Viscoelastic attenuation, anisotropy, and AVO

Abstract

The classical interpretation relating Amplitude Versus Offset (AVO) to Poisson’s ratio and other petrophysical properties is based on the assumptions of elasticity and isotropy. We extend this interpretation to a layered medium with anisotropic and/or viscoelastic properties, using a Fourier Pseudo‐Spectral method to solve the wave equation. Both viscoelasticity and anisotropy are key factors for the quantitative interpretation of AVO trends, because they contribute to the seismic energy partition at geological interfaces (the reflection coefficients), and because they continually induce propagation effects. We show that: 1) Reflection coefficients at an interface are strongly dependent on the elastic anisotropy of both the overlying and the underlying media. The AVO effect is further complicated by materials with viscoelastic properties. 2) Propagation effects are due to elastic anisotropic energy focusing and viscoelastic dissipation that distort the energy and phase distribution of the incident and reflected wavefronts. These two phenomena can be of the same order of magnitude as variations in reflection amplitudes with offset and can make it difficult to recover reflection coefficients along an interface from seismic data. In theory, they make the amplitude determination of a seismic event somewhat dependent on wavelet phase changes that occur continually as the wavefront propagates. In practice, they create anisotropic radiation patterns and differentially focus the seismic energy distribution along the wavefront. For these reasons, all detailed reservoir characterizations based on modeling and interpretation work should attempt to account for anisotropy and viscoelastic attenuation; this is not an easy task in the real world because of the difficulty in prescribing appropriate physical parameters.

Attenuation of P‐ and S‐waves as a method of distinguishing gas and condensate from oil and water

“…Additionally, it affects the wave propagation via the route source-reflector-receiver. Numerical results from various workers (Bourbie, 1982;Samec and Blangy, 1992) showed an amplitude jump in the AVO trend as a result of an increase in the apparent impedance contrast between the two layers. The impedance change is caused by the viscoelastic properties of the attenuating layers.…”

“…It alters the reflection coefficients at the boundary between two layers (Bourbie, 1982, Samec andBlangy, 1992). Additionally, it affects the wave propagation via the route source-reflector-receiver.…”

The acoustic signature of fluid flow in complex porous media

“…Most previous attempts to develop effective medium models for clayey sandstones have focused on the wave speeds (e.g., Xu and White, 1995;Sams and Andrea, 2001) and ignored the attenuations. This situation should definitely change since seismic attenuation is a potentially useful parameter for characterizing and monitoring hydrocarbon reservoirs in conjunction with seismic velocity (see Klimentos and McCann, 1990;Samec and Blangy, 1992;Carcione et al, 1998b;Koesoemadinata and McMechan, 2001). Fig.…”

“…If the goal is to predict velocity and attenuation spectra in rocks as a function of their petrophysical parameters (or visa versa), as is may be for an application within seismic exploration or reservoir monitoring (e.g., Samec and Blangy, 1992;McCann et al, 1997;Koesoemadinata and McMechan, 2001), one should probably focus on inclusion-based models. Hudson et al (1996), Pointer et al (2000 and Tod (2001Tod ( , 2002 have shown that progress along the inclusion-based line can be obtained if an extremely simple model of the porous microstructure is employed: namely, that where an anisotropic porous/ permeable medium is containing a dilute concentration of small-aspect ratio spheroidal cracks that are distributed in space in accordance with an isotropic correlation function.…”

A method for absorption compensation based on adaptive molecular decomposition

“…The commonly used methods for enhancing vertical resolution of seismic data are based on the assumption that the seismic data is stationary (i.e., the wavelet does not vary with travel time). Moreover, one of the assumptions of the classical interpretation method relating AVO to Poisson's ratio and other petrophysical properties is that the media is elastic (Samec and Blangy, 1992;Gu et al, 2002), but in fact the earth is viscoelastic. So to resolve these problems, absorption compensation is needed.…”