3-D VISCOELASTIC ANISOTROPIC MODELING OF DATA FROM A MULTICOMPONENT, MULTIAZIMUTH SEISMIC EXPERIMENT IN NORTHEAST TEXAS

Three-component seismic data were acquired along six azimuths at a site in northeast Texas. Sources were inclined impacts combined to produce equivalent vertical and crossline horizontal components. The data were analyzed by iterative modeling using a new 3-D, staggered-grid, finite-differencing scheme. The 3-D model is defined in terms of spatially variable anisotropic, viscoelastic parameters (P-velocity, S-velocity, density, Qp, and Qs) and converted to elastic tenser components and relaxation times for computation. Simultaneous fitting of amplitudes of near-surface reverberations, surface waves, converted waves, and deeper reflections gives an anisotropic, viscoelastic model that provides constraints on earth properties. The assumption of transverse isotropy in previous experiments is supported by the results of this experiment. The final model consists of a stack of flat layers, each of which has isotropic P-velocity, Qp, and Qs. The Wills Point and Kincaid shales are strongly transversely isotropic, with shear propagation velocity averaging 28% higher horizontally than vertically. The geometry of the Nacatoch Sandstone is similar, with approximate to 19% anisotropy. The marls and chalks are essentially isotropic. Observed mixing between crossline and vertical components is incorporated by simulating a small (4 degrees) tilt of the axis of the impact source from vertical.