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 by military forces to detect submarines (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field); military MAD equipment is a descendent of geomagnetic survey or aeromagnetic survey instruments used to search for minerals by detecting their disturbance of the normal earth-field.

Geoexploration by measuring and studying variations in the Earth's magnetic field has been conducted by scientists since 1843. The first uses of magnetometers were for the location of ore deposits. Thalen's "The Examination of Iron Ore Deposits by Magnetic Measurements", published in 1879, was the first scientific treatise describing this practical use.

Magnetic anomaly detectors employed to detect submarines during World War II harnessed the fluxgate magnetometer, an inexpensive and easy to use technology developed in the 1930s by Victor Vacquier of Gulf Oil for finding ore deposits. 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 (磁気探知機, "Magnetic Detector"). After the war, the U.S. Navy continued to develop MAD gear as a parallel development with sonar detection technologies.

Satellite, near-surface and oceanic data from detectors was used to create the World Digital Magnetic Anomaly Map published by the Commission for the Geological Map of the World (CGMW) in July 2007.

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 on 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 anomaly, according to the Inverse Square Law (or Law of Inverse Squares). The size of the submarine, its hull composition and orientation determine the detection range. MAD devices are usually mounted on aircraft.

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