| Currently the research of anisotropy mostly focuses on transversely isotropy(TI), but seismic data based on multicomponent recording indicate that orthorhombic responses are widespread in sedimentary basins; the study of orthorhombic or other more general anisotropy will be one of the effective ways for many geophysics problems. Owing to the lower symmetry, the independent elastic constants that characterize orthorhombic media rises to 9, this increased the difficulty in forward and inverse problems which limited the technological theory development of anisotropy.The analysis of wave propagation in orthorhombic medium is the basis of knowing its property and premise of establishing the relationship between anisotropic parameters and the seismic responses, and parameters estimation. This thesis discussed the wave character of orthorhombic media by means of numerical simulation in order to development an efficient and accurate numerical simulation method aimed at anisotropic media.Lebedev fully staggered grid(LG) finite-difference simulation method was proposed for anisotropic media, it avoids wavefield interpolation of Virieux standard staggered grid(SSG)when its straightforward application to TI media with a tilted symmetry axis which have higher simulation precision, however, this scheme requires more memory and computation. To counter the problems above, this paper optimized LG scheme in two ways.Firstly, we proposed the dispersion improved difference coefficients which can keep the accuracy under lower spatial sampling, reducing the grid dispersion. And also do some work on boundary conditions for anisotropic media, given the selection criteria for choosing Cerjan absorption index which can minimize the width of the attenuation zones. Both of these two improvements can reduce the memory to some extent.Secondly, considering anisotropic formations comprise in a number of models which is usually lower than 20%, Lisitsa et al. coupled the respective advantages of LG and SSG, which the LG is used only inside the anisotropic formations which has lower symmetry, while the SSG is used in the major isotropy or TI, orthorhombic part under constitutive coordinate of the model. This coupled scheme could be efficient on the premise of keeping the accuracy. But the second-order coupling scheme is computationally expensive, requiring a heavy oversampling(about 20~40 in unit wavelength). This paper presents the high-order FD relations in the transitional region of the coupled scheme and a new method for variables interpolation. This high-order coupled scheme could efficiently control the reflection error and overall error produced by the coupling interface even with big space sampling interval(8~10), at the same time, it provides a geometric savings in memory and the computation time decreases as well, thereby simulating the peculiarities of the wavefields in heterogeneous anisotropic media with high efficiency and accuracy.At last, this paper derived 3D phase/group velocity calculation formulas of orthorhombic media, analysis the relationship between wavefield and elastic constants. Then we gave out three types of fracture model which can generate effective orthotropy based on Kachanov effective theory, and then using the improved numerical modeling algorithm, we studied the essential link between fracture parameters, such as density, aspect ratio, the type of filling liquid and so on, and the media’s anisotropic property, pointed out the mechanism of fracture parameters influences the velocity and strength properties of anisotropic media, the results can provided theories basis and practice guidance for the research of fractured carbonate reservoir. |
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