GeoModeller Inversion Workflow Details
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Transcript of GeoModeller Inversion Workflow Details
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Details of the Inversion
workflow
Des Fitzgerald
August 2007
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Geology Constrained Inversion
Simple Slab Tutorial
Part 1 Background
A statistical approach
2007
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Objectives
Demonstrate new approaches to inversion
Use all available geology, gravity and magnetics to constrain the result
Limit the infinite possibilities to a set that is consistent with all the facts
Bring latest inversion & probability thinking to geoscience
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Constraining Your Geology
Geomodeller has a set of geological rules to resolve and constrain the initial model Contacts Dip/Strike Pile, erode/onlap Fault network
Inversion does not make direct use of these. Instead, it uses a framework of geological units, important for producing realistic property models allows simultaneous inversion of multiple
geophysical data types
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Commonality Possessing like and interchangeable
characteristics
The number of voxels in common between the proposed models and the
reference model is controlled
statistically using a Commonality
constraint jRefjSame
j yCommonalit
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Geological Commonality Misfit
Say you want 90%
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Shape Ratio "Shape" is the square-root of surface area
divided by the cube-root of volume (to mimimize any scale dependence).
"ShapeRatio" is the Shape of the formation in the proposal divided by the Shape of the formation in the ReferenceModel.
Shape of a sphere = 2.2 No scale dependency
Shape of a cylinder h = r, Shape = 2.42 h = 2r, Shape = 2.35 h = 4r, Shape ~ 2.41
h = 20r, Shape ~ 2.89 Some aspect ratio dependency !
Cube Shape ~ 2.45
Prism square base r*r , height r/4 - Shape ~ 2.75
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Geophysical Grid Preparation
Detrending
Need to be able to fit the anomalies caused by signal in your model
External influences or deeper sources are not relevant
Geomodeller does this on demand,
either just once at the beginning
Or on a regular basis as inversion continues
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Grids continued
You create a Measured Grids List
Type, (eg Gravity)
Mean Elev., (eg 50m )
Precision, (eg 0.5 mGals)
Detrend, (eg Yes/No)
Detrend degree, (eg 0,1,2 etc)
detrend freq., (eg 0, or 100000)
grid URL (eg c:/data/bouguer.ers)
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Geophysical Precision
An estimation of the standard deviation of the measurement error in your geophysical data
All observed data has inherent error due to a variety of factors
Typically, ground gravity would be good to about 0.5 mGals
Airborne Total Magnetic Intensity to about 1 nT
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Overview of Cases
1. Forward Model Propose a start model, compute gravity response
2. Prior Only Propose a geological model, explore probability
space implied by the geological constraints
3. Fixed Geometry Solve for properties using a bounded least-squares
fit
4. Bi-Modal Property Only No proposed body, just a proposed density contrast
5. Constrained Geology Use gravity to constrain result
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Overview continued
7. Constrained Geology
Use magnetics assuming induced response only
8. Constrained Geology
Joint gravity and magnetics
9. Constrained Geology
Use magnetics with the possibility of remanence
10. Constrained Geology
Use observed tensor gradients
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Property Uncertainty
vs
Geological Modelling By default, we give equal importance to
these two differing aspects
You can speed convergence of your inversion, if you have a high confidence
in your properties and there in no
spread of values
Simply adjust all the effort towards the geological uncertainty
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Tools & Status
The Geomodeller Tool as used for creating your 3D geological model, can also perform Forward Modelling functions
The ManageLithoInversion Tool is a Batch Processing tool only at V1.2 This is used for all other aspects of Inversion
Voxets Can be examined by Voxler, other thirdparty tools
Movies Standard thirdparty products
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Geology Constrained Inversion
Simple Slab Tutorial
Part 2 Practical
A statistical approach
2007
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Test Problem
Find Buried Object Unknown dip / strike
Unknown density contrast
Unknown magnetic susceptibility
Observed Geophysics Vertical component of gravity
Total magnetic intensity
No remanence
Observed Gravity
Observed Magnetics
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Make initial Geology Model
Propose a simple vertical slab as a
starting model
Buried 100 m
Dimension of 100 * 200 * 400
Make model
Save project
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Initial Plan view of model
200m below surface
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Test Properties
Slab density 3.67
Host density 2.67
Slab susceptibility 0.001
Host susceptibility 0.000001
Optional Extras
Slab remanence (0.05,0.00,100,25,115)
Host remanence (0.00,0.00,100,0,00)
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Geophysics Observations
Data Collection height : 50m
Gravity Data
normal ground observations
Magnetic Data
Field Intensity: 50000
Inclination: -65
Declination: 25
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Conditioning the Geophysics
For both Gravaity and Magnetics, we recommend you turn detrending ON
A first order trend should be fine, so choose 1
Gravity detrending should not need to be repeated as the inversion evolves
Sometimes Magnetic data can benefit from a further detrending adjustment of the misfit say every 100000 iteration
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Actual Body
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Preparing a case NewCase Command
Create an Inversion Case
Within this case the following are treated as constant
The voxet resolution and extent
The reference geological model
The physical property laws
The voxels to hold fixed
The geological constraints
The observed geophysical grids
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Preparing a Run
Create an Inversion Run
Within this run the following are treated as constant
The Case
The number of iterations
The initial lithology and physical properties
The starting or initial geological model can be varied
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Case 1 - Forward Modelling
Before starting the inversion, run forward model first
This is done via the Geomodeller Tool
This way we check that the starting geological model is reasonable
The assumed physical properties can be checked
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Case 1 Instructions
Start Geomodeller
Load Simple slab
Geophysics menu
3d Forward/Inversion
Choose G00 (gravity)
Constant grid
Choose apply
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Gravity Grids
Observed Initial
Forward
Not bad but missing some plunge??
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Case 2 Prior Only
This is always recommended before full inversion
Do you have good geological control of proposed models ie are your geological models viable?
Methodology Ignore the geophysics
Constrain the geological arrangements with respect to a reference geology
Randomly perturb, evaluate model geologically using one or more of:
1. Shape ratio constraint
2. Commonality of new with reference
3. Volume ratio
Prior Only is an expression of geological uncertainty
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Load start litho- & property models
Modify models
Test against geology model criteria
(Modification accepted) (Modification rejected)
Test against geophysics dataset
criteria
Continue?
Finish
Start
Fails
Fails
PRIOR ONLY
computational scheme
No Geophysics used
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Mechanics
You describe your process via the file : Inversion.xml
This is both human and machine readable Try internet explorer on it!
A copy of Your Geomodeller Project is also made for each case this is also mostly an XML file
Current interface is via batch scripts
An inversion run can take time to complete
A voxet of 50m x 50m x 50m is produced
A voxel change log for every viable model is kept so that it is possible to reconstruct the evolving state after the event.
- Inversion.xml -
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Run Parameters 1
Iterations 100000
Commonality: Y
Slab Commonality Weibull(0.02,1.0)
Host Commonality Weibull(0.05,1.0)
Shape Ratio: Y
Slab ShapeRatio LogNormal(0.1,0.05) Host ShapeRatio LogNormal(0.0,0.05)
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Geology Constrained Inversion
Simple Slab Tutorial
Part 3 Reporting
A statistical approach
2007
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Recommended Discipline for
Project layout
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Querying Inversion Outputs
Once the inversion itself is complete various visuals can be produced to analyse the
results
Images along sections
Movies along sections
Geophysical grids of the computed geophysics
Surface mesh representing isovalues
Probability of lithology voxets
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Available Queries MakeSummaryStats
Work out the summary statistics of all models between a start and end iteration. From this we can derive products such as the probability of a given lithology at a given cell
MakeHistogram Generates a histogram of densities and/or susceptibilities for each
Formation in a Voxel dataset
MakeSectionImage Generates a section through a Voxel dataset and presents it as a jpg
image that can then be displayed from within 3D GeoModeller
MakeDerivedVoxet Various operations on Voxel datasets. Ex. Searches though a list of
Sujmary stats datasets and can find the most probable formation
MakeEvolutionMovie On a nominated section can create an avi movie of the evolution of
the lithology model though the iterations
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Example of 2 SuperSummary Stats Outputs
The names of the various queried output properties are defined in the voxet
header: eg 85% & 95% probabilty effects only property 5 below.
SuperSummaryStats_Case_4_Run_1_iterations_3000000_1000000_85_super.vo,
AND identically also in:
SuperSummaryStats_Case_4_Run_1_iterations_3000000_1000000_95_super.vo
Property 1 = Change Count
Property 2 = Mean Density
Property 3 = Std Dev Density
Property 4 = Most Probable
Property 5 = Most Probable Thresholded
Property 6 = probability 0 (First lithology from model)
Property 7 = probability 1 (Second lithology from model)
Property 8 = probability 2
Property 9 = probability 3
Property 10 = probability 4
Property 11 = probability 5
Property 12 = probability 6
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Definition: Most probable thresholded
means
For the entire inversion after burn in, and considering
all of the accepted, proposed geology models Report back all the voxets which remain a single lithology for a given %, or more, of the total # of
proposals.
Report back the location and lithology of those voxets.
Black volumes indicate areas of greater changeability
of lithology, such that the threshold % was not met.
Section 130_000_N: Most probable geology
Section 130_000_N: Most probable geology Thresholded 85%
Section 130_000_N: Most probable geology Thresholded 95%
Example section
Each type of output statistic generated by MakeSuperSummaryStats
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ProbabilityResults
Run1 - All random changes allowed
Run 2 - Body constrained by commonality and shape ratios
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Mean Density
Predicted density
distributions
based purely
on geology
constraints
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Raw Statistics of Prior Only Run
AcceptanceCount="73365 AcceptanceRatePerHour="434214" CommonalityRejectCount="12969" CommonalityTestCount="99999" ComputerName="RAYS-PC" Counter="100000" FinishedDateTime="24/ 8/2007 9:24:18.122000"
InterAcceptanceDifferentFormationCount="8926" InterAcceptanceSameFormationCount="64439" InterProposalDifferentFormationCount="35560" InterProposalSameFormationCount="64439
ProposalRatePerHour="591849" ShapeRejectCount="13665" ShapeTestCount="87030" StartedDateTime="24/ 8/2007 9:14:9.443000
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Super Summary Stats
Statistical History of each voxel
Change count
Mean property
Standard deviation
Probability for each lithology
Most probable lithology over certain threshhold ( say > 90%)
Visualise via a voxet viewer or sectional images or movies
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Run Parameters 2
We can show how the predicted bodies can be tightened up to honour the original shapes by changing the commonality
Iterations 100000
Commonality: Y Slab Commonality Weibull(0.1,1.0) Host Commonality Weibull(0.1,1.0)
Shape Ratio: Y Slab ShapeRatio LogNormal(0.10,0.05) Host ShapeRatio LogNormal(0.0,0.05)
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Case 3 - No Geology Body Shape
Bi-Modal Properties
Another case to show the tool working in a way that is closest to existing
deterministic methods
Prepare a voxet with no buried geological body
Propose the existing of a second population via a bi-modal property law
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Create Physical Properties 1
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Create Bi-Modal Density for Host
80%
background
20% more
dense
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Gravity Inversion of Bi-Modal
Case Final Density Section Image Since we
have no
geological
control the
density
anomaly is
biased to the
near surface
NB takes
500000
iterations
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Case 4 - Gravity Inversion
Enforce commonality
Enforce Shape Ratio
Use observed ground gravity
Run for 100000 iterations
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Load start litho- & property models
Modify models
Test against geological model
(Modification accepted) (Modification rejected)
Test against gravity dataset
Continue?
Finish
Start
Fails
Fails
Gravity plus geology
constraints
computational scheme
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Dipping Slab
Gravity Grids
Final
Observed
Initial
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Case 5 Magnetics Inversion
Assume a small vertical slab body as a starting model
Constrain the geology via commonality and shape ratio
Use observed magnetic data to refine aspects of the bodys geometry and properties
Assume the magnetics is induced only
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Dipping Slab
Magnetic Grids
Final
Observed
Initial
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Load start litho- & property models
Modify models
Test against model criteria
(Modification accepted) (Modification rejected)
Test against dataset criteria
Continue?
Finish
Start
Fails
Fails
Overview of the
computational
scheme
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Case 6 - Joint Inversion
We have shown the dipping slab case using the observed gravity
Now we add observed magnetics to our case and run joint inversion
Run for 300000, with a burn-in of 200000
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Most Probable Lithology
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Joint Miss-fit evolution
Gravity and magnetic misfit during inversion
0
1
2
3
4
5
6
7
8
9
10
0 1000000 2000000 3000000 4000000 5000000 6000000
# iterations
mis
fit
(nT
/mG
al)
gravity
magnetic
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Comments
This shows good adaption to the expected outcome
Both Mag. And Gravity contribute
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Over-view of Outcomes
Gravity Only inversion
does not do depth estimates well
Volume/mass of geological anomaly is quite good
Induced Magnetics inversion
Depth to top of body is good
Orientation of body is also good
Poor estimate of body volume
Joint Magnetic & Gravity
Get both depth and volume OK