Anisotropic Waveform Inversion for Microseismic Source Parameters

Waveform inversion (WI), which has been used primarily for high-resolution velocity analysis, can also be employed to obtain the source parameters of microseismic events. Here, we implement WI to estimate the location, origin time, and seismic moment tensor of microseismic sources embedded in heterogeneous VTI (transversely isotropic with a vertical symmetry axis) media. The algorithm operates with multicomponent wavefields modeled using an elastic anisotropic finite-difference code. The gradient of the objective function for the three classes of parameters is calculated with the adjoint-state method under the assumption that the velocity field is known. Synthetic tests for data recorded by vertical receiver arrays show that it is possible to tightly constrain all source parameters, if a sufficiently accurate initial model and high-quality data are available. In particular, the source location in layered VTI media can be estimated simultaneously with the moment tensor. The resolution of event location, however, somewhat decreases when the origin time is unknown or there are errors in the velocity model.