Electromagnetic Exploration Method (Magnetotelluric Method)

Hello friends of Steemit!

In previous publications, I showed the surface seismic methods and the seismic well method based on the propagation of seismic waves for the detection of hydrocarbons and materials of interest to the industry.

This new publication is the first part of an electromagnetic exploration method known as magnetotelluric (MT).

The magnetotelluric method was used for the first time in 1950 by the geophysicist Louis Cagniard.

This method consits in the obtaining of the changes of resistivity of the subsoil from the variations of electric and magnetic fields registered in the surface. These fields coming from the ionosphere and caused by solar radiation are measured on the surface as shown in image 1 and 2.

Figure 1 - Configuration of the Magnetotelluric Method
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Figure 2 - Orientation of ground equipment
Surce

Sometimes micropulsations are used as a source of energy at frequencies near 1Hz.

Since the resistivity values of the different materials have been calculated in the laboratory, from the resistivity values obtained in the field, inferences about the lithological properties of the subsoil can be made.

Figure 4 - Resistivities of the materials
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The magnetic fields are measured through perpendicular induction coils, while the electric fields are obtained with unpolarized electrodes.

The depth of penetration of a wave in a medium is a function of the frequency given by the expression:

where:

ρ_a ➞ Apparent Resistivity (Ω.m)
δ ➞ depth of penetration (m)
E_y ➞ electric field component (V/m)
H_x ➞ magnetic field components (A/m)
μ ➞ magnetic permeability of the medium (H/m)
f ➞ frequency (Hz)
ω ➞ angular frequency (rad/s)

These expressions allow resistivity to be obtained as a function of the depth to be subsequently mapped through sections of resitivity.

Figure 5 - Visualization of the signal depending on the depth
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The low frequency waves are the most prevalent due to the attenuation they suffer in their journey from the ionosphere to the surface, allowing as shown by equation 1 to penetrate deeper than the higher frequency waves that only penetrate the shallower layers .

Below are sections of resistivity obtained in the field.

Figure 6 - Resistivity section
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Figure 7 - Resistivity section
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The magnetotelluric method is low cost and is applied in complex topographic zones and high impedance.

Thanks to advances in computational calculations and the creation of new software, the analysis of the results and their interpretation has been facilitated.

References

Vozoff, Keeva (1972). "The magnetotelluric method in the exploration of sedimentary basins". GEOPHYSICS 37 (1): 98–141
https://rmf.smf.mx/pdf/rmf/33/3/33_3_438.pdf
http://www.ruelsa.com/notas/tierras/pe70.html
http://www.ruelsa.com/notas/tierras/pe70.html
https://es.slideshare.net/PabloDiaz27/prospeccin-geoelctrica