The Analysis Of Anisotropy In Long-wave Equivalent Media Of Thin Interbed

The tight sandstone gas reservoir represents the thin interbed of weak impedance and the coal strata represent the thin interbed of strong impedance.They have the general characteristics of most continental sedimentary rocks in China.In this paper,the expressions of Thomsen coefficients for prosodic thin interbed are simplified and calculated.By calculating the Thomsen coefficient of the equivalent VTI medium,the influence factors of the anisotropy of the equivalent VTI medium are discussed.The results show that:(1)The equivalent velocity is affected by the velocity difference,thickness ratio and density difference of the two media and the true value of velocity and density does not affect the trend of equivalent velocity.(2)The anisotropy is mainly related to the difference of Poisson's ratio,thickness ratio and the Lame coefficient of the two media.It is linear that the variation of ??? with the difference of Poisson's ratio and the slope is related to the ratio of the Lame coefficient.Anisotropy is enhanced when Poisson's ratio and lame constant ratio change.(3)According to different stratification ways,the tight sandstone gas oil reservoirs and coal measures strata are divided and their anisotropy coefficients are calculated.The effect is better based on lithology to divide the thin interbed.In order to study the effect of anisotropy on the reflection coefficient AVO curve,the reflection coefficient of the periodic thin interbed is calculated by the propagation matrix theory and the reflection coefficient is related to the frequency,the angle of incidence and the thickness of the interlayer unit.(1)The intercept of AVO curve of PP wave and the slope of AVO curve of PS wave varies obviously with the interlayer thickness when the incident angle is less than 30.If the interlayer thickness is less than 1/8 of the main wavelength,it can be ignored that the AVO curve is influenced by the interlayer thickness.(2)With the change of N/G,the reflection coefficient of PP wave is more complex,while the AVO curve of PS wave has a certain regularity with N/G and this law is influenced by the frequency of the incident wave.(3)The reflection coefficient is influenced by physical parameters of thin interbed and physical properties of the bottom plate.The change of reflection coefficient is between 0.1~0.6 in the strong impedance thin interbed,while the reflection coefficient is less than 0.1 in the weak impedance difference thin interbed.(4)The reflection coefficient of the equivalent medium is influenced by the incident angle and the physical properties of the thin interbed,and it is independent of the physical properties of the bottom plate and the frequency.The intercept of AVO curve of PP wave and the slope of AVO curve of PS wave is positively correlated with N/G.(5)Finally,the reflection coefficient of thin interbed is compared with the reflection coefficient of equivalent medium,and the relative error between the two is calculated.The relative error of the two is less than 10% in some single frequency,and the maximum and minimum of relative errors alternate with frequency.