What have we learned from the Case Histories

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Basil Miller
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1 What have we learned from the Case Histories Earth materials have a range of physical properties. Application of geophysics is carried out in a 7 Step process. Physical property of the target must be different from host material Knowledge of a single physical property does not uniquely identify a material. Interpretation was aided by using multiple surveys. Examples: Gravel quaries: conductivity, elastic parameters Karst investigations: density, conductivity Mining: conductivity, chargeability slide 1

2 MAGNETIC SURVEYING slide 2

3 Principles for using Geophysics Geoscientific / Engineering Question What is the problem to be solved? Formulate problem in terms of physical properties Physical Properties Interpret geophysical results Geophysics: Survey Data Processing Inversion slide 3

4 Magnetic Survey slide 4

5 Some motivating examples for the use of magnetic susceptibility Geologic mapping Ore deposits Geotechnical problems Unexploded ordnance Buried foundations Archeology slide 5

6 Need for Magnetics: Geologic mapping Geology map Geology contacts can be inferred from mag maps. N 30 km Magnetic map N nt slide 6

7 Need for magnetics: Mineral exploration slide 7

Northing (m) Need for Magnetics: UXO 50 nt 40 30 20 10

8 Northing (m) Need for Magnetics: UXO 50 nt Easting (m) slide 8

9 Magnetic materials and Magnetization Earth materials are built up of minerals that behave as small magnets slide 9

10 Bar Magnet North pole and South pole (dipole) Dipole moment m is related to the strength of the magnet Field lines extend from the north pole to the south poles Slide 10 slide 10

11 Magnetic materials and Magnetization Earth materials are built up of minerals that behave as small magnets Strength of each magnet is given by the magnetic moment Magnetization Units: A/m dipole moment per unit volume When the particles are randomly distributed M can be zero slide 11

12 Small magnets in presence of large ones Opposite poles attract Like poles repel Small magnets align with fields of a larger magnet slide 12

13 Magnetic Susceptibility: Ability for a rock to become magnetized when an external magnetic field is applied. Units: dimensionless Magnetization: Units: Dipole moment per unit volume slide 13

14 Magnetic Susceptibility of Rocks slide 14

15 Magnetic Survey slide 15

16 Earth s Magnetic field Geomagnetic dynamo. Complicated inside the earth near the core. Outside the earth it looks like a magnetic field due to a dipole. slide 16

17 Magnetics Earth s field slide 17

18 Magnetic fields and definitions slide 18

19 Magnetics Earth s field Link to GPG How is the field described anywhere? X, Y, Z Inclination, Declination, Magnitude Compass? Inclination? Declination? Earth s field strength vs. anomalies. slide 19

20 Earth s Magnetic Field slide 20

21 Earth s magnetic field: Strength B Inclination I Declination D Total Field Strength (nt) Declination (degrees) Inclination (degrees) B max = 70,000 nt H max = 55.7A/m B min = 20,000 nt H min = 15.9A/m slide 21

22 Magnetic Survey slide 22

Basics of Magnetics Surveying Earth s magnetic field is the source: Materials in the earth become magnetized (dipole moment per unit volume) Magnetized rocks

23 Basics of Magnetics Surveying Earth s magnetic field is the source: Materials in the earth become magnetized (dipole moment per unit volume) Magnetized rocks create anomalous (compact object looks like a magnetic dipole) The anomalous field is a vector. To view we project it onto specific directions (x, y, z) slide 23

24 Magnetic Applet slide 24

25 Simulating the field due to prisms Interactive and live click here slide 25

26 Learning from the applet Locating the prism at the pole and equator Plotting Bx,By,Bz fields (sign convention: positive when pointing in direction of axis) Map data Profile Profile over a magnetic dipole. Effects of depth of burial (half width) Data sampling slide 26

27 Survey Acquisition (with applet) Must sample data sufficiently often to capture the anomaly. Want 3-5 points in a halfwidth Width of the signal increases with depth of burial. Ground surveys generally choose line spacing and station spacing to be equal. slide 27

28 Magnetic Survey slide 28

29 Magnetic Sensors to acquire data slide 29

30 In-class magnetic surveying: Detection Magnetic material underneath the table tops. NO PEEKING!!!! Use magnetic compass on smartphones (download apps if not already done) Carry out a survey to detect the magnetic bodies. Flag them with tape. slide 30

31 If the magnet was a UXO Magnet: diameter: 6mm; UXO 20cm in diameter If our table: ~ 1m X 3m how large would a survey area equivalent to the table be? Length scale ratio ~1:33 Survey area ~ 10m x 30m slide 31

32 If the magnet was a UXO 20cm in diameter how large would a survey area equivalent to the table be? slide 32

33 If the magnet was a UXO 20cm in diameter slide 33

34 Team Exercise: Searching for $1B Cu deposit Magnet: diameter: 6mm; height=2mm vol 5.65e-8 m^3 For table size: ~ 1m X 3m Price: $310 USD/lb; $684 /kg $1 billion. I need:??? Kg Density of copper 8960 kg/m^3 Need??? m^3 of copper Assume 0.3% Cu by volume:??? m^3 Scale length of deposit: 1 ~???? Survey area:?? km x?? km slide 34

35 Student exercise Magnet: diameter: 6mm; height=2mm vol 5.65e-8 m^3 Table: 1m x 3m Find a copper resource worth $1B Price: $3.10 USD/lb; $6.84 /kg (Note decimal) $1 billion 1,461,305 kg Density of copper 8960 kg/m^3 Need 163 m^3 of copper Assume 0.3% Cu by volume: 54,330 m^3 (cube: 38 m on side) Scale length: Survey area: 10 km x 30 km slide 35

36 If the magnet was a billion dollar copper deposit slide 36

37 Readings GPG Magnetics Lab #2 (see course website) slide 37

38 The composite field B 0 Composite field: B A B is a vector: B = B 0 + B A Total field: B = {B x, B y, B z } B = B 0 + B A slide 38

39 The Anomalous field B 0 Measured field B = B0 + B A B A Link to GPG The total field anomaly: B = B - B 0 If B A << B 0 then That is, total field anomaly B is the projection of the anomalous field onto the direction of the inducing field. slide 39

40 Why is the total field anomaly: Vector Diagram B B A B - B A B 0 B A cos slide 40

41 Learning from the applet: Locating the prism at the pole and equator Plotting total field anomaly (sign convention: positive in direction of Earths field) Map data Profile Profile over a magnetic dipole. Effects of depth of burial (half width) Data sampling slide 41

42 Remanent Magnetization Magnetic material cooling through Curie temperature (~550 C) acquires a magnetic field in the direction of the earth s field. Final magnetization sum of induced and remanent magnetization: slide 42

43 Remanent Magnetism at different scales Small scale: UXO, rebar, drums Large scale: geologic units. Sea floor spreading slide 43

44 Magnetic map of North America slide 44

45 Notebook App: A single prism with uniform magnetization Arbitrary dimensions Arbitrary orientation of the body Arbitrary strength and orientation of remanent magnetization. Can model cubes, rods, sheets, dykes slide 45

46 Find by digging slide 46

47 Work as a team slide 47

48 But geophysics may be more efficient and easier! slide 48

49 But geophysics may be more efficient and easier! slide 49

50 Data Processing Removing time variations of the Earth s field (necessity for a base station) Removing a regional trend Slide 50 slide 50

51 Magnetics Earth s field Slide 51 slide 51

52 Time Variations of the Earth s Field External sources Solar wind (micro-seconds, minutes, hours) Solar storms (hours, days, months) Man made sources Power lines (50/60 Hz plus harmonics) DC Motors, generators All electronic equipment Internal sources Fluctuations in core (days millions of years) slide 52

53 Time variations due to a magnetic storm Adapted from NRC slide 53

54 Field procedures Earth s magnetic field varies as a function of time Necessary to record the magnetic field at a fixed location to determine the Earth s field slide 54

55 Base station correction Set out another magnetometer (base station) Assume time variations at the base stations are the same as at the observation location Synchronize the times Perform a correction by subtraction slide 55

56 Anomalous field We measure the field at the Earth s surface but we are interested in the anomalous features Estimate a region and subtract Regional removal (along a line) Regional removal (for a plane of data) slide 56

57 Removing a background field Background field is generally anything that is smoothly varying over your region of interest and is much larger than the footprint of the body you are interested in. Deciding what is background is a subjective decision. slide 57

Some geological structural information is shown as black lines.

58 Regional field removal for map data Airborne magnetic data gathered over a 25 square km area around a mineral deposit in central British Columbia. Some geological structural information is shown as black lines. The monzonite stock in the centre of the boxed region is a magnetic body, but this is not very clear in the data before removing the regional trend. slide 58

59 Examples of magnetic anomaly data These are the data that we want to use to extract information about our problem. slide 59

60 Status Slide Now have all of the material needed for the lab slide 60

61 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (monopole, dipole, dyke) Interpret as complex body (inversion) slide 61

62 Inferences from data images Magnetic signatures are complicated because the same object provides different data depending where it on the earth (ie depends upon strength and direction of the inducing field. Applet example This ambiguity can be reduced by processing the data by Reducing to the Pole slide 62

63 Reduction to Pole Same object buried at different locations on the earth yields different total field anomalies. Inclination=0 Inclination=45 Inclination=90 slide 63

64 Reduction to Pole Filter the data to emulate the response as if the survey was taken at the pole. (Earth s field is vertical; measure vertical component of the anomalous field) 2D Fourier Filter This simplifies interpretation. Causative body lies beneath the peak. slide 64

65 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (dipole, dyke, monopole) Interpret as complex body (inversion) slide 65

66 Northing (m) Remanent Magnetization: A complicating factor.. These look like dipoles but directions are not in consistent direction nt Easting (m) slide 66

67 Parameter Estimation: UXO as a Dipole We model the response as a dipole (equivalent to a bar-magnet): Position Depth Orientation Size 6 parameters slide 67

68 Parameter extraction Six parameters ( [x,y,z, strength and orientation of dipole]) Inversion or parameter extraction is used to estimate the parameters of an underlying model Model Parameters: m Forward Operator d =g [m] Sensor data: d m=g -1 [d ] Inverse Operator slide 68

69 Parameter extraction DATA PREDICTED DATA Residuals Easting = m Northing = 0.16 m Depth = 0.26 m Moment = Am 2 Azimuth = 37 o Dip = 28.8 o Fit quality = 0.95 Parameters of interest slide 69

70 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (dipole, dyke, monopole) Interpret as complex body (inversion) slide 70

71 Modelling objects with uniform magnetization. Simplification by working with charges. slide 71

72 Magnetic Monopole (GPG-Basic Principles) Charges generate a magnetic field B Q -Q slide 72

73 Magnetic Dipole In nature: magnetic poles always appear in pairs with a positive and negative pole yielding a dipole. Magnetic moment Q -Q L Magnetic field of dipole m slide 73

74 Beyond dipoles real targets When is a buried feature like a simple dipole? When it s diameter is much less than depth to it s centre. GPG Magnetics: Basic Principles Fields from some buried bodies, (cylinders, dykes) can be estimated by using charge concepts. Charge strength, τ = M nˆ Slide 74 slide 74

75 A simple model for a vertical pipe Fields from some buried bodies, (cylinders, dykes) can be estimated by using charge concepts. (Magnetization) Q - slide 75

76 A simple model for a vertical pipe Fields from some buried bodies, (cylinders, dykes) can be estimated by using charge concepts. A vertical pipe has anomaly like a single pole.(tbl 2) Q - slide 76

77 Magnetic fields from an extended pipe For the lab For the TBL Connect with GPG Connection with amplitude of anomaly Width of anomaly and depth of burial slide 77

78 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (dipole, dyke, monopole) Interpret as complex body (inversion) slide 78

79 Superposition for Magnetics Data (GPG d5) Magnetic field for one prism Prism own dipole field Magnetic field for 5 prisms Superposition slide 79

80 Earth can be complicated A complicated earth model Magnetic data for a complicated earth model. To interpret field data from a complicated earth we need to have formal inversion procedures that recognize non-uniqueness. Think about finding the causative magnetization of each prism. slide 80

81 Example: Raglan aeromagnetic data Select a region of interest. Keep data set size within reason. Digitized the Earth up to 10 6 cells. Slide 81 slide 81

82 Inversion: Finding an earth model that generated the data? Inversion Divide the earth into many cells of constant but unknown susceptibility Solve the large inverse problem to estimate the value of each cell Slide 82 slide 82

Compare predictions to these measurements.

83 Misfit: comparing predictions to measurements Once a model is estimated Calculate data caused by that model. Compare predictions to these measurements. Is comparison within errors? YES Compare NO Modify model and try again Proceed to check for acceptibility Slide 83 slide 83

84 Raglan aeromagnetic data Estimate a model for the distribution of subsurface magnetic material. Model will be smooth, and close to pre-defined reference. Display result as cross sections and as isosurfaces.?? Are sills connected at depth? Inversion result supports this idea. It helped justify a 1050m drill hole. 330m of peridotite intersected at 650m 10m were ore grade. Image shows all material which has k > 0.04 SI. Slide 84 slide 84

85 APPLICATIONS slide 85

86 Principles for using Geophysics Geoscientific / Engineering Question What is the problem to be solved? Formulate problem in terms of physical properties Physical Properties Interpret geophysical results Geophysics: Survey Data Processing Inversion slide 86

87 Framework for Applied Geophysics: 7 Steps slide 87

88 Some motivating examples for the use of magnetic susceptibility Geologic mapping Ore deposits Geotechnical problems Unexploded ordnance Buried foundations Archeology Slide 88 slide 88

89 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (monopole, dipole, dyke) Interpret as complex body (inversion) slide 89

90 Magnetics for Geologic Mapping Geology unit A? Geology unit B? When rock outcrops are sparse we must rely on other available techniques to denote changes in geologic units and/or structures. slide 90

91 Magnetic surveys Very common mineral exploration tool to aid with geologic mapping. One of the cheapest geophysical surveys to execute on land or with an aircraft. Used on regional and deposit scale to identify geologic boundaries and structures (such as faults or folds). Many mineral deposits are found on geology boundaries or faults so magnetic maps are useful for target prospective areas. slide 91

92 Geologic boundaries Geology map Geology contacts can be inferred from mag maps. N 30 km Magnetic map N nt slide 92

93 Identifying regional scale faults Geology map Magnetic map N N 55 km Mag map highlights faults within known gold bearing plutonic body (red) in west-central Yukon. slide 93

94 Example - Iron ore deposit Magnetite rich rock shows as mag high anomaly slide 94

95 Other processing Vertical/horizontal derivative maps Mount Dore, Australia Slide 95 slide 95

7,580,000 7,620,000 7,660,00 Northing ( m )

96 7,580,000 7,620,000 7,660,00 Northing ( m ) Mount Dore, Queensland, Australia Landsat Image Topography Source: Queensland Government, Austral Surface Geology 410, , , , , , , ,000 Easting (m) Easting (m) Easting (m) slide 96

97 7,580,000 7,620,000 7,660,00 Northing ( m ) Magnetic data Total Magnetic Anomaly (nt) 1th Vertical Derivative Source: Queensland Government, Austral 410, , , , , , , ,000 Easting (m) Easting (m) Easting (m) slide 97

98 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (dipole, dyke, monopole) Interpret as complex body (inversion) slide 98

99 The Munitions Problem There are over 3,000 sites suspected of contamination with military munitions They comprise 10s of millions of acres The current annual cleanup effort is on the order of 1% of the projected total cost To make real progress on this problem, we need a better approach slide 99

100 The Munitions Problem slide 100

101 The Munitions Problem slide 101

102 The Munitions Problem slide 102

103 The Munitions Problem slide 103

104 The Munitions Problem slide 104

105 Environmental: How do we find UXO?? slide 105

106 Northing (m) Environmental : Magnetic Survey 50 nt Ferrex Easting (m) -200 TM4 slide 106

107 Northing (m) Northing (m) Northing (m) Northing (m) Examples of good data High SNR 210 nt Medium SNR 22 nt -140 nt -21 nt Easting (m) Easting (m) Medium SNR 8.5 nt Low SNR 4.3 nt -7.5 nt -2.7 nt Easting (m) Easting (m) slide 107

108 Northing (m) What is the formula for a magnetic dipole These look like dipoles but how do we analyze the signal? 50 nt Easting (m) EOSC Slide 108 slide 108

109 The model We model the response of buried items by a dipole (equivalent to a bar-magnet): Position Depth Orientation Size 6 parameters Position DEPTH slide 109

110 Parameter extraction Need six parameters (location, strength and orientation) Inversion or parameter extraction is used to estimate the parameters of an underlying model that encapsulates some useful attributes of the buried object Model Parameters: m Forward Operator d =g [m] Sensor data: d m=g -1 [d ] Inverse Operator slide 110

Parameter extraction DATA PREDICTED DATA Residuals Easting = -0.13 m Northing = 0.16 m Depth = 0.

111 Parameter extraction DATA PREDICTED DATA Residuals Easting = m Northing = 0.16 m Depth = 0.26 m Moment = Am 2 Azimuth = 37 o Dip = 28.8 o Fit quality = 0.95 Parameters of interest slide 111

112 Magnetics Magnetics fundamentals Sensor systems Data examples and demo Parameter extraction Concepts Real-world examples Classification Using the parameters to make discrimination decisions EM surveys will also be needed. slide 112

113 Possible routes to extracting information Inference from data images. Interpret with a simple body of uniform magnetization (dipole, dyke, monopole) Interpret as complex body (inversion) slide 113

114 Ekati Property, Northwest Territories slide 114

115 Ekati Property, Northwest Territories slide 115

116 Ekati Property, Northwest Territories slide 116

117 Ekati Property, Northwest Territories slide 117

118 Misery Pipe Property owned by BHP Billiton Pre-stripping under way Production expected to start in 2016 Borrowed from Nowicki et al. (2004) slide 118

119 Misery Pipe Local anomaly showing reversely magnetized body Removal of the regional field to enhance the target slide 119

120 Misery Pipe Magnetic Inversion Geology from drilling Inverted model Borrowed from Nowicki et al. (2004) slide 121

121 Summary: Magnetics interpretation 1. Qualitative: Correlate magnetic patterns to geology 2. Quantitative interpretation Determine shapes, volumes, contacts, materials 3. Direct interpretation of patterns 4. Forward modelling 1. Guess geology 2. Calculate result - Compare to data 3. Iterate. 5. Inversion: Given data, estimate possible configurations of susceptible material that could cause those data. Slide 122 slide 122

122 The End of Magnetics Slide 123 slide 123

September 16, 2010 Magnetic surveying

September 16, 2010 Magnetic surveying After today, you will be able to Sketch anomalies over objects at any location, and explain how you derived the pattern. Explain the relation between dipoles and real