- An electromagnetic method used to map the spatial variation of the Earth's resistivity by measuring naturally occurring electric and magnetic fields at the Earth's surface. These natural EM fields are generated (at all frequencies) in the Earth's atmosphere mainly by lightning strokes and by interactions between the solar wind and the ionosphere. In the most general MT method, the horizontal components of the electric field and all three components of the magnetic field are measured at the surface. The measurements are used to determine specific ratios of electric to magnetic field components called tensor impedances. The technique was introduced the French geophysicist Louis Cagniard in the 1950s and has been popular for mineralexploration and regional geophysical mapping. It is used in oil exploration for low-cost reconnaissance of sedimentary basins and for exploration in areas where seismic surveys are difficult because of severe topography or the presence high-impedancevolcanic rocks near the surface. The resolution of MT surveys is limited by the diffusive nature of EM propagation in the earth; it is usually on the order of hundreds of meters to kilometers. But the MT method can probe the Earth to depths of several tens of kilometers.
- Occam's inversion