Magnetic anomaly

A local deviation of a planet’s magnetic field from a reference field, caused by variations in field sources or magnetized material such as rocks.

Geomagnetic field: The magnetic field generated mainly by electrical currents in a planet’s interior. Anomaly measurements are deviations from a model of this broader field.
Magnetic survey: A geophysical survey that measures spatial variations in magnetic-field strength or direction. Survey instruments map anomalies across land, sea, or air.
Magnetometer: An instrument that measures the strength or direction of a magnetic field. Magnetometers provide the observations from which anomalies are calculated.
Bouguer anomaly: A gravity anomaly corrected for the gravitational effect of terrain and elevation. It is a gravity-field departure, unlike the magnetic-field departure considered here.
Mid-Atlantic Ridge magnetic anomaly: A pattern of alternating magnetic stripes on the seafloor flanking the Mid-Atlantic Ridge. Its symmetric bands record changes in Earth’s field as new ocean crust formed.
Magnetic induction: The production of magnetization in a material by an applied magnetic field. Induced magnetization adds a field component that can vary with local rock properties.
Aeromagnetic survey: A magnetic survey conducted from an aircraft to map variations over broad areas. Aircraft measurements reveal regional anomaly patterns beyond ground access.
Total-field magnetometer: A magnetometer that measures the magnitude of the magnetic field vector. Total-field surveys detect changes in field magnitude over magnetic sources.
Free-air gravity anomaly: A gravity anomaly corrected for elevation but not for the mass between the station and sea level. Its measured field and correction differ from magnetic anomaly analysis.
Seafloor spreading: The formation of new oceanic crust at ridges and its movement away from them. Spreading carries magnetized crust outward, arranging anomalies into parallel stripes.

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