No Slide TitleThe University of British ColumbiaThe University of British Columbia Geophysical Inversion FacilityGeophysical Inversion Facility
http://www.eos.ubc.ca/ubcgif
Doug Oldenburg
slide 2 © UBC-GIF 2007
slide 4 © UBC-GIF 2007
• Time varying transmitter current generates a time varying magnetic field.
• Time varying magnetic field generate an EMF (ie. electric field) in the earth.
• Currents are generated (J= E).
• The currents in the conductor generate magnetic fields.
• We measure these (secondary) fields and the (primary) fields of the transmitter
σ
primary
primary
secondary
secondary
primary
Transmitter (multiple frequencies) Grounded electrodes, or magnetic loops.
slide 7 © UBC-GIF 2007
• 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
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Source types: grounded, inductive, natural
Tx
ground
electrode
Source waveform: – Harmonic (frequency domain)
– Impulsive (time domain)
E
H
dH/dt
gives rise to different survey equipment, data and analysis.
BUT they all want to provide information about conductivity
Data
Waveform
Time
I
• Skin depth
where is resistivity in and f is frequency in Hz.
ρ mΩ
( ) ( ) ( ) tAtAtA ωφωφφω sinsincoscoscos +=+ In-phase
Z Z
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Airborne or ground system
skin depth
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Cross section
Plan view
Primary Currents: uniform earth
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Total currents: with a block
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Currents at 90 meters depth: Example of induction Secondary currents: Total currents – primary currents
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X
Y
Z
(0,0,0)
625 m
Target (x, y, z) = (250, 500, 100) Conductivity = 1.0 S/m
• The “Cominco” model : Loop source, conductive target in a host
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Re(Hy)
Re(Hx)
Re(Hz)
Re(Ey)
Im(Hz)
Im(Hx)
Im(Hy)
Im(Ey)
3D inversion
Inversion methodology is same as for DC resistivity but implementation is harder!
Invert E and H fields at 5 frequencies 0.1 – 1000 Hz
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Recovered iso-surface Recovered cross-section
True iso-surface True cross-section
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Transmitter (multiple frequencies) Grounded electrodes, or magnetic loops.
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2
0
175m
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Isosurface view of the same 3D conductivity model
San Nicolás: CSAMT 1D inversion results
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Iso-surface cutoff 10 Ohm-m
3D CSEM Inversion
3D CSEM
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Waveform
Time
Waveform: Time Domain
– Depth of investigation • (maximum 150-400 m)
– Large areas at low cost • low environmental impact
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Inversion: McMurray Tar Sands
– 2 km by 1.5 km
• dB/dt receivers – mainly z component
• transmitter waveform – 30 Hz sawtooth wave – dI/dt constant over half cycle
• Rx measures dB/dt
31.0
1.0
Observed
Predicted
1000
5
Ωm
Recovered cross section at 450 S Recovered cross section at 1380 W
easting northing-1000 -500-2500 -500
1000
5
Ωm
Resistivity from drilling at 1380 WResistivity from drilling at 450 S
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1D CSEM
3D CSEM
0
200
400
600
800
0
200
400
600
800
0
200
400
600
800
0
200
400
600
800
•3D (frequency and time domain) conductivities show deposit at the same location and same conductivities
slide 46 © UBC-GIF 2007
• detailing structure (3D ground or borehole surveys)
slide 47 © UBC-GIF 2007
• Scott Napier
• Rob Eso
• Nigel Phillips
• Eldad Haber
• Colin Farquharson
Grounded source (CSAMT)
Frequency Domain
Currents at 90 meters depth: Example of induction
Currents at 90 meters depth: Example of induction
Currents at 90 meters depth: Example of induction
Fields depend upon frequency
Geology and Physical Properties: San Nicolas deposit
San Nicolás: CSAMT 1D inversion results
3D CSEM Inversion
3D TEM Setup
Time Domain EM:
San Nicolas UTEM data
Summary:
Acknowledgments