Seismic anisotropy in microseismic event location analysis

Seismic velocity anisotropy has been known for many years to be an important factor in correctly imaging subsurface structures with reflection seismic methods (Alkhalifah et al., 1996). Fine vertical velocity layering naturally gives rise to VTI (vertical transverse isotropic) velocity in which seismic-wave velocity is faster in the horizontal direction than in the vertical direction. Similarly, vertically oriented natural fractures and stress fields influence seismic velocities in an azimuthal or compass direction which is described as HTI (horizontal transverse isotropic) velocity variation (Schoenberg and Sayers, 1995). HTI velocity analysis in seismic reflection imaging commonly has two main objectives: placing geologic features in the correct spatial location and characterizing fracture orientation and density to optimize oil and gas production strategy.