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George R. Jiracek San Diego State University

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LIGHTNING SOLAR WIND MT DATA BLACK BOX EARTH THE INPUT THE OUTPUT

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MT Data Collection

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Marlborough, New Zealand

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Southern Alps, New Zealand

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Taupo, New Zealand

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The “Banana” Southern Alps, New Zealand

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( Jiracek et al., 2007) Southern Alps, New Zealand

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New Zealand Earthquakes vs. Resistivity in Three-Dimensions Southern Alps, New Zealand

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Three-Dimensional MT Taupo Volcanic Geothermal Field, New Zealand (Heise et al., 2008)

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MT Phase Tensor Plot at 0.67s Period from the Taupo Volcanic Field

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Magnetotellurics (MT) Low frequency (VLF to subHertz) Natural source technique Energy diffusion governed by ρ(x,y,z) Techniques - MT (Ack. Paul Bedrosian, USGS)

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Magnetotelluric Signals Techniques - MT (Ack. Paul Bedrosian, USGS)

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Always Must Satisfy Maxwell’s Equations Quasi-static approx, σ >> εω Magnetotellurics (Ack. Paul Bedrosian, USGS) f is free charge density

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Quasistatic Approximation (Ack. Paul Bedrosian, USGS) is skin depth

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Graphical Description of Skin Depth,

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E x ( ) = Z( ) H y ( ) After Fourier transforming the E(t) and H(t) data into the frequency domain the MT surface impedance is calculated from: Magnetotelluric Impedance

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Note, that since E x ( ) = Z( ) H y ( ) is a multiplication in the frequency domain, it is a convolution in the time domain. Therefore, this is a filtering operation, i.e., H y (t) E x (t) Z(t)

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Apparent resistivity, a and phase, Apparent resistivity is the resistivity of an equivalent, but fictitious, homogeneous, isotropic half-space Phase is phase of the impedance = tan -1 (Im Z/Re Z)

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The goal of MT is the resistivity distribution, x,y,z of the subsurface as calculated from the surface electromagnetic impedance, Z s Dimensionality: One-Dimensional Two-Dimensional Three-Dimensional 11 22 33 44 55 66 77

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Geoelectric Dimensionality 1-D 3-D 2-D

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aa Period (s) Log y x z a Z 2 | Shallow Resistive Layer Intermediate Conductive Layer Deep Resistive Layer 1-D MT Sounding Curve

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Layered (1-D) Earth Longer period deeper penetration ( )m Using a range of periods a depth sounding can be obtained ExEx HyHy Apparent resistivity Impedance Phase Period (s) Degrees Ohm-m (Ack., Paul Bedrosian, USGS)

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MT “Screening” of Deep Conductive Layer by Shallow Conductive Layer (Ack., Martyn Unsworth, Univ. Alberta)

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When the Earth is either 2-D or 3-D: E x ( ) = Z( ) H y ( ) Now E x ( ) = Z xx ( ) H x ( ) + Z xy ( ) H y ( ) E y ( ) = Z yx ( ) H x ( ) + Z yy ( ) H y ( ) This defines the tensor impedance, Z( )

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3-D MT Tensor Equation

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2-D –Assumes geoelectric strike 3-D – No geoelectric assumptions 1-D, 2-D, and 3-D Impedance 1-D [ ] is Tensor Impedance (Ack., Paul Bedrosian, USGS)

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3- D MT Data Estimate transfer functions of the E and H fields. Measure time variations of electric (E) and magnetic (H) fields at the Earth‘s surface. Subsurface resistivity distribution recovered through modeling and inversion. Techniques - MT Impedance Tensor:App Resistivity & Phase: (Ack. Paul Bedrosian, USGS)

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aa Period (s) Log 2-D MT (Tensor Impedance reduces to two off- diagonal elements) y x z a Z 2 | Geoelectric Strike

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1.E-Fields parallel to the geoelectric strike are continuous (called TE mode) 2.E-Fields perpendicular to the geoelectric strike are discontinuous (called TM mode) Boundary Conditions TM TE Map View Log a Log Period (s) E-Parallel E- Perpendicular

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TE (Transverse Electric) and TM (Transverse Magnetic) Modes -2-D Earth structure -Different results at MT1 (Ex and Hy) and MT2 (Ey and Hx) TRANSVERSE ELECTRIC MODE (TE)TRANSVERSE MAGNETIC MODE (TM) MT1 MT2 (Ack., Martyn Unsworth, Univ. Alberta) Visualizing Maxwell’s Curl Equations

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The MT Phase Tensor and its Relation to MT Distortion (Jiracek Draft, June, 2014) Described as “elegant” by Berdichevsky and Dmitriev (2008) and a “major breakthrough” by Weidelt and Chave (2012) “Despite its deceiving simplicity, students attending the SAGE program often have problems grasping the essence of the MT phase tensor” (Jiracek et al., 2014) MT Phase Tensor

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X and Y are the real and imaginary parts of impedance tensor Z, i.e., Z = X + iY Ideal 2-D, β=0 Recommended β <3° for ~ 2-D by Caldwell et al., (2004)

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Ellipses are traced out at every period by the multiplication of the real 2 x 2 matrix from a MT phase tensor, (f) and a rotating, family of unit vectors, c( ), that describe a unit circle. MT Phase Tensor Ellipse 2-D Tensor Ellipse p 2D ( ) is:

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1-D TP T c 2-D TP T c 2-D TP Phase Tensor Example for Single MT Sounding at Taupo Volcanic Field, New Zealand (Bibby et al., 2005)

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1-D TP 2-D TP 2-D TP TcTc TcTc Phase Tensor Determinations of Dimensionality (1-D. 2-D), Transition Periods (TP), and Threshold Periods (T c )

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SAGE MT Caja Del Rio

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W E Elevation (m) Basin Basement Geoelectric Section From Stitched 1-D TE Inversions (MT Sites Indicated by Triangles) Resistive Basement Conductive Basin Distance (m) Elevation (m) W E

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2-D MT Inversion/Finite-Difference Grid M model parameters, N surface measurements, M>>N A regularized solution narrows the model subspace Introduce constraints on the smoothness of the model Techniques - MT (Ack. Paul Bedrosian, USGS)

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W E Elevation (m) Basin Basement Geoelectric Section From 2-D MT Inversion (MT Sites Indicated by Triangles) Conductive Basin Resistive Basement Distance (m) Elevation (m) W E

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(Winther, 2009) SAGE – Rio Grande Rift, New Mexico

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Resistivity Values of Earth Materials

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MT Interpretation Geology Well Logs

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(Winther, 2009) SAGE – Rio Grande Rift, New Mexico

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MT-Derived Midcrustal Conductor Physical State Eastern Great Basin (EGB), Transition Zone (TZ), and Colorado Plateau (CP) (Wannamaker et al., 2008)

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Field Area Now The Future?

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Bibby, H. M., T. G. Caldwell, and C. Brown, 2005, Determinable and non- determinable parameters of galvanic distortion in magnetotellurics, Geophys. J. Int., 163, Caldwell, T. G., H. M. Bibby, and C. Brown, 2004, The magnetotelluric phase tensor, Geophys. J. Int., 158, Heise, W., T. G. Caldwell, H. W. Bibby, and C. Brown, 2006, Anisotropy and phase splits in magnetotellurics, Phys. Earth. Planet. Inter., 158, Jiracek, G.R., V. Haak, and K.H. Olsen, 1995, Practical magnetotellurics in continental rift environments, in Continental rifts: evolution, structure, and tectonics, K.H. Olsen, ed., Jiracek, G. R., V. M Gonzalez, T. G. Caldwell, P. E. Wannamaker, and D. Kilb, 2007, Seismogenic, Electrically Conductive, and Fluid Zones at Continental Plate Boundaries in New Zealand, Himalaya, and California- USA, in Tectonics of A Continental Transform Plate Boundary: The South Island, New Zealand, Amer. Geophys. Un. Mono. Ser. 175, References

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Palacky, G.J., 1988, Resistivity characteristics of geologic targets, in Investigations in Geophysics Volume 3: Electromagnetic methods in applied geophysics theory vol. 1, M.N. Nabighian ed., Soc. Expl. Geophys., 53–129. Winther, P. K., 2009, Magnetotelluric investigations of the Santo Domingo Basin, Rio Grande rift, New Mexico, M. S thesis, San Diego State University, 134 p. Wannamaker, P. E., D. P. Hasterok, J. M. Johnston, J. A. Stodt, D. B. Hall, T. L. Sodergren, L. Pellerin, V. Maris, W. M. Doerner, and M. J. Unsworth, 2008, Lithospheric Dismemberment and Magmatic Processes of the Great Basin-Colorado Plateau Transition, Utah, Implied from Magnetotellurics: Geochem., Geophys., Geosys., 9, 38 p.

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