Abstract
We identify two interrelated but independent species of microcracks with different origins and different distributions. One species is the classic high-stress microcracks identified in laboratory stress-cells associated with acoustic emissions as microcracks open with increasing stress. The other species is the low-stress distributions of closely-spaced stress-aligned fluid-saturated microcracks that observations of shear-wave splitting (SWS) demonstrate pervade almost all in situ rocks in the upper crust, the lower crust, and the uppermost 400 km of the mantle. On some occasions these two sets of microcracks may be interrelated and similar (hence ‘species’) but they typically have fundamentally-different properties, different distributions, and different implications. The importance for hydrocarbon exploration and recovery is that SWS in hydrocarbon reservoirs monitors crack alignments and preferred directions of fluid-flow. The importance for earthquake seismology is that SWS above small earthquakes monitors the effects of increasing stress on the pervasive low-stress microcrack distributions so that stress-accumulation before, possibly distant, earthquakes can be recognised and impending earthquakes stress-forecast.
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This study was partially supported by the National Natural Science Foundation of China (No. 41174042).
Stuart Crampin (PhD, ScD, FRSE, FAGU, Conrad Schlumberger Award, EAGE, Virgil Kauffman Gold Medal, SEG) at British- Geological-Survey, Edinburgh, pioneered theory, observation, and interpretation of seismic anisotropy and shear-wave splitting in stress-aligned fluid-saturated microcracks pervasive throughout the Earth’s crust and upper-mantle. In 1988, Crampin founded Edinburgh-Anisotropy-Project (EAP) oil-company consortium, currently led by Professor Xiang-Yang Li. EAP is a research consortium supported by 10∼15 oilcompanies, which received the 2012 SEG Distinguished Achievement Award. Crampin (www.geos.ed.ac.uk/homes/scrampin/opinion) and EAP (www.eap.bgs.ac.uk) have extensive experience (∼40 years, ∼350 research papers) of the theory, calculation, and interpretation of shear-wave splitting in both exploration and earthquake seismology.
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Stuart, C., Gao, Y. Two species of microcracks. Appl. Geophys. 11, 1–8 (2014). https://doi.org/10.1007/s11770-014-0415-7
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DOI: https://doi.org/10.1007/s11770-014-0415-7