Relocation of clustered earthquakes in the Groningen gas field

Lisanne Jagt,Elmer Ruigrok,Hanneke Paulssen

doi:10.1017/njg.2017.12

Abstract

AbstractPrevious locations of earthquakes induced by depletion of the Groningen gas field were not accurate enough to infer which faults in the reservoir are reactivated. A multiplet analysis is performed to identify clusters of earthquakes that have similar waveforms, representing repeating rupture on the same or nearby faults. The multiplet analysis is based on the cross-correlation of seismograms to assess the degree of similarity. Using data of a single station, six earthquake clusters within the limits of the Groningen field were identified for the period 2010 to mid-2014. Four of these clusters were suitable for a relocation method that is based on the difference in travel time between the P- and the S-wave. Events within a cluster can be relocated relative to a master event with improved accuracy by cross-correlating first arrivals. By choosing master events located with a new dense seismic network, the relocated events likely not only have better relative, but also improved absolute locations. For a few clusters with sufficient signal-to-noise detections, we show that the relocation method is successful in assigning clusters to specific faults at the reservoir level. Overall, about 90% of the events did not show clustering, despite choosing low correlation thresholds of 0.5 and 0.6. This suggests that different faults and/or fault segments with likely varying source mechanisms are active in reservoir sub-regions of a few square kilometres.

Highlights

Gas extraction from the Groningen gas field in the northern part of the Netherlands has caused seismicity
We show that besides improving the relative locations of events in a cluster, we can improve their absolute locations by relocating with respect to accurately located master events that were detected with a denser seismic network that was deployed after 2014
The N-component-based multiplet analysis yielded five clusters, where clusters 2–5 are identical to those obtained for the Z-component, and cluster 1 has four of the five events in common (Table 1)

Summary

Introduction

Gas extraction from the Groningen gas field in the northern part of the Netherlands has caused seismicity. It is generally accepted that depletion of a hydrocarbon reservoir causes changes in the state of stress, which, in turn, can reactivate existing faults. Prior to 2015, the Groningen gas field events were recorded by a monitoring network in the northeastern Netherlands consisting of 14 boreholes with geophones at four different depth levels and 18 accelerometers (Dost et al, 2012; Kraaijpoel & Dost, 2013). The accuracy of the locations is insufficient to associate them with active faults in the reservoir. This is where a multiplet analysis can be useful

Methods

Results

Conclusion