Making use of the global sources of natural EM field (magnetospheric and ionospheric currents, lightning activity, etc.) allows the MT method to study the Earth's structure up to the depths of tens and even hundreds of kilometers, which are by far unreachable for any of the modern controlled-source methods of EM geophysics. However, relying only on the natural sources of EM signal also makes the MT method very vulnerable to the industrial EM noise, thus making it inapplicable in cities and other densely populated areas.
Modifications of the MT method
Magnetotellurics utilizes natural sources of the electromagnetic field: for higher frequency band (from about 1 Hz to 10 000 Hz) the main source is remote lightning activity, while for the longer periods (from about 10 sec) the MT signal comes from geomagnetic pulsations and magnetic substorms, i.e. from the various magnetosphere-ionosphere current systems.
There are two “dead bands” in the natural source spectrum. The first one (“AMT dead band”) is from 1 to 5 kHz, and the second one (“MT dead band”) lies in the gap between the two main types of MT sources (1 – 10 s), where the intensity of natural EM field is low. As a result, the MT data quality within the above dead bands is more susceptible to noise and thus is often rather poor. To increase the overall quality of the acquired MT data it is highly recommended to make use of the overnight records, since the most intensive natural EM field variations usually occur during local morning and evening time.
Two main types of natural sources of alternating EM field inducing currents in the Earth.
The MT method utilizes a broad spectrum of naturally occurring geomagnetic variations as a power source for EM induction in the Earth. The integrated EM response of the earth material between the ground surface and any desired depth of investigation (DOI) could be obtained by choosing relevant periods of the MT signal. This principle is embodied in the skin depth relation, which describes the exponential decay of EM fields as they diffuse down into the conductive Earth:
h = √(Tρ/πμ),
where h is the skin depth, T is the period of the MT signal, ρ is the electrical resistivity and μ - the magnetic permeability of the medium.
Dependence of the skin depth on the period of MT signal and Earth resistivity
Magnetotellurics workflow
