Download Shear-Wave Splitting

Introduction to
Shear Wave Splitting
Erin Elliott
Guofeng Yuan
February 26, 2010
Outline
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Concept
Definitions
History
Applications
Common Misunderstandings
Conclusions
What is Shear Wave Splitting?
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When a polarized shearwave enters an anisotropic
medium, it splits into two
quasi shear-waves (fast
and slow).
Shear Wave Splitting
(Seismic birefringence)
http://garnero.asu.edu/research_images/anis/garnero_splitting.gif
History
Measurements of P-wave azimuthal
velocity variations - Hess, 1964.
S-wave anisotropy identified in
upper mantle - Ando, 1980
Azimuthally-aligned shear wave
splitting- Crampin, 1981
Definitions
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Microcrack
Azimuthally Aligned
Polarization Diagrams
Microcracks
Crack density
Azimuthally Aligned
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the angles that the arrivals or waves make with a
reference point are all equal.
The waves are approximately parallel:
http://en.wikipedia.org/wiki/Azimuth
Polarization Diagrams
(Hodograms)
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Displays a 2-D projection of the trajectory of a
point moving in space.
a cross plot of two components of motion over a
time window.
http://www.crewes.org/ResearchLinks/ExplorerPrograms/Hodogram/Hodogram.html
Applications
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Possible earthquake prediction
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for earthquakes M 1.7 to M 7.7
CO2 Injection mapping
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in fractured carbonate reservoirs
Common Misunderstandings
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Polarizations of split shear waves are orthogonal
Crack anisotropy always decreases with depth as
fluid filled cracks are closed by lithostatic
pressure.
Signal-to-noise ratios of shear-wave splitting
above small earthquakes can be improved by
stacking.
Conclusion
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Stress aligned fluid-saturated microcracks are the
predominant cause of the nearly universal
observations of azimuthally aligned shear wave
splitting both in the earth’s crust and in the upper
mantle.
Questions?