A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...

. The term refers specifically to

magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...

s used by military forces to detect

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s (a mass of

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

material creates a detectable disturbance in the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

); military MAD equipment is a descendant of

geomagnetic Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...

survey or

aeromagnetic survey An aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger ar ...

instruments used to search for

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

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. 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 World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

harnessed the

fluxgate magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...

, an inexpensive and easy to use technology developed in the 1930s by Victor Vacquier of

Gulf Oil Gulf Oil was a major global oil company in operation from 1901 to 1985. The eighth-largest American manufacturing company in 1941 and the ninth-largest in 1979, Gulf Oil was one of the so-called Seven Sisters oil companies. Prior to its merger ...

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 The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...

continued to develop MAD gear as a parallel development with

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...

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

Operation

The magnetic anomaly from a submarine is usually very small. One source estimates that it is only about 0.2 n T at a distance of 600 m. Another source estimates that a 100m long and 10 m wide submarine would produce a magnetic flux of 13.33 nT at 500m, 1.65 nT at 1km and 0.01 nT at 5km. To reduce interference from electrical equipment or metal in the

fuselage The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraft t ...

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 In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals t ...

of 500 meters. 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 An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...

. For example, one study showed that a horizontal detection range of 450-800m, when aircraft was 200m above a submarine, decreased to less than 150m when the aircraft was 400m above the submarine. 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 impacted by meteorological conditions; indeed above

sea state In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and Swell (ocean), swell—at a certain location and moment. A sea state is characterized by statistics, including the ...

5, MAD may be the only reliable method for submarine detection.

Other uses

ZK-SWA geological survey aircraft Upernavik Airport 2007-08-01

For

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 A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

and geophysicists can study to determine the distribution and concentration of magnetic

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed ...

which are related to

geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

and

mineral deposits In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

References

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

History

Operation

Other uses

See also

References