1. UTAM 2004 Travis Crosby

2. UTAM 2004 Travis Crosby Very Low Frequency EM Surveys for the Purpose of Augmenting for the Purpose of Augmenting Near-Surface Seismic Studies 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. Instrument may record excessive noise. Ground may not provide sufficient contrast. Ground may not provide sufficient contrast. Overlapping anomalies may hinder Overlapping anomalies may hinder interpretation. interpretation.

5. UTAM 2004 Travis Crosby For Single Method Surveys: Instrument may record excessive noise. Instrument may record excessive noise. Ground may not provide sufficient contrast. Ground may not provide sufficient contrast. Overlapping anomalies may hinder Overlapping anomalies may hinder interpretation. 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 ). 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 At distance, EM field behaves as a plane wave with predictable magnetic and electrical components. 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 Eddy currents are generated when field passes through a buried conductor, creating a secondary magnetic field. 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. Instrument measures anomalous response to the induced current. Surface Location of Anomaly Secondary EM Field Vertical Anomaly H z /H x

10. UTAM 2004 Travis Crosby VLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with 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 (D n ) to convert complicated anomalies into simple peaks. complicated anomalies into simple peaks. Surface Location of Anomaly Secondary EM Field Vertical Anomaly H z /H x

11. UTAM 2004 Travis Crosby VLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with 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 (D n ) to convert complicated anomalies into simple peaks. complicated anomalies into simple peaks. Filtered Data n = - 0.102 D n-3 + 0.059 D n-2 – 0.561 D n-1 + 0 D n + 0.561 D n+1 - 0.059 D n+2 + 0.102 D n+3 + 0.561 D n+1 - 0.059 D n+2 + 0.102 D n+3 Where: D n = H z / H x, as measured by the instrument

12. UTAM 2004 Travis Crosby VLF Theory 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). 42 transmitting stations worldwide (15-30 kHz, 10-20 km ). At distance, EM field behaves as a plane wave with 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 (D n ) to convert complicated anomalies into simple peaks. By increasing filter spacing (…D n-2, D n, D n+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 (D n ) to convert complicated anomalies into simple peaks. By increasing filter spacing (…D n-2, D n, D n+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. VLF used to look for bodies of low electrical resistance, i.e. vertical fractures and ore deposits up to depths of 300 m. vertical fractures and ore deposits up to depths of 300 m.

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

15. UTAM 2004 Travis Crosby Site Location 20 km N

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

17. UTAM 2004 Travis Crosby Seismic Tomogram Ray Path Density

18. UTAM 2004 Travis Crosby Seismic Tomogram VLF Data (Karous & Hjelt Filtered) SENW ?

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

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

21. UTAM 2004 Travis Crosby Water WellStream VLF Data (Karous & Hjelt Filtered) SE NW

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

23. UTAM 2004 Travis Crosby Other VLF Studies: Ore Deposits VLF 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

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

25. UTAM 2004 Travis Crosby Future Work 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.

26. UTAM 2004 Travis Crosby