sage mt
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
George R. JiracekSan Diego State University
LIGHTNING
SOLAR WIND
MT DATA
BLACK BOX EARTH
THE INPUT
THE OUTPUT
MT Data Collection
Marlborough, New Zealand
Southern Alps, New Zealand
Southern Alps, New Zealand
Southern Alps, New Zealand
Taupo, New Zealand 2010-12
LOG (-M)DISTANCE (KM)
South Island , New Zealand Geoelectric SectionSouthern Alps Canterbury PlainNW
WSE
DE
PT
H (
KM
)
1
1.5
2
2.5
3
3.5
4
West-landndAFF
The “Banana”
Southern Alps, New Zealand
(Jiracek et al., 2007)
Southern Alps, New Zealand
New Zealand Earthquakes vs. Resistivity in Three-Dimensions
Southern Alps, New Zealand
Three-Dimensional MTTaupo 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)
Techniques - MT(Ack. Paul Bedrosian, USGS)
Magnetotelluric Signals
Techniques - MT(Ack. Paul Bedrosian, USGS)
Always Must SatisfyMaxwell’s Equations
0
f
J t
t
H E
E H
H
E
Quasi-static approx, σ >> εω
Magnetotellurics(Ack. Paul Bedrosian, USGS)
rf is free charge density
Quasistatic Approximation
metersf
ikwhere
eEzE
i
tt
kzx
5002
)1(
),(
),(
),(
0
2
2
ErE
ErE
(Ack. Paul Bedrosian, USGS)
d is skin depth
Graphical Description of Skin Depth, d
Ex(w) = Z(w) Hy(w)
After Fourier transforming the E(t) and H(t) data into the frequency domain the MT surface
impedance is calculated from:
Magnetotelluric Impedance
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.,
Hy(t) Ex(t)Z(t)
Apparent resistivity, ra and phase, f
2
0
1a Z
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
Dimensionality:
• One-Dimensional
• Two-Dimensional
• Three-Dimensional
r1
r2
r3
r4
r5
r6
r7
Geoelectric Dimensionality
1-D 3-D2-D
r a
Period (s)
Log
Log
y
x
z
ra a |Z2|
x
y
EH
=xyZ Shallow Resistive Layer
Intermediate Conductive Layer
Deep Resistive Layer
1-D MT Sounding Curve
Layered (1-D) Earth
Longer period deeper penetration ( )m
Using a range of periods a depth sounding can be obtained
Ex Hy1 1000 m
2 30 m
3 500 m
Apparent resistivity
Impedance Phase
20
40
60
80
0
101
102
103
104
100
30
10-2 102100 104
Period (s)
De
gre
es
Oh
m-m 500
1000
500 T
(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)
3-D MT Tensor Equation
y
x
yyyx
xyxx
y
x
H
H
ZZ
ZZ
E
E
• 2-D– Assumes geoelectric strike
• 3-D– No geoelectric assumptions
41 )(/)()(
)(/)()(
iD
jiij
eHEZ
HEZ
| |
yyyx
xyxxD ZZ
ZZZ 3
0
02
yx
xyD Z
ZZ
1-D, 2-D, and 3-D Impedance
• 1-D
[ ] is Tensor Impedance
(Ack., Paul Bedrosian, USGS)
3- D MT Data
x xx xy x
y yx yy y
H
E Z Z H
E Z Z H
E Z
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:
)()(
)(1
)(2
ZArg
Za
(Ack. Paul Bedrosian, USGS)
r a
Period (s)
Log
Log
2-D MT(Tensor Impedance reduces to two off-diagonal elements)
xy
yx
Z
Z
0
0Z
æ öç ÷=ç ÷ç ÷ç ÷è øy
x
z
ra a |Z2|
Geoele
ctric
Strik
e
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 r
a
Log Period (s)
E-Parallel
E- Perpendicular
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)
MT1MT2
(Ack., Martyn Unsworth, Univ. Alberta)
Visualizing Maxwell’s Curl Equations
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
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)
YXΦ 1
http://www-rohan.sdsu.edu/~jiracek/DAGSAW/Rotation_Figure/
tan( )cos( )
( )tan( ) sin( )
yx2D 2D
xy
p c( )
Ellipses are traced out at every period by the multiplication ofthe real 2 x 2 matrix from a MT phase tensor, F(f) and
a rotating, family of unit vectors, c(w), that describe a unit circle.
MT Phase Tensor Ellipse
2-D Tensor Ellipse p2D(w) is:
1-D TP Tc 2-D TP Tc 2-D TP
Phase Tensor Example for Single MT Sounding
at Taupo Volcanic Field, New Zealand
(Bibby et al., 2005)
1-D TP 2-D TP 2-D TP
Tc Tc
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)
Resistive Basement
Conductive Basin
Distance (m)
E
leva
tion
(m
)
W E
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)
Geoelectric Section From 2-D MT Inversion (MT Sites Indicated by Triangles)
Conductive Basin
Resistive Basement
Distance (m)
E
leva
tion
(m
)W E
(Winther, 2009)
SAGE – Rio Grande Rift, New Mexico
Resistivity Values of Earth Materials
MT Interpretation
Geology
Well Logs
(Winther, 2009)
SAGE – Rio Grande Rift, New Mexico
MT-Derived Midcrustal Conductor Physical StateEastern Great Basin (EGB), Transition Zone (TZ), and Colorado
Plateau (CP) (Wannamaker et al., 2008)
Field Area Now
The Future?
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.
References
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.