Magnetic anomaly
Magnetic anomalies (more rarely magnetic anomalies ) are local and regional disturbances in the strength of the earth's magnetic field . They are caused by magnetized rocks in the upper crust of the earth (petromagnetics), i. H. due to their proportion of ferromagnetic and ferrimagnetic minerals (especially magnetite ). The measurement and interpretation of the anomalies is carried out by geomagnetics , a branch of applied geophysics .
Small-scale anomalies can also originate from artificial magnetic disruptive bodies, e.g. B. iron bodies or archaeological objects underground.
Before analyzing magnetic interference fields , the so-called normal field must be subtracted from the measured field strengths . About 95% of it is caused by the earth's core (the rest of the ionosphere and magnetosphere ) and can be approximated as an eccentric dipole field with a 12 ° inclination to the earth's axis. More details are given in the article on the earth's magnetic field .
The local magnetic anomalies can make up a few percent of the normal field. They show an increased occurrence of disruptive bodies , even magnetized minerals ( remanent magnetization ) or parts of rock that have a high magnetic susceptibility (induced magnetization). These can be up to 20 km deep. Rocks deeper than about 20 km exceed the Curie temperature , above which static ferromagnetism ( magnetization ) is no longer possible.
On the earth's surface, disruptive bodies can generate geomagnetic anomalies of around 200 to 1000 nanotesla . The physical-mathematical interpretation of the interference potential takes place through targeted modeling of the disruptive bodies, which is refined until its effect corresponds as closely as possible to the anomalies measured.
However, the sole interpretation of physical fields is never unambiguous (see inversion problem of potential theory ), so that geomagnetics must also make use of the results of related fields ( gravimetry , geoseismics , geology ). Its aim is to determine the position and size of the hidden disruptive bodies, which should be compatible with geological assumptions about the local earth's crust.
The largest magnetic anomaly is the Kursk Magnetic Anomaly in the vicinity of a huge iron deposit in southwestern Russia. Its maximum is 190,000 nT . Further anomalies are those of Kiruna (northern Sweden) with 70,000 nT, in Central Europe (see Central European Belt of Magnetic Anomalies ) z. B. those on the Styrian Erzberg (Austria) and the Münchberg gneiss , which Alexander von Humboldt discovered around 1800. The latter was caused by the superficial magnetization of serpentinites by strong lightning strikes .
See also
- Magnetostatics , multipole field
- Bai disorder (short-term disruption of the magnetic field)
- Magnetic storm (by solar wind)
- Gravity anomaly , geological anomaly