SOFAR channel

The SOFAR channel ( SO and F ixing A nd R anging) is an intermediate layer in the ocean in which water- borne noise emitted there can propagate over long distances, similar to a waveguide .

Physical background

A condition for a SOFAR channel is a minimum sound velocity within the water column. The depth at which the minimum is located forms the axis of the SOFAR Canal.

The speed of sound in the ocean depends on the density and thus on temperature, salinity and pressure. Since the pressure increases linearly with the depth and the influence of the salinity is only slight, the temperature profile is decisive for the sound velocity profile . Temperature profiles for which there is a minimum in the sound velocity profile can be found mainly between 50 ° north and 50 ° south latitude, where a warm surface layer is present. The minimum there is typically at depths between 500 and 1500 m below the sea surface.

left: schematic sound velocity profile (blue) with a minimum at 1000 m depth and the radiation angle (black) right: beam paths for the given sound velocity profile and various radiation angles as well as the axis of the SOFAR channel (dotted)

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"Fixing"

According to Snellius' law of refraction : a sound beam that is emitted at an angle to the horizontal at the minimum speed of sound is refracted as follows: The angle of radiation, the speed of sound in the radiation depth , the refracted angle and the speed of sound in the adjacent depth layer. ${\ displaystyle \ cos (\ alpha _ {2}) = {\ frac {c_ {2}} {c_ {1}}} \ cos (\ alpha _ {1})}$ ${\ displaystyle \ alpha}$ ${\ displaystyle \ alpha _ {1}}$ ${\ displaystyle c_ {1}}$ ${\ displaystyle \ alpha _ {2}}$ ${\ displaystyle c_ {2}}$

At the beginning, and thus , the sound beam is refracted towards the horizontal until it comes to total reflection and the sign of the angle is reversed. Now and with it , the sound beam is refracted away from the horizontal and moves again in the direction of the depth in which the sound velocity minimum is. From there it is again and , the sound beam is refracted towards the horizontal again until total reflection occurs. ${\ displaystyle c_ {2}> c_ {1}}$ ${\ displaystyle \ left | \ alpha _ {2} \ right | <\ left | \ alpha _ {1} \ right |}$ ${\ displaystyle c_ {2} <c_ {1}}$ ${\ displaystyle \ left | \ alpha _ {2} \ right |> \ left | \ alpha _ {1} \ right |}$ ${\ displaystyle c_ {2}> c_ {1}}$ ${\ displaystyle \ left | \ alpha _ {2} \ right | <\ left | \ alpha _ {1} \ right |}$

The depth at which total reflection occurs depends on the gradient of the speed of sound and the radiation angle . The area between the depths where total internal reflection occurs is called the SOFAR channel. The larger the gradient and the flatter the beam angle, the narrower the SOFAR channel. ${\ displaystyle \ alpha}$

"Ranging"

Since the sound through the fixing spreads not spherical here, but cylindrical, taking acoustic energy not square with the distance to the sound source, but this linearly with from. In addition, there is no absorption on the surface or on the sea floor. This means that a large part of the sound energy remains in the SOFAR channel and the sound can thus travel over several thousand kilometers.

Use and application

Humans have also learned to use the SOFAR channel. During the Second World War, Maurice Ewing suggested using this channel for communication in emergencies. In the event of a pilot's crash, a metal ball that automatically exploded at a depth of 1 km (i.e. in the area of the SOFAR canal) due to the pressure caused a sound pulse. This signal could then be picked up with the help of three hydrophones set up on the American coast at about the same depth but in different locations . By precisely determining the point in time at which the sound waves arrived at the respective hydrophones, the position of the crash could be calculated with a relatively high degree of accuracy due to the transit time differences (and assuming a constant speed of sound ).

In the Mercury space program , so-called Sofar bombs were used - in addition to numerous other measures - to locate the return capsule that had landed. The Mercury capsules were each equipped with two such explosive devices, weighing around 450 grams. The first was dropped during landing when the main parachute deployed. The second remained on the return capsule and should only explode in the event of the capsule sinking. Both Sofar bombs were set to a detonation depth of about 1200 meters. The sound waves were picked up by measuring stations on both sides of the Atlantic and by ships and evaluated for position determination.

In addition, the SOFAR channel is used by SOSUS to locate submarines and they can also communicate via it.

The SOFAR channel can also be used to determine temperature using acoustic tomography .

literature

Günter Dietrich u. a .: General oceanography . Borntraeger Verlag, Berlin 1992, ISBN 3-443-01016-4
John R. Apel: Principles of Ocean Physics . Academic Press, London 1999, ISBN 0-12-058866-8

Web links

Recovery procedures in the Mercury project (from: NASA - This New Ocean)