Downhole microseismics has gained in popularity in recent years as a way to characterize hydraulic fracturing sources and to estimate in-situ stress state. Conventional approaches only utilize part of the information contained in the microseismic waveforms such as the P/S amplitude ratio and/or P first motion polarity to determine the microearthquake focal mechanisms and infer stress state. Thus, additional constraints like double-couple assumption must be made to stabilize the inversion for conventional methods. The situation becomes even worse for downhole monitoring where only limited azimuthal coverage is available. In this study, we have developed a full-waveform based approach to invert for complete moment tensor. We use the discrete wavenumber integration approach as the fast forward modeling tool to calculate the synthetic waveforms for one-dimensional layered velocity models. By matching full three-componentwaveforms across the array, a stable moment tensor solution can be obtained without imposing additional constraints. We also derive the source radius from the far-field displacement spectrum with the Madariaga's model and determine the stress drop afterwards. We test our method on a downhole microseismic dataset from hydraulic fracturing treatments in East Texas. The result indicates the existence of the isotropic component in some events. A clear difference is observed that non-double-couple events tend to have smaller stress drops, which is consistent with other studies. The derived fracture plane direction also agrees with that derived from multiple event location.
Microseismic downhole monitoring is a valuable tool for mapping the fractures and evaluating the effectiveness of hydraulic fracturing. The locations of microseismic events, with sufficient resolution, provide information on fracture geometry and properties (Warpinski et al. 1998, Phillips et al. 2002). Although moment tensor inversion has been applied in downhole hydraulic fracturing monitoring, most of them rely only on P- and S-wave amplitudes and/or Pwave first motion polarities.