Imaging a Fault with Magnetotellurics By Peter Winther

Outline Science of Magnetotellurics Instrumentation MT data TEM Static Shift Conclusion

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

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)

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!

Stratagem Up to ~500 m depth (based on skin depth) Frequency range: 10 Hz – 100 kHz

MT Line 100m

MT raw data TEM MT Log  a (  ) Log T (s) Phase

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

1D Modeling Depth (m) Log  a (  )

MT Cross section Elevation (m) m Resistive Conductive

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

2D Inversion Model Elevation (m) Resistive Conductive 100m

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.