Electromagnetic Induction

Transcription

1 Electromagnetic Induction Recap the motivation for using geophysics We have problems to solve Slide 1

2 Finding resources Hydrocarbons Minerals Ground Water Geothermal Energy SEG Distinguished Lecture slide 2

3 Natural Hazards Volcano Landslide Tsunami Earthquake SEG Distinguished Lecture slide 3

4 Geotechnical engineering Tunnels and highways In-mine safety Slope stability Subsurface voids SEG Distinguished Lecture slide 4

5 Environmental Water contamination Saline water intrusion UXO UXO detection? SEG Distinguished Lecture slide 5

6 Surface or Underground Storage CO 2 sequestration Aquifer storage and recovery Industrial and radioactive waste SEG Distinguished Lecture slide 6

7 How do we distinguish bodies? Characterize materials by physical properties: Density Magnetic susceptibility Electrical conductivity Chargeability Electrical permittivity Elastic moduli If we know the physical properties then we might be able to answer our question SEG Distinguished Lecture slide 8

8 Electrical conductivity can be diagnostic SEG Distinguished Lecture slide 9

9 Electrical conductivity: units and range SEG Distinguished Lecture slide 10

10 Electromagnetic Induction Everybody at an airport goes through a security scan. How does it work? Slide 11

11 Electromagnetic Induction Everybody at an airport goes through a security scan. How does it work? Slide 12

12 Electrical conductivity: basic equations Faraday s law: Ampere s law: are sources Other important relationships: Solution depends upon the sources and boundary conditions Frequency: Time-domain: SEG Distinguished Lecture slide 13

13 Basic principles of EM induction Time-varying transmitter current generates a time-varying magnetic field transmitter loop primary Time-varying magnetic field generates an EMF (i.e. electric field) in the earth Currents are generated ( ) secondary Currents in the conductor generate magnetic fields (secondary) primary secondary Measure the secondary fields and the primary fields of the transmitter receiver loop primary SEG Distinguished Lecture slide 14

14 EM induction example: small scale Transmit alternating primary magnetic field Induces eddy currents in conducting object Eddy currents produce secondary magnetic field Induces current in receiver coil SEG Distinguished Lecture slide 15

15 EM induction example: larger scale SEG Distinguished Lecture slide 16

16 Electromagnetic induction Survey involves a transmitter and receiver Frequency ~ Hz (GPR is ~ Hz) Tx Rx EOSC Slide 17

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18 EM in phase and quad phase?

19 EM 31 Data from Expo Site EOSC Slide 20

20 Electromagnetics Faraday s Law: A time varying magnetic field generates an electric field Electromagnetic induction E: electric field B: magnetic field Tx Rx Think about electric field as voltage in a circuit. Units of E are Volts/meter

21 Electromagnetics Ohm s Law: Electromagnetic induction Tx Rx J : current density (Amp/m^2) electrical conductivity E J V: voltage (Volts) I: current (Amperes). R: Resistance (Ohms) E: electric field (Volts/meter) J: current density (Amperes/meter^2). : Resistivity (Ohm-meters)

22 EM induction Faraday s law Time varying magnetic fields cause electric fields Electric fields produce currents in a conductor Hence current flows in conductors that are near an oscillating magnetic field EOSC Slide 23

23 Electromagnetics Amperes Law: A current generates a magnetic field Electromagnetic induction Tx Rx H: magnetic field J: current source density

24 EM induction Ampere s law - Currents generate magnetic fields Oscillating current will cause an oscillating magnetic field Current in wire causes a magnetic field to surround it (iron filings). EOSC Slide 25

25 Direction of the Field of a Long Straight Wire Right Hand Rule Grasp the wire in your right hand Point your thumb in the direction of the current Your fingers will curl in the direction of the field

26 EM induction Lens law - The direction of the induced currents will be in such a direction as to oppose any change in magnetic flux. Current in wire causes a magnetic field to surround it (iron filings). EOSC Slide 27

27 Basic principles of EM induction Time-varying transmitter current generates a time-varying magnetic field transmitter loop primary Time-varying magnetic field generates an EMF (i.e. electric field) in the earth Currents are generated secondary Currents in the conductor generate magnetic fields (secondary) primary Measure the secondary fields and the primary fields of the transmitter receiver loop secondary primary SEG Distinguished Lecture slide 28

28 EM induction example: metal detector An alternating primary magnetic field induces eddy currents in a conducting object moved through the detector. The eddy currents in turn produce an alternating secondary magnetic field and this field induces a current in the detector s receiver coil.

29 Important elements Primary field must couple with the target Strength of the induced currents must be big enough to generate signal Need to choose which fields to measure

30 Important elements Primary field must couple with the target Strength of the induced currents must be big enough to generate signal Need to choose which fields to measure

31 Airborne (Inductive source)

32 Basic principles of EM induction Time-varying transmitter current generates a time-varying magnetic field transmitter loop primary Time-varying magnetic field generates an EMF (i.e. electric field) in the earth Currents are generated secondary Currents in the conductor generate magnetic fields (secondary) primary Measure the secondary fields and the primary fields of the transmitter receiver loop secondary primary SEG Distinguished Lecture slide 33

33 Elements of EM Induction Transmitter and primary magnetic field Magnetic flux and coupling Target and induced currents Secondary magnetic fields Receiver Data

34 Generic EM system Tx: transmitter Rx: receiver Tx, Rx, target body are represented as circuits

35 Magnetic fields from a circular current produce a magnetic dipole Dipole moment: m = IA I : Current (amperes) A: area of loop (meter^2) Slide 36

36 Transmitter: Time varying current Tx Magnetic field of a loop of current is like a magnetic dipole Orientation of loop shows direction of primary field Time varying current generates time varying magnetic field

37 Couple with the target Max flux Zero flux

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40 Couple with the target Max flux Zero flux

41 Secondary Magnetic Fields Currents in the target generate magnetic fields If target is modelled by a current circuit then secondary magnetic fields are like those of a magnetic dipole.

42 Receiver Receiver is a coil. A time varying flux generates a voltage. For some instruments Hp is known and subtracted. Then receiver measures only Hs.

43 Understanding the data Simple circuit Sketch induced currents Secondary magnetic fields (Hs) at receiver EOSC Slide 44

44 Instrument, fields, and target Effect of buried objects See GPG Ch3.h. Source field moves with receiver. DATA Graph measurements vs line position FIELD PHYSICS EOSC Slide 45

45 Understanding the data Simple circuit Plotted But data are complex. EOSC Slide 46

46 Frequency domain EM data Transmitter I(t) cos t Receiver A V(t) -A amplitude Acos t Measure amplitude and phase (A, ) In-phase Real Out-of-phase Imaginary

47 Data Primary field: Secondary field: Decompose secondary field In-phase Out of-phase

48 Relative amplitudes of In-Phase and Out-of-phase components. See GPG In-Phase: Out-of-phase

49 Instrument, fields, and target Effect of buried objects See GPG Ch3.h. Source field moves with receiver. DATA Graph measurements vs line position FIELD PHYSICS Slide 51

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51 Relative sizes for a good conductor Slide 53

52 EM Loop Demo Slide 55

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66 Instrument, fields, and target Effect of buried objects See GPG Ch3.h. Source field moves with receiver. DATA Graph measurements vs line position FIELD PHYSICS Slide 71

67 Relative sizes for a good conductor Slide 72

68 Earth is also a conductor Depth of investigation depends upon skin depth source receiver geometry

69 EM waves inside the earth Vertical dimension is not to scale

70 amplitude Frequency Domain EM waves decay when propagating in a conducting earth Skin depth 506 f meter 1 e depth where is resistivity in and f is frequency in Hz. m

71 Sensitivity: horizontal coplanar system (HCP) Sometimes called vertical magnetic dipole system φ is the sensitivity Horizontal axis scaled by s (separation between source and receiver Note: System responds most strongly to conductivity at 0.4s

72 Obtaining apparent conductivity For s << δ Horizontal Dipole Apparent conductivity is Vertical Dipole General Formula Note: Apparent conductivity depends upon height and orientation of the instrument

73 Multilayered Earth Horizontal Dipole Vertical Dipole Note: Integration begins at the plane of the instruments! Sensitivity is carried with the instrument height.

74 Inphase/Quadrature The EM-31 gives two measurements called the In-phase and Quadrature In-phase: (also called real ) Particularly useful for find good conductors (metal pipes, drums) Quadrature: (also called imaginary or (out of phase) Yields apparent conductivity (if s>δ)

75 CH2: Sand and Gravel Quarries Setup: Find sand and gravel quarries. Area has granitic mountains, rolling hills and lakes. Glacial deposits are responsible for potential sand and gravel resources. Some of the area is bog and agricultural land. (Picture) Properties: Bog material is wet and conductive. Gravel deposits are resistive (low conductivity). Gravels are unconsolidated and have a low seismic velocity. Survey: Preliminary EM survey (EM31) Logistically easy and gives an estimate of ground conductivity in the top few meters. Good reconnaissance tool. More detailed follow-up using DC resistivity to get 2D conductivity structure and seismic to find the base of the gravel. Data: EM31data. Also DC and seismic are acquired along selected line profiles.

76 Apparent Conductivities from EM31

77 CH2: Sand and Gravel Quarries Processing: EM31 data is converted to ground conductivity. (Picture). DC resistivity data is inverted to get a 2D cross section. Seismic data are inverted to provide location of refracting interfaces. Interpretation: Areas of low conductivity are identified from the EM survey. The inversion of DC and seismic data outline a gravel lens along one of the transects. Gravel lens is 5-8 meters in thickness and meters in length. Synthesis: Seems successful. Have found gravel lenses and results have helped assess the potential tonnage across the site.

78 Case History Project: Expo Site Integrated site investigation of contaminated waste site in Vancouver Combines all the geophysical methods covered in EOSC 350: Magnetics, GPR, Seismic refraction and EM induction

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81 EM31

82 EM in phase and quad phase?

83 Other Frequency Domain Systems EM34 Different frequencies 10m at 6.4kHz 20m at 1.6kHz 40m at 0.4kHz Operate at different orientations Mapping deeper groundwater contaminant plumes, and groundwater exploration. Very sensitive to vertical geological anomalies. spacing.

84 EM34

85 GEM 3 Frequency range: 30 Hz to 24 khz Single or multiple, frequencies

86 Airborne Surveys: RESOLVE Horizontal coplanar coils: 400Hz, 1600Hz, 6400Hz, 25kHz, 100kHz Coaxial coils at 1600Hz

87 DIGHEM: RESOLVE

88 Solar Storm and Earth s field EOSC Slide 94

89 Auroral electrojet expansion: Currents EOSC Slide 95

90 Auroral Electrojets and Aurora EOSC Slide 96

91 Storm 1989 EOSC Slide 97

92 Storm 1989 EOSC Slide 98

93 Quebec power grid Closed loops of grid 500km x 300 km EOSC Slide 99

94 Some calculations B changes 400 nt in 2 minutes One grid loop in Quebec is 300 x 500 km Voltage induced: 500 Volts Equivalent circuit: V=IR Area of wire 4 cm^2 Length of loop: 1600km Conductivity of copper: ~10^7 S/m Induced current = 1 Amp EOSC Slide 100

95 Next EM quiz TBL EM-31 Geophysical Applications to Solid Waste Analysis Hutchinson