MAGNETO-TELLURIC METHOD
What is the Magnetotelluric (MT) method?
Magnetotelluric (MT) method involves measuring the response at the surface of the Earth to time-varying electromagnetic waves in the subsurface generated by natural and man made (artificial) sources.
We can consider the Magnetotelluric (MT) method as the electromagnetic method, since it comprises two components such as magnetic and electric fields, with the electric field in Magnetotelluric (MT) method is associated with telluric currents.
However electromagnetic and magnetotelluric methods have proved slightly some similarity and clear differences. If you want to know these comparisons in clear and simplified way check and visit to Comparative Geophysics eBook.
Magnetotelluric (MT) signals
Based on the type of sources of Magnetotelluric (MT) fields, Magnetotelluric (MT) signals can be categorized into two (2) forms.
1. Natural Magnetotelluric (MT) signals
These originated naturally from external fields interactions origin without any man influence. Then they are Passive MT signals. The word Magnetotelluric (MT) always is used to refer that the method or the signal is natural such as passive Geophysical method
2. Man-made Magnetotelluric (MT) signals
As the term described, these are signals that are generated by using artificial (man made) sources. They are Active signals, for example the Controlled Source Audio-Magnetotelluric (CSAMT) and Audio - Magnetotelluric (AMT) method.
The Magnetotelluric (MT) methods, especially using natural sources, are very good for probing great depths inside the Earth.
Deep crustal (> 2km) which include the upper Mantle (25 km - 100 km) depths. CSAMT or AMT method concentrates on frequencies that are much higher than MT method but enable resolution of smaller structures (resistivity and magnetic susceptibility anomalies) at relatively shallow depths.
Measurements of amplitude variations in the telluric field (Ex) and the associated magnetic field (Hy) determine earth resistivity.
Because of relationship between magnetic and Electric signals in this technique, the difference in Amplitude and phase can be used to compute the impedance (Z) which is then used to estimate the apparent resistivity of layers as shown below,
Apparent resistivity = [1/2πfµ] × |Z|2
Magnetotelluric measurements at several frequencies provide information on the variation of resistivity with depth because the depth of penetration of EM waves is a function of frequency.
Basic Operation Principle of Magnetotelluric (MT) Method
Magnetotelluric (MT) method relied on the basic electromagnetic induction principle that when natural source of external origin, known as the primary electromagnetic field, reaches the earth’s surface, part of it is reflected back and remaining part penetrates into the earth subsurface. Since the Earth acts as a good conductor, the electric currents known as telluric currents are induced within the subsurface by electromagnetic induction. The telluric currents in turn produce a secondary magnetic field which is then can be measured by receivers at which one of them is known as magnetometer.
More details on the basis for the MT method and the development of the theory that determines the depth of penetration is found by Tikhonov (1950) and Cagniard (1953).
The magnetotelluric receivers measure the total electromagnetic field, such as time variation of both magnetic field B (t) and induced electric field E (t). This means that the electrical properties (electrical conductivity) of the underlying material can be determined from the relationship between the components of the measured electric (E) and magnetic field (B) variations, or transfer functions.
The components of the measured electric (E) and magnetic field (B) variations are the horizontal electric field (Ex and Ey) and magnetic field (Bx and By) and vertical magnetic field (Bz).
Magnetotelluric techniques measure amplitudes of both Electric (E) and magnetic (B) field components as one must understand the basic concepts governing the interaction of these fields with Earth (Bastani, 2001). The MT method was introduced by Tikhonov (1950) and is based on the relationship between the measured magnetic and electric fields that is governed by Maxwell's equations. In the frequency domain and with the assumption of an eiωt time dependency the Maxwell’s equation are given as
∇ × E = − iωB Faraday’s law
∇ × H = J + iωD Ampere’s law
∇ . D = ρ Gauss’s law
∇ . B = ρ
Where, E is the electric field intensity (V/m),
H is the magnetic field intensity (A/m),
D is the dielectric displacement (C/m2),
B is the magnetic induction (Wb/m2 or T),
J is the electric current density (A/m2),
ω is the angular frequency (Hz),
ρ is the electric charge density (C/m3).
But Maxwell’s equations are related to one another by the constitutive equations. In the frequency domain they are:
D = εE
B = µH
J = σE
Where, ε is the electric permittivity (F/m),
σ is the conductivity (S/m),
µ is the magnetic permeability (H/m),
Applications of Magnetotelluric (MT) method
Earth's resources (mining such as Base metals and sulphides, oil and gas and geothermal) exploration.
Deep Crustal and upper mantle studies example Controlled source Audio Magnetotelluric (CSAMT). However for probing the depth more than 2km the Magnetotelluric (MT) method of natural source signals are frequently applied.
Geology structural and Lithological mapping. CSAMT
Geotechnical structural analysis of mine planning. CSAMT
That is the brief basics regarding Magnetotelluric (MT) method.
Check for details regarding Magnetotelluric (MT) method in comparison styles with other Geophysical methods in Comparative Geophysics eBook.
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References
Bastani, M (2001), A new controlled source/audio magneto-telluric system, EnviroMT
Cagniard L (1953), Basic theory of the magneto-telluric method of geophysical prospecting. Geophysics 18:605–635
Tikhonov AN (1950), The determination of the electrical properties of deep layers of the earth’s crust, SSR 73:295–297 (in Russian).
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