Traveltime Analysis of VSP Seismograms in a Horizontal Transversely Isotropic Medium: A Physical Modeling Result

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- wave 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 largesource-to-well offset VSP survey (offset/depth ≥ 0.35 in this study), special attention must be paid on the effects of anisotropy.

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