Imaging a Fault with Magnetotellurics

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Imaging a Fault with Magnetotellurics

• By Peter Winther

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Outline

• Science of Magnetotellurics
• Instrumentation
• MT data
• TEM Static Shift
• Conclusion

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Magnetotellurics

• Record natural occurring EM fields
• Sources are lightning and solar wind
• Currents are then induced within the Earth
• Apparent resistivity is calculated
• Depth of exploration is related to the skin depth
  

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MT Theory

• Helmholtz Equation
• Quasistic Approx.
• Characteristic Impedance
• Apparent Resistivity
• Skin Depth

• ?2E (???2 - i???)E 0
• d2Ex/dz2 k2Ex 0
• ? ?? ??
• Z Ex/Hy ??0/k
• ?a (1/??0) ?Ex/Hy?2
• ? (2/???)1/2
• ? 0.5(?T)1/2 (km)

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Imaging with MT

• MT theory assumes incident plane waves
• Need an apparent resistivity contrast
• Structures located within layers of the same
  resistivity can not be imaged!

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Stratagem

• Up to 500 m depth (based on skin depth)
• Frequency range 10 Hz 100 kHz

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MT Line
100m
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MT raw data
100
TEM
Log ?a (?)
MT
10
Log T (s)
90
Phase
-90
0.0001
0.001
0.01
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TEM Static Shift
TEM
100
MT
Log ?a (?)
10
Log T (s)
90
Phase
-90
0.0001
0.001
0.01
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1D Modeling
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Depth (m)
100
1000
Log ?a (?)
10
100
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MT Cross section
Elevation (m)
Resistive
1700
Conductive
Resistive
1600
Conductive
1500
100 m
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TEM Cross section
Elevation (m)
Resistive
1700
Conductive
Resistive
1600
1500
Conductive
100 m
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2D Inversion Model
Elevation (m)
Resistive
1700
Conductive
1600
1500
100m
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Conclusion

• Large conductive layer in canyon related to
  saturated clays in the Santa Fe Group.
• Resistive layer further out due to dry basin fill
  with a more conductive layer underneath.
• Fault is imaged between Station 500 and Station
  600.