magnetotelluric acquisition & processing, with examples ... · magnetotelluric acquisition...
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MT acquisition & processing
MagnetotelluricMagnetotelluric acquisition & processing, acquisition & processing, with examples from the Gawler with examples from the Gawler CratonCraton, , CurnamonaCurnamona Province and Province and CurnamonaCurnamona--
Gawler Link transects in South AustraliaGawler Link transects in South Australia
Peter R. MilliganGeoscience Australia
Stephan ThielThe University of Adelaide
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MT acquisition & processing
Acknowledgements
• Australian Government Onshore Energy Security Program for funding
• AuScope (NCRIS) for access to Magnetotelluric equipment
• Seismic Acquisition & Processing Project of Geoscience Australia
• Geodynamic Framework Project of Geoscience Australia
• School of Earth & Environmental Sciences, University of Adelaide
• Primary Industries & Resources South Australia (PIRSA) Minerals
• Quantec Geoscience (via Terrex Seismic)
Graham Heinson, Goran Boren, Jonathon Ross, Hamish Adam The University of Adelaide
Jenny Maher, Jingming Duan, Tanya Fomin, Steven Curnow Geoscience Australia
Tania Dhu, Emily Craven Primary Industries and Resources South Australia
Ted Lilley Australian National University
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MT acquisition & processing
Contents
1. Introduction2. MT Theory3. Field data acquisition4. Processing5. Analysis6. Modelling7. Conclusions
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MT acquisition & processing
1. Introduction
The Magnetotelluric Method (MT) records time variations of Earth’s magnetic and electric fields over a wide frequency range at arrays of ground sites to measure Earth electrical resistivity (conductivity) structure with depth (near-surface to core/mantle boundary)
• Magnetic field variations are the source signals
• Electric field variations are the response signals
• Ratio of Electric to Magnetic provides resistivity measurement
• Complimentary Earth physical property measurement to deep seismic imaging
• Excellent at mapping sedimentary basins
Three collaborative projects along seismic transects in South Australia
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MT acquisition & processing
1. IntroductionMT in context with EM techniques Frequency (Hz)
Ground penetrating
radar
EM InductionMagnetotellurics
Diurnals, ocean circulation,
secular variations
EM Induction Techniques
Depth of Investigation
10-6 (11.6days)109 106 103 100 (1s) 10-3 (17min)
Near surface (< 100 m) Environmental Studies
Upper Crust Exploration and Environment
Mid-Lower Crust
Upper Mantle Mantle Transition
Zone
Core-Mantle
Boundary
Dea
d ba
nd
Source fields
Transmitter Transmitter Lightning Magnetic storms Solar and ocean tides, core-mantle tides
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MT acquisition & processing
1. IntroductionGA collaborative SA projects
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MT acquisition & processing
1. IntroductionGA collaborative SA projects
Survey specifications
Gawler ― AuScope (University of Adelaide):
Long-period (LP) (3-component Fluxgate, sampling at 10 Hz, bandwidth .1 to .0001 Hz)
Broadband (BB) (2-component Lemi induction coils, sampling at 250 Hz, bandwidth 100 to .001 Hz)
12 Long-period sites (20 km spacing) & 24 Broadband sites (10 km spacing) in 2008
16 Broadband sites in 2009, with some repeat and some infill to 5 km
50 m dipole separation
Curnamona-Gawler Link ― AuScope (PIRSA):
15 LP & BB sites, 10 km spacing, 50 m dipole separation in 2009
Curnamona ― Quantec Geoscience (through Terrex Seismic) (Geoscience Australia): Quantec REF-TEK system
3-component BB, 25 BB sites, 10 km spacing, 100 m dipole separation, bandwidth 250 to .001 Hz,
in 2008 - 2009
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MT acquisition & processing
2. MT Theory
• Passive surface measurement of the Earth’s natural electric (E) and magnetic (H) fields
• Assume planar horizontal magnetic source field (reasonable assumption in mid-latitudes, far from external source regions)
• This is a diffusive process, the physics based on Maxwell’s equations of electromagnetic induction
• Measure time changes of E and H at arrays of sites
• Frequency range 10 KHz to .0001 Hz (0.0001 s to 10000 s)
• Ratio of E / H used to derive resistivity structure of sub-surface
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MT acquisition & processing
2. MT Theory
0.1 1 10 100 1000 10000 100000Resistivity Ohm.m
10 1 .1 .01 .001 .0001 .00001Conductivity mho.m-1
Igneous Rocks
Metamorphic Rocks
Dry Sedimentary Rocks
Wet Sedimentary Rocks
Molten Rock
Saline Water + Heat
Graphite and Sulphides
Why measure resistivity?
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MT acquisition & processing
2. MT Theory
Depth of Investigation – Skin Depth
3 concepts:
1. Low frequencies penetrate deeper than high frequencies
2. High frequencies image the near-surface
3. Signals penetrate further in resistive material
Depth
Conductive Resistive
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MT acquisition & processing
2. MT TheorySource fields
High frequencies >1 Hz from Spherics, generated by world-wide thunderstorms
Low frequencies <1 Hz from Earth’s magnetic field variations
- solar wind interactions
- variations with periods from seconds, minutes, hours, days to yearly cycles (eg. micropulsations, bays, storms)
Period (s)
Power
Dead Band
Dead Band
101 to 10-1 Hz
0.1 to 10 s
Little energy
Skin depths 1.5 to 15 km, upper-middle crust
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MT acquisition & processing
2. MT TheoryImpedance tensor
Measure two orthogonal components of electric field and two orthogonal components of magnetic field (usually north, x and east, y).
Apparent resistivity is determined from their ratios.
The magnetotelluric impedance tensor is defined as:
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛
y
x
y
x
yyyx
xyxx
EE
BB
ZZZZ The impedance tensor
transfer function values Z are complex values of frequency.
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MT acquisition & processing
2. MT TheoryDimensionality
TE & TM modes
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MT acquisition & processing
2. MT Theory
Geomagnetic Depth Sounding – Parkinson Arrows
• Geomagnetic depth sounding relates vertical magnetic field variations to horizontal magnetic field variations
• Ratios of Z to H are complex functions of period
• Ratio is always zero for a 1D Earth, so ratio senses 2D & 3D structure
• Parkinson Arrows “point” to subsurface electric currents – provide lateral information
H
ZInduced electric current
Vertical response magnetic field
Horizontal source magnetic field
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MT acquisition & processing
3. Field data acquisition
Equipment layout
15-100 m
15-100 mData logger
Magnetic sensor
North electrode
Central electrode
7 Days
Quiet Days
Magnetic Storm
East electrode
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MT acquisition & processing
3. Field data acquisitionInduction coils for broadband acquisition - AuScope
1.2 m
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MT acquisition & processing
3. Field data acquisitionFluxgate sensors for long-period acquisition - AuScope
Bartington sensor
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MT acquisition & processing
3. Field data acquisitionCopper/copper sulphate electrodes - AuScope
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MT acquisition & processing
3. Field data acquisition
Data acquisition systems
Quantec REF-TEK system (Curnamona transect)
AuScope Earth Data Logger system (Gawler & Curnamona-Gawler Link transects)
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MT acquisition & processing
3. Field data acquisition
Field locations
Curnamona-Gawler Link transect
Gawler transect
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MT acquisition & processing
3. Field data acquisitionExample time-series as recorded in the field
Magnetic Z
Electric N
Magnetic E
Electric E
Magnetic N
nT
Gawler transect 21 August 2008
uV/m
Hours
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MT acquisition & processing
3. Field data acquisitionExample time-series as recorded in the field
Magnetic Z
Electric N
Magnetic E
Electric E
Magnetic N
Gawler transect 21 August 2008
Record width 30 minutes
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MT acquisition & processing
4. ProcessingProcessing steps
• Read data
• Calibrate
• Rotate to geographic coords
• Edit
• Calculate spectra & impedance tensor components
• Store in EDI files
• Calculate apparent resistivity (δ) & phase (θ) from impedance tensor
• Display δ and θ graphically and as pseudosections
• Display Parkinson arrows
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MT acquisition & processing
4. Processing
Calculation of impedance tensor values (AuScope processing)
• Time series data are converted to the frequency domain
• Program BIRRP5 of Alan Chave is publicly available for non-commercial use (Bounded Influence Remote Reference Processing)
• Remote referencing with other sites (or observatory data) to remove uncorrelated noise
• For each frequency, the impedance equation is solved for Z with noise in E and B
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛
y
x
y
x
yyyx
xyxx
EE
BB
ZZZZ
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MT acquisition & processing
4. ProcessingCalculation of Apparent Resistivity & Phase from tensor elements
3 2 1 0 -1 -2 -3-2
-1
0
1
2
3
4
Apparent Resistivity bcf_002LO
G R
HO
(OH
M-M
)
LOG Frequency (Hz)RhoXY RhoYX
3 2 1 0 -1 -2 -3
-180
-135
-90
-45
0
45
90
135
180
Phase bcf_002
PH
AS
E A
NG
LE (D
EG
)
LOG Frequency (Hz)PhsXY PhsYX
3 2 1 0 -1 -2 -3
-1
0
1
Tipper Transfer Functions bcf_002
TRA
NS
FER
FU
NC
TIO
N
LOG Frequency (Hz)Txr Txi
3 2 1 0 -1 -2 -3
-1
0
1
Tipper Transfer Functions bcf_002TR
AN
SFE
R F
UN
CTI
ON
LOG Frequency (Hz)Tyr Tyi
Curnamona transect example (Quantec Geoscience)
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MT acquisition & processing
4. Processing
Pseudosections of apparent resistivity & phaseCurnamona transect example
Apparent resistivity XY
Phase XY
Section 240 km long
Freq
uenc
y
high
high
low
low
south north
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MT acquisition & processing
4. ProcessingParkinson arrows Curnamona-Gawler Link transect example
Long-period in-phase arrows (red) and strike symbols (black). Arrows point mainly east to southeast, indicating a current system in that direction (perhaps the Flinders Conductivity Anomaly).
Lake Torrens
Freq
uenc
y
high
low20 km
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MT acquisition & processing
4. ProcessingParkinson arrows Curnamona transect example
In-phase arrows (white) and strike symbols (black). Arrows point mainly northeast to east, indicating a current system in that direction (perhaps the Flinders Conductivity Anomaly).
Lake Frome
Frequencyhigh low
50 km
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MT acquisition & processing
5. Analysis
Analysis of MT tensor
The impedance tensor is the Earth filter, relating E response to H source
However, there are complicating factors:
• Dimensionality of Earth (1D, 2D state of art, 3D in infancy)
• Strike direction (from impedance tensor and phase tensor if 2D)
• Static shift
• Electric field distortion (eg. current channelling)
• Magnetic field distortion (eg. uniform source field assumption not true)
• Noise (from various sources, both natural & cultural)
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MT acquisition & processing
5. Analysis
Analysis of impedance tensor for dimensionality and strike
• Rotational invariants of the impedance tensor can be analyzed for dimensionality & strike
• If a well-defined 2D strike can be determined, then the tensor can be rotated so that the TE mode is parallel to strike, and the TM mode perpendicular to strike.
• A phase tensor can be defined & presented as an ellipse – less subject to distortions
Dimensionality example of Curnamona traverse data using WALDIM program
South North
Freq
uenc
y
High
Low
1D
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MT acquisition & processing
5. AnalysisPhase tensor ellipses
For periods up to a few 100 s, there is a clear distinction between the western and eastern sites with varying major current flow as depicted by the major orientation of the ellipses. Skew values indicate mostly 2D for periods up to 300 s with increasing complexity for longer periods.
Gawler transect, 12 long-period responses
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MT acquisition & processing
5. AnalysisPhase tensor ellipses Curnamona Traverse, 25 BB sites
Ellipses coloured by skew
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MT acquisition & processing
6. Modelling
• 1D – forward & inverse are easy and straightforward, but most data not 1D. Good when structures relatively wide compared with depth, such as aquifers & sedimentary basins.
• 2D – forward & inverse codes well-developed, this is “state of the art”. Resolution best for conductive structures. Can model TE, TM & Hz modes separately or jointly.
• 3D – still mainly in the research phase, codes not generally available, but much of the Earth is 3D!
• Models can be unconstrained, or constrained by known features, and degree of smoothness controlled
• The more known rock property information the better
• Future challenge is joint inversions with seismics, magnetics & gravity, and constraints of structures & properties
Modelling – a complex subject
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MT acquisition & processing
6. Modelling
1D modelling – Bostick transform of Curnamona traverse data
Show major features of data – low accuracy
TE mode
TM modeSouth North
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MT acquisition & processing
6. Modelling2D modelling
Gawler
Curnamona
West East
South North
Using the Rodie, Mackie finite difference NLCG method as implemented in WinGLink software
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MT acquisition & processing
7. Conclusions
• MT data acquired along 3 seismic transects in SA in collaboration with U of A and PIRSA
• Earth conductivity is complimentary to information from the seismic method, and the MT method has been briefly described
• Examples of display & analysis of data
• Analysis confirms major features of Earth conductivity, useful prior to inverse modelling
• Top of section sediments are well imaged by MT
• Curnamona data show correlations with interpreted seismic structures, but also reveal other resistive and conductive regions which show no obvious correlations with the seismic data. Also show a response to the Flinders Conductivity Anomaly.
• Gawler data show a distinct conductive anomaly in the crust in mid-section (perhaps the extension north of the Eyre Peninsula Conductivity Anomaly), and a deeper conductive region in the west
• Processing and modelling of all three sets of data are on-going – results presented here are preliminary
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MT acquisition & processing
7. ConclusionsPast, Present & Future Work