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UTAM 2004

UTAM 2004. Travis Crosby. UTAM 2004. Travis Crosby. Very Low Frequency EM Surveys for the Purpose of Augmenting Near-Surface Seismic Studies. UTAM 2004. Travis Crosby. Introduction Instrument Theory Geophysical Results Other Instrument Applications Future Work. UTAM 2004.

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UTAM 2004

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  1. UTAM 2004 Travis Crosby

  2. UTAM 2004 Travis Crosby Very Low Frequency EM Surveys for the Purpose of Augmenting Near-Surface Seismic Studies

  3. UTAM 2004 Travis Crosby Introduction Instrument Theory Geophysical Results Other Instrument Applications Future Work

  4. UTAM 2004 Travis Crosby For Single Method Surveys: • Instrument may record excessive noise. • Ground may not provide sufficient contrast. • Overlapping anomalies may hinder interpretation.

  5. UTAM 2004 Travis Crosby For Single Method Surveys: • Instrument may record excessive noise. • Ground may not provide sufficient contrast. • Overlapping anomalies may hinder interpretation. Reconciliation of multiple data sets often provides a more true interpretational picture.

  6. UTAM 2004 Travis Crosby VLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ).

  7. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components.

  8. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field.

  9. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. • Instrument measures anomalous response to the induced current. Hz/Hx Surface Location of Anomaly Secondary EM Field Vertical Anomaly

  10. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. • Instrument measures anomalous response to the induced current. • Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. Surface Location of Anomaly Hz/Hx Secondary EM Field Vertical Anomaly

  11. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. • Instrument measures anomalous response to the induced current. • Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. Filtered Data n = - 0.102 Dn-3 + 0.059 Dn-2 – 0.561 Dn-1 + 0 Dn + 0.561 Dn+1 - 0.059 Dn+2 + 0.102 Dn+3 Where: Dn = Hz / Hx, as measured by the instrument

  12. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. • Instrument measures anomalous response to the induced current. • Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. • By increasing filter spacing (…Dn-2, Dn, Dn+2,…), information about deeper apparent depths can be obtained, and used to produce cross-section plots of current density.

  13. UTAM 2004 Travis Crosby VLF Theory • 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). • At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. • Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. • Instrument measures anomalous response to the induced current. • Karous & Hjelt filter applied to each data point (Dn) to convert complicated anomalies into simple peaks. • By increasing filter spacing (…Dn-2, Dn, Dn+2,…), information about deeper apparent depths can be obtained, and used to produce cross-section plots of current density. • VLF used to look for bodies of low electrical resistance, i.e. vertical fractures and ore deposits up to depths of 300 m.

  14. UTAM 2004 Travis Crosby Seismic Refraction VLF 595 m profile length 120 Geophones – 5 m Spacing 41 Shot Points – 15 m Spacing Source used – 550 lb EWG First arrival P-wave measured • 595 m profile length • 3 m station spacing • Instrument used – Abem Wadi • Frequency used – 25.1 kHz (Tx located in North Dakota)

  15. UTAM 2004 Travis Crosby Site Location N 20 km

  16. UTAM 2004 Line Location Travis Crosby N Legend Water Well Mapped Fault, dotted where inferred Seismic or VLF Line VLF Detected Fault Seismic Detected Fault 600 m

  17. UTAM 2004 Travis Crosby Seismic Tomogram VLF Data (Karous & Hjelt Filtered) SE NW ?

  18. UTAM 2004 Site Interpretation Travis Crosby N Legend Water Well Mapped Fault, dotted where inferred Seismic or VLF Line VLF Detected Fault Seismic Detected Fault 600 m

  19. UTAM 2004 Travis Crosby Test Profile N ? ? Legend Water Well Mapped Fault, dotted where inferred Seismic or VLF Line VLF Detected Fault 600 m Seismic Detected Fault

  20. UTAM 2004 VLF Data (Karous & Hjelt Filtered) Travis Crosby NW SE Water Well Stream

  21. UTAM 2004 ? Travis Crosby Site Interpretation N ? Legend Water Well Mapped Fault, dotted where inferred Seismic or VLF Line VLF Detected Fault 500 m Seismic Detected Fault

  22. UTAM 2004 Travis Crosby Other VLF Studies: Ore Deposits • Survey area 90 km northeast of Yellowknife, NW Territories, Canada. • Original defined strike limits Ag-Pb-Au banded sulfide lens was 160 m. • VLF survey complementing other data sets suggested a greater strike length of 400 m. Data from: Fugro Airborne Surveys VLF

  23. UTAM 2004 Future Work Travis Crosby • To augment seismic refraction tomography studies by conducting smaller scale, higher resolution VLF to detect normal and antithetic faults bounding larger colluvial wedge structures.

  24. UTAM 2004 Future Work Travis Crosby • To augment seismic refraction tomography studies by conducting smaller scale, higher resolution VLF to detect normal and antithetic faults bounding larger colluvial wedge structures. • Multi-electrode, high-resolution, 2-D, DC resistivity imaging of colluvial wedges for comparison with seismic tomograms.

  25. UTAM 2004 Travis Crosby

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