Layer-induced seismic anisotropy from full-wave sonic logs: Theory, application, and validation

Abstract

Abstract Thin isotropic elastic layering in the earth is one cause of seismic VTI anisotropy, along with intrinsic anisotropy and fractures. An important issue related to the routine use of VTI seismic data processing is the estimation of the necessary parameters. The full set of layer-induced VTI anisotropy parameters can be computed from full-wave sonic and density log data using Backus averaging. Intrinsic anisotropy can be incorporated if it is known from laboratory analysis. The isotropic layering method is applied to six wells in eastern Saudi Arabia, and the estimated anisotropy parameters are persistent across distances of many kilometers. This leads to the possibility of parameter estimation at sparse well locations for use in seismic data processing. Validation is demonstrated by direct numerical simulation of elastic wavefields in original and Backus-averaged earth models with various window lengths. We observe precise equivalence of the full wavefield when the averaging length is less than or equal to one-third of the minimum dominant wavelength. First arrival information, used in depth migration, is preserved with much longer averaging windows, up to twice the minimum dominant wavelength.