Ingeophysics, amagnetic anomaly is a local variation in theEarth'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 (geomagnetic reversals) in successive bands of ocean floor parallel withmid-ocean ridges was important evidence forseafloor spreading, a concept central to the theory ofplate tectonics.
Magnetic anomalies are generally a small fraction of the magnetic field. The total field ranges from 25,000 to 65,000 nanoteslas (nT).[1] To measure anomalies,magnetometers need a sensitivity of 10 nT or less. There are three main types of magnetometer used to measure magnetic anomalies:[2]: 162–164 [3]: 77–79
In ground-based surveys, measurements are made at a series of stations, typically 15 to 60 m apart. Usually a proton precession magnetometer is used and it is often mounted on a pole. Raising the magnetometer reduces the influence of small ferrous objects that were discarded by humans. To further reduce unwanted signals, the surveyors do not carry metallic objects such as keys, knives or compasses, and objects such as motor vehicles, railway lines, and barbed wire fences are avoided. If some such contaminant is overlooked, it may show up as a sharp spike in the anomaly, so such features are treated with suspicion. The main application for ground-based surveys is the detailed search for minerals.[2]: 163 [3]: 83–84
Airborne magnetic surveys are often used in oil surveys to provide preliminary information for seismic surveys. In some countries such as Canada, government agencies have made systematic surveys of large areas. The survey generally involves making a series of parallel runs at a constant height and with intervals of anywhere from a hundred meters to several kilometers. These are crossed by occasional tie lines, perpendicular to the main survey, to check for errors. The plane is a source of magnetism, so sensors are either mounted on a boom (as in the figure) or towed behind on a cable. Aeromagnetic surveys have a lower spatial resolution than ground surveys, but this can be an advantage for a regional survey of deeper rocks.[2]: 166 [3]: 81–83
In shipborne surveys, a magnetometer is towed a few hundred meters behind a ship in a device called afish. The sensor is kept at a constant depth of about 15 m. Otherwise, the procedure is similar to that used in aeromagnetic surveys.[2]: 167 [3]: 83
Sputnik 3 in 1958 was the first spacecraft to carry a magnetometer.[5]: 155 [6] In the autumn of 1979,Magsat was launched and jointly operated byNASA andUSGS until the spring of 1980. It had acaesium vapor scalar magnetometer and a fluxgate vector magnetometer.[7]CHAMP, a German satellite, made precise gravity and magnetic measurements from 2001 to 2010.[8][9] A Danish satellite,Ørsted, was launched in 1999 and is still in operation, while theSwarm mission of theEuropean Space Agency involves a "constellation" of three satellites that were launched in November, 2013.[10][11][12]
There are two main corrections that are needed for magnetic measurements. The first is removing short-term variations in the field from external sources; e.g.,temporal variations which include diurnal variations that have a period of 24 hours and magnitudes of up to 30 nT, probably from the action of thesolar wind on theionosphere.[3]: 72 In addition,magnetic storms can have peak magnitudes of 1000 nT and can last for several days. Their contribution can be measured by returning to a base station repeatedly or by having another magnetometer that periodically measures the field at a fixed location.[2]: 167
Second, since the anomaly is the local contribution to the magnetic field, the main geomagnetic field must be subtracted from it. TheInternational Geomagnetic Reference Field is usually used for this purpose. This is a large-scale, time-averaged mathematical model of the Earth's field based on measurements from satellites, magnetic observatories and other surveys.[2]: 167
Some corrections that are needed forgravity anomalies are less important for magnetic anomalies. For example, the vertical gradient of the magnetic field is 0.03 nT/m or less, so an elevation correction is generally not needed.[2]: 167
| Type | Susceptibility ( SI) |
|---|---|
| Sedimentary | |
| Limestones | 0-3 |
| Sandstones | 0-20 |
| Shales | 0.01-15 |
| Igneous | |
| Basalt | 0.2-175 |
| Gabbro | 1-90 |
| Granite | 0-50 |
| Rhyolite | 0.2-35 |
| Metamorphic | |
| Gneiss | 0.1-25 |
| Serpentine | 3-17 |
| Slate | 0-35 |
| Minerals | |
| Graphite | 0.1 |
| Quartz | -0.01 |
| Coal | 0.02 |
| Clays | 0.2 |
| Pyrrhotite | 1-6000 |
| Magnetite | 1200-19200 |
The magnetization in the surveyed rock is the vector sum of induced andremanent magnetization:
The induced magnetization of many minerals is the product of the ambient magnetic field and theirmagnetic susceptibilityχ:
Some susceptibilities are given in the table.
Minerals that arediamagnetic orparamagnetic only have an induced magnetization.Ferromagnetic minerals such asmagnetite also can carry a remanent magnetization or remanence. This remanence can last for millions of years, so it may be in a completely different direction from the present Earth's field. If a remanence is present, it is difficult to separate from the induced magnetization unless samples of the rock are measured. The ratio of the magnitudes,Q =Mr/Mi, is called theKoenigsberger ratio.[2]: 172–173 [13]
Interpretation of magnetic anomalies is usually done by matching observed and modeled values of the anomalous magnetic field. An algorithm developed by Talwani and Heirtzler(1964) (and further elaborated by Kravchinsky et al., 2019) treats both induced and remnant magnetizations as vectors and allows theoretical estimation of the remnant magnetization from the existingapparent polar wander paths for different tectonic units or continents.[14][15]
Magnetic surveys over the oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges. They involve a series of positive and negative anomalies in the intensity of the magnetic field, forming stripes running parallel to each ridge. They are often symmetric about the axis of the ridge. The stripes are generally tens of kilometers wide, and the anomalies are a few hundred nanoteslas. The source of these anomalies is primarily permanent magnetization carried by titanomagnetite minerals inbasalt andgabbros. They are magnetized when ocean crust is formed at the ridge. Asmagma rises to the surface and cools, the rock acquires athermoremanent magnetization in the direction of the field. Then the rock is carried away from the ridge by the motions of thetectonic plates. Every few hundred thousand years, the direction of the magnetic fieldreverses. Thus, the pattern of stripes is a global phenomenon and can be used to calculate the velocity ofseafloor spreading.[16][17]
In theSpace Odyssey series byArthur C. Clarke, a series ofmonoliths are left by extraterrestrials for humans to find. One near the craterTycho is found by its unnaturally powerful magnetic field and namedTycho Magnetic Anomaly 1 (TMA-1).[18] One orbiting Jupiter is named TMA-2, and one in theOlduvai Gorge is found in 2513 and retroactively named TMA-0 because it was first encountered by primitive humans.
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