Editing Magnetic anomaly detector

{{Short description|Instrument for detecting variations in the Earth's magnetic field}}
[[File:Lockheed P-3C (JMSDF)02.jpg|thumb|MAD rear boom on [[P-3C]]]]
[[File:SH-60B Seahawk.jpg|thumb|The [[SH-60B Seahawk]] helicopter carries a yellow and red towed MAD array known as a "MAD bird", seen on the aft fuselage]]

A '''magnetic anomaly detector''' ('''MAD''') is an instrument used to detect minute variations in the [[Earth's magnetic field]].<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=183}}</ref> The term typically refers to [[magnetometer]]s used by military forces to detect [[submarine]]s (a mass of [[ferromagnetic]] material creates a detectable disturbance in the [[magnetic field]]). Military MAD equipment is a descendant of [[geomagnetic]] [[geological survey|survey]] or [[aeromagnetic survey]] instruments used to search for [[mineral]]s by detecting their disturbance of the normal earth-field.

==History==
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.  [[Robert Thalén|Thalen's]] "The Examination of Iron Ore Deposits by Magnetic Measurements", published in 1879, was the first scientific treatise describing this practical use.<ref>[https://web.archive.org/web/20050310171755/http://www.geo.ucalgary.ca/~wu/Goph547/CSM_MagNotes.pdf Note Outline: Geophysical Surveying Using Magnetics Methods, January 16, 2004, University of Calgary]</ref>

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.<ref>Dunmore, Spencer, Lost Subs, Chartwell Books, Edison NJ, 2007, p.120 {{ISBN|978-0-7858-2226-4}}</ref><ref name="lat">{{citation|url=http://www.latimes.com/news/science/la-me-vacquier24-2009jan24,0,3328591.story|journal=[[Los Angeles Times]]|title=Victor Vacquier Sr., 1907–2009: Geophysicist was a master of magnetics|date=January 24, 2009|page=B24}}.</ref>  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 [http://projects.gtk.fi/export/sites/projects/WDMAM/project/perugia/WDMAM_1.02_2007_Edition_low_resolution_reduced1.pdf World Digital Magnetic Anomaly Map] published by the [[Commission for the Geological Map of the World]] (CGMW)  in July 2007.

==Operation==
[[File:Tu-142M&P-3C-Orion-1986-DN-SC-87-00265.jpg|thumb|A Soviet [[Tupolev Tu-142|Tu-142MK]] (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 n[[Tesla (unit)|T]] at a distance of 600 m.<ref>{{cite journal|title=Magnetic Anomaly Detection Based on Full Connected Neural Network|year=2019 |page=182198|publisher=[[IEEE]]|doi=10.1109/ACCESS.2019.2943544|last1=Liu |first1=Shuchang |last2=Hu |first2=Jiafei |last3=Li |first3=Peisen |last4=Wan |first4=Chengbiao |last5=Chen |first5=Zhuo |last6=Pan |first6=Mengchun |last7=Zhang |first7=Qi |last8=Liu |first8=Zhongyan |last9=Wang |first9=Siwei |last10=Chen |first10=Dixiang |last11=Hu |first11=Jingtao |last12=Pan |first12=Xue |journal=IEEE Access |volume=7 |bibcode=2019IEEEA...7r2198L |s2cid=204082945 |doi-access=free }}</ref> Another source estimates that a 100{{nnbsp}}m long and 10 m wide submarine would produce a magnetic flux of 13.33 nT at 500{{nnbsp}}m, 1.65 nT at 1{{nnbsp}}km and 0.01 nT at 5{{nnbsp}}km.<ref>{{cite conference
  |title=Methods of Differential Submarine Detection Based on Magnetic Anomaly and Technology of Probes Arrangement
  |url=https://www.atlantis-press.com/proceedings/iwmecs-15/25840634
  |page=446
  |work=Proceedings of the 2015 2nd International Workshop on Materials Engineering and Computer Sciences
  |author1=Yuqin Chen |author2=Jiansheng Yuan
  |year=2015
  |doi=10.2991/iwmecs-15.2015.88
|doi-access=free
  }}</ref> 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.<ref name=":0">{{Cite web|url=https://fas.org/man/dod-101/navy/docs/fun/part09.htm|title=Fundamentals of Naval Weapons Systems - Chapter 9 Underwater Detection and Tracking Systems}}</ref> 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 [[Magnet#Fields of a magnet|inverse cube of distance]], one source gives a detection [[slant range]] of 500{{nnbsp}}m.<ref name=":0" /> The size of the submarine, its hull composition and orientation, as well as the water depth and complexity of the natural magnetic field, determine the detection range. MAD devices are usually mounted on [[aircraft]].<ref name=":0" /> For example, one study showed that a horizontal detection range of 450–800{{nnbsp}}m, when aircraft was 200{{nnbsp}}m above a submarine, decreased to less than 150{{nnbsp}}m when the aircraft was 400{{nnbsp}}m above the submarine.<ref name=chengjing>{{cite journal|title=Detection Range of Airborne Magnetometers in Magnetic Anomaly Detection|author=Chengjing Li|journal=Journal of Engineering Science and Technology Review |year=2015 |volume=8 |issue=4 |pages=105–110 |doi=10.25103/JESTR.084.17 |s2cid=117237628 |display-authors=etal|doi-access=free }}</ref>

If the sea floor has sunken ships, then submarines may operate near them to confuse magnetic anomaly detectors.<ref>{{cite book|title=China's Rising Sea Power|page=93-94|author=Peter Howarth}}</ref>

MAD has certain advantages over other detection methods. It is a passive detection method. Unlike sonar it is not impacted by meteorological conditions; indeed above [[sea state]] 5, MAD may be the only reliable method for submarine detection.<ref name=chengjing/>

==Other uses==
[[File:ZK-SWA geological survey aircraft Upernavik Airport 2007-08-01.jpg|thumb|[[PAC P-750 XSTOL]] 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==
* [[Submarine detection system]]
* [[Autolycus (submarine detector)|Autolycus]], exhaust plume detector

==References==
{{commons category|Magnetic anomaly detectors}}
{{reflist}}

{{DEFAULTSORT:Magnetic Anomaly Detector}}
[[Category:Anti-submarine warfare]]
[[Category:Military electronics]]
[[Category:Geophysics]]
[[Category:Naval weapons of the United States]]
[[Category:Military sensor technology]]