Seismic anisotropy estimated from P-wave arrival times in crosshole measurements

P>Seismic anisotropy is evidenced in the inner core, upper mantle and the lower crust in large scale, and the evidence is generally provided by shear wave splitting analysis. Here this paper searches for the evidence of anisotropy in the uppermost crust, by using P-wave arrival times from crosshole seismic measurement to directly estimate velocity anisotropy associated with the fine-layering effect of multiple sedimentary beds. Conceptually fine layering causes the so-called VTI (vertical transverse isotropy) anisotropy with a vertical symmetry and the effect is parametrized by the horizontal and vertical velocity ratio. It is found however that the VTI anisotropic parameter does not have a simple vertical symmetry but is also azimuth dependent. This azimuthal anisotropy may reflect the fracture orientation due to large-scale tectonic movements, and is very important in the production of oil reservoirs, as the seismically fast directions can indicate preferred directions of fluid flow. This paper presents innovative methods for anisotropy analysis in both vertical and horizontal plane. Integrated seismic anisotropy interpretation clearly indicates distinguished strain orientations forming fractures in Oligocenic, Miocenic and Pliocenic sediment, in the edge of the extensional basin immediately next to Tan-Lu Fault, an active continental strike-slip fault zone.