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Wave-equation migration velocity analysis for VTI models with geological and rock physics constraints
Full thesis PDF
Table of contents
- Chapter 1: Introduction
- Chapter 2: WEMVA for VTI models
- Chapter 3: Rock physics constrained anisotropic WEMVA
- Chapter 4: 3-D field data test
- Chapter 5: Conclusions
- Appendix A: Implicit finite differencing
- Appendix B: Rock physics modeling
- Appendix C: RTM-based WEMVA for VTI models
Abstract Wave-equation migration velocity analysis (WEMVA) is a powerful technique for robust velocity model building when the subsurface is complex and the starting model is far from true. However, this traditional isotropic WEMVA technique cannot explain anisotropic wave phenomenon, which has significant effects when the reflectors are steeply dipping and/or the waves are traveling at large angles. Furthermore, errors in the anisotropic parameters cause similar defocusing as errors in isotropic velocity. Any defocusing caused by anisotropy may be translated to false updates in the isotropic velocity, which lead to further mispositioning and misinterpretations. However, simple extension of the isotropic WEMVA to the anisotropic medium cannot provide an unique and reliable solution due to the nonlinear and underdetermined nature of the anisotropic model building problem. Many anisotropic models with vastly different geological interpretations may explain the surface seismic data equally well.
This thesis addresses these issues by including anisotropic effects in WEMVA and by integrating other useful information from geology and rock physics to better regularize the inversion. First, I extend the isotropic WEMVA method to the anisotropic medium to evaluate and update the anisotropic Earth models. Instead of the industry standard ray-based tomographic methods, the anisotropic WEMVA technique uses wavefields as information carrier to handle both the complex subsurface and the frequency-dependent behavior of the wave propagation. I include the geological information using steering filters to regularize the gradients. Both synthetic and field 2-D examples show that the anisotropic WEMVA technique can resolve the errors in both velocity and anisotropic parameters. Consequently, the migration images are