thesis
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Multiscale Seismology: the future of inversion W. MENKE Lamont-Doherty Earth Observatory Columbia University E. CHESNOKOV and R.L. BROWN Institute for Theoretical Geophysics University of Oklahoma. Thesis - PowerPoint PPT PresentationTRANSCRIPT
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Multiscale Seismology:
the future of inversion
W. MENKELamont-Doherty Earth Observatory
Columbia University
E. CHESNOKOV and R.L. BROWNInstitute for Theoretical Geophysics
University of Oklahoma
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Thesis
The past 15 years has seen a tremendous improvement in the fidelity of many types of seismic images.
This improvement was driven by, more and higher quality seismometers, faster computers, better data archiving and processing methodologies.
But our ability tointegrate knowledge from multiple data types
hasn’t kept up. Often different data types are telling us seemingly-contradictory things.
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Example:
Surface Wave Tomography
e.g. of North America
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Woodward and Snieder, 1993
275 seismograms
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Zhang & Tanimoto, 1993
18,000 seismograms globally, about 324 prorated for area
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685 seismograms
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400,000 seismograms globally, about 7,200 prorated for area
note inversion includes transverse anisotropy
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“Exponential”
growth
of
data !
Images better & better in evolutionary
way
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But how do these impressive images
connect with
other things we know about the earth?
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Connection 1
Continental Scale Body Wave Traveltimes
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Surface wave models have big asthenospheric LVZ’s that imply very large
shadow zones
Are such shadows actually observed in continental-scale P or S waves?
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Connection 2
SKS Shear Wave Splitting
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From Gaherty
North America has large amount of transverse anisotropy
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Predicts
Correctly predicts large Love-Rayleigh discrepancy along paths
parallel to MOMA Array
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But inconsistent with SKS splitting results along MOMA array
Fouch’s splitting data as plotted by Gaherty
No plausible anisotropic material can have fast-axis parallel to array and have large Love-Rayleigh discrepancy parallel to the array, too
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More overlap in parameters than length scale !
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Hypothesis:
differentlength scales
strongly influence interpretation
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EARTH OBSERVATION
INVERSION
StrongSpooky
Interactions
(seismic waves)
Length scale ofLength scale of
Length scale of
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we understand this interactionpretty well
(but only in very idealized media)
strong spooky interactionsOBSERVATION
(seismic waves)EARTHLength scale of
Length scale of
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WHAT IS THE STRUCURE OF THIS MEDIUM ?
,, ρμ
,, ρμ
bulk modulus 1
shear modulus 1,
density 1
thickness 1
bulk modulus 2
shear modulus 2
density 2
thickness 2
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WHAT IS THE STRUCTURE OF THIS MEDIUM ?
Inhomogenous with various properties of
isotropic layers?
YES !when <<thickness
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WHAT IS THE STRUCTURE OF THIS MEDIUM ?
Strongly Scattering?
YES !when ~thickness
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WHAT IS THE STRUCURE OF THIS MEDIUM ?
Effectively homogeneous and anisotropic?
YES !when >>thickness
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Theory for understanding this effectin 3-D media with random heterogeneitiesis well developed …
Elasticity and density written in terms of average and deviation from average
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• Chesnokov et al. 2000
weird! effective density is a tensor ..
1. elasticity and density are frequency-dependent
2. integrals embody interaction of wavefield with scale length of heterogeneities …
3. … through correlation functions
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“UPSCALING” ExampleReconciling Sonic Log with VSP
Collect Sonic Logs (500 Hz) of Vp, Vs1, Vs2, density
Infer all components of Cijkl(f=500 Hz)
Compute Correlation Functions
Predict Cijkl(f=50 Hz)
Compare with VSP (50 Hz) experiment
Is this Inversion?
Not quite …
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Sonic Log
Result for C55
Predicted VSPVSP
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theorycan be
extendedto include
more complicated
micro-physics
e.g.fluid/rock
interactions
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There’s been some interesting efforts on this side of the triangle, too
stro
ng s
pook
y in
tera
ctio
ns
OBSERVATIONLength scale of
INVERSIONLength scale of
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True Slip on Hypothetical Fault
ThreeInversionsThatFit the DataEquallyWell
Results of Slip Inversions Highly Dependent onScale of Model Representation
Courtesy of Morgan Page
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A Challenge of the Future
Create
Earth knowledge
that
practitioners using
different techniques
AGREE UPON
!
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EARTH
INVERSION
Joint Inversions thathandle multiple scales
in a Physics-Based way
OBSERVATION(seismic waves)
Length scale ofLength scale of
Length scale of
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The Future ?
Scale1 Data
physics-based parameterization
1
Assessment of underlying physics
Scale2 Data
Business as Usual
Scale1 Data
Ad-hoc parameterization
1
Ad-hoc parameterization
2
Speculative attempts to integrate and reconcile results
Scale2 Data
feedback
confusion ?!Improved knowledge of earth