Independent Microseismic Event Location Uncertainty Analysis

Monitoring microseismicity induced by hydraulic fracturing provides insight into an otherwise opaque process. Identifying the hypocenters of microseismic events can allow operators to avoid hazards, and judge the efficacy of employed stimulation strategies. Despite the demonstrated utility of microseismic monitoring, the technique faces significant cahllenges in providing reliable event locations. While conventional seismic surveys use known source and receiver locations to characterize the seismic properties, microseismic inversion requires a well-constrained velocity model for successful location and characterization of source events. The size of error ellipsoids are sensitive to a number of unknown factors, making the accuracy of event locations a subject of much debate. Here, we present the utilization of microseismic multiplets, events occuring in nearly the same location with common source parameters, as an independent method of evaluating microseismic event location uncertainty. We use forward modeling of seismic waveforms to evaluate commonly held assumptions regarding relative distances between events in a multiplet group. The results of this analysis confirm a conservative upper bound of fifteen meters of hypocentral separation between events in a multiplet group.A dataset with nearly ten thousand located events is used as a case study. The results suggest that event location errors may be substantially larger than typical error analyses estimate. In some instances, the provided locations of events that occur within 15 meters of each other are over five hundred feet apart. Errors of this magnitude have important implications for the reliability of microseismic analyses.