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Wave-equation migration velocity analysis for VTI models with geological and rock physics constraints

by Yunyue (Elita) Li

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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 improved with higher resolution and correct depths for the reflectors.

Second, I mitigate the ambiguities among the anisotropic parameters using the rock physics prior information. I model shale anisotropy in the seismic scale from well log measurements and the interpretation results from a previous isotropic processing workflow . By varying the key parameters to the rock physics models, I include different geological scenarios and parameter uncertainties in the rock physics modeling. As a result, multiple realizations of the anisotropic parameters are obtained and used as the rock physics constraints for the seismic data inversion. On a synthetic example, I show the correct rock physics regularization accelerates the convergence of the well constrained parameter and brings extra information for the poorly constrained parameters. Anisotropic WEMVA inversion of a 3-D field data set yields models that are consistent with seismic data, geological knowledge, as well as rock physics information. The migration image based on the inverted models shows better-resolved faulting discontinuities and better imaged salt flanks.

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