Natural Field Electromagnetics

The most advanced natural-source electromagnetic method is Magnetotellurics (MT). Its energy sources are naturally occurring electromagnetic variations generated by global lightning activity (audio-magnetotelluric range, AMT) and by solar–magnetospheric interactions (MT range).

Because these sources originate at very large distances, the electromagnetic field arriving at the survey area can be approximated as a plane wave. This makes MT a far-field method, where the depth of investigation is primarily controlled by signal frequency — lower frequencies probing greater depths.

Magnetotellurics is a passive sounding technique capable of investigating structures to depths of several kilometers. It provides critical information about subsurface electrical conductivity contrasts, revealing geological layering, structural features, and potential target zones.

A key advantage of the MT method is its exceptional depth of penetration — no other electromagnetic technique can investigate comparable depths. However, because MT relies on naturally occurring signals, data must be recorded over extended periods (typically overnight) to ensure sufficient signal strength and stability.

One of the main limitations of the method is the increasing level of electromagnetic noise associated with modern infrastructure. Power lines, electrified railways, proximity of mining sites, and radio transmissions can significantly affect data quality. As a result, MT surveys are often constrained near artificial infrastructure and are generally impractical within densely populated urban environments.