Magnetotelluric Method

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Magnetotelluric Method
Stephen Park IGPP UC Riverside magneto_at_ucrmt.ucr.e
du
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So, what is the magnetotelluric method?
The magnetotelluric (MT) method determines the
tensor electrical impedance of the earth through
measurement of naturally varying EM fields, and
then uses computer modeling to find cross
sections of electrical resistivity that yield
theoretical responses similar the observed ones.
And why is it abbreviated MT?

• It is the empty method because
• of the long waiting times in the field
• needed for data collection (MIT field
• camp students, 1981).
• It describes the look on the faces in the
• audience when the above description is
• given.
• The initials stand for MagnetoTelluric
• (Cagniard, 1953).

But seriously.. What can it tell us about the
Earth?
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MT is one of the few techniques capable of
sensing through the Earths crust to upper
mantle.
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IN THE CRUST

• Silicate minerals comprise 95 of the crust
• and silicate minerals are very resistive
• (lt 10-6 S/m). Electrical currents do not like
• resistors!
• The observed finite conductivity (10-4 - 1 S/m)
• of the crust is due to small fractions (ppm-10)
• of interconnected conductive material.
• aMT cannot be used to determine mineralogy
• but can be used to identify small fractions of
• aqueous fluids (0.1-10 S/m)
• partial melt (2-10 S/m)
• graphite (106 S/m)
• metallic oxides and sulfides (104 S/m)
• MT has been used successfully to locate
• Underthrust sediments
• Regions of metamorphism and partial melting

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IN THE MANTLE

• Temperatures are sufficiently high (gt 800C)
• that mobilities of crystal defects and impurities
• are enhanced.
• Ionic mobility ? ? Electrical conductivity!
• Enhanced mantle conductivity is caused by
• higher temperatures
• partial melt (gt 0.01 S/m)
• hydrogen (and carbon?) diffusion
• MT has been used successfully to identify
• partial melt
• variations in lithospheric temperature
• asthenosphere

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What IS MT?.
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ionosphere
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Not all MT signals are from interactions with the
solar wind
Micropulsations
Global lightning
Range of frequencies used to probe lower crust
Murphys law is hard at work!!
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Lets look at the governing equations
These break down into components
         
Consider a halfspace and a vertically
incident plane wave Is there any difference
between one point and another 1 km away?
NO!
So, what terms vanish above?
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Note lack of vertical fields and similarity
of equations for (Hx,Ey) and (-Ex,Hy).
Assume solutions of form exp(jkz), and get k/-
(j?µs)½ and final result of
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Note that both of these contain an undetermined
constant, A, that is set by the strength of the
source field. in order to get rid of this
constant, we examine the impedance of the
Earth ZE/H
Note that phase is constant at -45 and amplitude
is proportional to frequency and resistivity
(1/s). This leads to the concept of
apparent resistivity
MT responses are represented by phase and
amplitude (apparent resistivity)
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Assignment Derive equations for Ex, Hy and Zxy.
What similarities or differences do you see with
Zyx?
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SAME apparent resistivity and phase is 135 (-1
is 180) different from Zyx.
Summary Layered halfspace characteristics
apparent resistivity is independent of frequency
phase is either 45 or 135 apparent
resistivities for two modes (Ex,Hy
and (Ey,Hx) are equal NO vertical fields.
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Asssignment
In a 1-D earth (layered geology) and a
vertically incident plane wave source, what terms
can be eliminated?
x
z
In a 2-D earth (variations in conductivity in x
and z only) and a vertically incident plane wave
source, what terms can be eliminated?
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0 Z1 -Z1 0
0 Z1 Z2 0
Z1 Z2 Z3 Z4
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T
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When we have multiple sites, we plot a
pseudosection
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• Interpretation
• 1-D modeling, inversion fast, easy,
• readily available, almost always WRONG!
• 2-D modeling, inversion slower, more
• difficult, programs usually available, may
• have 3-D effects in data.
• 3. 3-D modeling used to verify 2-D results,
• programs available but only simple models
• possible. Inversion not yet available.
• ?2-D inversion is standard tool for
• interpretation.

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A system of equations for Ex, Ez, and Hy (called
the TM mode)
and a system of equations for Hx, Hz, and Ey
(called the TE mode)
Note similarities in equations if E, H switched
and ?, -j?? switched. This leads to some
simplifications in programming the
forward solution! Each mode is simply excited by
an equivalent current sheet in the appropriate
direction at the surface (Jx for the TM mode and
Jy for the TE mode).
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These sources lead to problems in solving both
sets of equations with one forward solution!
In EM, basic boundary conditions at Interfaces
are 1)continuity of tangential fields
2)continuity of normal current density
Consider the TM case (with Jx source)
Jx
Because Jx at the surface must be continuous
both across the air-Earth interface and between
the adjacent prisms, Jx is constant everywhere on
the surface and therefore is a equivalent to an
MT source with a uniform plane wave. Thus, the
current sheet is placed at z0.
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Consider the TE case (with Jy source)
Ey1
Ey2
Jy
Continuity of tangential E at the
surface requires that Ey be continuous across the
air-Earth interface AND at the edges of the
prisms. Because Jy ?Ey, Jy must be
DIScontinuous at the edges of the prism. This
means that Jy varies in the x direction across
the model and does NOT represent a uniform source!
SOLUTION Add air layers to top of model to a
sufficient height that Jy is once again uniform
(typically 8-10 layers to a height of 100 km or
more).
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• Typical steps for interpretation
• Identify TE, TM modes based on
• a. comparison to geologic strike
• b. decomposition of impedance tensor
• c. similarity of Hz with Hhorizontal

TE mode
Hhorizontal
Induction arrows
I
H
Hz
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MT can provide resistivity sections at many
scales from the uppermost crust
High resolution MT profile in Krygyzstan to
determine neotectonic structure
to the entire crust.
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MT profile across Sierra Nevada and eastern
California
MT modeling and inversion are regional
problems! Data in the Sierra Nevada are affected
by the highly conductive Pacific Ocean (and all
of the structure in between). Mackie et al.
(1996) showed with a 3-D model of California that
the Transverse Ranges resistivity affected
electric field levels in Death Valley.
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However, what you really need not electrical
resistivity..