Exact reflection coefficients of orthorhombic anisotropic media
ZHANG Xueying1, SUN Pengyuan2, MA Xuejun3, LU Jun4, LI Mengqi4
1. School of Energy Resources, China University of Geosciences(Beijing), Beijing 100083, China; 2. Geophysical Research and Development Center, BGP, CNPC, Zhuozhou, Hebei 072751, China; 3. Petroleum Exploration and Production Research Institute, Sinopec Northwest Oilfield Company, Urumqi, Xinjiang 830011, China; 4. School of Geophysics and Information Technology, China University of Geosciences(Beijing), Beijing 100083, China;
Abstract：Most of continental sedimentary formations in China are characterized by thin interbeds.Under the influence of tectonic movement,in sedimentary formations,vertical or nearly vertical high-dip fractures are developed,which result in typical orthorhombic anisotropy.This kind of orthorhombic anisotropy can be regarded as the superposition of VTI and HTI.Therefore,the study on AVO responses of orthorhombic anisotropic media is significant for fine characterization of fractured reservoirs.To the orthor-hombic anisotropy induced by a set of upright frac-tures developed in a VTI background,Tsvankin aniso- tropy parameters are used to construct the stiffness coefficient matrix.Then,the method for calculating exact reflection and transmission coefficients is presented based on the Christoffel equations and the correspon-ding boundary conditions.Model test shows that: ①if P-wave is incident from a low-impedance isotropic medium to a high-impedance orthorhombic anisotropic medium,the increase of the VTI anisotropy coefficient in the background media will lead to the increase of the PP-wave reflection coefficient and the decrease of the PS1- and PS2-wave reflection coefficients; and the increase of fracture weakness will lead to the decrease of the PP-wave reflection coefficient and the increase of the PS1- and PS2-wave reflection coefficients; ②if P-wave is incident from a high-impedance isotropic medium to a low-impedance orthorhombic anisotropic medium,the reflection coefficients of PP-,PS1- and PS2-waves present the opposite characteristics; ③the azimuthal anisotropy of PP-wave reflection coefficients are much smaller than those of PS1- and PS2-waves.By comparing with the results of Rüger approximate reflection coefficient formulas,the accuracy of our calculation method is verified.
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