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Applied Geophysics 4 credits SP07 call#84362/#87487 Instructor Dr. John Louie, LME 217, ph 784-4219 and GEOL 493/693 Mining Exploration.

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Presentation on theme: "Applied Geophysics 4 credits SP07 call#84362/#87487 Instructor Dr. John Louie, LME 217, ph 784-4219 and GEOL 493/693 Mining Exploration."— Presentation transcript:

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

2 Electrical Properties of Rocks and Electrical Resistivity Methods

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

4 Ohm’s Law

5 Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivity is 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 Conductivity = σ = 1/ρ (mho/meters) Resistance = ρ (ohm-meters)

7 Calculating Resistance from Resistivity

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

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. ElectricalHydraulic Transverse resistance: T =  h i  i = H  l Transmissivity: T h =  h i k i = K l H Longitudinal conductance: S=  h i  i = H/  l Leakance: L h =  k i /h i = K t /H Average aquifer resistivities:  l,  t Average hydraulic conductivities: K l, K t

14 Electrical resistivity of rocks with various wt % of sulfide. Metallic Sulfide Mineral Content

15 Effect of Water Temperature

16 Conductivity Ranges of Various Materials

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20 Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivity is 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

22 Four Electrode Resistivity Measurement on rock sample … are used to avoid electrode contact resistance effects seen in two electrode measurements. C1 C2 current I P1P2

23 Four electrode resistivity arrays

24 The Basic Concept of an Earth Resistivity Measurement

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

26 Fig. 5.4g Electrode Contact Resistance is 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

29 Pole-Dipole Array

30 Pole-Dipole Array

31 Wenner Array

32 Schlumberger

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)

51 Fig. 5.14g

52 Fig. 5.15g (a)

53 Fig. 5.15g (b)

54 Fig. 5.18g (a)

55 Fig. 5.18g (b)

56 Fig. 5.18g (c)

57 Fig. 5.19g

58 Fig. 5.21g (a)

59 Fig. 5.20g

60 Fig. 5.21g (b)

61 Fig. 5.21g (c)

62 Fig. 5.22g

63 Fig. 5.23g (a)

64 Fig. 5.23g (c)

65 Fig. 5.24g (a)

66 Fig. 5.24g (c)

67 Fig. 5.25g (a)

68 Fig. 5.25g (b)

69 Fig. 5.26g

70 Fig. 5.27g (a)

71 Fig. 5.27g (b)

72 Fig. 5.28g

73 Fig. 5.29g

74 Fig. 5.30g (a,b)

75 Fig. 5.30g (c,d)

76 Fig. 5.31g

77 Fig. 5.33g (a)

78 Fig. 5.33g (b)

79 Fig. 5.33g (c)

80 Fig. 5.39g

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82 One point removed

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84 C:\Shortcut to Geo-CD-ROM.exe

85 Airborne EM Resistivity

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