Velocity anisotropy observed in wellbore seismic arrivals: Combined effects of intrinsic properties and layering

A method of estimating velocity anisotropy by the τ-p inversion of seismic traveltimes observed in a multi-depth and offset VSP experiment is developed. In the method, a set of seismic traces observed at a fixed depth in the wellbore are transformed to the τ-p domain. Incidence-angle-dependent interval velocities are extracted from the resulting curves using coherency measures after accounting for the effects of the overlaying layers. To test the method, we obtained seismic records from a shallow borehole drilled through a flat lying sedimentary sequence. One depth interval that consists of a homogeneous bioturbated shale is nearly isotropic. A second interval characterized by an alternation of thin (~1 m) sands and shales displays an anisotropy of 15% with velocities increasing from the near vertical (0 < 30°) to the oblique (30° < 0 < 55°) angles of incidence. Oblique velocities were always larger. The observation that the shale-sand depth interval displaying the greatest anisotropy also is the most heterogeneous in the sonic logs suggests that the thin layering produces the anisotropy. The theoretical vertical-to-horizontal velocity an isotropy for this interval, computed assuming isotropic layering, is 14.2%. Comparing this theoretical value to the vertical-to-oblique value obtained over a smaller range of incidence angles indicates that the theoretical value is too small. This discrepancy is possibly because of the intrinsic anisotropy of the shale layers themselves that is not considered in the theoretical calculations.