Introduction. Electrical surveying. Resistivity method Induced polarization method (IP) Self-potential (SP) method

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2 Introduction Electrical surveying Resistivity method Induced polarization method (IP) Self-potential (SP) method Higher frequency methods (electromagnetic surveys): Electromagnetic induction methods Ground penetrating radar (GPR) 2

3 Resistivity method The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties (resistivity) of the subsurface 3

4 Application Exploration of bulk mineral deposit (sand, gravel) Exploration of underground water supplies Engineering/construction site investigation Waste sites and pollutant investigations Cavity, karst detection Glaciology, permafrost Geology Archaeological investigations 4

5 Structure of the lecture The next two lectures 1. Resistivity of rocks 2. Equations in resistivity surveying 3. Survey strategies and interpretation 4. Summary of resistivity methods: case histories 5. Conclusions 5

6 1. Resistivity of rocks 6

7 Resistivity and units ρ resistivity in ohm.m (Ωm) σ =1/ ρ conductivity in Siemens per meter (S/m) δ L δr = ρ δ A δ A ρ = δr δ L Resistivity is the physical property which determines the aptitude of this material to be opposed to the passage of the electrical current 7

8 Electronic conductibility The current flows by displacement of electrons. Known as electronic conductibility or metallic because it is a similar conductibility to that of metals. This solid conductibility is really significant only for certain massive mineral deposits. 8

9 Electrolytic conductibility The current is carried by ions. The electrical resistivity of rocks bearing water is controlled mainly by the water which they contain. 9

10 Electrolytic conductibility The resistivity of a rock will depend : on the resistivity of the natural pore water and consequently the quantity of dissolved salts in the electrolyte 1g/liter=1000 ppm on the quantity of electrolyte contained in the unit of rock volume (saturation) on the mode of electrolyte distribution, porosity 10

11 Effect of temperature ρ = t 1+ ρ ( t 18) A rock totally frozen is infinitely resistant and it is impossible to implement resitivity methods (use EM methods) 11

Archie s Law ρ = ρ a φ w m S n ρ resistivity of the rock ρ w resistivity of the fluid (water) Φ porosity S saturation in water a factor which depends of the lithology (varies between 0.

12 Archie s Law ρ = ρ a φ w m S n ρ resistivity of the rock ρ w resistivity of the fluid (water) Φ porosity S saturation in water a factor which depends of the lithology (varies between 0.6 and 2) m cementation factor (depends of the pores shape, of the compaction and varies between 1.3 for unconsolidated sands to 2.2 for cimented limestone n about 2 for majority of the formations with normal porosities containing water between 20 and 100 %. 12

13 Formation factor F ρ = ρ aφ ρ = ρ w w FS m n S n For sand and sandstones: F 0.62/φ 2.15 For well cemented rocks: F 1/φ 2 13

14 Permeability There is no direct relationship between resistivity and permeability. 14

15 Resistivity of rocks and minerals Air, gas or oil: infinite or very high resistivity! Liquid materials from landfills are generally conductive (<10 ohm.m) 15

16 Effect of clay and graphite Clay has a high ionic exchange capacity, therefore the conductivity of the pore fluid largely increases Archie s Law is not valid if clay is present! Graphite, often associated with pyrite, makes the resistivity decrease 16

17 Summary The conductivity of a rock increases if The quantity of water increases The salinity increases (quantity of ions) The quantity of clay increases The temperature increases 17

18 Current flow in the ground 21

20 Two current electrodes 26

21 Potential field between two current electrodes A and B A B 27

22 Potential difference V p1 is the sum of the potential contribution from the current electrodes C 1 and C 2 V P = Iρ / 2πr 1 + ( Iρ / 2πr ) = ( ) 2 Iρ / 2π r1 2 1 r

29 Two potential electrodes 1 1 1 1 1 2 1 1 1 1 2 1 1 2 1 1 2 + = + = = = = NB AN

23 29 Two potential electrodes = + = = = = NB AN MB AM I V NB AN MB AM I V V V NB AN I V MB AM I V MN a N M MN N M π ρ π ρ π ρ π ρ

24 Apparent resistivity In a heterogeneous medium, the measured resistivity is an apparent resistivity, which is a function of the form of the inhomogeneity and of the electrode spacing and surface location. K is named the geometric factor. ρ a = V I MN K 30

25 Geometric factor For a half-space, a general definition for the geometric factor can be written: K = 4π AM AN BM BN AM AN BM BN 31

26 Electrode spreads 32

27 Electrode spreads ρ = 2πa a V I V ρa = πnn ( + 1) a I Wenner array Schlumberger array ρ a = πn( n + 1)( n + 2) a V I dipole-dipole array 33

28 Current penetration 2 1 2z I f = tan π AB z depth AB distance between current electrodes I f fraction of current penetrating below a depth z 34

29 2 1 2z I f = tan π AB 35

30 Principle of reciprocity 36

31 Heterogeneous Earth 37

38 Reflection and transmission ( ) ( ) 2 1 1 2 ρ ρ ρ ρ + = = k V V N M For r

32 38 Reflection and transmission ( ) ( ) ρ ρ ρ ρ + = = k V V N M For r 1 =r 2 =r 3 = + = r k r I V r k I r I V N M π ρ π ρ π ρ

33 39 Modified Snell s Law / tan / tan ρ ρ θ θ = / tan / tan / tan / tan z z x z x z L L L L L L = = = θ θ θ θ / tan / tan / 1/, 1/ / ρ ρ θ θ ρ ρ ρ ρ = = = = z z z z L L L L j V L L j V

34 tan tanθ θ 1 = 2 ρ2 ρ 1 ρ 1 < ρ 2 ρ 1 > ρ 2 40

35 Current distribution 41

36 Current distribution 42

37 Current distribution 43

Anisotropy S n = S + S + + S = 1 2... n i=1 h i ρ = i H ρ l T n = T + T + + Tn = 1 2... i=1 h ρ = i i Hρ t λ = ρt ρ l e.g.

38 Anisotropy S n = S + S + + S = n i=1 h i ρ = i H ρ l T n = T + T + + Tn = i=1 h ρ = i i Hρ t λ = ρt ρ l e.g. λ 1 for alluvium λ >2 for graphitic slates ρ l longitudinal resistivity ρ t transverse resistivity 44 λ coefficient of anisotropy

39 Effect of topography Equipotential: dashed lines 45

40 3. Survey strategies and interpretation 46

41 Resistivity survey equipment 47

42 48

43 Device Current source: batteries in series Voltmeter and ammeter (resistivimeter) Electrodes: metallic stakes current electrodes: stainless steel potential electrodes: stainless steel or impolarizable electrodes Polarization occurs at the contact electrode/ground: this creates an additional potential difference. 49

44 Polarization and skin depth Use an alternating current to avoid polarization Very low frequency (<10 Hz) Skin depth: depth δ at which the amplitude of the field reaches 1/e of its original value a the source δ 503 ρ f 50

45 Contact resistance dr = dl ρ = s dl ρ 2πL 2 R = ρ 1 2π r 1 L L = distance to the centre of the electrode [m] r = radius of the electrode [m] R = resistance [ohm] ρ = resistivity of the surrounding ground [ohm.m] 51

46 To decrease the contact resistance Add electrodes in parallel Increase the current intensity Increase the diameter of the current electrodes Put electrode deeper into the ground Add water (with salt) near the electrodes About 90% of the contact resistance contribution comes from a portion of the ground around the electrode that is equal to 10 times the diameter of the electrode 52

47 Equivalent circuit 53

48 Survey strategies Resistivity mapping, constant separation traversing (CST): used to determine lateral variations of resistivity. The current and potential electrodes are maintained at a fixed separation and moved along profiles Vertical electrical sounding (VES): used in the study of near-horizontal interfaces. The electrode spread is progressively expanded about a central point Resistivity tomography (ERT): is a mix between CST and VES. Also named electrical imaging 54

49 Constant separation traversing (CST) 55

50 Constant separation traversing (CST) 56

51 Constant separation traversing (CST) 57

52 Constant separation traversing (CST) 58

53 Constant separation traversing (CST) 59

54 Interpretation of CST 60

55 61

56 62

57 63

58 64

59 65

60 66

61 Pontis Nappe Siviez- Mischabel Nappe Unstable area Water infiltration 68

62 Small scale resistivity map (archaeology) AB=4m 69

63 Mobile arrays Source: Geocarta, Paris 100 data points/seconde 70 1 data point each 20cm

64 Mobile arrays Source: Geocarta, Paris Vineyards investigations 71

65 Mobile arrays Current injection A B Resistivity measurement (three investigation depths) M2 M1 N1 N2 M3 N3 72 Source: Geocarta, Paris

66 Mapping example with mobile array (spacing 2m) Surface: 155 hectares Apparent resistivity 30 ohm.m 160 ohm.m 73 Source: Geocarta, Paris

67 Mapping example with mobile array (spacing 2m) Surface: 140 hectares Apparent resistivity 15 ohm.m 150 ohm.m 74 Source: Geocarta, Paris

68 Profile spacing 6m Profile spacing 12m Profile spacing 24m Apparent resistivity 10 ohm.m 90 ohm.m 75 Source: Geocarta, Paris

69 Ecartement 0.5m Ecartement 1m Ecartement 2m Apparent resistivity 10 ohm.m 60 ohm.m 76 Source: Geocarta, Paris

70 Vertical electrical sounding (VES) 77

71 Vertical electrical sounding (VES) 78

72 Vertical electrical sounding (VES) 79

73 Vertical electrical sounding (VES) 80

74 81

75 GEOPHYSIC INSTITUT - UNIVERSITY OF LAUSANNE - SWITZERLAND STUDY : SOUNDING: DATE: COORDINATES. OPERATOR: COTE: ρ = K V I K = AM.AN MN π Marks OA en m MN 1m MN 10m MN 60m MN 200m Ven mv I en ma ρ en.m / / / / / / / / / / / / / / / / / /

76 One layer and two layers 82

77 83

78 84

79 Three layers and more 85

80 86

81 Equivalence R = hρ R = h ρ 87

82 Parametric sounding A parametric sounding is a VES carried out on an outcrop or near a borehole to precisely determine the resistivity of a geological formation. A precise determination of resistivity reduce the problem of equivalence 88

83 Suppression 89

84 90

85 91

86 Interpretation of VES 92

87 Interpretation of VES 93

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93 99

A simple 1-D analysis does not often yield results that are satisfactory. C7.1.1 Cavity detection Studies in karst terrain.

94 C7 Application of DC resistivity exploration C7.1 Mapping resistivity structures in 2-D and 3-D Most modern studies using DC resistivity collect data for generating a 2-D or 3-D resistivity model of the Earth. A simple 1-D analysis does not often yield results that are satisfactory. C7.1.1 Cavity detection Studies in karst terrain. Caves show up as very high resistivity zones in a Wenner array profile. Do you think this provide a better way of detecting tunnels than using gravity exploration? Why? In the second example, the DC resistivity survey detected both a known cave and discovered a new (larger) cave that was called the Sting Cave. Figure courtesy of M.H. Loke

Contaminated fluids leak into two layers that are characterized by a low resistivity. Locating and mapping landfills.

95 C7.1.2 Environmental geophysics Conductive plume: (low resistivity) often due to saline water, heavy metals Resistive plume: hydrocarbons, CCl 4 and DNAPLS (dense non-aqueous phase liquids) Example from a landfill near Utrecht in the Netherlands. Contaminated fluids leak into two layers that are characterized by a low resistivity. Locating and mapping landfills. In this example the landfill is higher resistivity than the surroundings. In other cases the landfill will be a low resistivity zone. Why?

96 Note that contaminants leak from surface at the edge of a metal loading dock (shows up as a low resistivity zone). Figure courtesy of M.H. Loke C7.1.3 Hydrocarbon exploration Shallow gas exploration Example from Alberta. Data courtesy of Komex International

C7.1.4 Geothermal exploration A geothermal reservoir is

low resistivity clay cap above the reservoir.

but DC resistivity is not always effective at locating the

Also note that when a geothermal reservoir is depleted, the clay

97 C7.1.4 Geothermal exploration A geothermal reservoir is generally a low resistivity zone, owing to the presence of saline fluids. The hydrothermal circulation and high temperatures often form a low resistivity clay cap above the reservoir. DC resistivity exploration can be used to locate the clay cap, but DC resistivity is not always effective at locating the underlying reservoirs. Also note that when a geothermal reservoir is depleted, the clay cap will remain. Electromagnetic exploration can be used to map geothermal reservoirs, as discussed in Geophysics 424. Bacman geothermal field Tongonan geothermal field Mayon volcano, Philippines

Country 1990 1995 2000 Argentina 0.67 0.67 0 Australia 0 0.17 0.17 China 19.2 28.78 29.17 Costa Rica 0 55 142.5 El Salvador 95 105 161 Ethiopia 0 0 8.

4 Iceland 45 50 170 Indonesia 145 310 590 Italy 545 632 785 Japan 215 413 547 Kenya 45 45 45 Mexico 700 753 755 New Zealand 283 286 437 Nicaragua 35

98 Country Argentina Australia China Costa Rica El Salvador Ethiopia France Guatemala Iceland Indonesia Italy Japan Kenya Mexico New Zealand Nicaragua Philippines Russia (Kamchatka) Thailand Turkey USA Total More details Hot dry rock projects

99 C7.1.5 Geotechnical applications Evaluating the hazards posed by landslides. Figure courtesy of M.H. Loke

100 C7.2 Studying the time variation of subsurface resistivity structures. Monitoring the flow of water in the ground and hydrogeology (TLE, October 1998)

101 Monitoring in-situ vitrification of radio active waste Spies and Ellis, Cross-borehole resistivity tomography of a pilot scale, in-situ vitrification test, Geophysics, 60, , 1995) (a) Pre-melt (b) maximum amount of melting. The melt body has a low resistivity but is surrounded by a high resistivity halo. Why? (c) post-melt. The melt has frozen to glass and has a high resistivity. MJU November 2005

Electrical Surveying (part A)

Electrical Surveying (part A) Dr. Laurent Marescot Course given at the University of Fribourg (2009) Contact: laurent@tomoquest.com www.tomoquest.com 1 Introduction Electrical surveying Resistivity method