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. 8.16 Fig. 8.17


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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. 8.23 Fig. 8.24 (a)

Fig. 8.24 (b) Fig. 8.25

Fig. 8.1 left - The Nevada Seismological...

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