Natural source
MT/AMT — Magnetotellurics / Audio-Magnetotellurics
Natural-field soundings from the near surface to the mantle
Frequency band
10 kHz - 0.001 Hz
Depth range
50 m - 100 km
Resolution
Decreases with depth
Source
Lightning and solar-wind fields
Typical cost
Moderate
Survey speed
Moderate - hours to days per station
Best terrain
Any quiet site away from cultural noise
Domain
Natural source
Overview
MT records orthogonal natural electric and magnetic fields and forms the impedance tensor. Because no transmitter is needed, the only limit on depth is how long you record. AMT is the same technique in the audio band for shallower targets.
How the physics works
The Cagniard apparent resistivity is |Z|^2 / (omega x mu). Longer periods penetrate deeper, so a sounding curve read left to right is effectively a resistivity-versus-depth profile through a layered earth.
Where it is used
- Deep crustal and lithospheric imaging
- Geothermal reservoir and clay-cap delineation
- Mineral systems beneath thick cover
- Hydrocarbon exploration in thrust belts