Building a 3-D anisotropic model: Its implication to traveltime calculation and velocity analysis

Since 3-D transversely isotropic (TI) synthetic models are rare, we construct an example model to test our 3-D anisotropic traveltime algorithm from the SEG/EAGE 3-D salt body model. The resulting TI model includes a stratification dependent variable axis of symmetry. The strength and location of anisotropy in this model are based on our experience with probable anisotropy behavior observed in field data. The steps used to build the anisotropic model are sufficiently general to derive initial anisotropic velocity models in real settings. Traveltimes for the model are calculated using a 3-D algorithm for transverse isotropic media with a tilted axis of symmetry. Ray-tracing equations, based on the acoustic assumption for anisotropy, efficiently calculates extremely accurate traveltime data. In fact, the acoustic based traveltime calculation is far more efficient than those based on conventional elastic ray-tracing equations. In addition, ray tracing provides us with the flexibility of generating traveltime information corresponding to the most energetic arrivals, an important feature for imaging complex media. The algorithm uses interpolation techniques that are highly stable an efficient to transform the traveltime information from the rays onto a regular grid in 3-D. This traveltime algorithm is sufficiently flexible and efficient to make repetitive traveltime calculations for purposes of prestack anisotropic migration and velocity estimation practical.