Geomagnetics

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Under geomagnetic those methods are the geophysics (in particular the applied geophysics ) understood that with the natural geomagnetic field (commercially produced) or the effect of artificial magnetic fields in the Earth's crust deal.

Similar to gravimetry , geomagnetics is one of the potential methods and, like these, can be used universally and inexpensively. For many applications it can even be done from an aircraft ( aeromagnetics ). The interpretation (interpretation of the measurements) can be ambiguous, as with other physical fields .

In the scientific umbrella organization of the IUGG (Union for Geodesy and Geophysics), magnetics is represented by the IAGA (International Association of Geomagnetism and Aeronomy). With the branch of magnetotellurics , it is also at the transition to geoelectrics .

Earth's magnetic field: induction and normal field

The terrestrial magnetic field induces magnetic properties (induced magnetization ) in all substances . However, the influence can differ by many orders of magnitude because the magnetic susceptibility strongly depends on the substance (various rocks , ores , metals or ceramics in a landfill , etc.). Induction in the earth's magnetic field turns these bodies into magnets themselves . Their magnetic field is superimposed on that of the earth as an interference field that creates " anomalies " in the normal field.

Before interpreting such anomalies, the so-called normal field must be subtracted from the measured field strengths . It can be viewed approximately as a dipole field , whereby the fictitious dipole is not exactly in the center of the earth , but about 400 km eccentric (in the direction of New Guinea) and its axis is inclined by almost 12 ° to the earth's axis. About 10% of the normal field is due to atmospheric induction, which can vary with solar activity ( solar wind ).

Areas of application of geomagnetics

Calibration of a gauss meter with the help of a zero Gauss chamber made of Magnoshield- Mu-metal

Suitable measuring devices can find geophysical anomalies, some of which make up less than 1 per thousand of the main earth magnetic field. To evaluate the measurements, modeling methods are used that allow statements about the position, depth, shape and composition of the disruptive bodies .
However, these conclusions about the magnetization or mass distribution are often ambiguous - see the inverse problem of potential theory .

Geomagnetics is used, among other things:

Measuring instruments

Measurements are made with magnetometers . Earlier mechanical magnetometers ( magnetic field balances , torsion magnetometers ) have been replaced by systems that work electronically or atomically (saturation nucleus, fluxgate, Förster probes, proton, cesium).

The measuring systems of the gradiometer measure with two probes at intervals of less than 1 m, e.g. B. to detect fine vertical changes in the earth's magnetic field . Magnetic signatures as well as fluctuations over time can be interesting.

With the help of a so-called zero Gaussian chamber , an almost field-free space can be created. It serves z. B. for the calibration of measuring devices and sensors in conditions that are almost free from earth magnetic fields. A fully annealed Mu-metal shield is used as the material. So u. a. enables reproducible measurement results at any location. Conversely, field-free spaces or certain variants of the magnetic field can also be actively generated and adjusted by the action of electromagnetic coils.

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