Magnetotellurics

Summary

Magnetotellurics is a method to determine the electrical conductivity of the Earth from natural variations of the electric and magnetic fields at the surface. The variations come from currents in the ionosphere and magnetosphere and from lightning. They induce currents in the conducting Earth by Faraday's law of induction. The ratio of the horizontal electric field to the horizontal magnetic field is the impedance. It depends on the conductivity down to a skin depth that grows with the period, so long periods probe deep structure. It is used to image the crust and mantle, geothermal reservoirs and mineral deposits, from depths of tens of metres to hundreds of kilometres.