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SAGE MT. George R. Jiracek San Diego State University. "Understanding is More Important Than Knowledge". THE INPUT. THE OUTPUT. MT DATA. LIGHTNING. SOLAR WIND. BLACK BOX EARTH. MT Data Collection. Marlborough, New Zealand. Southern Alps, New Zealand. Southern Alps, New Zealand.

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## SAGE MT

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**SAGE MT**George R. Jiracek San Diego State University "Understanding is More Important Than Knowledge"**THE INPUT**THEOUTPUT MT DATA LIGHTNING SOLAR WIND BLACK BOX EARTH**Southern Alps, New Zealand**The “Banana”**Southern Alps, New Zealand**(Jiracek et al., 2007)**Southern Alps, New Zealand**New Zealand Earthquakes vs. Resistivity in Three-Dimensions**Three-Dimensional MT**Taupo Volcanic Geothermal Field, New Zealand (Heise et al. , 2008)**MT Phase Tensor Plot at 0.67s Period from the Taupo Volcanic**Field**Magnetotellurics (MT)**• Low frequency (VLF to subHertz) • Natural source technique • Energy diffusion governed by ρ(x,y,z) (Ack. Paul Bedrosian, USGS) Techniques - MT**Magnetotelluric Signals**(Ack. Paul Bedrosian, USGS) Techniques - MT**Quasi-static approx, σ >> εω**Always Must SatisfyMaxwell’s Equations rfis free charge density (Ack. Paul Bedrosian, USGS) Magnetotellurics**Quasistatic Approximation**d is skin depth (Ack. Paul Bedrosian, USGS)**Magnetotelluric Impedance**After Fourier transforming the E(t) and H(t) data into the frequency domain the MT surface impedance is calculated from: Ex(w) = Z(w) Hy(w)**Note, that since**Ex(w) = Z(w) Hy(w) is a multiplication in the frequency domain, it is a convolution in the time domain. Therefore, this is a filtering operation, i.e., Ex(t) Hy(t) Z(t)**Apparent resistivity, ra and phase, f**Apparent resistivity is the resistivity of an equivalent, but fictitious, homogeneous, isotropic half-space Phase is phase of the impedance f = tan-1 (Im Z/Re Z)**The goal of MT is the resistivity distribution, r(x,y,z), of**the subsurface as calculated from the surface electromagnetic impedance, Zs r7 r1 r2 Dimensionality: r3 • One-Dimensional • Two-Dimensional • Three-Dimensional r4 r5 r6**Geoelectric Dimensionality**3-D 1-D 2-D**x**Shallow Resistive Layer Intermediate Conductive Layer Deep Resistive Layer y ra Log z Log Period(s) 1-D MT Sounding Curve raa |Z2|**104**Ex 100 103 Hy 500 102 Ohm-m 30 101 Apparent resistivity 80 Impedance Phase 60 Degrees 40 20 0 10-2 100 102 104 Period (s) Layered (1-D) Earth 1000 Longer period deeper penetration ( )m Using a range of periods a depth sounding can be obtained (Ack., Paul Bedrosian, USGS)**MT “Screening” of Deep Conductive Layer**by Shallow Conductive Layer (Ack., Martyn Unsworth, Univ. Alberta)**When the Earth is either 2-D or 3-D:**Ex(w) = Z(w) Hy(w) Now Ex(w) = Zxx(w) Hx(w) + Zxy(w) Hy(w) Ey(w) = Zyx(w) Hx(w) + Zyy(w) Hy(w) This defines the tensor impedance, Z(w)**1-D, 2-D, and 3-D Impedance**• 1-D • 2-D • Assumes geoelectric strike • 3-D • No geoelectric assumptions [ ] is Tensor Impedance (Ack., Paul Bedrosian, USGS)**3- D MT Data**Measure time variations of electric (E) and magnetic (H) fields at the Earth‘s surface. Estimate transfer functions of the E and H fields. Subsurface resistivity distribution recovered through modeling and inversion. Impedance Tensor: App Resistivity & Phase: (Ack. Paul Bedrosian, USGS) Techniques - MT**x**y ra Log z Log Period (s) 2-D MT (Tensor Impedance reduces to two off- diagonal elements) Geoelectric Strike raa |Z2|**TM**TE Boundary Conditions • E-Fields parallel to the geoelectric strike are continuous (called TE mode) • E-Fields perpendicular to the geoelectric strike are discontinuous (called TM mode) Map View E-Parallel Log ra E- Perpendicular Log Period (s)**TE (Transverse Electric) and TM (Transverse Magnetic) Modes**MT2 • 2-D Earth structure • Different results at MT1 (Ex and Hy) • and MT2 (Ey and Hx) MT1 TRANSVERSE ELECTRIC MODE (TE) TRANSVERSE MAGNETIC MODE (TM) Visualizing Maxwell’s Curl Equations (Ack., Martyn Unsworth, Univ. Alberta)**MT Phase Tensor**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) The MT Phase Tensor and its Relation to MT Distortion (Jiracek Draft, June, 2014)**MT Phase Tensor**• 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)**MT Phase Tensor Ellipse**• Ellipses are traced out at every period by the multiplication of • the real 2 x 2 matrix from a MT phase tensor, F(f) and • arotating, family of unit vectors, c(w), that describe a unit circle. 2-D Tensor Ellipse p2D(w) is: http://www-rohan.sdsu.edu/~jiracek/DAGSAW/Rotation_Figure/**Phase Tensor Example for Single MT Sounding at Taupo**Volcanic Field, New Zealand (Bibby et al., 2005) 1-D TP Tc 2-D TP Tc 2-D TP**Tc**Tc 1-D TP 2-D TP 2-D TP Phase Tensor Determinations of Dimensionality (1-D. 2-D), Transition Periods (TP), and Threshold Periods (Tc)**SAGE MT**Caja Del Rio**Geoelectric Section From Stitched 1-D TE Inversions (MT**Sites Indicated by Triangles) E W Conductive Basin Elevation (m) Resistive Basement Distance (m) W E Basin Elevation (m) Basement**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 (Ack. Paul Bedrosian, USGS) Techniques - MT**Geoelectric Section From 2-D MT Inversion (MT Sites**Indicated by Triangles) W E Conductive Basin Elevation (m) Resistive Basement Distance (m) W E Basin Elevation (m) Basement**SAGE – Rio Grande Rift, New Mexico**(Winther, 2009)**MT Interpretation**Geology Well Logs**SAGE – Rio Grande Rift, New Mexico**(Winther, 2009)**MT-Derived Midcrustal Conductor Physical State**Eastern Great Basin (EGB), Transition Zone (TZ), and Colorado Plateau (CP) (Wannamaker et al., 2008)**Field Area Now**The Future?**References**Bibby, H. M., T. G. Caldwell, and C. Brown, 2005, Determinable and non-determinable parameters of galvanic distortion in magnetotellurics, Geophys. J. Int., 163, 915 -930. Caldwell, T. G., H. M.Bibby, and C. Brown, 2004, The magnetotelluric phase tensor, Geophys. J. Int., 158, 457- 469. Heise, W., T. G. Caldwell, H. W. Bibby, and C. Brown, 2006, Anisotropy and phase splits in magnetotellurics, Phys. Earth. Planet. Inter., 158, 107-121. 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., 103-129. 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, 347-369.

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