fig. 8.1 left - the nevada seismological...
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Electromagnetic Surveying Methods
Fig. 8.1 left
10-1
300 m30 km
http://ece.uprm.edu/~pol/waves_review.pdf
Skin-depth
Fig. 8.2 top
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Fig. 8.2 bottom
Lenz's Law
• When an emf is generated by a change in magnetic flux according toFaraday's Law, the polarity of the induced emf is such that it produces acurrent whose magnetic field opposes the change which produces it.
Ampere's &Biot-Savart Laws
.
Fig. 8.3g
Amperes Law
Fig. 8.4g
Frequency Domain:
Two approaches to EM Surveying
Time Domain:
Fig. 8.5g
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Fig. 8.6g (a) Fig. 8.7 top
Fig. 8.7 bottom Fig. 8.8g
Fig. 8.10g Fig. 8.11 top
Time Domain Primary Field
Induced voltage in air and ground
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Fig. 8.11 bottom
Time Domain Secondary MagneticField
Fig. 8.12g top
“Smoke Ring” Eddy Current simultaneously expandsand travels downward into the earth
Its rate of travel is proportional to earth resistivity
Fig. 8.12g bottom
Fig. 8.6g (b)
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DIGHEM DIGHEM
Airborne EM Resistivity 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 commerciallyavailable!). It is a system designed to provide the optimum conductivity mappingrange and resolution for horizontally layered geology. With a full 7.9 m coil separationon all frequencies it is the optimum system for geologic mapping and manyenvironmental and engineering type problems.
•
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RESOLVE - The World's Most Advanced HEM System
Airborne EM systems developed by theCentre are deployed on Skyvan aircraftoperated by Fugro Airborne Surveys.A "bird" containing receiver coils beingdeployed for a survey.
TEMPEST- Digital Time DomainElectromagnetics
•• TEMPESTs TEMPESTs fixed wing configuration makes it costfixed wing configuration makes it costeffective for large scale reconnaissance projects, whereeffective for large scale reconnaissance projects, whereground and helicopter EM surveys would be prohibitivelyground and helicopter EM surveys would be prohibitivelyexpensive. The quality detail and accuracy of TEMPESTexpensive. The quality detail and accuracy of TEMPESTdata reveals geological and structural information thatdata reveals geological and structural information thatcannot be matched by other airborne techniques.cannot be matched by other airborne techniques.
TEMPEST- Digital Time Domain Electromagnetics
TEMPEST
1 secondGPS Cycle Rate
0.1 nTTypical noise level
0.001nTMagnetometer Resolution
200 ms (~12m)Magnetometer output interval
Fully digitalCompensation
Stinger-mounted cesium vapourMagnetometer
15 windows from 13 microseconds to 16.2 millisecondsWindow centre times
15Number of output windows
200 ms (~12 m)Stacked data output interval
37 m (nominal, actual value determined)Tx-Rx vertical separation
122 m (nominal, actual value determined)Tx-Rx horizontal separation
Towed bird with 3 component dB/dt coilsEM sensor
120 m (subject to safety considerations)Flying Height
25 Hz to 37.5 kHzSystem bandwidth
1500Samples per half-cycle
13 microsecondsSample interval
75 kHzSample rate
27,900 Am2Average moment
55,800 Am2Peak moment
300 APeak current
10 msTransmitter off-time
10 msTransmitter pulse width
50%Duty cycle
SquareWaveform
1Transmitter turns
186 m2Transmitter area
25 HzBase frequency HeliGEOTEM
EM system SpecificationsDipole Moment: 230,000Am2
Waveform Frequency: 30 Hz / 4 msWaveform: Half Sine
Receiver: GEOTEM® Multi-Component
Fugro Airborne Surveys offersthe HeliGEOTEM™ System,incorporating the provencharacteristics of the GEOTEM®system.
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GEOTEM
• Fugro Airborne Surveys operates three GEOTEM aircraft out of bases in Perth, Australia andOttawa, Canada. However, these systems have operated on five continents, in just about everyclimate and geological/geographical environment. Survey targets have ranged from mineraldeposits and petroleum reservoirs to groundwater resources.
.
•Measurement in the receiver bird ofthree orthogonal components of thesecondary EM field (two horizontaldirections and one vertical).Recording of both B-field and dB/dTresponse data.•Lower transmitted waveformfrequencies of 12.5, 15, 25, and 30hertz.•Measurements during the on-time aswell as the off-time of the transmitterpulse.
MEGATEM
• MEGATEM System SpecificationsA time-domain airborne EM system to meet the challenge ofexploration areas located at high altitude, in remote areas, orrequiring deep target detection.
MEGATEM & GEOTEM
• Time domain systems
de HavillandDHC-7 (Dash 7)C-GJPI (Canadian)Pratt & Whitney PT6A-50 turboprop (4) 1050 HPeach
Manufacturer Type Registration Engines
Survey Aircraft:
Exploranium256 channel, self-stabilising, downward/upwardGR-820Nal (TI), 33.6 litres minimum
Manufacturer Type Model Detector
Spectrometer:
ScintrexCesium vapour optically pumped split beamCS-2 in towed bird10 Hz0.01 nT<0.25 nT
Manufacturer Type Model Sampling rate Sensitivity Noise envelope
Magnetometer:
all raw data channelsDigital recording
4 stacks per secondStacking rate
base frequency to 10 kHzBandwidth
voltage (dB/dt) and B-fieldMeasured response
3-component induction coil sensorReceiver
64 per pulse192 per pulsePrestack ("stream") data sampling rate
128 per pulse384 per pulse1/2 Waveform sampling rate
840 amperes1340 amperes Peak transmitter current
1.71 x 106 Am22.18 x 106 Am2Dipole moment (approx.)
406 m2406 m2Loop area
5 turns4 turnsLoop
2 ms4 msPulse width
90 or 75 Hz30 or 25 HzWaveform frequency
MEGATEMIIModel
595 amperes665 amperesPeak transmitter current
0.97 x 10 Am21.08 x 106Am2Dipole moment (approx.)
406 m2406 m2Loop area
4 turns4 turnsLoop
2 ms4 msPulse width
90 or 75 Hz30 or 25 HzWaveform frequency
MEGATEMModel
Fugro Airborne SurveysManufacturerAirborne Electromagnetic:
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MEGATEM
3 orthogonal (2 horizontal, 1 vertical)16 bit at up to 80 kHz/channel4standard 20 programmableup to 36 millisecondsB-field and dB/dt
Coil axis: Digitising: Samples per second: Gate Distribution: Off-time: Recorded data:
Receiver:
verticalalternating half-sine-wavecurrent pulse25, 30, 75, 90, 125, 150 Hz1 to 8 millisecondsup to 6.9 x 105 Am2
Coil axis: Waveform: Waveform frequencies: Pulse width (delta T): Tx loop dipole moment:
Transmitter:
CASA 212-200 Aircraft:
Survey Platform:
Specifications
DIGHEMOverburden mapping forpipeline construction
Overburden mapping for pipelineconstruction
DIGHEM
• DIGHEM
Fig. 8.26 bottom Fig. 8.26 top
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Fig. 8.26 middle
Magnetotellurics Method
Skin-depth
http://hendrix.uoregon.edu/~dlivelyb/LPE_talk/mt_sources.jpg
http://hendrix.uoregon.edu/~dlivelyb/LPE_talk/mt_sources.jpg http://hendrix.uoregon.edu/~dlivelyb/LPE_talk/mt_xformer.gif
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Impedance:
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Airborne EM Resistivity Airborne EM Resistivity
Fig. 8.13 (a) Fig. 8.13 (b)
Fig. 8.14 top Fig. 8.14 bottom
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Fig. 8.15 top Fig. 8.15 bottom
Fig. 8.16 Fig. 8.17
Fig. 8.18 top Fig. 8.18 bottom
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Fig. 8.19 Fig. 8.20g top
Fig. 8.20g middle Fig. 8.20g bottom
Fig. 8.21g top Fig. 8.21g bottom
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Fig. 8.22 top Fig. 8.22 bottom
Fig. 8.23 Fig. 8.24 (a)
Fig. 8.24 (b) Fig. 8.25