Quantification of the Impact of Seismic Anisotropy in Microseismic Location

Abstract

Seismic anisotropy in sedimentary rocks, due to either layered bedding or fracturing, may bias microseismic locations if unaccounted in velocity models. To quantitatively assess such biases, we have applied a nonlinear location method to synthetic travel time data in seven models from isotropy to different levels of VTI (Vertical Transverse Isotropy) and HTI (Horizontal Transverse Isotropy) cases. Synthetic waveforms are recorded at two vertical receiver arrays in a three-layer velocity model using a pseudo-spectral method. Both P and S wave arrivals are used to locate three events assuming an effective isotropic velocity model. The average location error is 59 m for isotropic case, about the size of the grid interval in the velocity model, and is 156 m (158 m), 237 m (244 m), and 258 m (265 m) for 5%, 10% and 15% VTI (HTI) cases, respectively. These results suggest that even 5% seismic anisotropy, if not properly accounted, can cause significant biases in microseismic event locations.