Effect of viscoelasticity and anisotropy on amplitude‐versus‐offset interpretation

Samec, Pierre; Blangy, J. P.; Nur, Amos

doi:10.1190/1.1890030

Cited by 1 publication

(1 citation statement)

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“…Direct measurements using vertical seismic profiling (VSP) (Hauge, 1981;Hedlin et al, 2001), well logs (Schmitt, 1999), and rock samples (Behura et al, 2007;Winkler & Nur, 1982) show that attenuation and velocity dispersion can be significant, especially within hydrocarbon-saturated zones. Therefore, some failures of conventional AVO analysis can be attributed to the influence of attenuation (Luh, 1988;Samec et al, 1990).…”

Section: Introductionmentioning

confidence: 99%

Reflection coefficients in attenuative anisotropic media

Behura

Tsvankin

2009

GEOPHYSICS

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Reservoir rocks such as heavy oils are characterized by significant attenuation and, in some cases, attenuation anisotropy. Most existing attenuation studies are focused on plane-wave attenuation coefficients, which determine the amplitude decay along the raypath of seismic waves. Here, we discuss the influence of attenuation on PP-and PS-wave reflection coefficients for anisotropic media with main emphasis on models with VTI (transversely isotropic with a vertical symmetry axis) symmetry. Concise analytic solutions obtained by linearizing the exact plane-wave reflection coefficients are verified by numerical modeling. To make a substantial contribution to reflection coefficients, attenuation has to be strong, with the quality factor Q not exceeding 10. For such highly attenuative media, it is also necessary to take attenuation anisotropy into account if the magnitude of the Thomsen-style attenuation-anisotropy parameters is relatively large. In general, the linearized reflection coefficients in attenuative media include velocity-anisotropy parameters but have almost "isotropic" dependence on attenuation. Our formalism also helps to evaluate the influence of the inhomogeneity angle (the angle between the real and imaginary parts of the slowness vector) on the reflection coefficients. A nonzero inhomogeneity angle of the incident wave introduces additional terms into the PP-and PS-wave reflection coefficients, making conventional AVO (amplitude-variation-with-offset) analysis inadequate for strongly attenuative media. It is interesting that an incident P-wave with a nonzero inhomogeneity angle generates a mode-converted PSwave at normal incidence, even if both halfspaces have a horizontal symmetry plane. This phenomenon can provide an alternative explanation for substantial PS-wave energy at zero offset observed on field data. The linearized solutions developed here can be used in AVO inversion for highly attenuative (e.g., gassand and heavy-oil) reservoirs.

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