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méthode electrique et methode magnétique dans la prospection géophysique.ppt
- 4. Skin Depth
- 8. Lenz's Law • When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.
- 10. Amperes Law
- 11. Frequency Domain: Two approaches to EM Surveying Time Domain:
- 18. Time Domain Primary Field Induced voltage in air and ground
- 19. Time Domain Secondary Magnetic Field
- 20. “Smoke Ring” Eddy Current simultaneously expands and travels downward into the earth Its rate of travel is proportional to earth resistivity
- 27. DIGHEM
- 28. DIGHEM
- 30. DIGHEM-VRES •The DIGHEM-VRES is a 5-coplanar system with frequencies of 380 Hz, 1400 Hz, 6200 Hz, 25,000 Hz, and 101 000 Hz (the highest HEM frequency commercially available!). It is a system designed to provide the optimum conductivity mapping range and resolution for horizontally layered geology. With a full 7.9 m coil separation on all frequencies it is the optimum system for geologic mapping and many environmental and engineering type problems. •
- 32. RESOLVE - The World's Most Advanced HEM System
- 33. Airborne EM systems developed by the Centre are deployed on Skyvan aircraft operated by Fugro Airborne Surveys. A "bird" containing receiver coils being deployed for a survey.
- 34. TEMPEST- Digital Time Domain Electromagnetics • TEMPESTs fixed wing configuration makes it cost effective for large scale reconnaissance projects, where ground and helicopter EM surveys would be prohibitively expensive. The quality detail and accuracy of TEMPEST data reveals geological and structural information that cannot be matched by other airborne techniques.
- 35. TEMPEST Base frequency 25 Hz Transmitter area 186 m2 Transmitter turns 1 Waveform Square Duty cycle 50% Transmitter pulse width 10 ms Transmitter off-time 10 ms Peak current 300 A Peak moment 55,800 Am2 Average moment 27,900 Am2 Sample rate 75 kHz Sample interval 13 microseconds Samples per half-cycle 1500 System bandwidth 25 Hz to 37.5 kHz Flying Height 120 m (subject to safety considerations) EM sensor Towed bird with 3 component dB/dt coils Tx-Rx horizontal separation 122 m (nominal, actual value determined) Tx-Rx vertical separation 37 m (nominal, actual value determined) Stacked data output interval 200 ms (~12 m) Number of output windows 15 Window centre times 15 windows from 13 microseconds to 16.2 milliseconds Magnetometer Stinger-mounted cesium vapour Compensation Fully digital Magnetometer output interval 200 ms (~12m) Magnetometer Resolution 0.001nT Typical noise level 0.1 nT GPS Cycle Rate 1 second
- 36. HeliGEOTEM EM system Specifications Dipole Moment: 230,000Am2 Waveform Frequency: 30 Hz / 4 ms Waveform: Half Sine Receiver: GEOTEM® Multi-Component Fugro Airborne Surveys offers the HeliGEOTEM™ System, incorporating the proven characteristics of the GEOTEM® system.
- 38. GEOTEM • Fugro Airborne Surveys operates three GEOTEM aircraft out of bases in Perth, Australia and Ottawa, Canada. However, these systems have operated on five continents, in just about every climate and geological/geographical environment. Survey targets have ranged from mineral deposits and petroleum reservoirs to groundwater resources. . •Measurement in the receiver bird of three orthogonal components of the secondary EM field (two horizontal directions and one vertical). Recording of both B-field and dB/dT response data. •Lower transmitted waveform frequencies of 12.5, 15, 25, and 30 hertz. •Measurements during the on-time as well as the off-time of the transmitter pulse.
- 39. MEGATEM • MEGATEM System Specifications A time-domain airborne EM system to meet the challenge of exploration areas located at high altitude, in remote areas, or requiring deep target detection.
- 40. MEGATEM & GEOTEM • Time domain systems
- 41. Airborne Electromagnetic: Manufacturer Fugro Airborne Surveys Model MEGATEM Waveform frequency 30 or 25 Hz 90 or 75 Hz Pulse width 4 ms 2 ms Loop 4 turns 4 turns Loop area 406 m2 406 m2 Dipole moment (approx.) 1.08 x 106Am2 0.97 x 10 Am2 Peak transmitter current 665 amperes 595 amperes Model MEGATEMII Waveform frequency 30 or 25 Hz 90 or 75 Hz Pulse width 4 ms 2 ms Loop 4 turns 5 turns Loop area 406 m2 406 m2 Dipole moment (approx.) 2.18 x 106 Am2 1.71 x 106 Am2 Peak transmitter current 1340 amperes 840 amperes 1/2 Waveform sampling rate 384 per pulse 128 per pulse Prestack ("stream") data sampling rate 192 per pulse 64 per pulse Receiver 3-component induction coil sensor Measured response voltage (dB/dt) and B-field Bandwidth base frequency to 10 kHz Stacking rate 4 stacks per second Digital recording all raw data channels Magnetometer:
- 43. MEGATEM Specifications Survey Platform: Aircraft: CASA 212-200 Transmitter: Coil axis: Waveform: Waveform frequencies: Pulse width (delta T): Tx loop dipole moment: vertical alternating half-sine-wave current pulse 25, 30, 75, 90, 125, 150 Hz 1 to 8 milliseconds up to 6.9 x 105 Am2 Receiver: Coil axis: Digitising: Samples per second: Gate Distribution: Off-time: Recorded data: 3 orthogonal (2 horizontal, 1 vertical) 16 bit at up to 80 kHz/channel 4 standard 20 programmable up to 36 milliseconds B-field and dB/dt
- 44. DIGHEM Overburden mapping for pipeline construction
- 45. Overburden mapping for pipeline construction DIGHEM
- 46. • DIGHEM
- 53. Skin-depth
- 58. Impedance:
- 64. 0.5-meter antenna separation (offset)
- 65. Low-freq. antenna, 100 MHz Med.-freq. antenna, 500 MHz
- 66. High-freq. antenna, 1.0 GHz
- 70. Electromagnetic wave velocity in meters per nanosecond
- 72. Cross-hole radar
- 73. Cross-hole radar
- 74. Cross-hole radar
- 75. Diffraction from a Void
- 80. Strong Impedance Mismatch at the Ground Surface