Utilizing multiplets as an independent assessment of relative microseismic location uncertainty

Abstract

Location uncertainty is a key factor affecting interpretation of microseismic data associated with multistage hydraulic fracturing. We provide an independent assessment of relative microseismic event location uncertainty using multiplets — microseismic events that occur in essentially the same place with the same source characteristics. We establish a relationship between waveform similarity and hypocentral separation using synthetic events in a representative velocity model, finding an upper bound of 15 m between events in any given multiplet group. This implies that greater separation of locations for events within a multiplet reflects relative location error. We identified hundreds of multiplet groups in a case study in the Barnett with unusually high-quality data. All stages were recorded with two downhole monitoring arrays. Although the events within each multiplet group must be within 15 m of each other to produce nearly identical waveforms on the recording arrays, the scatter in contractor-provided event locations from the multiplet centroid is about 60 m for the most well-located multiplet events with source receiver distances of 150–200 m, despite a stated absolute location uncertainty of about 30 m. The scatter in event locations increases to 120 m when the source receiver distance is more than 250 m.