Abstract
In order to investigate the effects of anisotropy in a VSP survey, the multi-component VSP data for observing the qP-, qSH- and qSV-waves were collected with two different source-to-well offsets in an elastic transversely isotropic medium (TIM) with a horizontal symmetry axis (HTI). The traveltimes of the events were then computed using two different algorithms: first with elastic constants of the medium, and second with “isotropic velocities”. Comparing the traveltimes of computations with the recorded data, a very clear and important consequence can be derived. For strongly anisotropic media (the anisotropy for the qP- and qS-waves are 25% and 29%, respectively, in our laboratory work), the effects of anisotropy increase with increasing source-to-well offset. As the source-to-well offset is small compared with the depth of the receiver (offset/depth ≦ 0.2 in our work), the effects of anisotropy can be ignored. However, in processing a large source-to-well offset VSP survey (offset/depth ≧0.35 in this study), special attention must be paid on the effects of anisotropy.
Highlights
Much of the earth's crust appears to be more or less anisotropic (Crarnpin 1981; Crampin and Lovell 1991; Helbig 1993), making seismic wave propagation more complicated than in isotropic media
Seismic exploration techniques are being extended from simple isotropic models to complex anisotropic models in order to construct the geological structures in more detail
We study the effects of anisotropy on vertical seismic profile (VSP) data using physical models
Summary
Much of the earth's crust appears to be more or less anisotropic (Crarnpin 1981; Crampin and Lovell 1991; Helbig 1993), making seismic wave propagation more complicated than in isotropic media. The effects of singularities in sedimentary basins were observed in examining the shear-wave splitting in multi-offset VSPs at a borehole site in the Paris Basin (Bush and Crampin 1991). Brown et al (1991) showed that the effect of the shear-wave window and the variation of the hyperbolic NMO parameter with offset are clearly observed in ultrasonic waves propagation through an orthorhombic medium. Chang et al (1994) clearly demonstrated the phenomena of shear wave splitting in a transversely isotropic medium (TIM). Chang and Gardner (1997) investi gated the effects of subsurface fractures in a three-layer medium and observed that the hori zontal moveout velocity decreases from the strike direction toward the transverse direction of the fractures. The observed anisotropic traveltimes in a strongly TIM (phenolite) with a horizontal symmetry axis (HTI) are compared with its theoretical anisotropic traveltimes and the isotropic traveltimes for two different source-to-well geometries
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