Applied Geophysics

1. GEOL 492/692 Applied Geophysics 4 credits SP07 call#84362/#87487Instructor Dr. John Louie, LME 217, ph 784-4219 louie@seismo.unr.edu and GEOL 493/693 Mining Exploration Geophysics 4 credits SP07 call #89118/88005Instructor Dr. Gary Oppliger, LME 304, ph 784-7056 oppliger@mines.unr.edu Office hours: 12:00-1 pm M,T,W

2. Electrical Properties of RocksandElectrical Resistivity Methods

3. Resistance • Definition of an OHM • An ohm is a resistance in a conductor that produces a potential differenceof one volt when a current of one ampere is flowing through it. R =

4. Ohm’s Law http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html#c1

5. Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivityis an intrinsic property of all physical materials Units: Ohm-Meters Apparent Resistivity is a resistivity estimate based on a assuming a half-space geometry. Units: Ohm-meters

6. Electrical Resistivity vs Electrical Conductivity Resistance =ρ (ohm-meters) Conductivity = σ = 1/ρ (mho/meters)

7. Calculating Resistance from Resistivity • http://www.cflhd.gov:80/agm/index.htm

8. Factors Influencing Electrical Conductivity in Rocks Porosity(connected/effective - fractures or pores) Pore saturation (% air or gas) Hydrocarbon Fluid Saturation Water salinity (TDS) Clay Content Metallic Sulfide Mineral Content Fluid temperature Rock Matrix intrinsic resistivity

9. Archie’s Law

10. Formation Factor

11. The conductivity of most geological formations can be fit to Archie’s Law

12. Influence of Permeability A rock with a non-conducting matrix must be permeable (connected pores) as well as porous to conduct electricity. Darcy's Law: Ohm's Law: where Despite the similarity between Darcy’s and Ohm’s Laws, electric currents have zero viscosity so even a narrow crack can provide an effective electrical connection between pores that not contribute to hydraulic permeability.

13. Comparison of electric and hydraulic properties. • http://www.cflhd.gov:80/agm/index.htm

14. Metallic Sulfide Mineral Content Electrical resistivity of rocks with various wt % of sulfide.  • http://www.cflhd.gov:80/agm/index.htm

15. Effect of Water Temperature http://appliedgeophysics.berkeley.edu:7057/dc/figures/fig43_7.jpg

16. Conductivity Ranges of Various Materials http://www.cflhd.gov:80/agm/index.htm

17. http://www.cflhd.gov:80/agm/index.htm

18. http://www.cflhd.gov:80/agm/index.htm

19. http://www.cflhd.gov:80/agm/index.htm

20. Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivityis an intrinsic property of all physical materials Units: Ohm-Meters Apparent Resistivity is a resistivity estimate based on a assuming a half-space geometry. Units: Ohm-meters

21. Calculating Resistance from Resistivity • http://www.cflhd.gov:80/agm/index.htm

22. current I C2 C1 P1 P2 Four Electrode Resistivity Measurement on rock sample … are used to avoid electrode contact resistance effects seen in two electrode measurements.

23. Four electrode resistivity arrays http://www.cflhd.gov/agm/images/fig90.jpg

24. The Basic Concept of an Earth Resistivity Measurement http://www.cflhd.gov/agm/images/fig91.jpg

25. Fig. 5.4g Electrode Contact Resistance is typically much higher than the intrinsic earth resistivity

26. Fig. 5.4g Electrode Contact Resistanceis concentrated around each electrode

27. Fig. 5.4g If a standard two electrode resistivity meter were used to measure the earth’s “resistance” we only obtain information on the quality of the electrode contacts – not the earth’s resistivity

28. Pole-Pole Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

29. Pole-Dipole Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

30. Pole-Dipole Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

31. Wenner Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

32. Schlumberger http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

33. Dipole-Dipole Array

34. Fig. 5.4g The electric potential varies as 1/r around a single current electrode on a homogeneous half-space

35. Fig. 5.5g

36. Fig. 5.9g

37. Fig. 5.6g Equal potential voltage surfaces between the electrodes

38. Depth of current flow between two current electrodes

39. Fig. 5.14g

40. Fig. 5.13g (a)

41. Fig. 5.13g (b)

42. Fig. 5.13g (c)

43. Fig. 5.12g (a)

44. Fig. 5.12g (b)

45. Fig. 5.12g (c)

46. Fig. 5.14g

47. Fig. 5.15g (a)

48. Fig. 5.15g (b)

49. Fig. 5.13g (b)

50. Fig. 5.13g (c)