Anisotropic velocity analysis of P-wave reflection and borehole data

Abstract

Efficient application of the modern imaging technology requires development of velocity-analysis methods that take anisotropy into account. In the thesis, I present time- and depth-domain algorithms for anisotropic parameter estimation using P-wave reflection and VSP (vertical seismic profiling) data. First, I introduce a nonhyperbolic moveout inversion technique based on the velocity-independent layer-stripping (VILS) method of Dewangan and Tsvankin (2006). The layer stripping of moveout parameters in the conventional method is replaced by the more stable stripping of reflection traveltimes. Then, to estimate the interval parameters of TTI (transversely isotropic with a tilted symmetry axis) models composed of homogeneous layers separated by plane dipping interfaces, I develop 2D and 3D inversion algorithms based on combining reflection moveout with borehole information. These algorithms help build an accurate initial TTI model for migration velocity analysis. To perform parameter estimation for more complicated heterogeneous TTI media, I develop a 2D P-wave ray-based tomographic algorithm. The symmetry-direction velocity VPO and the anisotropy parameters Epsilon and Delta are iteratively updated on a rectangular grid, while the symmetry-axis tilt Nu is obtained by setting the symmetry axis orthogonal to the reflectors. To ensure stable reconstruction of parameter fields, reflection data are combined with walkaway VSP traveltimes. To improve the convergence of the inversion algorithm, I propose a three-stage model-updating procedure that gradually relaxes the constraints on the spatial variations of Epsilon and Delta. Geologic constraints are incorporated into tomography by designing appropriate regularization terms Synthetic tests for models with a "quasi-factorized" TTI syncline (i.e., Epsilon and Delta are constant inside the TTI layer) and a TTI thrust sheet are used to identify conditions for stable parameter estimation. Then the performance of the regularized joint tomography of reflection and VSP data is examined for two sections of the more complicated TTI model produced by BP that contain an anticline and a salt dome. Finally, the algorithm is applied to a 2D line from 3D OBS (ocean bottom seismic) data acquired at Volve field in the North Sea.