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In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. The magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges is important evidence supporting the theory of seafloor spreading, central to plate tectonics. ==Measurement== Magnetic anomalies are generally a small fraction of the magnetic field. The total field ranges from 25,000 to 65,000 nanoteslas (nT).〔(【引用サイトリンク】title=Geomagnetism Frequently Asked Questions )〕 To measure anomalies, magnetometers need a sensitivity of 10 nT or less. There are three main types of magnetometer used to measure magnetic anomalies:〔 # The fluxgate magnetometer was developed during World War II to detect submarines.〔 It measures the component along a particular axis of the sensor, so it needs to be oriented. On land, it is often oriented vertically, while in aircraft, ships and satellites it is usually oriented so the axis is in the direction of the field. It measures the magnetic field continuously, but drifts over time. One way to correct for drift is to take repeated measurements at the same place during the survey.〔〔 # The proton precession magnetometer measures the strength of the field but not its direction, so it does not need to be oriented. Each measurement takes a second or more. It is used in most ground surveys except for boreholes and high-resolution gradiometer surveys.〔〔 # Optically pumped magnetometers, which use alkali gases (most commonly rubidium and cesium) have high sample rates and sensitivities of 0.001 nT or less, but are more expensive than the other types of magnetometers. They are used on satellites and in most aeromagnetic surveys.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Magnetic anomaly」の詳細全文を読む スポンサード リンク
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