Magnetic anomaly

Magnetic anomalies ( rare magnetic anomalies) are regional and local disturbances in the strength of the earth's magnetic field. They are caused by magnetized rocks of the upper crust ( Petromagnetik ), ie in their proportion of ferro-and ferrimagnetic magnetic minerals (mainly magnetite). The measurement and interpretation of the anomalies done by the Geomagnetism, a branch of applied geophysics.

Small-scale anomalies may also originate from artificial magnetic disturbing bodies, eg Iron bodies or archaeological objects underground.

Prior to the analysis of magnetic interference fields must be subtracted from the measured field strengths, the so-called normal field. It is caused about 95% of the Earth's core (the rest of the ionosphere and magnetosphere ) and can be approximated regarded as eccentric dipole field with 12 ° inclination to the Earth's axis.

The local magnetic anomalies may account for a few percent of the normal field. They show an increased incidence of disturbing bodies, even magnetized minerals ( remanent magnetization ) or from rock units that have a high magnetic susceptibility ( induced magnetization ). This can be in up to 20 km depth. Rocks deeper than 20 km beyond the Curie temperature above which no static ferromagnetism ( magnetization ) is possible.

On the surface can produce disruptive body geomagnetic anomalies of about 200 to 1000 nano Tesla. The physico- mathematical interpretation of the interference potential is carried out through specific modeling of the bluff body, which is refined until its effect closely corresponding to the measured anomalies.

However, the sole interpretation of physical fields is never clearly (see reverse problem of potential theory ), so that the geomagnetic must also use the results related subjects (gravimetry, Geoseismik, geology). As a destination it has the determination of the location and size of the hidden bluff body, which should be possible to arrange possible with geological assumptions of the local crust.

The largest magnetic anomaly is the Kursk magnetic anomaly in the context of a huge iron deposit in southwestern Russia. Your maximum is 190,000 nT. Other anomalies are those of Kiruna ( Northern Sweden ) with 70,000 nT, in Central Europe (see Central European belt of magnetic anomalies ), eg those at Erzberg ( Austria ) and the Münchberger gneiss mass, the Alexander von Humboldt discovered around 1800. The latter was developed by surface magnetization of serpentinites by strong lightning strikes.