Magnetotellurics
The magnetotelluric (MT) is a method of geophysics . By time-varying magnetic fields electrical [ telluric; tellus: concerning the earth ] Eddy currents induced in conductive structures in the earth's interior. The electric field generated by the currents in turn results in a secondary magnetic field. The stimulating primary magnetic fields can be of natural origin (current systems in the ionosphere and magnetosphere , radiation of thunderstorm lightning ) as well as artificially generated (long-wave transmitter). Due to a very wide range of periods of the alternating fields (from s to s), statements about the distribution of electrical conductivity within the various parts of the earth's crust up to the upper mantle area can be made with this method via the frequency-dependent penetration depth (skin effect) .
Due to the different periods or frequency ranges, a distinction is made between:
- Audio Magnetotellurics (AMT), audio frequency range
- Controlled Source AMT (CSAMT), artificial transmitter
- Magnetotellurics (MT)
- Radiomagnetotellurics (RMT), longest wave transmitter
For some years now, marine MT (MMT) measurements on the sea floor have also been used for hydrocarbon exploration.
The electrical field is linked to the primary magnetic field via the impedance . In practice, the horizontal components of the electric and magnetic fields are measured on the earth's surface. To measure the electric field, non-polarizable electrodes are placed in the ground at a distance of about 50–100 m and the voltage between the electrodes, which varies over time, is recorded. The measurement of the horizontal magnetic field is usually done with induction coils or fluxgate magnetometers . If the vertical magnetic field is also measured, the magnetic transfer functions can be determined. These provide information about changes in conductivity over a large area. This method is called geomagnetic depth sounding (ETS) or geomagnetic depth sounding (GDS).
AN Tichonow (1950) and Louis Cagniard (1953) showed, independently of one another, how the electrical conductivity of a layered subsurface can be derived from the recordings of the temporal variations of the electrical and magnetic fields . In the period that followed, the methods were further improved in order to be able to better reconstruct spatially complex conductivity structures.
