| Generally, fluid saturated pore, fracture and crack with multi-scaled, multi-orientation are widely exist in earth media. And the true earth media is anisotropic visco-elastic rather than isotropic elastic. Pore, fracture, crack and the fluid saturated in them have two main effects on the earth media. On one hand, the elastic module (such as shear and bulk modulus) of the earth media is decreased, and the fluid transport property is changed. On the other hand, the seismic wave propagating in such medium shows anisotropy, attenuation and dispersion.The two major difficulties on study of geophysics in modern times are two-phase and anisotropy of the earth media. In case of general anisotropy media, there are 21 independent parameters in elastic tensor which satisfy with the Christoffel equation. Customarily, there is no analytic solutions for general anisotropy media. Ray theory is suitable to high frequency situation. Moreover, as the most of the earth media is weakly anisotropic, perturbation theory can be applied to get approximate solutions (such as phase velocities or energy velocities, polarization vectors) by using WA (Weakly Anisotropic) parameters. We have got linear inversion equations of WA parameters in the TTI media. To explain frequency dispersion and attenuation of seismic wave, we also take the visco-elastic into account.Since 1970s, scientists have studied seismic anisotropy by the following 4 main method: (1) azimuth anisotropy of seismic wave velocities; (2) shear wave splitting; (3) receiver function method; (4) surface wave polarization method. Among them, as an important method to analyze anisotropic media as well as mega tectonics (such as lithosphere), shear wave splitting is considered to be a relatively simple and the most intuitive one, with advantages of high lateral resolution, sensitive to crack density and ratio.This thesis has introduced EDA (Extensive Dilatancy Anisotropy), APE (Anisotropic Poro-Elasticity) as well as Chapman multi-scale model as the example of equivalent medium model. Among them, EDA and APE model study on the single-scale aligned, fluid-situated, parallel penny-shaped micro-cracks with density of ε= Na3 /V(ε<< 1). And if shear-wave propagates through EDA media it will split into fast and slow waves. EDA is widely accepted as the primary cause of crust anisotropy. In addition, the dominant direction of fast wave polarization coincides with the maximum horizontal principal compressive stress. By studying the shear-wave splitting at different period of time, we can study various local stresses. Although EDA model has acquired important achievements, it cannot explain the various and the scatter of shear-wave splitting before great earthquakes. APE model indicates that shear-wave is sensitive to geometry characteristics (such as crack density and ratio), which is sensitive to various stress fields. That is how the shear-wave splitting got immediately response to changes of local stress before and after big earthquakes. The response could be applied to monitor the local stress. Since the local stress varies at different geological tectonic period, the micro-cracks of earth media are multi-scales and probably with multi preferred directions. Chapman multi-scale model gets frequency dependent elastic matrix by adding multi-scale fractures into the bulk media with micro-cracks.By taking complex elastic matrix into account, we can consider the wave theory of anisotropic two-phase media. The real and imagine part of elastic matrix are frequency dependent. And the computational complexity will increase because of introducing of the imagine part into elastic matrix and Christoffel equation. At high frequency approximation, ray tracing theory and perturbation theory of weakly anisotropic elastic media could be applied to weakly anisotropy attenuation (WAA) media by replacing solid isotropic bulk and real value perturbation of elastic matrix with anisotropic bulk and complex perturbation, respectively. And we get the linear inversion formula of P-wave attenuation with azimuth in WAA media.Applying liner inversion functions of TTI media to 2,3 and 6 sets of walkaway VSP profiles, we have verified the validity and reliability of inversion method. And then we applied the inversion method to a real data set of 3 profiles from Java Sea.Based on the linear inversion formula of P-wave attenuation in WAA media, we analyzed earthquakes occurred in a 200km region around Wenchuan Ms8.0 earthquake center by Meyer-Yamada wavelet (MYW), and obtained average attenuation of wavelet spectrum and azimuth anisotropic attenuation at 9 stations within 140km around Wenchuan Ms8.0 earthquake center. This method can make use of the most of the small earthquakes other than the shear-wave splitting method. It will find much more applications in the future. |
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