China
Africa
Explore chapters and articles related to this topic
A case example of integrated stress profile evaluation
Published in Katsuhiko Sugawara, Yuzo Obara, Akira Sato, Rock Stress, 2020
Shear wave splitting, namely thearrival of two shear waves, is associated with anisotropic properties of the rock. It is generally accepted that, for isotropic rock matrix, this anisotropy is induced by the effect of the stress field on the opening of micro-fissures (Crampin and Lowell, 1991). Schematically, the principle is that micro-fissures oriented perpendicularly to the maximum principal stress are more closed than those which are perpendicular to the principal minimum stress. Hence velocity is faster in the direction of the maximum principal stress than in the direction of the minimum principal stress. When the three principal stress magnitudes are different, the rock mass exhibits three axis of symmetry (9 constants of elasticity) but when two principal stress magnitudes are equal, the material exhibits planar isotropy with only 5 constants of elasticity.
Seismic wave propagation in fractured rock
Published in Hans-Peter Rossmanith, Mechanics of Jointed and Faulted Rock, 2018
Rocks contain discontinuities at all scales from grain boundaries and cracks at the microscale to joints and fractures at the macroscale to faults at the mesoscale. It has also been recognized for a long time that the effect of these discontinuities, if averaged over some volume, is a reduction in the stiffness of the rock. From the theory of elasticity for a propagating seismic wave, this reduction in stiffness translates into a reduction in wave velocity. If discontinuities are preferentially oriented in one direction the average stiffness will be reduced more in one direction than another, resulting in an effective anisotropic medium. One of the important characteristics of wave propagation in anisotropic media is that a shear wave propagating at an oblique angle to an axis of symmetry will be split into components traveling at different velocities related to the degree of anisotropy. Thus, shear wave splitting is often taken as diagnostic of fracture orientation and density. These concepts constitute the most common approach in seismic geophysics to modeling wave propagation in fractured rock (e.g. O’Connell and Budiansky 1974, Crampin 1981, Hudson 1981, Thomsen 1996 and others).
Location and temporal variations of shear wave splitting in a South African gold mine
Published in H. Ogasawara, T. Yanagidani, M. Ando, Seismogenic Process Monitoring, 2017
N. Nagai, M. Ando, H. Ogasawara, T. Ohkura, Y. Iio, A. Cho
Two parameters are necessary to describe the nature of shear wave splitting, polarization direction of the faster shear wave, ϕ, and arrival time difference between two split shear waves, Δt. The two split shear waves should have the same waveform because they were decomposed from the wave that has traveled with the same waveform before entering an anisotropic medium. We use a cross-correlation method (Ando et al. 1983, Hiramatsu et al. 1998) to check whether two split shear waves have the same waveform or not. After this check, then we try to find optimum ϕ and Δt. The larger the absolute value of the cross-correlation is, the more similar the two split shear waveforms are.
Correlation between crustal anisotropy and seismogenic stress field beneath Shillong–Mikir Plateau and its vicinity in North East India
Published in Geomatics, Natural Hazards and Risk, 2021
Santanu Baruah, Antara Sharma, Chandan Dey, Sowrav Saikia, Goutam Kashyap Boruah, Ayodeji Adekunle Eluyemi, Prachurjya Borthakur, Nabajyoti Molia, Anwesha Dutta Hazarika, Saitluanga Sailo, Sebastiano D’Amico, Manoj K. Phukan, Saurabh Baruah, J. R. Kayal
The phenomenon of crustal anisotropy may be caused by the preferred alignment of micro-cracks or joints in the sedimentary bedding planes in the formation, or in the highly foliated metamorphic rocks (Margheriti et al. 1997). Sometime the cracks in the rocks are preferentially aligned in the maximum compressive stress direction and the anisotropy in the crust resulting from aligned cracks are used to estimate the crustal stress. Further, anisotropy can be observed near local active faults and lineaments with preferentially oriented cracks, which may indicate the presence of stress field (Pastori et al. 2012). In a region dominated by strike-slip mechanism, ơ1 is horizontal and is represented by ơH (maximum horizontal stress) and the minimum stress is represented as Ơh. Generally, effective elastic properties of rocks and the crustal stress field are related to the alignment of microcracks and the seismic shear waves in the direction of ơH are expected to travel faster than those in the direction of σh (Nur and Simmons 1969; Nur 1971; O’Connell and Budiansky 1974; Hudson 1981). The shear wave splitting is commonly characterized by two parameters: Polarization direction (ϕ) and time delay (δt) between the fast and slow shear waves. Shear wave splitting helps to understand characteristics of anisotropy in the crust below seismic stations. A detailed description of the method, analysis and results is given in our earlier work (Sharma et al. 2017).