Magnetic anomaly detector

A magnetic anomaly detector (MAD) is an instrument 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 (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field) or to a geomagnetic survey instrument used to search for minerals by the disturbance of the normal earth-field.


Magnetic anomaly detectors were first employed to detect submarines during World War II. MAD gear was used by both Japanese and U.S. anti-submarine forces, either towed by ship or mounted in aircraft to detect shallow submerged enemy submarines. The Japanese called the technology jikitanchiki. After the war, the U.S. Navy continued to develop MAD gear as a parallel development with sonar detection technologies.


To reduce interference from electrical equipment or metal in the fuselage of the aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamic device. Even so, the submarine must be very near the aircraft's position and close to the sea surface for detection of the change or anomaly. The size of the submarine and its hull composition determine the detection range. MAD devices are usually mounted on aircraft.


There is some misunderstanding of the mechanism of detection of submarines in water using the MAD boom system. Magnetic moment displacement is ostensibly the main disturbance, yet submarines are detectable even when oriented parallel to the earth's magnetic field, despite construction with non-ferromagnetic hulls. For example, the Soviet-Russian Alfa class submarine, whose hull is constructed out of titanium to give dramatic submerged performance and protection from detection by MAD sensors, is still detectable.

The Alfa's detectability has led some analysts to deduce that the MAD's name is an intentional deception, so effective that the Soviet Union decided to construct the Alfa and even consider building the Typhoon class submarine 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 sensitivity.

An electric field is set up in conductors experiencing a variation in physical environmental conditions, providing that they are contiguous and possess 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 creates an electric potential across the hull. An electric current then flows through the hull, between the laminae of seawater separated by depth and temperature.

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.

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.

During the Vietnam War the original AC-130A Spectre gunships employed the "Pave Mace"/"Black Crow" magnetic anomaly detection system to detect truck ignition coils in vehicles hidden under heavy jungle canopies. When the electric current flowing through such a coil changes it produces a magnetic flux, which is, in simplest terms, a change in the magnetic field over a given area. In particular, when the trucks were turned on the current flowing through the coils went from zero to some maximum, producing a large and easily detectable flux.

In Popular Culture

Magnetometers have been flown at low altitude near the towns of Wallkill, Walden, Montgomery, and Pine Bush in Orange and Ulster counties, New York, a rural area in the Wallkill River valley. It has been asserted that many "anomalies" indicate that extraterrestrials had been present. This unusual use of magnetic anomaly measurement would generally be classified as pseudoscience.

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