AVO in transversely Isotropic media—An overview

Abstract

The amplitude variation with offset (AVO) responses of elastic transversely isotropic media are sensitive to contrasts in both of Thomsen’s anisotropic parameters δ and ε. The equation describing P-P reflections indicates that the smaller the contrasts in isotropic properties (compressional velocity, shear velocity, and density) and the larger the contrasts in δ and ε across an interface of reflection, the greater the effects of anisotropy on the AVO signature. Contrasts in δ are most important under small‐to‐medium angles of incidence as previously described by Banik (1987), while contrasts in ε can have a strong influence on amplitudes for the larger angles of incidence commonly encountered in exploration (20 degrees and beyond). Consequently, using Rutherford and Williams’ AVO classification of isotropic gas sands, type I gas sands overlain by a transversely isotropic (TI) shale exhibit a larger decrease in AVO than if the shale had been isotropic, and type III gas sands overlain by a transversely isotropic (TI) shale exhibit a larger increase in AVO than if the shale had been isotropic. Furthermore, it is possible for a “type III” isotropic water sand to exhibit an “unexpected) increase in AVO if the overlying shale is sufficiently anisotropic. More quantitative AVO interpretations in TI media require considerations of viscoelastic TI in addition to elastic TI and lead to complicated integrated earth models. However, when elastic and viscoelastic TI have the same axis of symmetry in a shale overlying an isotropic sand, both elastic and viscoelastic TI affect the overall AVO response in the same direction by constructively increasing/decreasing the isotropic component of the AVO response. Continued efforts in this area will lead to more realistic reservoir models and hopefully answer some of the unexplained pitfalls in AVO interpretation.

Constraints on elastic parameters and implications for lithology in VTI media

“…When the anisotropy is caused by thin layering, additional constraints apply: (Backus,19G2). The VTI medium can also be described using vertical ray velocities V p = and VSV = * (where p is the density) together with the parameters e, 6, and 7 defined by Thomsen (1986): The Thomsen parameters were originally defined for the purpose of quantifying weak anisotropy, but provide convenient notation for describing any VTI medium since the algebraic expressions in (8) through (10) appear in the equations for dip-moveout corrections, normal-moveout velocities, and variations of amplitude with offset for seismic reflection data (e.g., Thomsen, 1986;Berge, 1991;Tsvankin, 1994;Alkhalifah and Tsvankin, 1994;Tsvankin and Thomsen, 1994;Blangy, 1994). The factor E -6 is particularly important.…”

Naturally fractured tight gas reservoir detection optimization. Final report

“…Anisotropy has profound effects on AVO. Layering anisotropy (transversely isotropic with vertical symmetry axis, or TIv) in cap rocks has been studied extensively (Samec et al, 1990;Blangy, 1994;Hanitzsch et al, 1995;Waluyo et al, 1995;Kumar et al, 1996); AVO applications to azimuthal anisotropy (transversely isotropic with horizontal symmetry axis, or TIH) in the reservoir are more recent (Strahilevitz and Gardner, 1995;Ruegex and Tsvankin;Ruga, 19%;bugen and Ursiq 19%;Sayers and Rickett;1997). We ran 18 test cases for all combinations of isotropy, TIV, and TIH in the cap shale and reservoir sand, for both positive and negative bulk impedance contmts.…”

Integrated seismic study of naturally fractured tight gas reservoirs. Final report, September 1991--January 1995

“…However, since the early 1 9 8 0 '~~ people have been trying to use amplitude and amplitude versus offset (AVO) information as a hydrocarbon indicator (Ostrander, 1984(Ostrander, ,1985Sengupta, 1987;Blangy, 1992), because AVO can be quite sensitive to Poisson's ratio. In recent years several papers have been published on the effects of anisotropy on AVO (Wright, 1987;Pelissier et al 1991;Mallick and Frazer, 1991;Blangy, 1994). The results suggest the possibility of using P-wave data (azimuthal AVO, azimuthal stacking velocity) to study anisotropy.…”