Analysis of PS-Wave Birefringence from A 3-D/3-C Land Survey for Fracture Characterization

An important application of 3-D converted P to S-waves (PS-waves) is to exploit S-wave birefringence (splitting) for delineating reservoir fractures. In azimuthally anisotropic media, fracture intensities and orientations are directly related to traveltime differences between the fast and slow S-wave and the polarization direction of the fast S-wave, respectively. Analysis techniques to help characterize a fracture setting are demonstrated using a 3-D data set from the Green River basin, Wyoming. Ratios of fast (PS1) and slow (PS2) average velocity are particularly important to identify the vertical extent of overburden anisotropy, as well as reservoir horizons. Overburden effects can be removed by 2Cx2C Alford rotation and layer-stripping analyses before characterizing deeper horizons. In addition, less quantitative attributes, which are very sensitive indicators of S-wave splitting, can be employed such as residual off-diagonal 2Cx2C amplitudes after overburden removal and isochron differences between PS1 and PS2.