Transversely isotropic velocity analysis for lithology discrimination

Abstract

This paper presents a practical technique for estimating the five stiffness coefficients A, C, F, L and M from seismic traveltime measurements for multiple, horizontally-layered, transversely isotropic media. This technique is based on the construction of ray velocity surfaces of elastic wave propagation in terms of five measurement parameters — three from a skewed hyperbolic moveout formula for P waves and two from SH waves. The skewed hyperbolic formula is used for analyzing moveout of signals on multi-channel P-wave surface seismic or VSP data. A model-iterative optimization scheme is then used to invert the five measurement parameters for the five stiffness coefficients in a layer-stripping mode. Both synthetic model and field experiments are performed to demonstrate the feasibility of the method. Synthetic P-wave model experiments demonstrate that the skewed hyperbolic moveout formula yields an excellent fit to time-distance curves over a wide range of ray angles. The measurement parameters are shown to reflect adequately the characteristics of velocity dependency on ray angle, i.e., velocity anisotropy. Although inversion errors generally increase with increasing number of layers, the proposed method does provide a quantitative measure of velocity anisotropy as valuable additional information that can not be readily obtained from conventional methods. A field VSP data example is also provided to show the correlation between the anisotropy parameters with lithology. Chalk and shale exhibited high degrees of anisotropy, and sands showed low degrees of anisotropy.