Magnetic anomaly detector

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. ==Operation==

(with MAD located in aft-facing fin-top fairing) escorted by US Navy Lockheed P-3C (MAD located in projection at base of tail), March 1986 The magnetic anomaly from a submarine is usually very small. One source estimates that it is only about 0.2 nT at a distance of 600 m. Another source estimates that a 100m long and 10m wide submarine would produce a magnetic flux of 13.33nT at 500m, 1.65nT at 1km and 0.01nT at 5km. 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, because magnetic fields decrease as the inverse cube of distance, one source gives a detection slant range of 500m. If the sea floor has sunken ships, then submarines may operate near them to confuse magnetic anomaly detectors. MAD has certain advantages over other detection methods. It is a passive detection method. Unlike sonar it is not affected by meteorological conditions; indeed above sea state 5, MAD may be the only reliable method for submarine detection. In many target-based schemes, the measured anomaly is expanded in orthogonal basis functions (OBFs) that work by using the dipole model. OBF decomposition works by expanding the measured field into an orthogonal basis derived from dipole theory in which a detection statistic is constructed from the energy of the expansion coefficient, enhancing the signal-to-noise ratio for weak magnetic anomalies. ==Other uses==

geosurvey aircraft with a MAD stinger in Upernavik, Greenland For aeromagnetic survey applications the magnetic sensor can be mounted on an aircraft (typically on a long probe in front of or behind the aircraft to reduce the magnetic effects of the aircraft itself) or in a towed device. A chart is produced that geologists and geophysicists can study to determine the distribution and concentration of magnetic minerals which are related to geology and mineral deposits. ==See also==