Tilted TI models in surface microseismic monitoring

Abstract

A recently published analytic technique for computing locations of microseismic events jointly with velocities of homogeneous isotropic media was extended to surface microseismic monitoring and transversely isotropic models with tilted symmetry axes (TTI). The analysis of traveltimes of the direct P-, SV-, and SH-waves, conducted under the assumptions of homogeneity and weak anisotropy, indicated that the SV-wave data acquired in modern wide-azimuth surface microseismic surveys yield uniquely solvable joint inverse problems for an arbitrary symmetry-axis tilt, whereas the tilts should be close to 90° from the vertical for the P-waves propagating in anelliptically anisotropic media and strictly equal to 90° for the SH-waves to maintain the uniqueness of the joint inversion. These theoretical findings, confirmed on ray-tracing synthetic, were applied to a field microseismic data set. The P-waves excited by microseismic events were found to exhibit significantly flatter moveouts and better focused stacks when located in a constructed effective TTI model compared to those located in a horizontally layered isotropic model provided as a part of conventional microseismic service.