ship type

One of the main causes of hydro acoustic noise from fully submerged lifting surfaces is the unsteady separated turbulent flow near the surface's trailing edge that produces pressure fluctuations on the surface and unsteady oscillatory flow in the near wake.The relative motion between the surface and the ocean creates a turbulent boundary layer (TBL) that surrounds the surface. The noise is generated by the fluctuating velocity and pressure fields within this TBL.

J. Lamar Worzel

J. Lamar (Joe) 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.

Marine mammals and sonar

Active sonar, the transmission equipment used on some ships to assist with navigation, is detrimental to the health and livelihood of some marine animals. Research has recently shown that beaked and blue whales are sensitive to mid-frequency active sonar and move rapidly away from the source of the sonar, a response that disrupts their feeding and can cause mass strandings. Some marine animals, such as whales and dolphins, use echolocation or "biosonar" systems to locate predators and prey. It is conjectured that active sonar transmitters could confuse these animals and interfere with basic biological functions such as feeding and mating. The study has shown whales experience decompression sickness, a disease that forces nitrogen into gas bubbles in the tissues and is caused by rapid and prolonged surfacing. Although whales were originally thought to be immune to this disease, sonar has been implicated in causing behavioral changes that can lead to decompression sickness.

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 basins, ocean bottom photography, submarine sound transmission (including the SOFAR channel), deep sea coring of the ocean bottom, theory and observation of earthquake surface waves, fluidity of the Earth's core, generation and propagation of microseisms, submarine explosion seismology, marine gravity surveys, bathymetry and sedimentation, natural radioactivity of ocean waters and sediments, study of abyssal plains and submarine canyons.

Mesopelagic zone

The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1000 meters (~660 to 3300 feet) below the ocean surface. It hosts a diverse biological community that includes bristlemouths, blobfish, bioluminescent jellyfish, giant squid, and a myriad of other unique organisms adapted to live in a low-light environment. It has long captivated the imagination of scientists, artists and writers; deep sea creatures are prominent in popular culture, particularly as horror movie villains.

Ocean acoustic tomography

Ocean 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 instruments, one an acoustic source and one a receiver, separated by ranges of 100–5000 km. If the locations of the instruments are known precisely, the measurement of time-of-flight can be used to infer the speed of sound, averaged over the acoustic path. Changes in the speed of sound are primarily caused by changes in the temperature of the ocean, hence the measurement of the travel times is equivalent to a measurement of temperature. A 1 °C change in temperature corresponds to about 4 m/s change in sound speed. An oceanographic experiment employing tomography typically uses several source-receiver pairs in a moored array that measures an area of ocean.

RAFOS float

RAFOS floats are submersible devices used to map ocean currents well below the surface. They drift with these deep currents and listen for acoustic "pongs" emitted at designated times from multiple moored sound sources. By analyzing the time required for each pong to reach a float, researchers can pinpoint its position by triangulation. The floats are able to detect the pongs at ranges of hundreds of kilometers because they generally target a range of depths known as the SOFAR (SOund Fixing And Ranging) channel, which acts as a waveguide for sound. The name "RAFOS" derives from the earlier SOFAR floats, which emitted sounds that moored receivers picked up, allowing real-time underwater tracking. When the transmit and receive roles were reversed, so was the name: RAFOS is SOFAR spelled backward. Listening for sound requires far less energy than transmitting it, so RAFOS floats are cheaper and longer lasting than their predecessors, but they do not provide information in real-time: instead they store it on board, and upon completing their mission, drop a weight, rise to the surface, and transmit the data to shore by satellite.

Ray tracing (physics)

In physics, ray tracing is a method for calculating the path of waves or particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces. Under these circumstances, wavefronts may bend, change direction, or reflect off surfaces, complicating analysis. Ray tracing solves the problem by repeatedly advancing idealized narrow beams called rays through the medium by discrete amounts. Simple problems can be analyzed by propagating a few rays using simple mathematics. More detailed analysis can be performed by using a computer to propagate many rays.

When applied to problems of electromagnetic radiation, ray tracing often relies on approximate solutions to Maxwell's equations that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength. Ray theory does not describe phenomena such as interference and diffraction, which require wave theory (involving the phase of the wave).

SOSUS

SOSUS, an acronym for sound surveillance system, is a chain of underwater listening posts located around the world in places such as the Atlantic Ocean near Greenland, Iceland and the United Kingdom—the GIUK gap—and at various locations in the Pacific Ocean. The United States Navy's initial intent for the system was for tracking Soviet submarines, which had to pass through the gap to attack targets further west. It was later supplemented by mobile assets such as the Surveillance Towed Array Sensor System (SURTASS), and became part of the Integrated Undersea Surveillance System (IUSS).

Sofar

Sofar may refer to:

Sofar bomb (SOund Fixing And Ranging bomb), a long-range position-fixing system that uses explosive sounds in the deep sound channel of the ocean

SOFAR channel (SOund Fixing And Ranging channel), a horizontal layer of water in the ocean centered on the depth at which the speed of sound is minimum

Sofar bomb

In oceanography, a sofar bomb (Sound Fixing And Ranging bomb), occasionally referred to as a sofar disc, is a long-range position-fixing system that uses impulsive sounds in the deep sound channel of the ocean to enable pinpointing of the location of ships or crashed planes. The deep sound channel is ideal for the device, as the minimum speed of sound at that depth improves the signal's traveling ability. A position is determined from the differences in arrival times at receiving stations of known geographic locations. The useful range from the signal sources to the receiver can exceed 3,000 miles (4,800 km).

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 follow a curved path. The radius of curvature of the sound path is inversely proportional to the gradient.When the sun warms the Earth's surface, there is a negative temperature gradient in atmosphere. The speed of sound decreases with decreasing temperature, so this also creates a negative sound speed gradient. The sound wave front travels faster near the ground, so the sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. The opposite effect happens when the ground is covered with snow, or in the morning over water, when the sound speed gradient is positive. In this case, sound waves can be refracted from the upper levels down to the surface.In underwater acoustics, speed of sound depends on pressure (hence depth), temperature, and salinity of seawater, thus leading to vertical speed gradients similar to those that exist in atmospheric acoustics. However, when there is a zero sound speed gradient, values of sound speed have the same "isospeed" in all parts of a given water column (there is no change in sound speed with depth). The same effect happens in an isothermal atmosphere with the ideal gas assumption.

Sound speed profile

A sound speed profile shows the speed of sound in water at different vertical levels. It has two general representations:

tabular form, with pairs of columns corresponding to ocean depth and the speed of sound at that depth, respectively.

a plot of the speed of sound in the ocean as a function of depth, where the vertical axis corresponds to the depth and the horizontal axis corresponds to the sound speed. By convention, the horizontal axis is placed at the top of the plot, and the vertical axis is labeled with values which increase from top to bottom, thus reproducing visually the ocean from its surface downward.Table 1 shows an example of the first representation; figure 1 shows the same information using the second representation.

Although given as a function of depth, the speed of sound in the ocean does not depend solely on depth. Rather, for a given depth, the speed of sound depends on the temperature at that depth, the depth itself, and the salinity at that depth, in that order.The speed of sound in the ocean at different depths can be measured directly, e.g., by using a velocimeter, or, using measurements of temperature and salinity at different depths, it can be calculated using a number of different sound speed formulae which have been developed. Examples of such formulae include those by Wilson, Chen and Millero and Mackenzie. Each such formulation applies within specific limits of the independent variables.From the shape of the sound speed profile in figure 1, one can see the effect of the order of importance of temperature and depth on sound speed. Near the surface, where temperatures are generally highest, the sound speed is often highest because the effect of temperature on sound speed dominates. Further down the water column, as temperature decreases in the ocean thermocline, sound speed also decreases. At a certain point, however, the effect of depth, i.e., pressure, begins to dominate, and the sound speed increases to the ocean floor. Also visible in figure 1 is a common feature in sound speed profiles: the SOFAR channel. The axis of this channel is found at the depth of minimum sound speed. Sounds emitted at or near the axis of this channel propagate for very long horizontal distances, owing to the refraction of the sound back to the channel's center.Sound speed profile data are a necessary component of underwater acoustic propagation models, especially those based on ray tracing theory.

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 rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.

Depending largely on season, latitude, and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur, or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude, and local environmental conditions, such as tides and currents.

Tsunami warning system

A tsunami warning system (TWS) is used to detect tsunamis in advance and issue warnings to prevent loss of life and damage to property. It is made up of two equally important components: a network of sensors to detect tsunamis and a communications infrastructure to issue timely alarms to permit evacuation of the coastal areas. There are two distinct types of tsunami warning systems: international and regional. When operating, seismic alerts are used to instigate the watches and warnings; then, data from observed sea level height (either shore-based tide gauges or DART buoys) are used to verify the existence of a tsunami. Other systems have been proposed to augment the warning procedures; for example, it has been suggested that the duration and frequency content of t-wave energy (which is earthquake energy trapped in the ocean SOFAR channel) is indicative of an earthquake's tsunami potential.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

Waveguide

A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. There is a similar effect in water waves constrained within a canal, or guns that have barrels which restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space.

There are different types of waveguides for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves.

The geometry of a waveguide reflects its function. Slab waveguides confine energy in one dimension, fiber or channel waveguides in two dimensions. The frequency of the transmitted wave also dictates the shape of a waveguide: an optical fiber guiding high-frequency light will not guide microwaves of a much lower frequency.

Some naturally occurring structures can also act as waveguides. The SOFAR channel layer in the ocean can guide the sound of whale song across enormous distances.

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