Magnetotelluric Method Stephen Park IGPP UC Riverside

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”? 1.It is the “empty” method because of the long waiting times in the field needed for data collection (MIT field camp students, 1981). 2.It describes the look on the faces in the audience when the above description is given. 3.The initials stand for MagnetoTelluric (Cagniard, 1953). But seriously….. What can it tell us about the Earth?

MT is one of the few techniques capable of sensing through the Earth’s crust to upper mantle.

Silicate minerals comprise 95% of the crust… and silicate minerals are very resistive* (< S/m ). Electrical currents do not like resistors! The observed finite conductivity ( S/m) of the crust is due to small fractions (ppm-10%) of interconnected conductive material.  MT cannot be used to determine mineralogy but can be used to identify small fractions of: aqueous fluids ( S/m) partial melt (2-10 S/m) graphite (10 6 S/m) metallic oxides and sulfides (10 4 S/m) MT has been used successfully to locate: Underthrust sediments Regions of metamorphism and partial melting Fault zones (fractured, fluid-filled rock) *At crustal temperatures! IN THE CRUST…

IN THE MANTLE… Temperatures are sufficiently high (>  C) that mobilities of crystal defects and impurities are enhanced. Ionic mobility   Electrical conductivity! Enhanced mantle conductivity is caused by higher temperatures partial melt (> 0.01 S/m) hydrogen (and carbon?) diffusion MT has been used successfully to identify: partial melt variations in lithospheric temperature asthenosphere

What IS MT?….

ionosphere

Not all MT signals are from interactions with the solar wind: Micropulsations Global lightning Range of frequencies used to probe lower crust Murphy’s law is hard at work!!

Let’s 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?

Note lack of vertical fields and similarity of equations for (Hx,Ey) and (-Ex,Hy). Assume solutions of form exp(jkz), and get k=+/- (jωμσ) ½ and final result of:

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: Z=E/H Note that phase is constant at -45° and amplitude is proportional to frequency and resistivity (1/σ). This leads to the concept of apparent resistivity: MT responses are represented by phase and amplitude (apparent resistivity)

Assignment: Derive equations for Ex, Hy and Z xy. What similarities or differences do you see with Z yx ?

SAME apparent resistivity and phase is 135° (-1 is 180°) different from Z yx. 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.

In a 1-D earth (layered geology) and a vertically incident plane wave source, what terms can be eliminated? y z x y z x 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? Asssignment:

| 0 Z1| | -Z1 0 | | 0 Z1| | Z2 0 | | Z1 Z2| | Z3 Z4|

T

When we have multiple sites, we plot a pseudosection:

Interpretation: 1.1-D modeling, inversion – fast, easy, readily available, almost always WRONG! 2.2-D modeling, inversion – slower, more difficult, programs usually available, may have 3-D effects in data 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.

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).

In EM, basic boundary conditions at Interfaces are: 1)continuity of tangential fields 2)continuity of normal current density These sources lead to problems in solving both sets of equations with one forward solution! 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 z=0.

Consider the TE case (with Jy source): 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! Ey 1 Ey 2 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).

Typical steps for interpretation: 1.Identify TE, TM modes based on a. comparison to geologic strike b. decomposition of impedance tensor c. similarity of H z with H horizontal 2.Design starting model based on a. geologic structure b. other geophysical data c. guesses 3.Run inversion and try to fit data 4.Perform sensitivity analysis to determine which bounds on modeled structure. I H Hz H horizontal Induction arrows TE mode:

High resolution MT profile in Krygyzstan to determine neotectonic structure MT can provide resistivity sections at many scales from the uppermost crust… to the entire crust….

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.

However, what you really need not electrical resistivity…..