If you are looking for natural magnetotelluric (MT) frequencies that are nearly always observable then you can count on the Schumann resonances. You can assume very low signal strength in the low micro Volt range. The foundational Schumann Resonance is the strongest at 7.83 Hz (around 8Hz) and it generally has a strength measured around 1 microV/root Hz in the San Francisco Bay Area.
The Schumann and other lightening generated frequencies are propagated into the atmosphere, the atmosphere acting as a wave guide due to the electromagnetic signal reflecting off the ionosphere. As electromagnetic waves interact with the Earth’s surface they act as displacement currents going vertically into the Earth. These displacement currents then create secondary currents that flow horizontally in the Earth. MT signals are assumed to be plane waves since the source is far enough away to be several skin depths distant. The assumption of plane wave and multiple polarizations of the signal allows magnetotelluric calculations to be made without consideration of the source parameters.
The general MT signals will come and go depending on atmospheric conditions, time of day, time of year, location, and general distant lightning activity. Signals below about 0.1 Hz are typically from the ionosphere, generated by variations in the solar winds and how they press on the ionosphere, and not by lightning strikes. Traditional deep MT measurements will use natural magnetotelluric signals from 0.001 Hz (1,000 second period) and even lower frequencies with instruments that are capable. The dominant Schumann Resonant frequencies are 7.83Hz, 14.3, 20.8, 27.3, and 30.8Hz. There exist a magnetotelluric “dead-zone” in the 800 Hz to 4 kHz range, and this dead-zone is the result of certain frequencies not being contained in the atmospheric wave guide, instead simply dissipating into space.
If you are working with AMT measurements generally the limit is somewhere between 0.1 Hz to around 2 Hz but that is because of instrumentation not the existence of the fields. The Stratagem EH4 went to 10 Hz and the Geode EM3D goes to 0.1 Hz.
All the power line harmonics of 60 Hz in North America and 50 Hz in other parts of the world will give strong signal but are considered noise as far as MT measurements go and need to be avoided and filtered out. Another noise problem are the world-wide very low frequency (VLF) signals that also need to be filter and avoided. VLF signals are military signals from stations around the world and they swamp out the much lower natural magnetotelluric fields. There is a geophysical method that actually uses the man-made VLF signal to detect linear conductive geologic structures but VLF are a problem for MT measurements.
For more information on the Schumann Resonances, which are the predominant natural magnetotelluric currents that exist, watch the video by Geophysicist Stefan Burns below: