Introduction to Shear Wave Splitting Erin Elliott Guofeng

Introduction to Shear Wave Splitting Erin Elliott Guofeng Yuan February 26, 2010

Outline l l l Concept Definitions History Applications Common Misunderstandings Conclusions

What is Shear Wave Splitting? l 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 l l l Microcrack Azimuthally Aligned Polarization Diagrams

Microcracks Crack density

Azimuthally Aligned l l 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) l l 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/Research. Links/Explorer. Programs/Hodogram. html

Applications l Possible earthquake prediction l l for earthquakes M 1. 7 to M 7. 7 CO 2 Injection mapping l in fractured carbonate reservoirs

Common Misunderstandings l l l 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 l 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?