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Applicability of Electrical Methods in Deep Detection and Monitoring of Conductive Lixiviants

By Hanson, Jay C.

Excerpt
ABSTRACT: Various electrical and electromagnetic (EM) geophysical techniques are currently being evaluated by the U.S. Bureau of Mines for their effectiveness in the detection and monitoring of electrically conductive (1 to 5 S/m) liiviant (leach solution) to depths of 600 m, either above or below the water table. These techniques include magnetotellurics (MT), controlled source audiofrequency magnetotellurics (CSAMT), resistivity and focused resistivity, ground-penetrating radar (GPR), frequency-domain electromagnetics (FEM), and time-domain electromagnetics (TEM). Of these techniques, TEM may be the most effective, but CSAMT and focused resistivity also hold promise. Geophysical computer modeling of the borehole TEM technique was conducted based on an idealiid geoelectric section (layered earth) with characteristics based on the Santa Cruz porphyry copper deposit near Casa Grande, AZ. Layer resistivities and thicknesses were obtained from geophysical data and geologic logs available from the site: Modeling of borehole TEM in both preleach and during leach situations was conducted. Simulating the TEM method using computer modeling proved to be encouraging since there were substantial differences between leached and nonleached responses. The modeling does not prove the effectiveness of TEM in the field, but does indicate that detection of deep livviant zones is theoretically possible.

Table of Contents
CONTENTS Abstract , . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Natural source methods 4 Magnetotellurics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Controlled source audiofrequency magnetotellurics 4 Electrical methods 6 Conventional resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Focused resistivity 6 Borehole resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Ground-penetrating radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Electromagnetic methods 11 Frequency-domain electromagnetics 11 Time-domain electromagnetics 13 Time-domain electromagnetic computer modeling 15 Conclusions 30 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 ILLUSTRATIONS 1. Geoelectric section and leach zone of Santa Cruz site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Equipment configuration for CSAMT surveying 5 3. Three common resistivity arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Comparison of conventional and focused current injection as used in resistivity surveying .. . . . . . . . . . . 7 5. Transmitted wavelet from GPR antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Comparison of reflected pulses from nonconductor and conductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Borehole GPR used to monitor lixiviant zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Example of borehole GPR data .... '.' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. EM field behavior near subsurface conductor ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 10. Current waveforms and resultant secondary field observed in TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 11. Principle of operation of commerical borehole induction logger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 12. Equipment used in TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 13. Surface profiling over fluid-filled fracture zone 14 14. Layered-earth model and leach zone 17 15. Plan view of leach zone and borehole locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 16. Central induction configuration 18 17. Offset loop configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 18. TEM response in borehole 1 at depth of 660 m 19 19. TEM response in borehole 1 at depth of 690 m 20 20. TEM response in borehole 1 at depth of 720 m' '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 21. TEM response in borehole 2 at depth of 660 m 22 22. TEM response in borehole 2 at depth of 690 m 23 23. TEM response in borehole 2 at depth of 720 m 24 24. TEM response in borehole 3 at dept

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Title: Applicability of Electrical Methods in Deep Detection and Monitoring of Conductive Lixiviants  
Author: Hanson, Jay C.
Volume:
Language: English
Subject: Health., Medical research, Medical reports
Collection: Medical Library Collection
Subcollection:
Historic
Publication Date:
Publisher: Centers for Disease Control and Prevention
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