CWP-714 Multiparameter TTI tomography of P-wave reflection and VSP data

Transversely isotropic models with a tilted symmetry axis (TTI media) are widely used in depth imaging for complex geologic structures. Here, we present a modification of a previously developed 2D P-wave tomographic algorithm for building heterogeneous TTI models and apply it to synthetic data. The symmetry-direction velocity VP0, anisotropy parameters ǫ and δ, and the symmetry-axis tilt ν are defined on a rectangular grid. To ensure stable re-construction of the TTI parameter fields, reflection data are combined with walkaway VSP (vertical seismic profiling) traveltimes in joint tomographic in-version. To improve the convergence of the inversion algorithm, we propose a three-stage model-updating procedure that gradually relaxes the constraints on the spatial variations of the anisotropy parameters, while the symmetry axis is kept orthogonal to the reflectors. Only at the final stage of the inversion the parameters VP0, ǫ, and δ are updated on the same grid. We also incorporate geologic constraints into tomography by designing regularization terms that pe-nalize parameter variations in the direction parallel to the interfaces. First, the reflection tomography without borehole constraints is tested on a model that includes a bending TTI layer with a wide range of dips. Then we examine the performance of the regularized joint tomography of reflection and VSP data for two sections of the BP TTI model that contain an anticline and a salt dome. The TTI parameters in the shallow part of both sections (down to 5 km) are well-resolved by the three-stage model-updating process. Due to the limited constraints from reflection events and sparse coverage of VSP rays at depth, the velocity field in the deeper part of the section is estimated with larger er-rors. These results provide useful guidance for building accurate TTI models for prestack depth imaging.