Download MT - WordPress.com

H. SAIBI
November 26, 2015
Outline
1.
2.
3.
4.
what is the magnetotelluric method?
Magnetotelluric theory
Interpretation
Equipment and Applications
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.
IN THE CRUST…
Silicate minerals comprise 95% of the crust… and silicate minerals are very
resistive*
(< 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
• Fault zones (fractured, fluid-filled rock)
*At crustal temperatures!
IN THE MANTLE…
Temperatures are sufficiently high (> 800C) 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
Range of frequencies
used to probe lower crust
Murphy’s law is hard at work!!
Global
lightning
| 0 Z1|
| -Z1 0 |
| 0 Z1|
| Z2 0 |
| Z1 Z2|
| Z3 Z4|
T
Magnetotelluric Theory
Electric field
Magnetic field
Impedance
Conductivity
Resistivity
Skin Depth
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.
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.
A system of equations for Ex, Ez, and Hy (called the TM mode):
Hy
  Ez
x

Hy
  Ex
z
Ez
Ex

  jHy
x
z
and a system of equations for Hx, Hz, and Ey (called the TE mode):
Ey
  jHz
x
Ey
 jHx
z
Hz
Hx

  Ey
x
z
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).
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 z=0.
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).
Typical steps for interpretation:
1.
Identify TE, TM modes based on
a. comparison to geologic strike
b. decomposition of impedance tensor
c. similarity of Hz with Hhorizontal
TE mode:
Induction
arrows
Hhorizontal
H
2.
3.
4.
I
Hz
Design starting model based on
a. geologic structure
b. other geophysical data
c. guesses
Run inversion and try to fit data
Perform sensitivity analysis to determine which bounds on modeled structure.
Applications
- Mainly in geothermal
Stratagem EH4
Stratagem EH4
Hybrid-Source Magnetotellurics
•
•
•
•
•
Frequency range of 10 Hz to 90k Hz
Approx. depth of investigation from 5m to 1km
Portable with rapid setup and teardown
Full tensor MT and CSAMT measurements
In-field display and printout of 1D inversion and 2D depth
section
• In-field display and print out of sounding curves
• In-field display and print out of signal amplitudes, phase,
and coherency curves
Stratagem Theory of Operation
• Stratagem EH4 is a magnetotellurics instrument used to measure ground
resistivity. Frequency domain EM instrument.
• Ground resistivity can be calculated from the ratio of the amplitudes of the
magnetic and electrics fields generated by currents in the ground (telluric
currents). Resistivity in Ohm-meters is ρ = (0.2/f)*(E/H)2 where ρ is
apparent resistivity, E is amplitude of the electric field, and H is amplitude
of the orthogonal magnetic field.
• Currents generated by natural fields (lightning strikes) and artificial source
(transmitter antenna).
• Electrode stakes used to measure electric fields and highly sensitive
magnetic coils used to measure magnetic fields.
Stratagem Theory 2
• Time series from electric and magnetic fields are converted to
frequency domain measurements by Fourier transformation.
• Calculating apparent resistivities at multiple frequencies
provides sounding curve of apparent resistivity vs frequency.
• Inverting frequency sounding curves gives true resistivity and
depth.
• 2-D depth sections derived from inversion data and filtering.
Equipment Setup
• Electric fields measured with galvanic stakes (can use
porous pot for low-freq. measurements below 10 Hz)
• Magnetic fields measured with induction coils
• Natural field measurements from 10 Hz to 90k Hz
• Transmitter intended to fill in the natural field gaps in
the range from 1k Hz to 70 k Hz.
Four channel operation (two electric and two
magnetic) allows for rapid deployment and data
collection.
Induction-Loop Antenna for high-frequency,
controlled-source operation. Dual-loops allow full
tensor CSAMT measurements.
Case Studies
Geothermal Exploration
Test Survey for geothermal exploration in China. Red (conductive) area is
location of a know fault with geothermal activity. Note high resolution of fault
dip. Depth of 250 meters.
Minerals Exploration
Minerals Exploration: Resistive (blue) areas correspond to silicification zones
with high concentration of hard-rock gold.
Groundwater Exploration
Groundwater Exploration: Resistive (darker) areas correspond to paleochannel
acting as a confining aquifer in high clay content sediments. A well drilled
over the resistive body was highly productive.
Ground water exploration in China: The two red lines indicate suspected
faults/fracture zones. A well was drilled in the second (deeper) fracture and
provided the most productive fresh water well in the region.
MT application in groundwater exploration in Egypt (M.A. Zaher et al., pure and applied geophysics)
MT can provide resistivity sections
Minerals Exploration
Mining Applications -Diamonds
Mining - Platinum