fulltext - Pure appl. geophys. 164 (2007) 21412156...

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Anisotropy Effects on Microseismic Event Location A NDREW K ING 1 and S HAHRIAR T ALEBI 2 Abstract— Seismic anisotropy in sedimentary environments is signiFcant—microseismic waveforms often show strong shear-wave splitting, with differences reaching 40% between horizontally and vertically-polarized shear-wave velocities. ±ailure to account for this anisotropy is shown to result in large microseismic event location errors. A method is presented here for determining the Fve elastic parameters of a homogeneous, vertical transverse-isotropic (VTI) model from calibration shot data. The method can also use data from mining- induced seismic events, which are then simultaneously located. This simple model provides a good Ft to arrival times from coal-environment data, and results in dramatic shifts in interpreted event locations. Key words: Anisotropy, transverse isotropy, coal mine, microseismic, source location. 1. Introduction An isotropic material is one whose properties do not vary with the axis along which they are considered. The elasticity of a homogeneous solid in general is expressed by a 6 · 6 stiffness matrix relating strain and stress components. ±or an isotropic medium, the elements of the matrix can be deFned by two independent parameters. Most rocks, however, have material properties that do depend on the axis of observation, hence they are called anisotropic. The stiffness matrix being symmetric due to simple thermody- namic considerations, the number of independent elements of the stiffness matrix is reduced to 21. Thus anisotropy in its most general case can be expressed by 21 independent elastic moduli. This number is further reduced if the material has any axes or planes of symmetry. The combination of such elements of symmetry can result in eight anisotropy systems (e.g., G IBOWICZ and K IJKO , 1994). Transverse Isotropy (TI) is one type of anisotropy that has been extensively studied in soil and rock mechanics (e.g., T SVANKIN , 1996; L IU et al., 2000; R ASOLO±OSAON , 2003). The existence of one axis of symmetry in this case, often along the vertical direction (VTI), reduces the number of independent elastic parameters to Fve. Most seismological modelling assumes that the earth is isotropic, though anisotropy studies are becoming an increasingly important part of seismology research (S HEARER , 1 CSIRO Exploration and Mining, P.O. Box 883, Kenmore, QLD 4069, Australia. E-mail: Andrew.king@csiro.au. 2 CANMET-MSSL, 555 Booth St, BCC 10, Ottawa, ON K1A 0G1, Canada. Pure appl. geophys. 164 (2007) 2141–2156 Ó Birkha ¨user Verlag, Basel, 2007 0033–4553/07/112141–16 DOI 10.1007/s00024-007-0266-8 Pure and Applied Geophysics
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1999). The situation is identical in the feld oF induced seismicity as most studies are Focused on hard-rock mines where material anisotropy is small and generally neglected.
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This note was uploaded on 02/10/2011 for the course GEOL 4330 taught by Professor Chesnokov during the Spring '11 term at University of Houston - Downtown.

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fulltext - Pure appl. geophys. 164 (2007) 21412156...

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