Study On AVO And Elastic Impedance In Azimuthal Anisotropic Media

Fractured reservoirs are becoming more and more important in petrol exploration and development .To find fracture is main task in fracture formation .Identify and prediction of fracture and its developmental zone is meaningful to reservoir exploration and development .According to solid mechanics ,fracture strata is anisotropic media .For this reason ,the theory of anisotropy is the foundation of the theory of fracture .VTI and HTI model are the ideal anisotropic model . Horizontally transverse isotropic with the horizontal axis of symmetry (HTI) media is the simplest azimuthal anisotropic model used to describe realistic fractured reservoirs, which is one of the most effective model for studying seismic characterization of fractures .Recent advances show that it is therefore important to obtain fracture developmental zones , fracture strike and crack density from PP wave and PS converted wave seismic data .However ,the development of PS wave AVO analysis in azimuthal anisotropic media is slow. The main problem is lack of the simple and brief PS wave reflection coefficient equation .Still ,the most inversion of combined the PP wave and PS wave , and inversion of elastic impendence in anisotropic media are limited for this reason.Cherepanov and Nefedkina (2004)have derived the approximate linearized solution for PS wave reflection coefficient for the first time using assumption that elastic parameters contrast at separating interface and anisotropy are weak. But the parameters in the equation are not easy to use. In order to resolve this problem, the new approximate PS wave reflection coefficient has been obtained by the Thomsen anisotropic parameters. The simple relationship between the reflection amplitudes and anisotropic coefficients given here can be regarded as helpful rules of thumb in quickly evaluating the importance of anisotropy in a particular application.Then we discuss the effect of new reflection coefficient to three kind of gas sand AVO model .It is visible tools to identify the AVOⅢgas sand .The effect of three Thomsen parameters to PS wave reflection show that the effect of shear-wave splitting parameterγ(V) is strongest and the effect of anisotropy coefficientε(V) weakest in three parameters. And the effect of three parameters vary with azimuthal angle .The less of azimuthal angle ,the stronger of the effect .Based on the new approximation reflection coefficient in HTI media by the Thomsen anisotropic parameters, the attributes of AVO and characteristics of AVO background trends crossplotting of density ,S-wave velocity contrast and shear modulus have been described . It will be helpful to AVO interpretation in complicate condition.Petrophysics theory is the key of AVO technology .It is the bridge of well ,geology and seismic .we can get the geological information from the seismic data,such as lithological character ,physical property and hydrocarbon showing. Wherefore ,we introduce the Gassmann theory , postulation condition and the method of fluid substitution .Furthermore ,we study the effect of the porosity ,saturation and Thomsen anisotropic parameters to PS wave reflection coefficients in HTI-Isotropic pore reservoir model. The results show that the effect of porosity and anisotropic parameters is much stronger than that of saturation.And effect of porosity is a little stronger than that of anisotropic parameters.These conclusion are of significance in PS wave AVO analysis and elastic AVO theoretical investigation.Acoustic impedance (AI) will produce the ambiguity as the hydrocarbon indictor in the complicated reservoir .Using a simple linear formalism, Connolly (1999) introduced the concept of elastic impedance(EI) to allow investigating AVO anomalies at wide-angle regions of incidence. Because the resulting formula explicitly depends on medium parameters, the advantage of EI is often regarded as a rock attribute analogous to AI but for varying incidence angle ranges. Then S impedance(SEI) and PS impedance are obtained .This integration strategy is great helpful to the effect of AVO. Then ,the PP and PS wave elastic impendence in anisotropic media is derived for the first time in this paper. The advantage of new elastic impedance is that introduced the concept of elastic impedance(EI) to allow investigating AVO anomalies at wide-angle regions of incidence and azimuthal angle with the effect of anisotropic parameters .Then ,we can analysis the difference of elastic impedance with azimuthal angle and anisotropic parameters .Especially , we can study the variation of elastic impedance in 3D space .Furthermore , the effect of Thomsen anisotropic parameters to elastic impendence and fluid factor are described .On the basis of these results,we have obtained PP wave elastic impedance is sensitive to the variation of anisotropic parameters.on the contrast , PS wave elastic impedance is immunity to the anisotropic parameters.It has been carried out that variation of PP and PS wave elastic impedance to three kind of typical AVO model.Our previous work provide that new PP wave elastic impedance is only sensitive to AVOⅢ。New PS wave elastic impedance is sensitive to all of three typical AVO model.