Magnetic anomaly

For more details on this topic, see Magnetic striping.
Satellite measurements of the radial magnetic field on Mars.

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).[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

  1. The fluxgate magnetometer was developed during World War II to detect submarines.[3]:75[4] 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.[2]:163–165[3]:75–77
  2. 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.[2]:163–165[3]:77–78
  3. 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.[3]:78–79

Data acquisition

Ground-based

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, they do not carry objects such as keys, knives or compasses. In addition, objects such as motor vehicles, railway lines, and barbed wire fences are avoided. If some such contaminant is overlooked, it often shows up as a sharp spike in the anomaly, so such features are treated with suspicion. The main application for ground-based surveys is detailed search for minerals.[2]:163[3]:83–84

Aeromagnetic

Main article: Aeromagnetic survey
This helicopter is equipped with a magnetometer array. It flies six feet above ground at speeds of 30 to 40 mph.

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 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 of ground surveys, but this can be an advantage for a regional survey of deeper rocks.[2]:166[3]:81–83

Shipborne

In shipborne surveys, a magnetometer is towed a few hundred meters behind a ship in a device called a fish. 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

Spacecraft

American and Russian satellites began measuring the Earth's field in the 1960s.[5] In the fall of 1979, Magsat was launched and jointly operated by NASA and USGS until the spring of 1980. It had a cesium vapor scalar magnetometer and a fluxgate vector magnetometer.[6] CHAMP, a German satellite, made precise gravity and magnetic measurements from 2001 to 2010.[7][8] A Danish satellite, Ørsted, was launched in 1999 and is still in operation, while the Swarm mission of the European Space Agency involves a "constellation" of three satellites that were launched in November, 2013.[9][10][11]

Data reduction

There are two main corrections that are needed for magnetic measurements. The first is remove short-term variations in the field from external sources. There are diurnal variations that have a period of 24 hours and magnitudes of up to 30 nT, probably from the action of the solar wind on the ionosphere.[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

The anomaly is the local contribution to the magnetic field, so the main geomagnetic field must be subtracted from it. Usually the International Geomagnetic Reference Field is 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 for gravity 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

Interpretation

The magnetization in the surveyed rock is a vector sum of induced and remanent magnetization:

\mathbf{M} = \mathbf{M}_\text{i} + \mathbf{M}_\text{r}.

The induced magnetization of many minerals is the product of the ambient magnetic field and their magnetic susceptibility χ:

\mathbf{M}_\text{i} = \chi \mathbf{H}.

Some susceptibilities are given in the table.

Minerals that are diamagnetic or paramagnetic only have an induced magnetization. Ferromagnetic minerals such as magnetite 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 the Koenigsberger ratio.[2]:172–173[12]

Applications

Ocean floor stripes

Magnetic anomalies around the Juan de Fuca and Gorda Ridges, off the west coast of North America, color coded by age.

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 in basalt and gabbros. They are magnetized when ocean crust is formed at the ridge. As magma rises to the surface and cools, the rock acquires a thermoremanent magnetization in the direction of the field. Then the rock is carried away from the ridge by the motions of the tectonic plates. Every few hundred thousand years, the direction of the magnetic field reverses. Thus, the pattern of stripes is a global phenomenon and can be used to calculate the velocity of seafloor spreading.[13][14]

In fiction

In the Space Odyssey series by Arthur C. Clarke, a series of monoliths are left by extraterrestrials for humans to find them. One near the crater Tycho is found by its unnaturally powerful magnetic field and named Tycho Magnetic Anomaly 1 (TMA-1).[15] One orbiting Jupiter is named TMA-2, and one in the Olduvai Gorge is found in 2513 and retroactively named TMA-0 because it was first encountered by primitive humans.

See also

References

  1. ↑ "Geomagnetism Frequently Asked Questions". National Geophysical Data Center. Retrieved 21 October 2013.
  2. 1 2 3 4 5 6 7 8 9 10 Mussett, Alan E.; Khan, M. Aftab (2000). "11. Magnetic surveying". Looking into the earth: an introduction to geological geophysics (1. publ., repr. ed.). Cambridge: Cambridge Univ. Press. pp. 162–180. ISBN 0-521-78085-3.
  3. 1 2 3 4 5 6 7 8 9 10 Telford, W. M.; L. P. Geldart; R. E. Sheriff (2001). "3. Magnetic methods". Applied geophysics (2nd, repr. ed.). Cambridge: Cambridge Univ. Press. pp. 62–135. ISBN 0521339383.
  4. ↑ Murray, Raymond C. (2004). Evidence from the earth: forensic geology and criminal investigation. Missoula (Mont.): Mountain press publ. company. pp. 162–163. ISBN 978-0-87842-498-6.
  5. ↑ Purucker, Michael E.; Whaler, Kathryn A. "6. Crustal magnetism". In Kono, M. Geomagnetism (PDF). Treatise on Geophysics 5. Elsevier. p. 195–236. ISBN 978-0-444-52748-6.
  6. ↑ Langel, Robert; Ousley, Gilbert; Berbert, John; Murphy, James; Settle, Mark (April 1982). "The MAGSAT mission". Geophysical Research Letters 9 (4): 243–245. doi:10.1029/GL009i004p00243.
  7. ↑ "The CHAMP mission". GFZ German Research Centre for Geosciences. Retrieved 20 March 2014.
  8. ↑ Reigber, Christoph, ed. (2005). Earth observation with CHAMP : results from three years in orbit (1st ed.). Berlin: Springer. ISBN 9783540228042.
  9. ↑ Staunting, Peter (1 January 2008). "The Ørsted Satellite Project" (PDF). Danish Meteorological Institute. Retrieved 20 March 2014.
  10. ↑ "Swarm (Geomagnetic LEO Constellation)". eoPortal Directory. European Space Agency. Retrieved 20 March 2014.
  11. ↑ Olsen, Nils; Stavros Kotsiaros (2011). "Magnetic Satellite Missions and Data". IAGA Special Sopron Book Series 5: 27–44. doi:10.1007/978-90-481-9858-0-2.
  12. ↑ Clark, D. A. (1997). "Magnetic petrophysics and magnetic petrology: aids to geological interpretation of magnetic surveys" (PDF). AGSO Journal of Australian Geology & Geophysics 17 (2): 83–103. Retrieved 20 March 2014.
  13. ↑ Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1996). The magnetic field of the earth : paleomagnetism, the core, and the deep mantle. San Diego: Acad. Press. pp. 172–185. ISBN 0124912451.
  14. ↑ Turcotte, Donald L. (2014). Geodynamics. Cambridge University Press. pp. 34–39. ISBN 9781107006539.
  15. ↑ Nelson, Thomas Allen (2000). Kubrick : inside a film artist's maze (New and expanded ed.). Bloomington: Indiana University Press. p. 107. ISBN 9780253213907.

Further reading

  • Constable, Catherine G.; Constable, Steven C. (2004). "Satellite Magnetic Field Measurements: Applications in Studying the Deep Earth". In Sparks, Robert Stephen John; Hawkesworth, Christopher John. The state of the planet frontiers and challenges in geophysics. Washington, DC: American Geophysical Union. pp. 147–159. ISBN 9781118666012. 
  • Hinze, William J.; Frese, Ralph R.B. von; Saad, Afif H. (2013). Gravity and magnetic exploration : principles, practices, and applications. Cambridge: Cambridge University Press. ISBN 9780521871013. 
  • Hinze, R. A. Langel, W. J. (2011). The magnetic field of the earth's lithosphere : the satellite perspective (1st pap. ed.). Cambridge, U.K.: Cambridge University Press. ISBN 978-0521189644. 
  • Kearey, Philip; Brooks, Michael; Hill, Ian. "7. Magnetic surveying". An Introduction to Geophysical Exploration. John Wiley & Sons. ISBN 9781118698938. 
  • Maus, S.; Barckhausen, U.; Berkenbosch, H.; Bournas, N.; Brozena, J.; Childers, V.; Dostaler, F.; Fairhead, J. D.; Finn, C.; et al. (August 2009). "EMAG2: A 2-arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements". Geochemistry, Geophysics, Geosystems 10 (8). doi:10.1029/2009GC002471. 

External links

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