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magnetic anomaly pattern
magnetic anomaly pattern (magnetic age) Phenomenon, discovered originally in the north-eastern Pacific Ocean, of linear magnetic anomalies which lie parallel to oceanic ridges where spreading has occurred. The magnetic striping, which is determined by instrumental measurements, results from repeated reversals of the Earth's magnetic field (see GEOMAGNETIC FIELD): it exists as corresponding patterns on either side of an oceanic spreading centre (see MID-OCEAN RIDGE). These can be correlated with the magnetostratigraphic time-scale (polarity time-scale), so assigning a magnetic age to some of the individual magnetic anomalies. See also SEA-FLOOR SPREADING.
aeromagnetic survey Survey of the Earth's magnetic field, based on data from magnetometers towed behind aircraft or suspended below helicopters. These instruments measure the total intensity of the geomagnetic field or, occasionally, components of this field. The resulting measurements can then be compared with theoretical models for the value of the field and the differences (magnetic anomalies) can be interpreted in terms of changes in the magnetic properties of the rocks below the survey line or grid. The magnetometers are usually flown with other instrumentation, e.g. radiometric and electromagnetic, at the lowest practicable constant height above the ground. Usually the magnetometer is housed in a ‘bird’ towed behind the aircraft, or in a wing-tip pod, or in a ‘stinger’ in the tail. In cases where the magnetometer is on board, in-board coil systems compensate for the aircraft's own magnetic field.
magnetic anomaly Any magnetic field remaining after allowance for some particular model. Normally this is the field remaining after allowance for the International Geomagnetic Reference Field, but sometimes it is the field remaining after further removal of either regional or near-surface anomalies.