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Anisotropic Inversion for VTI Media Based on a Modified Approximation of the PP-Wave Reflection Coefficient: Theory AndNormal access

Authors: F. Zhang, T. Zhang and X. Li
Event name: 80th EAGE Conference and Exhibition 2018
Session: AVO/AVA Analysis - Case Histories
Publication date: 11 June 2018
DOI: 10.3997/2214-4609.201800670
Organisations: EAGE
Language: English
Info: Extended abstract, PDF ( 798.78Kb )
Price: € 20

Summary:
Anisotropy has significant influence on the seismic amplitude variation with offset (AVO) response. Although analytical solutions and approximations of the PP-wave reflection coefficient for tranversely isotropic media have been explicitly studied, it is diffcult to apply these equations to anisotropic AVO inversion, because five parameters need to be estimated, and such an inverse problem is highly ill-posed. A modified approximation of the PP-wave reflection coefficient is proposed in this paper based on Rüger’s formula for VTI (tranverse isotropy with a vertical symmetry axis) media. The derived equation is formed using the attributes of acoustic impedance, anisotropic shear modulus, and an anisotropic P-wave velocity, which is proportional to the vertical P-wave velocity with the natural exponential function of Thomsens’s ε. Numerical tests show that the approximation has sufficient accuracy over a wide range of angles. The field-data application in a lower Silurian-age shale formation reveals that the inverted attributes are very useful for the characterization of a shale-gas reservoir. The Thomsen’s anisotropy parameter ε can be recovered subsequently. Inverted ε not only provides a reliable model for the anisotropic imaging and full waveform inversion, but also acts as an indicator of gas-saturated shale, which shows lower anisotropy than surrounding shales.


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