Audio Magnetotelluric Analysis of the Tanos Fault Area

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Audio Magnetotelluric Analysis of the Tanos Fault
Area

• SAGE 2007
• Española Basin, NM

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Overview

• Area of Study
• Introduction to Magnetotellurics
• Data Acquisition
• 2-D Modeling
• Analysis
• Conclusions

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Area of Study

• Vulcan Quarry Site, Sandoval County, NM

Proposed Tanos Fault
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Magnetotellurics

• The MT method uses natural variations in the
  magnetosphere to probe deep into the earth.
• There are two main sources of these variations
• Lightning Higher Frequency Variations
• Solar Phenomena Lower Frequency Variations

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Data Aquisition

• To supplement natural field variations, a
  Stratagem 400 A-m2 transmitter was used to boost
  field strength in the 1-50 kHz range.
• We acquired data from 10 Hz- 100 kHz

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Data Aquisition

• Data was obtained at 6 stations.
• Station spacing was 100m.
• Station line was centered across the Tanos fault.
• In each data set noise was masked manually.
• Data was rotated through 30 degrees.
• No other data editing techniques were used.

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2-D Inversion Models

• Many different models were run
• Smoothness of models are governed by equation
• ? ?d ???m
• ?d misfit
• ?m roughness
• ? balancing term
• In addition, gradient smoothing (as opposed to
  Laplacian smoothing) was used to better fit data
  sets.
• We smoothed ?? instead of ?2?

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Why smooth ?? instead of ?2??
Smoothing ??
Smoothing ??2
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Models of Varying Smoothness ? 1
10
Models of Varying Smoothness ? 3
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Models of Varying Smoothness ? 10
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Models of Varying Smoothness ? 50
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Models of Varying Smoothness ? 10
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Conductive Anomaly (? 10)
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Picking Accurate Ranges

• h?.357vT?a km
• This simple approximation was used to verify the
  existence of the conductive anomaly that appears
  throughout the models.

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Range of Depth Resolution (? 10)
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Conductive Anomaly (? 10)
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Ranges of Resistivity in Various Materials
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Models of Varying Smoothness ? 10
S
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Evidence of a resistive anomaly

• A resistive layer appears in the beginning of
  data sets 4,5,6.
• Again estimated depth using h?.357vT?a km

Resistive Anomaly?
Apparent Resistivity ?a (?-m)
Period T (s)
Tanos Station 5
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Range of Data Sets Resistor (? 10)
N
S
Depth (m)
100 m
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Final Cross Section (? 10)
Fault?
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Conclusions

• Probable Anomalous Conductor
• Centered laterally between stations 2 and 3
• Centered vertically at 150m
• May be related to underground freshwater
• Possible evidence of faulting along previously
  mapped Tanos fault
• Discontinuity can be interpreted as suggestive,
  but evidence is far from conclusive