What it is
Magnetotellurics, or MT, is a passive electromagnetic geophysical method. It measures the natural, ambient variations in the Earth's electric (E) and magnetic (B) fields — caused primarily by solar-wind interactions with the magnetosphere and, at higher frequencies, by atmospheric lightning. The ratio of the electric to magnetic field variations, as a function of frequency, maps to the electrical resistivity structure of the subsurface.
MT receivers are deployed at surface stations for hours to days, recording the natural field variations. The frequency range spans many orders of magnitude; lower frequencies probe deeper.
Strengths
- Very deep imaging. MT can image to 30,000+ ft vertical depth — deeper than most active-source methods.
- Effective through salt and basalt. Where seismic struggles due to complex overburden, MT's resistivity response often provides complementary information.
- Passive. No emitted signal; no environmental footprint beyond the ground-station cabling and batteries.
Limitations
- Poor shallow resolution. MT's strength is at depth; it's not the right tool for near-surface targets.
- Structural not substantive. MT images resistivity structure, not fluid or mineral substance directly.
- Interpretation-intensive. Requires careful regional calibration and 2D or 3D inversion to extract usable results.
How it complements other methods
MT is particularly useful in sub-salt and sub-basalt plays (pre-salt Brazil, Faroe-Shetland, parts of Gulf of Mexico). Operators increasingly combine MT (deep structure), seismic (shallow structure, high resolution), and remote substance classification (direct hydrocarbon indicators) into a layered pre-drill workflow.