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Audio Magnetotelluric Analysis of the Tanos Fault Area

Audio Magnetotelluric Analysis of the Tanos Fault Area. SAGE 2007 Espa ñola Basin, NM. Overview. Area of Study Introduction to Magnetotellurics Data Acquisition 2-D Modeling Analysis Conclusions. Area of Study. Vulcan Quarry Site, Sandoval County, NM. Proposed Tanos Fault.

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Audio Magnetotelluric Analysis of the Tanos Fault Area

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  1. Audio Magnetotelluric Analysis of the Tanos Fault Area SAGE 2007 Española Basin, NM

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

  3. Area of Study • Vulcan Quarry Site, Sandoval County, NM Proposed Tanos Fault

  4. 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

  5. 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

  6. 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.

  7. 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

  8. 102 Apparent Resistivity a (-m) 101 Period T (s) 10-5 10-5 10-3 10-3 10-1 10-1 Why smooth  instead of 2? Smoothing 2 Smoothing 

  9. N S 100 m 1850 1400 2000  (-m) Depth (m) 200 5 Models of Varying Smoothness:  = 1

  10. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Models of Varying Smoothness:  = 3

  11. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Models of Varying Smoothness:  = 10

  12. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Models of Varying Smoothness:  = 50

  13. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Models of Varying Smoothness:  = 10

  14. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Anomalous Conductor? ~30 -m Conductive Anomaly ( = 10)

  15. 102 Apparent Resistivity a (-m) 101 Accurate Range Period T (s) 10-5 10-3 10-1 Picking Accurate Ranges • h.357√Ta [km] • This simple approximation was used to verify the existence of the conductive anomaly that appears throughout the models.

  16. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Range of Depth Resolution ( = 10)

  17. N S 100 m 2000 1850 1400 Water Table  (-m) Depth (m) 200 5 Anomalous Conductor ~30 -m Conductive Anomaly ( = 10)

  18. Ranges of Resistivity in Various Materials

  19. N 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Anomalous Resistor? ~150-200 -m Models of Varying Smoothness:  = 10 S

  20. A resistive layer appears in the beginning of data sets 4,5,&6. Again estimated depth using h.357√Ta [km] 102 101 10-5 10-3 10-1 Evidence of a resistive anomaly Resistive Anomaly? Apparent Resistivity a (-m) Period T (s) Tanos Station #5

  21. 1750m 1850m Range of Data Set’s “Resistor” ( = 10) N S Depth (m) 100 m

  22. N S 100 m 2000 1850 1400  (-m) Depth (m) 200 5 Anomalous Conductor ~30 -m Final Cross Section ( = 10) Fault?

  23. 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

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