at zero-offset

Jaime Ramos-Martı́nez, Andrey A. Ortega‡, and George A. McMechan Splitting of zero-offset reflected shear-waves is mea-sured directly from three-component finite-difference synthetic seismograms for media with intersecting verti-cal crack systems. Splitting is simulated numerically (by finite differencing) as a function of crack density, aspect ratio, fluid content, bulk density, and the angle between the crack systems. The type of anisotropy symmetry in media containing two intersecting vertical crack systems depends on the angular relation between the cracks and their relative crack densities, and it may be horizontal transverse isotropy (HTI), tetragonal, orthorhombic, or monoclinic. The transition fromone symmetry to another is visible in the splitting behavior. The polarities of the reflected quasi-shear waves polarized perpendicular and parallel to the source particle motion distinguish between HTI and orthorhombic media. The dependence of the mea-sured amount of splitting on crack density for HTI sym-metry is consistent with that predicted theoretically by the shear-wave splitting factor. In orthorhombic media (with two orthogonal crack systems), a linear increase is observed in splitting when the difference between crack densities of the two orthogonal crack systems increases. Splitting decreases nonlinearly with the intersection an-gle between the two crack systems from 0to 90. Surface and VSP seismograms are simulated for a model with several flat homogeneous layers, each con-taining vertical cracks with the same and with differ-ent orientations. When the crack orientation varies with depth, previously split shear waves are split again at each interface, leading to complicated records, even for sim-ple models. Isotropic and anisotropic three-component S-wave zero-offset sections are synthesized for a zero-offset sur-vey line over a 2.5-Dmodel of a carbonate reservoir with a complicated geometry and two intersecting, dipping crack sets. The polarization direction of the fast shear wave, propagating obliquely through the cracked reser-voir, is predictedby theoretical approximations for effec-tive properties of anisotropicmediawith twononorthog-onal intersecting crack sets