P. Schmoldt, PhD - MTNet - DIAS

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8. Recovering a synthetic 3D subsurface model using lower-dimensional inversion schemes small scale 3D bodies due to charge build up on the off-profile boundaries of these small scale bodies [e.g. Jones, 1983a; Wannamaker et al., 1984; Berdichevsky et al., 1998; Ledo et al., 2002; Ledo, 2005; Siripunvaraporn et al., 2005b]. Hence, 3D inversion appears to be required for the Pannonian Basin region. The circumstance that MT data are usually acquired along a profile (as opposed to an array of recording stations) does not prohibit 3D inversion. Siripunvaraporn et al. [2005b] illustrate that 3D inversion of profile data can account for off-profile features and as a result yields an improved model of structures beneath the profile. However, longer computational time and higher computational costs are associated with 3D inversion and it would be desirable to find a 2D inversion approach that can cope with certain types of 3D subsurface without these drawbacks of 3D inversion. One particular 3D subsurface case consists of lateral changes in electric conductivity along regional-scale interfaces with varying orientations of the interfaces at depth, e.g. at crustal and mantle depths. Such a case might emerge, for instance, where crustal faulting, originating from present day tectonics, is situated above a mantle where structures are dominated by earlier or current plate tectonic processes, e.g. continental collision from an oblique direction. Cases of oblique geoelectric strike directions for different subsurface regions are a known problem in MT investigation and have previously been reported, among others, by Eaton et al. [2004], and Miensopust et al. [2011]. Whereas recovery of crustal structures can usually be achieved in a straightforward manner by confining the modelled frequency range to crustal penetration depths, deriving mantle structures is more challenging. Presently, no silver bullet solution is known to the problem of recovering mantle structures for cases of significantly oblique strike directions. Miensopust et al. [2011] inferred varying strike directions for their profile in northeastern Botswana and used separate focussed inversions with a different geoelectric strike directions to enhance their model. Therein, the authors used a strike direction of 55 degrees clockwise from North (N55E) for a subset of their model, whereas a strike direction of N35E was used for the rest of the model. However, an extension of the approach by Miensopust et al. [2011] to a case with more oblique geoelectric strike directions is not straightforward. In such a case, TE and TM mode estimates for the deeper region will be related to incorrect depths because of the significantly erroneous decomposition of the impedance tensor in at least one of the regions. A simple ‘stitching’ of inversion models from different strike directions is therefore highly likely to yield a model in which structures of the deeper region are misrepresented. In the PICASSO Phase I investigation, varying geologic strike direction with depth and along the profile is deduced for the region of the Tajo Basin (cf. Sec. 9). Geoelectric strike direction in the Tajo Basin crust is approximately NW-SE, coinciding with the direction of the Iberian Range and Neogene faults, whereas at mantle depths a dominant NNE- SSW direction is observed (Figs. 8.1). The defined change in strike direction is supported by results from seismic tomography studies (cf. Sec. 7.3.2) inferring a NW-SE directed interface at crustal depth and a NE-SW direction for deeper regions (cf. Fig. 8.2). Based on their orientation, a correlation with alpine orogenies that formed the approximately 170
direction direction Depth: 12 – 30 km (crust) Depth: 35 – 120 km (lithospheric-mantle) pic020 pic019 pic017 pic015 pic013 pic011 pic009 8.2. Generating synthetic 3D model data Multi-strike (Av. RMS-misfit in brackets) Multi-strike (Av. RMS-misfit in brackets) pic007 Fig. 8.1.: RMS misfit for different geoelectric strike directions applied to data from magnetotelluric (MT) stations in the Tajo Basin recorded during PICASSO Phase I, using data in the Niblett-Bostick depth (cf. Sec. 6.3.1) ranges 12 - 30 km and 35 - 300 km. Empty spaces are due to lack of sufficient data for this depth range at the respective station. Also shown in the top right corner is the optimal common geoelectric strike direction calculated using the program strike by McNeice and Jones [2001]. NW-SE stretching Pyrenees and the NE-SW stretching Betics during Late Mesozoic – Cenozoic times (cf. Sec. 7) seems likely. Computational cost of 3D inversion is high, usually permitting detailed inversion of a region with the size of the Tajo Basin. 2D inversion, on the other hand, requires the investigator to commit to one strike direction to be used for the inversion process, hence to invert data of at least one region with an erroneous strike direction assumption. This problem motivated construction and investigation of a synthetic model case that contrasts results of different inversion schemes and parameter settings for the case of oblique strike directions at crust and mantle depths. In particular, advances of novel algorithms that incorporate effects of anisotropic structures in the subsurface are utilised to recover structures at mantle depth. The use of anisotropic inversion codes for the recovery of oblique strike directions has not been reported before, meaning that this study breaks new ground. 8.2. Generating synthetic 3D model data 8.2.1. Generating the synthetic 3D model According to published velocity distribution (Sec. 7.3.2) and derived distribution of electric resistivity for the Tajo Basin subsurface and surrounding regions (Chap. 9), a synthetic 3D model (Fig. 8.3) is created using the 3D forward modelling program of the WinGLink software package [WinGLink, 2005], based on the algorithm by Rodi and Mackie [2001]. pic006 pic005 pic004 pic003 N40.9W (0.8) N29.4E (0.8) pic002 pic001 RMS misfit RMS misfit 171
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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 156 and 157: 6. Using magnetotellurics to gain i
Page 168 and 169: Part II Geology of the study area I
Page 170 and 171: 7. Geology of the Iberian Peninsula
Page 205: Recovering a synthetic 3D subsurfac
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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 and 218: Model variation RMS misfit Optimal
Page 219 and 220: Profile: 3D-crust (TM-only) Depth (
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
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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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