The magnetotelluric (MT) method

The magnetotelluric (MT) method and its high-frequency version, audio-magnetotelluric (AMT), is a natural source electromagnetic (EM) technique that involves measuring fluctuations in the natural electric and magnetic fields in orthogonal directions at the surface of the Earth as a means of determining the conductivity structure of the Earth at depths ranging from a few tens of metres to several hundreds of kilometres.

The method uses the natural time variation of the Earth’s magnetic field as a source of signals. Electromagnetic fields that are naturally induced in the Earth and are exploitable for MT studies have wave periods ranging from  10^-3  to 10⁵ seconds. Short-period signals are generated by interactions in the Earth–ionosphere waveguide, whereas longperiod signals are generated by solar wind–magnetosphere interactions.

The MT method does not pose any damage to the environment since it does not generate any type of artificial signal and the stations can be installed without clearing of vegetation.