Petrofabric-derived seismic properties of a mylonitic quartz simple shear zone: implications for seismic reflection profiling
G. E. Lloyd, J. M. Kendall, 2005. "Petrofabric-derived seismic properties of a mylonitic quartz simple shear zone: implications for seismic reflection profiling", Petrophysical Properties of Crystalline Rocks, P. K. Harvey, T. S. Brewer, P. A. Pezard, V. A. Petrov
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The link between petrofabric (LPO) and seismic properties of an amphibolite-facies quartzo-feldspathic shear zone is explored using SEM/EBSD. The shear-zone LPO develops by a combination of slip systems and dauphine twinning, with a-maximum parallel to lineation (X) and c-maximum normal to mylonitic foliation (XY). The LPO are used to predict elastic parameters, from which the three-dimensional seismic properties of different shear-zone regions are derived. Results suggest that LPO evolution is reflected in the seismic properties but the precise impact is not simple. In general, the P-wave velocity (VP) minimum is parallel to the a-axis maximum; the direction of maximum shear-wave splitting (AVS) is parallel to mylonitic foliation; and the VP maximum and AVS minimum are parallel to the c-axis maximum. The seismic anisotropy predicted is significant and increases from shear zone wallrock to mature mylonite. The P-wave anisotropy ranges from 11 to 13%, fast and slow shear waves’ anisotropies range from 6 to 15% and the magnitude of shear-wave splitting ranges from 9 to 16%. Nevertheless, such anisotropy requires a considerable thickness of rock with this LPO before it becomes seismically visible (i.e. 100s of m for local earthquakes; 10s of m for controlled source experiments). However, reflections and mode conversions provide much better resolution, particularly across tectonic boundaries. The low symmetry and strong anisotropy due to the LPO suggest that multi-azimuth wide-angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones.
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Boreholes are commonly drilled into crystalline rocks to evaluate their suitability for various applications such as waste disposal (including nuclear waste), geothermal energy, hydrology, sequestration of greenhouse gases and for fault analysis. Crystalline rocks include igneous, metamorphic and even some sedimentary rocks. The quantification and understanding of individual rock masses requires extensive modelling and an analysis of various physical and chemical parameters. This volume covers the following aspects of the petrophysical properties of crystalline rocks: fracturing and deformation, oceanic basement studies, permeability and hydrology, and laboratorybased studies. With the growing demands for sustainable and environmentally effective development of the subsurface, the petrophysics of crystalline rocks is becoming an increasingly important field.