Introduction to Shear Wave Splitting

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Presentation transcript:

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 shear-wave 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 CO2 Injection mapping 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?