The document discusses the magnetotelluric (MT) method, which uses natural electromagnetic fields generated by solar winds and lightning to infer the conductivity and resistivity distribution of the subsurface. The MT method involves passive surface measurements of the earth's natural EM fields across a wide frequency range to investigate structures at intermediate to deep depths. Key aspects covered include skin effect, which causes exponential attenuation of EM waves with depth; MT data processing in the frequency domain; and 1D and 2D inversion modeling to estimate subsurface resistivity structures from measured impedance data.

1. MAGNETOTELLURIC (MT) METHOD

2. Dependence of the electric and magnetic phenomena on conductivity of the medium can be exploited to the study of the solid earth MT method employs naturally existing electromagnetic (EM) fields to infer the conductivity (or resistivity) distribution of the subsurface Natural EM fields are generated by interaction of Earth’s permanent magnetic field with particles from the solar wind and with atmospheric lightning MT method was developed independently in the 50s by Cagniard (France), Tikhonov (Russia), Wait (USA) Kato and Kikuchi (Japan) Magnetotellurics (MT ), background

3. Characteristics of MT method passive surface measurement of the earth’s natural EM fields,  no need for transmitter, simplifies the logistics frequency domain and wide frequency bands  intermediate to deep investigation depth wide range of applications  regional scale geological studies/tectonics  mineral, geothermal and oil exploration Magnetotellurics (MT ), background

4. electromagnetic induction

5. natural electromagnetic field

6.  B ( t ) _______  t E ( t ) electric - magnetic phenomena represented by the Maxwell’s equations

7. H ( t ) electric - magnetic phenomena represented by the Maxwell’s equations  D ( t ) _______  t J ( t ) +

8. Maxwell’s equations (with constitutive equations) EM wave or diffusion equation solution of wave or diffusion equation EM fields in the medium (MT response) MT response calculation, concept

9. EM induction ~ wave diffusion spectral content, not propagation parameters incident waves transmitted waves surface reflected waves

10. Simple resistivity distribution  homogeneous  1 – D    surface  1 h 1 z 1 = 0 . . . z 2 z 3 z N - 1 z N  2 h 2  N - 1 h N - 1  N  (Ohm.m) z (m)

11. __________ E x = A exp (- k z ) k = √ i   0 /  H y = ( A exp (- k z )) z = depth k ________ i    1 – D homogeneous solution to Maxwell’s equations in simple medium E x = A exp (- k z ) + B exp ( + k z ) H y = ( A exp (- k z ) - B exp ( + k z )) k ________ i   

12. Skin effect = exponential EM wave attenuation Skin depth (  ) is defined as a depth in a homogeneous medium where the EM wave amplitude has become 1/ e of its amplitude at the surface Skin depth is associated to penetration depth Lower frequency and higher resistivity = slower attenuation of EM wave  deeper penetration Skin Effect and Penetration Depth E x = A exp (- k z )  E x ( z = 0) = A  E x ( z =  ) = A / e _________  = √ 2  / (   0 )  = 2  f f = frequency

13. Lower frequency and higher resistivity = slower attenuation of EM wave  deeper penetration Principles of MT sounding  wide frequency band measurement probes different parts (depths) of the subsurface Skin Effect and Penetration Depth

14. Skin depth (meter) :  = 503 (  T ) 1/2 Effective depth (meter) :  eff =  /  2 = 355 (  T ) 1/2  = resistivity (Ohm.m) T = period (second) T = 1/ f ; f = frequency (Hertz) Skin depth and effective depth are used only for a rough estimation of the investigation depth of a frequency or period for a homogeneous equivalent medium Investigation depth from skin depth and effective depth is usually too “optimistic” Skin Effect and Penetration Depth

15. Skin Effect and Penetration Depth

16. MT field set-up

17. MT field set-up

18. MT Time series data

19. Data processing sequence

20. E x = Z xx H x + Z xy H y E y = Z yx H x + Z yy H y  E = Z H  sounding curves Data Processing (Frequency Domain)  a = 0.2 T | Z | 2  = atan Im Z _______ Re Z

21. Modeling and Interpretation

22. Recursive formula connecting the impedance at the surface of two successive layers  Z I, j = ( i   0  j ) 1/2 = intrinsic impedance  impedance at j –th layer as function of the layer’s parameters (  j  , h j ) and impedance at the subsequent layer Z j+ 1  Impedance at the surface of a 1-D Earth model is calculated from the surface at the last layer and recursively upward 1-D MT modeling 1 – R j exp ( – 2 k j h j ) _____________________ 1 + R j exp ( – 2 k j h j ) Z j = Z I,j R j = Z I,j – Z j + 1 __________ Z I,j + Z j + 1

23. Impedance at the surface of the Earth . . . Impedance at the ( N -1) th layer impedance at the surface of the last ( N th ) layer, intrinsic impedance 1-D MT modeling, algorithm  surface  1 h 1 z 1 = 0  z 2  2 h 2 . . .  z N - 1  N - 1 h N - 1 z N  N

25. 2-D resistivity section from 1-D models on a profile  correlation of resistivity units from station to station  correlation of resistivity units with geology and lithology

26. pseudosection (data) 2-D smooth model hot-spring hot-spring