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A magnetic anomaly detector (MAD) is a piece of equipment that is used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used either by military forces to detect submarines—such a mass of ferromagnetic material disturbs the magnetic field and can be detected—or to a geomagnetic survey instrument used to search for minerals by the disturbance of the normal earth-field.
To reduce interference from electrical equipment aboard anti-submarine aircraft the 'MAD head' is placed out from the aircraft on a boom or is a towed aerodynamic device. Even so the submarine must be very near the aircraft's position (and close to the sea surface) to detect the change or anomaly. The detection range is normally related to the distance between the sensor and the submarine. The size of the submarine, its orientation in the Earth's magnetic field, and its hull material composition also determines the detection range. MAD devices are usually mounted on aircraft.
Function
There is some misunderstanding of the actual detection of submarines in water using the MAD boom system - ostensibly magnetic moment displacement is the main disturbance, yet submarines are detectable even when lying parallel with the earth's magnetic field, and despite construction with non-ferromagnetic hulls , in particular the Soviet-Russian Alfa-class attack submarine, whose hull is constructed out of Titanium to both give dramatic submerged performance and protection from detection by MAD sensors - to which it is still detectable.
The Alfa's detectability has led analysts not privy to the actual workings of the MAD sensor to deduce that its very name is a form of intelligence smokescreen, so effective that the Soviet Union to construct the Alfa and even consider building the Typhoon-class SSBN out of titanium at one point. Since titanium structures are detectable, MAD sensors do not directly detect deviations in the earth's magnetic field. Instead, they may be described as long-range electric and electromagnetic field detector arrays of great sensisitivity.
A electric field is set up in conductors experiencing a variation in physical environmental conditions, providing that they are contiguous and posess sufficient mass. Particularly in submarine hulls, there is a measurable temperature difference between the bottom and top of the hull producing a related salinity difference, as salinity is affected by temperature of water. The difference in salinity sets up an electric potential, with the submarine hull acting as a conductor - and an electric field flows through the hull, as ions exchange charge as a flow of metal nuclei-bound free electron flow in the metal between the otherwise depth and temperature seperated laminae of sea-water.
The resulting dynamic electric field produces an electromagnetic field of its own, and thus even a titanium hull will be detectable on a MAD scope, as will a surface ship for the same reason.
In the case of the "Pave Mace"/"Black Crow" system (see below), the same principle applies - the electric field of the truck coil is visible, rather than its magnetic disturbance, which is in itself a strong indication as to the true nature of the MAD sensor system's functions.
Other uses
For geomagnetic survey work the detector can be mounted on a long probe in front of the aircraft or be a towed device. A chart is produced that geologists can study to determine the location and extent of mineral deposits.
Another, similar use was by the original AC-130A Spectre gunships, which employed the "Pave Mace"/"Black Crow" magnetic anomaly detection system to detect truck ignition coils (hidden in heavy overhead jungles) during the Vietnam War.
See also: Antisubmarine warfare
"Magnetic anomaly detector"