Seismic modeling in fractured media with applications to multicomponent multiazimuth data from southwest Venezuela

The phenomenon of shear wave splitting has been observed from ultrasonic to seismic frequencies and provides evidence that most crustal rocks are anisotropic. The splitting of S-to-S and P-to-S converted wave reflections from the bottom of a layer containing a biplanar distribution of vertical fractures is measured from three-component (3-C) surface data generated by raytracing and by eighth-order finite differencing on a three-dimensional (3-D) staggered grid. A new polygonal shooting raytrace algorithm was developed to synthesize multicomponent source gathers and zero-offset P-wave and converted P-to-S wave sections in 3-D heterogeneous layered anisotropic media of any symmetry and any amount of anisotropy. Responses of general triclinic symmetry systems can be simulated by this program. At normal incidence, the splitting increases non-linearly as a function of the crack density and decreases non-linearly as a function of the crack angle in the transitions from orthorhombic to tetragonal anisotropy, from azimuthal to orthorhombic anisotropy, and from monoclinic to orthorhombic anisotropy.; For propagation along the symmetry planes, the splitting of converted P-to-S waves is zero in tetragonal anisotropic media. As a result, the fast and slow converted waves interfere destructively in the transverse component, and constructively in the radial component; no energy is recorded in the component transverse to one of the symmetry planes, and one-converted P-to-S reflection per layer is recorded in the radial component.; At zero-offset the splitting of converted P-to-S wave modes in transversely isotropic media is mainly controlled by the inclination of the symmetry axis and the amount of anisotropy. As they are both increased, significant azimuthal time delay variations are produced by the orientation of the isotropy planes relative to the alignment of a recording line.; Three-component multiazimuth surface seismic data collected in the Maporal field in southwest Venezuela are analysed by inverting for the (anisotropic) elastic constants from P-wave reflection moveouts, time delays of split P-to-S converted wave modes, dipolar sonic logs, and check-shot information. Simultaneous fitting of amplitudes and traveltimes using the elastic constants with a 3-D shooting polygonal anisotropic raytracing algorithm provides constraints on the Earth properties. Results are interpreted to indicate the existence of five transversely isotropic layers with vertical and horizontal axes of symmetry, with intensities of vertical fracturing varying from 0.10-0.58%. The average strike of the fracture systems is 111{dollar}spcirc{dollar} and appears to be constant with depth.