Natural-source Audio-frequency Magnetotellurics (AMT) is an electromagnetic survey technique that uses naturally-occurring ionospheric currents (passive energy sources) to electrically map geologic structure to depths of 1000-2000 meters or more.
Natural-source electromagnetic (EM) signals are generated in the atmosphere and magnetosphere.
The time-varying electric and magnetic fields induce currents into the earth and oceans, which produce magnetotelluric (MT) signals, which are measured by AMT and MT data acquisition systems.
Low-frequency magnetotelluric EM signals (< 1 Hz) are generated by the interaction between the earth’s magnetosphere and solar wind.
High-frequency sources in the audio range (> 1 Hz) are generated by thunderstorms worldwide.
The AMT and MT geophysical methods combine measurements made of site-specific electric and magnetic fields using grounded dipoles and magnetic field antennas over a wide band of frequencies.
Low frequencies sample deep into the earth and high frequencies (AMT) correspond to shallow samples.
Ground resistivity values are calculated from the magnitude and ratios of these components and then mapped using inversion and modeling software. Ground resistivity is relates to the geology.