Viability of shear‐wave amplitude versus offset studies in anisotropic media

Abstract

The study of shear‐wave splitting can yield information about crack strike, crack density, and other parameters of the cracks and aligned inclusions within the in situ rockmass. Such information is important for reservoir characterization and other hydrocarbon production applications. Crack density is usually inferred from the percentage shear‐wave anisotropy which is measured from time delays between the two split shear waves. Reservoirs may be too thin for discernible time delays to build up, and the time delay (and crack density) in a reservoir layer may be unresolvable in conventional arrival‐time analysis of reflection surveys or vertical seismic profiles. Shear‐wave amplitude versus offset (AVO) techniques are studied to see if they provide a more viable method of determining anisotropic parameters in thin reservoir layers. The behaviour of reflected shear‐wave amplitudes with angle of incidence is investigated in simple two‐layer models. The variations in shear‐wave AVO are calculated for a range of percentage shear‐wave anisotropies for water‐filled cracks and dry cracks in the second layer (representing a reservoir) to see if characteristic information about reservoir properties can be extracted from the shear‐wave AVO signatures. The shear‐wave AVO curves, for thin cracks, are sensitive to changes in anisotropy and crack content. Most of the information about crack content is contained in wide offset reflection data which suggests applications in crosshole monitoring of enhanced oil recovery. The variations in shear‐wave AVO may be distorted by the acquisition system. Most of the information about the percentage anisotropy is contained in the amplitudes of shear waves reflected at near‐vertical angles of incidence. Vertical incidence reflection amplitude methods are reviewed and a simple graphical procedure is suggested for determining their viability in different reservoir environments. Copyright © 1991, Wiley Blackwell. All rights reserved