Simultaneous inversion for velocity model and microseismic sources in layered anisotropic media

Abstract

Microseismic monitoring is widely applied in fracturing operations, reservoir delineation and water front monitoring. Microseismic inversion generally uses an initial velocity model based on well logs and seismic data from perforation shots. This model is generally not suitable to locate microseismic sources. To reduce errors, we have developed a method that simultaneously updates the velocity model and locates events in space and time. 1-D layered anisotropy with a transversely-isotropic (TI) symmetry is assumed, where the symmetry axis can be arbitrarily oriented. The arrival times and ray paths of the qP, qSV and qSH waves are calculated with a modified multistage shortest path algorithm. Combined with the conjugate gradient method, a damped, minimum-norm, least-squares and constrained problem is solved. The numerical examples show that the proposed algorithm can be used to invert the anisotropic velocity model (elastic moduli and interface depth for each layer) and locate the microseismic sources and their onset times, simultaneously. It is shown that the algorithm is not very sensitive to random noise, which may be contained in arrival times and model anisotropy heterogeneities.