P. Schmoldt, PhD - MTNet - DIAS

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8. Recovering a synthetic 3D subsurface model using lower-dimensional inversion schemes Depth (km) 0 50 100 150 200 250 300 With crustal and mantle tear zone -80 -40 0 40 80 Distance (km) Lower global smoothing (t = 1) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Higher horizontal smoothing (a = 3) El. Resistivity (Log 10(Wm)) Depth (km) Depth (km) Depth (km) 0 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100 150 200 250 300 -80 -40 0 40 80 Distance (km) 200 Wm 50 Wm 500 Wm 1000 Wm -80 -40 0 40 80 Distance (km) -80 -40 0 40 80 Distance (km) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 El. Resistivity (Log 10(Wm)) El. Resistivity (Log 10(Wm)) El. Resistivity (Log 10(Wm)) Depth (km) 0 50 100 150 200 250 300 -80 -40 0 40 80 Distance (km) Without static shift correction Using the “true crust” -80 -40 0 40 80 Distance (km) Fig. 8.8.: Selection of isotropic 2D inversion models for a synthetic 3D model with orthogonal strike direction at crust and mantle depth using the profile 3D-mantle and the stations syn001 – syn020 (cf. Figs. 8.3 and 8.4). The true resistivity distribution beneath the profile is indicated on the central model together with the northern and southern ends of the profile, denoted by the inverted triangles. See text for details about the inversion settings. (“TM-only inversion”) may therefore yield superior results. This hypotheses is tested here with the synthetic 3D model using datasets decomposed according to the crustal strike direction (N45W, profile: 3D-crust) as well as the mantle strike direction (N45E, profile: 3D-mantle) for the whole period range (10 −3 – 10 5 s) with the same smoothing parameters determined for the isotropic 2D inversion with both modes (Tab. 8.3). Results of the TM-only inversion indicate that this approach is not appropriate for the subsurface model used in this study since respective models (Figs. 8.9 and 8.10) differ significantly from the electric resistivity distribution of the true model (Fig. 8.3). In conclusion, it has been found that approximating the crust by a 30 km layer and applying static shift correction during isotropic 2D inversion yields models that are closest to the synthetic 3D model (cf. central graphic in Figure 8.8). The preferential combination of parameters used for an isotropic 2D inversion of the 3D subsurface model with oblique geoelectric strike directions at crust and mantle depths is given in Table 8.3. However, even the model with the relatively best agreement with the true subsurface distribution suffers from a lateral shift of the resistivity interface towards the North (i.e. to the right 182 Depth (km) 0 50 100 150 200 250 300 4.0 3.5 3.0 2.5 2.0 1.5 1.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 El. Resistivity (Log 10(Wm)) El. Resistivity (Log 10(Wm))
Profile: 3D-crust (TM-only) Depth (km) TM Log 10(periods) RMS -2 -1 0 1 2 3 4 0 50 100 150 200 250 300 pic020 pic019 Apparent resistivity 8.3. Inversion of 3D model data S N 0 17 34 51 68 85 102 119 136 153 Distance (km) log10 (Wm) 1.6 2.4 3.2 pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic006 pic005 pic004 pic003 pic002 pic001 pic017 pic015 pic013 pic011 pic009 pic007 Misfit (Total = 0.26) pic006 pic005 pic004 Phase pic003 pic002 pic001 Log 10(Wm) degrees pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic006 pic005 pic004 pic003 pic002 pic001 Fig. 8.9.: Result of isotropic 2D inversion for the 3D-crust profile (cf. Fig. 8.5) using only data from the TM mode (“TM-only”) of stations pic001 – pic020, decomposed according to the geoelectric strike direction of the crust (N45W). Misfit values for this model are of lesser significance since error and scatter levels of the data are not realistic. in Figure 8.8) and the introduction of a resistive body in the South of the model. Introduction of such artefacts needs to be kept in mind when interpreting results of isotropic 2D inversions for similar subsurface cases. Using only data from the TM mode for the inversion did not result in a better agreement of inversion models with the synthetic 3D model, hence the TM-only inversion approach is found not appropriate for this special case of electric resistivity distribution with oblique strike directions. Furthermore, the decision about the best subsurface model, hence the optimal combination of inversion parameters, is achieved through comparison with the true model. Selecting the best model for a case in which the subsurface is not a priori known will be more challenging and the doubt of ambiguity regarding the chosen model will remain, due to the non-uniqueness of MT inversion. 183
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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
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Part I Theoretical background of ma
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2. Sources for magnetotelluric reco
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Mathematical description of electro
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yields 3.2. Deriving magnetotelluri
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3.2. Deriving magnetotelluric param
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3.3. Magnetotelluric induction area
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Depth d s d 1 d 2 d n-2 d n-1 t 1 t
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3.4. Boundary conditions materials
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3.5. The influence of electric perm
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Distortion of magnetotelluric data
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4.1. Types of distortion Fig. 4.1.:
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J s 0 s 0 4.1. Types of distortion
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Scale Type Terminology Example Atom
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4.1. Types of distortion the use of
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4.2. Dimensionality Fig. 4.12.: The
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1D 2D local 3D/1D 3D/2D regional 4.
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4.3. General mathematical represent
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4.4. Removal of distortion effects
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Parameter Geoelectrical application
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4.4.5. Caldwell-Bibby-Brown phase t
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Method Applicability Swift angle 2D
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5. Earth’s properties observable
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6. Using magnetotellurics to gain i
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Page 168 and 169: Part II Geology of the study area I
Page 170 and 171: 7. Geology of the Iberian Peninsula
Page 205 and 206: Recovering a synthetic 3D subsurfac
Page 207 and 208: direction direction Depth: 12 - 30
Page 209 and 210: 8.2. Generating synthetic 3D model
Page 211 and 212: Distance from the centre of the mes
Page 213 and 214: 3D N45W 3D-crust TE Rho TE Phi Peri
Page 215 and 216: 8.3. Inversion of 3D model data sch
Page 217: Model variation RMS misfit Optimal
Page 221 and 222: Parameter Value 8.3. Inversion of 3
Page 223 and 224: Depth (km) 10 -2 10 -1 10 0 10 1 10
Page 225 and 226: Depth (km) 10 -2 10 -1 10 0 10 1 10
Page 227 and 228: Step 1: Isotropic 2D inversion Step
Page 229 and 230: 8.3. Inversion of 3D model data par
Page 231 and 232: 8.4. Summary and conclusions bution
Page 233 and 234: Regularisation order Smoothing para
Page 235 and 236: S N 1% 0 Depth (km) 3% Depth (km) 1
Page 237 and 238: 9.1. Profile location Data collecti
Page 239 and 240: Location (degrees) Recording period
Page 241 and 242: Geological region Stations Geologic
Page 243 and 244: 9.4. Segregation of data acquired w
Page 245 and 246: Phase (degrees) 135 90 45 0 Z xy -4
Page 247 and 248: 0 km 10 km 30 km 100 km 300 km Dept
Page 255 and 256: Pseudo-sections crustal strike dire
Page 257 and 258: 9.8. Analysis of vertical magnetic
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Page 262 and 263: 10. Data inversion WinGLink softwar
Page 264 and 265: a (horizontal smoothing) 10. Data i
Page 266 and 267: 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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P. Schmoldt, PhD - MTNet - DIAS

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