Relocation of Groningen seismicity using refracted waves

The Groningen gas field is a giant natural gas accumulation in the Northeast of the Netherlands. Decades of production have led to significant compaction of the reservoir rock. The (differential) compaction is thought to have reactivated existing faults and being the main driver of induced seismicity. Precise earthquake location is difficult due to a complicated subsurface. Likely for that reason, the current hypocentre estimates do not clearly correlate with the well-known fault network. The seismic velocity structure down to reservoir depth is quite well known. Most to date earthquake detections, however, were made with a sparse pre-2015 seismic network. For shallow seismicity (<5 km depth) horizontal source-receiver distances tend to be much larger than vertical distances. Consequently, preferred source-receiver travel paths are refractions over high-velocity layers below the reservoir. Seismic velocities of layers below the reservoir are poorly known. We estimated an effective velocity model of the main refracting layer below the reservoir. This refracting layer is then added to the known velocity model, and the combined model is used to relocate the past seismicity. From the resulting relocations we assess which of the faults are being reactivated.