P. Schmoldt, PhD - MTNet - DIAS

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4. Distortion of magnetotelluric data Fig. 4.6.: Distortion of magnetotelluric (MT) responses due to a highly conductive feature at depth, causing resistivities of the TM mode (solid squares) at periods greater than 0.2 s to decrease rapidly, followed by an adjustment to the values of the TE mode (open squares) for longer periods. The drop is caused by a deviation of electric currents into direction of the conductor, decreasing the electric field of the respective mode. This particular form of distortion is occasionally referred to as birdy, owing to the distorted mode’s similarity in shape with an infantile drawing of a flying bird. diagonal elements are increased accordingly. The degree of distortion on each mode, caused by the current channelling effect, is therefore dependent on the orientation of the distorter relative to the regional geoelectric strike direction. When the orientation of the conducting distorter is close to the direction of one coordinate-system axis, i.e. either in strike direction or orthogonal to it, the apparent resistivity of the respective mode will exhibit a rapid drop in resistivity. For longer periods, the distorted mode will then, due to mode mixing, approach the apparent resistivity level of the orthogonal mode. This behaviour results in a very peculiar shape of the related mode, occasionally referred to as a birdy owing to its similarity in shape with an infantile drawing of a flying bird (cf. solid symbols in Figure 4.6). In order to provide a quantitative description of the current channelling phenomenon Edwards and Nabighian [1981] considered the ratio of current flowing through a conducting dyke Jd to the total available current Jt, using the equation Jd Jt = α , (4.5) 1 + α where α is the current channelling number. The authors examine the case of a dyke with vertical extent h, infinitive lateral extent, and conductivity σd in a resistive host medium with conductivity σh. For controlled source measurements α can be calculated in dependence of the angular frequency ω and the distance down strike of the source L when ωh
4.1. Types of distortion Fig. 4.7.: The effective area of influence of a dyke (a) for a controlled source survey, and (b) for a natural source survey, from [Jones, 1983a] (see text for description of variables) dyke of infinite lateral extension (Fig. 4.7), i.e. α2 = ωh δ 2 h · σh and for a 3D structure of horizontal extensions h and Lh α3 = ωhLh δ 3 h · σd σh σd (4.7) , (4.8) showing that the ratio of lateral extension to skin depth is of major influence for the proportion of electric current channelled into the dyke. 4.1.2. Inductive distortion According to Faraday’s Law (Eq. 3.2), a changing magnetic field induces an orthogonal circular electric current in a conducting body, which in turn generates a secondary magnetic field (cf. Eq. 4.2 and Fig. 4.8). The contribution of inductive distortion caused by small heterogeneities is very much dependent on the frequency range (cf. Eqs. 4.2, 4.3), 57
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Multidimensional isotropic and anis
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Contents 2.3. Deviation from plane
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Contents 8.3. Inversion of 3D model
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List of Figures 2.1. Amplitude of t
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List of Figures 4.17. Visual repres
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List of Figures 8.2. Ambient noise
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List of Figures 10.10.RMS misfit va
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List of Figures A.15.Result of anis
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List of Tables xviii 5.5. Parameter
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List of Acronyms FE finite element
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List of Symbols Below is a list of
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Symbol SI unit Denotation φ · pha
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Abstract The Tajo Basin and Betic C
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Publications Poster presentations x
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Acknowledgements Team, namely Colin
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Introduction 1 The Iberian Peninsul
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ections from enhanced one-dimension
Page 41: Part I Theoretical background of ma
Page 44 and 45: 2. Sources for magnetotelluric reco
Page 67 and 68: Mathematical description of electro
Page 69 and 70: yields 3.2. Deriving magnetotelluri
Page 71 and 72: 3.2. Deriving magnetotelluric param
Page 73 and 74: 3.3. Magnetotelluric induction area
Page 75 and 76: Depth d s d 1 d 2 d n-2 d n-1 t 1 t
Page 77 and 78: 3.4. Boundary conditions materials
Page 79 and 80: 3.5. The influence of electric perm
Page 85 and 86: Distortion of magnetotelluric data
Page 87 and 88: 4.1. Types of distortion Fig. 4.1.:
Page 89 and 90: 4.1. Types of distortion Fig. 4.3.:
Page 91: J s 0 s 0 4.1. Types of distortion
Page 95 and 96: Scale Type Terminology Example Atom
Page 97 and 98: 4.1. Types of distortion the use of
Page 99 and 100: 4.2. Dimensionality Fig. 4.12.: The
Page 101 and 102: 1D 2D local 3D/1D 3D/2D regional 4.
Page 103 and 104: 4.3. General mathematical represent
Page 105 and 106: 4.4. Removal of distortion effects
Page 107 and 108: Parameter Geoelectrical application
Page 111 and 112: 4.4.5. Caldwell-Bibby-Brown phase t
Page 115: Method Applicability Swift angle 2D
Page 118 and 119: 5. Earth’s properties observable
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6. Using magnetotellurics to gain i
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Part II Geology of the study area I
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7. Geology of the Iberian Peninsula
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Recovering a synthetic 3D subsurfac
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direction direction Depth: 12 - 30
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8.2. Generating synthetic 3D model
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Distance from the centre of the mes
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3D N45W 3D-crust TE Rho TE Phi Peri
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8.3. Inversion of 3D model data sch
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Model variation RMS misfit Optimal
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Profile: 3D-crust (TM-only) Depth (
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Parameter Value 8.3. Inversion of 3
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Depth (km) 10 -2 10 -1 10 0 10 1 10
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Depth (km) 10 -2 10 -1 10 0 10 1 10
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Step 1: Isotropic 2D inversion Step
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8.3. Inversion of 3D model data par
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8.4. Summary and conclusions bution
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Regularisation order Smoothing para
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S N 1% 0 Depth (km) 3% Depth (km) 1
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9.1. Profile location Data collecti
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Location (degrees) Recording period
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Geological region Stations Geologic
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9.4. Segregation of data acquired w
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Phase (degrees) 135 90 45 0 Z xy -4
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0 km 10 km 30 km 100 km 300 km Dept
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Pseudo-sections crustal strike dire
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9.8. Analysis of vertical magnetic
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9.8. Analysis of vertical magnetic
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10. Data inversion WinGLink softwar
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a (horizontal smoothing) 10. Data i
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10. Data inversion Short period ran
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10. Data inversion TM+TE Depth (km)
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10. Data inversion (a) Constrained
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10. Data inversion the model into u
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10. Data inversion Depth (km) S N 0
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Depth (km) 10. Data inversion S N 0
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10. Data inversion Group velocity m
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10. Data inversion ductivity of thi
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10. Data inversion Shtrikman upper
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10. Data inversion in the lithosphe
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10. Data inversion isotropic 2D lay
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10. Data inversion Depth (km) Depth
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10. Data inversion tigation is usua
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Depth (km) Depth (km) 10. Data inve
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Modelled Observed Modelled Observed
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Depth (km) 10. Data inversion 0 30
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10. Data inversion Depth off LAB (k
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10. Data inversion Depth (km) S 0 5
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10. Data inversion 10.3. Summary an
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10. Data inversion owing to availab
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11 Summary and conclusions The key
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11.2. PICASSO Phase I investigation
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A. Appendix Eocene 54 Ma 42 Ma 36 M
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A. Appendix A.2. Auxiliary informat
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A. Appendix 292 Fig. A.3.: Issues i
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A. Appendix A.2.4. Computation time
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296 3D-mantle profile Inversion res
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298 07-centre profile The profile 0
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300 3D-crust profile The profile 3D
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302 J-centre profile The J-centre p
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A. Appendix Anisotropy Resistivity
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A. Appendix A.4. Auxiliary figures
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A. Appendix 314 ρ TE(Ω−m) φ T
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A. Appendix 320 pic003 (off-diagona
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A. Appendix 322 pic013 (off-diagona
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Bibliography Abalos, B., J. Carrera
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Bibliography Artemieva, I. M. (2006
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Bibliography Berdichevsky, M., V. D
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Bibliography Cebriá, J.-M., and J.
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Bibliography de Vicente, G., J. Gin
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Bibliography Egbert, G. D., and J.
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Bibliography Ganapathy, R., and E.
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Bibliography Haak, V., and R. Hutto
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Bibliography Hutton, R. (1972), Som
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Bibliography Jones, A. G., and R. W
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Bibliography Kurtz, R. D., J. A. Cr
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Bibliography Lviv Centre of Institu
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Bibliography Merrill, R. T., and M.
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Bibliography Newman, G., and G. Hoh
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Bibliography Pádua, M. B., A. L. P
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Bibliography Prácser, E., and L. S
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Bibliography Ritter, J. R. R., M. J
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Bibliography Serson, P. H. (1973),
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Bibliography Spitzer, K. (2006), Ma
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Bibliography Tikhonov, A. N., and V
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Bibliography Wanamaker, B. J., and
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Bibliography Xu, Y., C. McCammon, a
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