Stacking-velocity Tomography with Borehole Constraints for Tilted TI Media

Abstract

We present a 2D inversion methodology for transversely isotropic media with a tilted symmetry axis (TTI) based on combining reflection moveout with borehole information. The input data include P-wave normal-moveout (NMO) velocities, zero-offset traveltimes and reflection time slopes, as well as check-shot traveltimes, reflector depths and dips measured in a borehole. For a dipping TTI layer, estimation of all the medium parameters proves to be ambiguous despite the borehole constraints. However, under the assumption that the symmetry axis is orthogonal to the reflector (a model typical for dipping shale layers), Vp0 and delta can be recovered with high accuracy, even when the symmetry axis deviates by 5 degrees from the reflector normal. The parameter epsilon cannot be constrained for dips smaller than 60 degrees without using nonhyperbolic moveout. To invert for the interval parameters of layered TTI media, we apply 2D stacking-velocity tomography supplemented with the same borehole constraints. Numerical testing on noise-contaminated data shows that VP0 and delta are well resolved, if the range of reflector dips does not exceed 30 degrees. Our algorithm can be used to build an accurate initial TTI model for migration velocity analysis.