Magnetotellurics

The electromagnetic method ‚magnetotellurics’ (MT) refers geoelectric (telluric) fields on geomagnetic fields and thus figures out the electrical conductivity of the earth’s underground down to great depths. Distinguished by their triggering sources, there are:

• Classic magnetotellurics (MT), using as a source the natural current systems in ionosphere and magnetosphere in a period range of 1 s to 10⁵ s,
  
  
• Audiomagnetotellurics (AMT), based mainly on radiation and reflection of atmospheric thunderstorm events in a period range of 10^-4 s to 1 s,
  
  
• Radiomagnetotellurics (RMT) using the electromagnetic waves emitted by radio transmitters in a period range of about 10^-4 s to 10^-6 s.
  
  
• and Controlled Source Audiomagnetotellurics (CSAMT), using some transmitters in the AMT range.

The exploration depth, ranging from some meters in RMT to several kilometres in MT, depends on the period range used. In all MT methods, the electrical fields on the earth’s surface are registered by pairs of probes along two paths orthogonal to one another. The probes can consist of metal spits or special telluric probes (hard to polarize) that are driven into the ground or buried and register two horizontal field directions (as a rule NS and EW) orthogonal to one another. An additional vertical inductor allows to refer vertical magnetic fields on horizontal ones. This is called ‘geomagnetic depth sounding’ (EDS).

For the determination of the electrical conductivity in the earth’s underground the relations of field components of electric and magnetic fields (impedances) orthogonal to one another are evaluated for discreet measuring frequencies. With the penetration depth depending on the frequencies, soundings are possible. In combination with measurements along profiles or at the surface, the spatial distribution of the electrical conductivity in the earth’s underground can be deduced.

Literature:

Vozoff, K., 1987. The magnetotelluric method. In: Nabighian, M.N. (ed.), Electromagnetic methods in applied geophysics, Vol. 2, Application. Society of Exploration Geophysicists, Tulsa, OK, 641-711.

Technical contributions:

Source Rock Detection by Magnetotellurics

Exploration of Geothermal High Enthalpy Resources using Magnetotellurics