Wave Propagation In A Vertical Transversely Isotropic Medium: Field Experiment and Model Study

Abstract

A down-hole experiment was carried out in the transversely isotropic Oxford Clay outcropping in the south of England. Different moveout curves for the two shear wave types and anomalous amplitude features for the SV-wave were found in the field data. Based on velocity measurements carried out formerly at the site a model study was performed to explain the results. Phase velocity and group velocity curves computed analytically with the method of characteristics, and synthetic seismograms computed with the AlekseevMikhailenko method, are presented. The field experiment and the model studies demonstrate that the occurrence of cuspidal triangles in the qSV-wavefront is an essential feature of wave propagation in transversely isotropic media. Even for weak transversely isotropic media there is a focusing effect into the direction of the cusp which leads to prominent shear wave amplitudes in this direction. Furthermore, we examined the effect of numerical anisotropy which can contaminate the synthetic seismograms. Velocity errors are one order of magnitude higher for shear waves than for compressional waves and increase with increasing Poisson's ratio. It was found that the error can be restricted to less than 1 percent only if using a spatial sampling of three times higher than a value that would generally be regarded as sufficient in finite difference computations.

Wave propagation in a general anisotropic poroelastic medium with anisotropic permeability: phase velocity and attenuation

“…Crampin (1987) discussed the geological and industrial implications of EDA. In exploration studies (Helbig, 1984;Leary and Henyey, 1985;Kerner et al, 1989;Corrigan, 1989; and many others), the velocity anisotropy measured from travel times has suggested the presence of significant anisotropy in sedimentary basins.…”

Three-dimensional wave propagation in a general anisotropic poroelastic medium: phase velocity, group velocity and polarization

“…Volumes of inherently isotropic rocks may be microfractured by regional tectonic stresses, resulting in a system of oriented fractures along principal axes which could lead to global anisotropy (Bamford & Nunn 1979; Leary et al 1987). In exploration studies (Helbig 1984; Leary & Henyey 1985; Corrigan 1989; Kerner et al 1989, and many others), velocity anisotropy measured from traveltimes has suggested the presence of significant anisotropy in sedimentary basins. Similar results for Earth's crust and mantle have been obtained in earthquake studies (Gupta 1973; Crampin 1985; Thomsen 1986; and many others).…”

Inventory of Shale Formations in the US, Including Geologic, Hydrological, and Mechanical Characteristics

“…Single values are for normal to bedding unless marked with * indicating orientation is unknown. # saturated bulk density based on porosity and grain density ## saturated bulk density based on porosity and dry bulk density + extrapolated to zero confining stress $ computed using Poisson ratio and Young's modulus Jougnot et al (2010); (17) Burland et al (1977); (18) Ghorbani et al (2009);(19) Bastiens et al (2007); (20) Kerner et al (1989); (21) Niandou et al (1997) Similar data are available from Horsrud (2001), who also developed several correlations between sonic velocity and petrophysical parameters. These data are mainly from formations in the North Sea that are of interest for petroleum resource development.…”