Electrical Surveying (part A)

Transcription

1 Electrical Surveying (part A) Dr. Laurent Marescot Course given at the University of Fribourg (2009) Contact: 1

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 Resistivity of Rocks Equations in Resistivity Surveying Survey Strategies and Interpretation Conclusions 5

6 1. Resistivity of Rocks 6

7 Resistivity and Units Resistivity is the physical property which determines the aptitude of this material to be opposed to the passage of the electrical current resistivity in ohm.m (m) =1/ conductivity in Siemens per meter (S/m) 7

8 Electrolytic Conductibility The current is carried by ions. The electrical resistivity of rocks bearing water is controlled mainly by the water which they contain. 8

9 Electrolytic Conductibility The resistivity of a rock will depend : on quality of the electrolyte, i.e., on the resistivity of the natural pore water and consequently the quantity of dissolved salts in the electrolyte 1g/liter=1000 ppm on the mode of electrolyte distribution, porosity on the quantity of electrolyte contained in the unit of rock volume (saturation) on the temperature 9

10 Porosity Total porosity: t Total volume of voids Total volume of rock Effective porosity: e Total volume of communicating voids Total volume of rock 10

11 Saturation Sw Volume of saturated voids Total volume of voids 11

12 Effect of Temperature t t 18 A rock totally frozen is infinitely resistant and it is impossible to implement resitivity methods (use EM methods) 12

13 Archie s Law w a m S n resistivity of the rock w resistivity of the fluid (water) porosity S saturation in water a factor which depends of the lithology m cementation factor (depends of the pores shape, of the compaction) n about 2 for majority of the formations with normal porosities containing water between 20 and 100 %. 13

14 Formation factor F a w w FS m n S n For sand and sandstones: F 0.62/ 2.15 For well cemented rocks: F 1/ 2 14

15 Permeability There is no direct relationship between resistivity and permeability. This table shows also the problem in identifying rocks due to overlapping resistivity values (no contrast) 15

16 Resistivity of Rocks and Minerals Air, gas or oil: infinite or very high resistivity! Liquid materials from landfills are generally conductive (<10 ohm.m) 16

17 Effect of Clay Clay has a high ionic exchange capacity, therefore the resistivity of the pore fluid largely decreases. Archie s Law is not valid if clay is present! 17

18 Summary The resistivity of a rock decreases if The quantity of water increases (more water) The salinity increases (more ions) The quantity of clay increases The temperature increases (less water viscosity) 18

19 2. Equations in Resistivity Surveying 19

20 Current Flow in the Ground r 20

21 Potential from a single electrode r V I 2 r 21

22 Two current electrodes 22

23 Potential Difference V p1 is the sum of the potential contribution from the current electrodes C 1 and C 2 V P I / 2r 1 I / 2r 2 I / 2 r1 2 1 r

24 Potential field between two current electrodes A and B A B Note the fast decrease near A and B (contact resistance) 24

25 Current Distribution 25

26 Current Distribution This has an influence on the depth of investigation! 26

27 Current Distribution 27

28 Heterogeneous Earth 28

29 Effect of Topography 29

30 3. Survey Strategies and Interpretation 30

31 31

32 32 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

33 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. 33

34 Geometric Factor For a half-space and electrodes on the surface, a general definition for the geometric factor K can be written: a VMN I AM MB AN NB 1 a V I MN K 34

35 Device Current source: batteries in series Voltmeter and ammeter (resistivimeter) Electrodes: metallic stakes current electrodes: stainless steel potential electrodes: stainless steel or nonpolarizing electrodes Polarization occurs at the contact electrode/ground: this creates an additional potential difference. 35

36 To Decrease 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 36

37 Origine of Noise Telluric currents Man-made currents Metallic conductors in the ground (short-circuits) Solutions: Use of alternating current Stacking operations Rejection filters (16-20 Hz, Hz) 37

38 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 Electrical Resistivity Tomography (ERT): is a mix between CST and VES. Also named electrical imaging 38

39 Constant Separation Traversing (CST) 39

40 Constant Separation Traversing (CST) 40

41 Constant Separation Traversing (CST) 41

42 Constant Separation Traversing (CST) 42

43 Constant Separation Traversing (CST) Demo during the lecture 43

44 Electrode Spreads 44

45 Electrode spreads 2a a V I V a n( n 1) a I Wenner array Schlumberger array a n( n 1)( n 2) a V I dipole-dipole array 45

46 Penetration Depth 46

47 Interpretation of CST 47

48 48

49 49

50 50

51 51

52 Multiple Twin Probes RM15 resistance meter with multiplexer 52

53 Sanctuary of Poseidon (island of Poros, Greece) Papadopoulos et al., Archeological Prospection, 13,

54 Ancient Royal Site of Rathcroghan, Ireland Barton & Fenwick, Archeological Prospection, 12,

55 Peristyle villa Gallo-romaine Yvonand (Vaud) AB=4m wall fountain? 55

56 Manually Dragged Systems Dabas et al., 2000, Archeological Prospection, 7,

57 Roman city, Wroxester (UK) Dabas et al., 2000, Archeological Prospection, 7,

58 Mobile Arrays with Vehicle Source: Geocarta, Paris 100 data points/seconde 1 data point each 20cm 58

59 59

60 Mobile Arrays Source: Geocarta, Paris Vineyards investigations 60

61 Mobile Arrays Current injection A B Resistivity measurement (three investigation depths) M2 M1 N1 N2 M3 N3 61 Source: Geocarta, Paris

62 Mapping example with mobile array (spacing 2m) Surface: 155 hectares Apparent resistivity 30 ohm.m 160 ohm.m 62 Source: Geocarta, Paris

63 Mapping example with mobile array (spacing 2m) Surface: 140 hectares Apparent resistivity 15 ohm.m 150 ohm.m 63 Source: Geocarta, Paris

64 Profile spacing 6m Profile spacing 12m Profile spacing 24m Apparent resistivity 10 ohm.m 90 ohm.m 64 Source: Geocarta, Paris

65 Ecartement 0.5m Ecartement 1m Ecartement 2m Apparent resistivity 10 ohm.m 60 ohm.m 65 Source: Geocarta, Paris

66 Inaccuracy in Location 66

67 Electrostatic Mobile Arrays 67

68 XVII and XVIII centuries structures (La Rochelle, France) Panissod et al., 1998, Archeological Prospection, 5,

69 Vertical Electrical Sounding (VES) 69

70 Vertical Electrical Sounding (VES) 70

71 Vertical Electrical Sounding (VES) 71

72 Vertical Electrical Sounding (VES) Demo during the lecture 72

73 73

74 One Layer and Two Layers 74

75 75

76 76

77 Three layers and more 77

78 Interpretation Field data Model Calculated data (response of model B) Comparison between data A and C and modification of model B 78

79 Interpretation of VES Demo during the lecture 79

80 Interpretation of VES 80

81 Equivalence h constant h constant 81

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 82

83 Suppression 83

84 84

85 85

86 86

87 87

88 88

89 89