P. Schmoldt, PhD - MTNet - DIAS

List of Figures
viii
2.17. Deviation of phase φ and apparent resistivity ρa curves from true values
for magnetotelluric and magnetovariational data dependent on the distance
between recording point and line electrojet . . . . . . . . . . . . . 28
2.18. Apparent resistivity ρa and phase φ variation curves obtained for electrojets
of finite length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1. Induction depth δs for an MT station over a conductive half-space. . . . . 38
3.2. Change of wave magnitude with depth for the case of a 1D subsurface
with n-layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3. Apparent resistivity and phase behaviour in the complex plane for a horizontal
interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.4. Behaviour of TE and TM mode for the same period in the presence of a
conductivity contact zone . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.5. Electric resistivities of rocks and other common Earth materials . . . . . 45
3.6. Electric conductivity profile of the deep Earth . . . . . . . . . . . . . . . 46
3.7. Difference between the impedance calculated with and without neglecting
the permittivity term for different frequencies . . . . . . . . . . . . . . . 47
4.1. Model of the potential galvanic and inductive effects in a magnetotelluric
survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2. Orientation-dependent effects of a small-scale 3D body onto the modes of
the magnetotelluric measurement . . . . . . . . . . . . . . . . . . . . . . 52
4.3. The galvanic distortion effect . . . . . . . . . . . . . . . . . . . . . . . . 53
4.4. Magnetotelluric responses under the influence of static shift (electric galvanic
distortion) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5. The principle of current channelling . . . . . . . . . . . . . . . . . . . . 55
4.6. Distortion of magnetotelluric responses due to a highly conductive feature
at depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.7. The effective area of influence of a dyke for a controlled source survey,
and for a natural source survey . . . . . . . . . . . . . . . . . . . . . . . 57
4.8. The inductive effect in a vertical sheet conductor . . . . . . . . . . . . . 58
4.9. Types of electric anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.10. Basic anisotropy parameters . . . . . . . . . . . . . . . . . . . . . . . . 60
4.11. Models of (isotropic) subsurface dimensionality . . . . . . . . . . . . . . 62
4.12. Induction area for different period ranges at the same station . . . . . . . 63
4.13. Two subsurface models used to illustrate the effect of frequency-dependent
dimensionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.14. Responses for a magnetotelluric station on top of a 3D/2D model . . . . . 65
4.15. Responses for a magnetotelluric station over a 2D/2D regional model . . . 66
4.16. Visualisation of Weaver-Agarwal-Lilley (WAL) tensor invariants using
Mohr circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73