P-wave imaging for VTI media
Published:January 01, 2012
If one does not look for the existence of anisotropy in P-wave data, it can often go unnoticed. Still, where the subsurface is anisotropic (e.g., in the presence of shale formations), conventional P-wave processing based on the assumption of isotropy yields errors in seismic images and interpretations. Such anisotropy-induced distortions as mispositioning of both horizontal and dipping reflectors and misstacking of dipping events were discussed in chapters 3, 6, and 7, and will be further analyzed below.
The most critical step in correcting for anisotropy in seismic processing is estimation of the parameters of anisotropic velocity fields. As shown in chapters 6 and 7, P-wave time processing for VTI media with a laterally homogeneous overburden requires knowledge of two parameters [Vnmo (0) and η], which can be obtained from surface P-wave data. To estimate the vertical velocity VP0 and build VTI models suitable for depth imaging, P-wave reflection moveout has to be combined with borehole data (e.g., check shots) or traveltimes of mode-converted or pure shear waves (see chapter 7). Note that if the overburden contains dipping interfaces or other kinds of lateral heterogeneity, P-wave reflection traveltimes become dependent on the individual values of VP0, ∈, and δ and, for a certain class of models, can be used to reconstruct the velocity field in depth (Le Stunff et al., 2001).
Once the needed medium parameters have been estimated, conventional isotropic P-wave processing algorithms can be extended to vertical transverse isotropy by using phase-velocity equations introduced in chapter 1 and equations for reflection moveout from chapters 3 and 4. Here, we describe several efficient dip-moveout (DMO) and migration techniques for VTI media based on well-established isotropic imaging algorithms. This discussion of VTI processing is by no means exhaustive, as it does not include, for example, Kirchhoff migration. The main goal of this chapter is to show how analytic results developed in the previous chapters can be applied in seismic processing and to provide practical recipes for devising anisotropic imaging algorithms. Although DMO correction is seldom used in modern seismic processing, it retains a certain relevance because moveout of dipping events provides essential information for anisotropic parameter estimation.
Figures & Tables
Seismic Signatures and Analysis of Reflection Data in Anisotropic Media, Third Edition
This is a new edition of Ilya Tsvankin’s reference volume on seismic anisotropy and application of anisotropic models in reflection seismology. Seismic Signatures and Analysis of Reflection Data in Anisotropic Media, Geophysical References Series No. 19, provides essential background information about anisotropic wave propagation, introduces efficient notation for transversely isotropic (TI) and orthorhombic media, and identifies the key anisotropy parameters for imaging and amplitude analysis. To gain insight into the influence of anisotropy on a wide range of seismic signatures, exact solutions are simplified in the weak-anisotropy approximation.