SOFAR channel

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

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 Dr. Maurice Ewing and Stanley Wong in the 1940s.^[3]

Details

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

• Bathythermograph
• Underwater acoustics

References

External links

• The SOFAR or deep sound channel, from NOAA
• A sound pipeline, from the National Academy of Sciences
• 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

Hydroacoustics
Sonar	Active acoustics Baffles (submarine) Bistatic sonar Echo sounding Fessenden oscillator GLORIA sidescan sonar Multibeam echosounder Passive acoustics Scientific echosounder Side-scan sonar Sonar beamforming Sonobuoy Surveillance Towed Array Sensor System Synthetic aperture sonar Towed array sonar Upward looking sonar
Ocean acoustics	Acoustic network Acoustic release Acoustic Doppler current profiler Acoustic seabed classification Acoustical oceanography Hydrophone Long baseline acoustic positioning system Ocean acoustic tomography Short baseline acoustic positioning system Sofar bomb SOFAR channel Sound speed gradient Sound velocity probe Ultra-short baseline Underwater acoustics Underwater acoustic communication Underwater acoustic positioning system
Acoustic ecology	Acoustic survey in fishing Acoustic tag Animal echolocation Beached whale Deep scattering layer Fishfinder Fisheries acoustics Hearing range of marine mammals Marine mammals and sonar Whale song
Related topics	Acoustic signature Bioacoustics Biophony Geophysical MASINT Hydrographic survey Noise map Soundscape

Physical oceanography
Waves	Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Sneaker wave Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami megatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water one-dimensional Saint-Venant equations shallow water equations Wind wave model
Circulation	Atmospheric circulation Baroclinity Boundary current Coriolis effect Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño Southern Oscillation General Circulation Model Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Maelstrom Modular ocean model Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulation shutdown Upwelling Whirlpool World Ocean Circulation Experiment
Tides	Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline
Landforms	Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano
Plate tectonics	Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc
Ocean zones	Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash
Sea level	Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System
Acoustics	Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics
Satellites	Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3
Related	Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas
Category Commons