P. Schmoldt, PhD - MTNet - DIAS

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6. Using magnetotellurics to gain information about the Earth Horizoontal magnetic coil Electric dipoles VVertical ti l magnetic coil NN electrode Ground electrode Recording station GPS Horizontal magnetic coil Fig. 6.1.: Schematic layout of the broadband magnetotelluric (MT) recording system used during the PICASSO Phase I fieldwork campaign; see Section 6.1.2 for details. Lviv long-period LEMI-417M recording system [Lviv Centre of Institute for Space Research, 2009]. For the measurement of the electric field, Phoenix Geophysics and WOLF Pb-PbCl2, as well as Lviv Cu-CuSO4 non-polarising sensors (commonly referred to as electrodes) are used with both recording systems. To avoid polarisation effects only electrode pairs of the same material are used during a recording. Time-series data of electric and magnetic fields are stored internally by the two systems and can be transferred onto a computer via a removable solid state disc. The two different systems are employed to investigate both crustal and mantle structures; the limiting factor of a recording system, regarding depth of investigation, is usually the magnetic sensor. The MTC-50 induction coils are able to record data in the period range 10 −3 – 5 · 10 4 s whereas the fluxgate magnetometer, used with the Lviv system, is designed to record data with a period length of 30 s and above. The maximum usable period for a MT station is then dependent on recording duration and noise level. 6.1.2. Station setup A typical layout for a Phoenix broadband MTU-5 recording system and Lviv long-period LEMI-417M recording system is shown in Figure 6.1 and 6.2, respectively. The station setup starts by assigning the location of the electric field measurements. Two pairs of electrodes, each forming an electric dipole, are aligned to geomagnetic north-south and east-west respectively, using a compass. A dipole length of 90 m is usually chosen, however, the length may vary depending on the local environment. In addition, a fifth electrode is installed, acting as protective ground for the main recording unit; in the case 106 ~1m 1m ~1
Electric dipoles N 6.1. Recording of magnetotelluric signals Electrode Ground electrode Recording station Fluxgate magnetometer Fig. 6.2.: Schematic layout of the long-period magnetotelluric (MT) recording system used during the PICASSO Phase I fieldwork campaign; see Section 6.1.2 for details. of the new LEMI-417 design, the fifth electrode can be used as an additional source of information (see Sec. 9.3), requiring it to be located in the centre of the two dipoles. The above-described configuration may vary in certain situations, e.g. when topographic circumstances would force a significant shortening of a dipole in one geomagnetic direction. In such cases, the orientation of the electric dipoles can be rotated; however, the two electric dipoles must remain orthogonal. The electric field can then be derived for an arbitrary horizontal orientation through a simple coordinate transformation (cf. Sec. 4.3). To prevent polarisation effects due to electric charge build-up along the electrodeground interface, the metallic electrodes are placed in an electrolytic mud, i.e. Cuelectrodes in a CuSo4 solution and Pb-electrodes in saltwater brine (since exposure to PbCl2 may cause lead poisoning). Electrodes are buried in the ground to protect them from drying-out, human or animal interference, and effects of temperature variation. The ultimate depth is usually limited by ground condition and logistic considerations; a depth of an extended arm-length (approx. 1 m) is considered as a good compromise. The main difference between the systems, regarding their setup, is the installation of the magnetic sensors, i.e in case of the MTU-5 system, two (three, if the vertical magnetic field is recorded as well) magnetic coil-sensors are installed; whereas, in case of the LEMI-417 system, only one fluxgate magnetometer, recording all three magnetic components simultaneously, needs to be installed. For the MTU-5 system, the two (horizontal) coils are buried in the ground, with the depth controlled by the same factors as the electrode depth and 1 m again assumed an acceptable compromise. The two horizontal coils have to be accurately levelled and aligned to geomagnetic north–south and east–west. As GPS ~1m 107
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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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4.1. Types of distortion Fig. 4.3.:
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Page 95 and 96: Scale Type Terminology Example Atom
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Page 107 and 108: Parameter Geoelectrical application
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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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