magnetotelluric data for exploration · magnetotelluric data for exploration janelle simpson |...
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![Page 1: Magnetotelluric data for exploration · Magnetotelluric data for exploration Janelle Simpson | Geophysicist Geological Survey of Queensland Dep. of Natural Resources, Mines and Energy](https://reader033.vdocuments.mx/reader033/viewer/2022053109/607c89883328a871db254436/html5/thumbnails/1.jpg)
Magnetotelluric data for exploration
Janelle Simpson | Geophysicist
Geological Survey of QueenslandDep. of Natural Resources, Mines and Energy
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Overview
Workshop overview
12:50 pm Introduction to MT
1:10 pm Overview of data release products
1:20 pm MT for targeting – exercise
1:50 pm Targeting discussion
2:00 pm General Q&A • Sensitivity analysis
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Introduction to MT
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Introduction to MT outline
• MT basics
• Regional survey design
• Examples
• MT limitations
MT Basics
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What is MT?
Passive geophysical method
𝐸𝑥𝐸𝑦
=𝑍𝑥𝑥 𝑍𝑥𝑦𝑍𝑦𝑥 𝑍𝑦𝑦
𝐻𝑥
𝐻𝑦
Characterisation of crustal resistivity
MT Basics
Note – High conductivity = low resistivityElectric field Magnetic field
MT data
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Source fieldHigh frequencies (>1 Hz) = Spherics
thunderstorm activity world-wide
Low frequencies (<1 Hz) = Micropulsations
Solar wind interacting w/ magnetic field
Vary on hourly, daily,
yearly cycles
MT Basics
High frequency = short period
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MT Basics
Data typesAudiomagnetotelluric (AMT)• 10-4 – 100 s• Record time 2 hrsBroadband (BBMT)• 10-2 – 103 s• Record time 16 hrsLong Period MT• Record time 3 weeks• 101 – 104 s
Ap
par
ent
resi
stiv
ity
Period (s)10-4 104100
Increasing depth of investigation
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Depth of investigation
MT Basics
Skin depth characterises effective depth of penetration for MT
AMTApprox. 50 m to 5 km
BBMTApprox. 300 m to 150 km
Long PeriodApprox. 15 km to 500 km
Skin depth
ρ – resistivity (Ωm)T – period (seconds)
Conductive basin sediments reduce DOI
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Depth of investigation
MT Basics
Where does MT sit in the electric methods toolkit?
MT*
Active source ground/ airborne hybrid
Ground EM
AEM
Mantle depths
250 m
500 m
1 km
5 km
* Dependent on frequencies acquired
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Field setup
MT Basics
Electric field
Magnetic field
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Data formatsRaw Data (time
series)
• ascii
• proprietary
Processed Data
• .edi files
– Spectra
– Impedance
MT Basics
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MT workflow overview
Modelling
Inversion Sensitivity analysis Interpretation
Data analysis
Dimensionality Strike Noise and distortion
Data collection
Survey planning Data collection and processing Contractor models generated
MT Basics
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What is inversion?Inversion
Forward modelling
MT Basics
Observed data
Resistivity model
Resistivity model
Predicted data
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Sources of conductivity
MT Basics
Composition
Temperature
Pressure
Fluids
Hydrogen
Conductivity
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Regional surveys
Regional survey design
• 2-5 km station spacing
• BBMT / AMT
• Regional targeting
2007
2016
2009
2014-15
2014
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Long Period example
Examples
Wang et al. 2014
53 km depth slice
255 km spaced sites
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AusLAMP
Examples
Thiel and Heinson, 2010
~55 km spaced sites
Large crustal and upper mantle features
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BBMT data
Examples
Crustal features
10 km station spacing
Korsch et al. 2012
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BBMT data
Examples
Mount Isa
George Fisher
Crustal features
2 km station spacing
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AMT data
Examples
Features to approx. 5 km
500 m station spacing
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Relevance to exploration
Heinson et al. 2018
OD – Olympic DamWW – Wirrda WellVC – Vulcan
• Conductivity anomalies associated with major deposits
• Profile data only
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Relevance to exploration
• Conductivity anomalies associated with major deposits
• First example with a grid survey
Wang et al. 2018
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Introduction to MT outline
• MT basics
• Regional survey design
• Examples
• Limitations
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poor
MT limitations - vertical current gathering
Limitations
conductive
resistive
Electric current
Poor resolution
highly
conductive
Current flows through
conductive bodies
Areas beneath
conductors have low
current density
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Limitations
10 Ωm
100 Ωm
70
00
Ωm
10 Ωm
100 Ωm
300 Ωm
70
00
Ωm
10 Ωm
30 Ωm
100 Ωm
70
00
Ωm
3 models:Same 2 surface layers
*#
#
*
Additional basin units
Potential exploration target
Calculated MT responses almost identical
MT limitations - vertical current gathering
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Comparison to drilling
MT limitations - vertical current gathering
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• Models from the same data
– Different code
– Different parameters
Limitations
MT limitations – Non-uniqueness
WingLINK
Occam 2D
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• Is anomaly visible in data?
Limitations
MT limitations – Non-uniqueness
WingLINK
Occam 2D
?
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Take home messages• MT data provide information about crustal resistivity at a
variety of depths
• Regional MT surveys are designed to help target exploration rather than drilling
• Inversion is used to generate resistivity / depth models for interpretation
• Non-uniqueness and target resolvability can have significant impacts on interpretation
MT Basics
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ReferencesWang, L., Duan, J, Simpson J., 2018. Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry
Region. Record 2018/05. Geoscience Australia, Canberra.
Heinson, G., Didana, Y., Soeffky, P., Thiel, S., Wise, T., 2018. The crustal geophysical signature of a world-class
magmatic mineral system. Sci. Rep. 8. https://doi.org/10.1038/s41598-018-29016-2
Korsch, R.J., Huston, D.L., Henderson, R.A., Blewett, R.S., Withnall, I.W., Fergusson, C.L., Collins, W.J., Saygin, E.,
Kositcin, N., Meixner, A.J., Chopping, R., Henson, P.A., Champion, D.C., Hutton, L.J., Wormald, R., Holzschuh, J.,
Costelloe, R.D., 2012. Crustal architecture and geodynamics of North Queensland, Australia: insights from deep
seismic reflection profiling. Tectonophysics 572–573, 76–99. https://doi.org/10.1016/j.tecto.2012.02.022
Thiel, S., Heinson, G., 2010. Crustal imaging of a mobile belt using magnetotellurics: an example of the Fowler
Domain in South Australia. J. Geophys. Res. 115. https://doi.org/10.1029/2009JB006698
Wang, L., Hitchman, A.P., Ogawa, Y., Siripunvaraporn, W., Ichiki, M., Fuji-ta, K., 2014. A 3-D conductivity model of
the Australian continent using observatory and magnetometer array data. Geophys. J. Int. 198, 1171–1186.
https://doi.org/10.1093/gji/ggu188
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Magnetotelluric data release products
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Cloncurry MT datasetInitial release
• .edi files
• Acquisition reports
• Contractor models (if available)
• Times series data (offline delivery)
http://qdexdata.dnrm.qld.gov.au/flamingo/
MT Products
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Cloncurry MT dataset
Second data release
• Modelling results
• Modelling report
http://qdexdata.dnrm.qld.gov.au/flamingo/
MT Products
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Cloncurry MT dataset
MT Products
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Cloncurry MT datasetReadme contains:
• Folder contents
• File formats
• Suggested software (if applicable)
• Contact details
• Information about product generation
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Cloncurry MT dataset
• Modelling report
• x,y,z,restivity point file
• Located geotif depth slices
• Sgrid
• Inversion files
• Data (.edi files)
MT Products
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Targeting exercise
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Use provided MT data package to target 5 km2 exploration area(s)
• Cloncurry MT data package– Geoscience Analyst – sgrid/point set
– GIS package - depth slices
Exercise
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Use provided MT data package to target
5 x 5 km exploration area(s)
• Cloncurry MT data package– Geoscience Analyst – sgrid/point set
– GIS package - depth slices
Exercise
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Exercise
Some how to screenshots
• Geoscience Analyst
• GoCAD
View and rotate model only
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Exercise
HoloLens
• Augmented reality view of the Cloncurry MT model
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Targeting discussion
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2.5 km500 m50 m 4.5 km 6.5 km
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9500 m 14 km
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Exercise 1
• Targets supported by several sites
• Targets on edges
• Anomalies associated with 1 site
• Targets consistent in the sensitivity analysis
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Q&A
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Take home messages
• Regional surveys are designed for exploration targeting NOT drill targeting
• MT inversions produce variable results – Edge effects
– Poorly constrained features
• Petrophysical properties of cover can play a significant role in target resolvability
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Sensitivity Analysis:why should you care?
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What is sensitivity analysis for?
• Inversion testing to establish sensitivity and resolvability of key features
• Generally included in modelling report
Sensitivity analysis
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Cloncurry example
Sensitivity analysis
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Cloncurry example Well resolved Not well resolved
Sensitivity analysis
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Cloncurry example Ernest Henry here
Sensitivity analysis
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Isa Extension example
Sensitivity analysis
Isosurface models for 15 inversions
Comparable data fits
Up to 4 conductive features
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Isa Extension example
Sensitivity analysis
Variability distribution with depth
Blue features low variability = low uncertainty
Red features high variability = high uncertainty
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Sensitivity analysis provides a quick evaluation about the reliability of inversion features
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What are phase tensor plots?
Plots of data rather than interpretative products of the data
Used to assess • Data noise• Dimensionality• Gain high order understanding of the
dataset
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What are induction arrows?
Indication of the presence of subsurface conductors
Derived from 3 component measurement of the magnetic field
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Extra exercise
• What is hiding beneath conductive cover?
• 1D modelling exercise
Exercise
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Exercise 2
• 30% rule of thumb
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Regional surveys
Regional survey designCloncurry MT survey
– 2 km grid
– 1000 Hz to 1000 s
Isa Extension MT survey
– BBMT sites 2 km x 5 km spacing
– AMT 500 m sites
– AMT 10,000 Hz to 1 Hz
– BBMT 250 Hz to 2000 s
2007
2016
2009
2014-15
2014
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Limitations
MT limitations – inappropriate inversion
• MT data have inherent dimensionality
• 1D data can be modelled by 3D inversion
• 3D data cannot be modelled by 1D or 2D inversion
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MT limitations – out of array effects
MT data can be sensitive to a feature but unable to resolve it
Isa Extension dataset- Long period data unable to be modelled
Limitations
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Limitations
MT limitations – modelling noise
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Finding inversion
features in data
?