Transcript Document
Introduction to
Shear Wave Splitting
Erin Elliott
Guofeng Yuan
February 26, 2010
Outline
Concept
Definitions
History
Applications
Common Misunderstandings
Conclusions
What is Shear Wave Splitting?
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
Microcrack
Azimuthally Aligned
Polarization Diagrams
Microcracks
Crack density
Azimuthally Aligned
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)
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
Possible earthquake prediction
for earthquakes M 1.7 to M 7.7
CO2 Injection mapping
in fractured carbonate reservoirs
Common Misunderstandings
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
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?