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.
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.
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.