Magnetotellurics

The magnetotelluric (MT ) is a method of geophysics. Due to time-varying magnetic electric [ are telluric; tell us: the earth concerning ] induces eddy currents in conductive structures of the Earth. The electric field generated by the currents in turn result in a secondary magnetic field. The stimulating primary magnetic fields can be either of natural origin ( current systems in the ionosphere and magnetosphere, radiation from lightning storm ) and artificially generated ( long wave transmitter ). By a very wide period range of fields ( from s to s ) ( skin effect) may contain statements about the distribution of electrical conductivity within the various Erdkrustenanteile into the upper Erdmantelbereich be made with this method, the frequency dependent penetration depth.

Due to the different periods or frequency ranges it may be:

• Audiomagnetotellurics (AMT ), audio frequency range
• Controlled Source AMT ( CSAMT ), artificial channels
• Magnetotelluric (MT )
• Radiomagnetotellurics ( RMT), VLF transmitter

For several years, marine MT ( MMT) used measurements on the seabed for hydrocarbon exploration.

The electric field is connected via the impedance with the primary magnetic field. In practice, each of the horizontal components of the electric and magnetic field at the surface are measured. For the measurements of the electric field non-polarizable electrodes are placed at a distance of approximately 50-100 m in the ground and registering the time-varying voltage between the electrodes. The measurement of the horizontal magnetic field is usually performed with induction coils or fluxgate magnetometers. If in addition we measured the vertical magnetic field, the magnetic transfer functions can be determined. These provide information on areal conductivity changes. This method is called Geomagnetic depth sounding (ETS) or Geomagnetic Depth Sounding (GDS ).

As the electrical conductivity of the layered substrate can be derived from the registrations of the temporal variations of the electric and magnetic fields showed independently on Tikhonov (1950 ) and Louis Cagniard ( 1953). In the period following the methods were further improved in order to reconstruct spatially complex conductivity structures better.

• Geophysics