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Published byAllyson Douglas Modified about 1 year ago

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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 2 E + ( 2 - i )E = 0 d 2 E x /dz 2 + k 2 E x = 0 Z = E x /H y = 0 /k a = (1/ 0) E x /H y 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 TEM MT Log a ( ) Log T (s) Phase

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TEM Static Shift TEM MT Log a ( ) Log T (s) Phase

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1D Modeling Depth (m) Log a ( )

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MT Cross section Elevation (m) m Resistive Conductive

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TEM Cross section Elevation (m) Resistive Conductive Resistive 100 m

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2D Inversion Model Elevation (m) Resistive Conductive 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.

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