AVO Forward Modeling Of Anisotropic Media

AVO(Amplitude variation with offset)is a seismic exploration method to study lithology and detect oil and gas by amplitude information.In conventional AVO analysis,it is often assumed that the underground medium is isotropic.However,in practice,this assumption is not strict,and wave anisotropy exists widely in the earth medium.Based on petrophysical analysis and logging data,the accuracy of Zoeppritz equation and its approximation in isotropic media is discussed.VTI(transverse isotropic media model with vertical symmetry axis),HTI(transverse isotropic media model with horizontal symmetry axis)and TTI(transverse isotropic media model with inclined symmetry axis)are established.The influence of these three media on AVO characteristics is discussed by qualitative model.For VTI media,the effects of ?? and ?(variation of anisotropic parameters)on the reflection coefficients of P-wave are studied in this paper.VTI medium is a very common anisotropic medium model.The influence of anisotropy of VTI medium on AVO can not be ignored.From the forward results,it can be seen that VTI medium does not change the intercept of P-wave reflection coefficient.?? has a great influence on P-wave reflection coefficient at a small angle,while ? has almost no effect on P-wave reflection coefficient at a small angle.With the increase of incident angle,the influence of ? on P-wave reflection coefficient increases gradually.In the theory of homogeneous isotropic media,the reflection coefficient which should be gradually reduced with the incident angle may be gradually increased because of the influence of VTI medium.For HTI media,the effect of azimuth angle on AVO characteristics is studied in this paper.When the incident angle is relatively small,the change rate of P-wave reflection coefficient is almost zero no matter how the azimuth angle changes,but when the incident angle is large,P-wave reflection coefficient becomes sensitive to the change of azimuth angle gradually.The difference between P-wave reflection coefficient when the incident direction is parallel to the direction of fracture development and P-wave reflection coefficient when the incident direction is perpendicular to the direction of fracture development is the greatest.Accordingly,the fracture can be predicted theoretically.In addition,the characteristics of AVO are greatly influenced by the properties of filling materials and the density of cracks.For TTI media,the article focuses on the effect of crack dip on AVO characteristics.If the fracture density is fixed,when the fracture layer is filled with fluid,the P-wave reflection coefficient curve has a similar trend with the change of the fracture inclination angle,and the intercept change rate does not exceed 10%;when the fracture layer is a gas-bearing fracture,With the increase of the dip angle of the crack,the intercept of the P-wave reflection coefficient generally increases,and the gradient also changes greatly.Under the condition that the fracture property is gas-bearing fracture,the intercept of the P-wave reflection coefficient decreases gradually with the increase of the fracture density,and the P-wave reflection coefficient also has a different trend with the incident angle.