Microseismic Source Inversion in Anisotropic Media

Sedimentary rocks and shales in particular are known to be anisotropic, sometimes strongly so, and since hydraulic fracturing is now common practice to enhance the extraction of hydrocarbons from sedimentary rocks it is of interest to study the impact that anisotropy may have on hydraulically induced seismicity. This thesis is concerned with the inclusion of anisotropy into the geophysical forward and inverse problems of induced seismic sources – in a phrase, “anisotropic moment tensor inversion”.
Ray theory in transversely isotropic media with a vertical axis of symmetry (VTI) is used for the forward problem; inversion includes vector waveform fitting in the frequency domain and recovery of the source-time function as well as the moment tensor. Theoretical details of the forward problem, layered anisotropic VTI ray tracer, inversion technique and moment tensor decomposition are discussed. The impact of anisotropy on radiation patterns and moment tensor decomposition is investigated. New approaches are introduced for inversion, moment tensor decomposition and visualization.
The real waveform data analyzed come from a dual-well downhole monitoring of a lateral hydraulic stimulation from central Alberta. Initial model building is based on sonic logs and anisotropic model calibration uses arrivals picked on sliding sleeve events. Model calibration includes a soft, rock physics constraint to enable recovery of a plausible model with depth-dependent anisotropy. 182 events from a central stage are analyzed, and high quality inversions are shown in detail for selected events. The event collection suggests an unexpected type of source mechanism but is supported by results from an independent inversion code. Also shown is a force source inversion of a sleeve event.
Future directions include extensions to more general forms of anisotropy, non-direct arrivals, unbalanced force sources and application to surface arrays and regional earthquakes.