Microseismicity during geothermal stimulation at the Ngatamariki geothermal field: New detections via a matched- filter method
The high-temperature, ﬂuid-dominated Ngatamariki geothermal ﬁeld is located in the central Taupo Volcanic Zone, North Island, New Zealand, and is used to generate electricity via an 82 MW power plant. Injection wells have been in operation since June 2012. During June and July 2012, injection well NM8 was injected with with cold water in order to improve reservoir permeability. Geothermal stimulation and production may trigger microearthquakes by ﬂuid ﬂow through the reservoir. Close clustering of microseismic events’ hypocentres relative to the source-receiver distance results in many events having similar waveforms. We capitalize on this relationship by using a matched-ﬁlter detection method in which high-quality seismograms corresponding to a well-recorded earthquake (“templates”) are cross-correlated against continuous data to reveal additional earthquakes with similar characteristics. Clustering of the detections’ hypocenters also implies that small variations in travel times between two events corresponds to small diﬀerences in hypocentral locations, which is the foundation of the double-diﬀerence relocation method. Using an 11 station seismic network, we detect 863 events via cross-correlation of 110 matched-ﬁlter templates during the two months stimulation testing. We locate each of these detections using a double-diﬀerence relocation method by which events are relocated based on relative travel times. The locatable seismicity delineates: a northern Ngatamariki cluster, a southern Ngatamariki cluster, and a cluster to the south, at the neighboring Rotokawa ﬁeld. Seismicity in the northern Ngatamariki cluster (522 events) is of greatest interest for this project due to its proximity to well NM8 and temporal signature relative to injection. The seismicity cluster centers around well NM8 at a depth of 2.1 km below sea level. Events in this cluster extend to up to 2.5 km from the injection well. An increase in seismicity near NM8 lags behind the onset of injection by 4–8 days. In contrast, a seismicity-rate decrease coincides with injection shut-in without any time lag. Local magnitudes in this cluster span the range −0.09 ≤ Ml ≤ 1.66 with a completeness magnitude of 0.25. Seismicity within 200 m of NM8 is induced by thermal stresses caused by the diﬀerence in temperature between the injectate and the reservoir. Seismicity further than 200 m, but still within this cluster, from NM8 is induced via pore ﬂuid pressure increases from the injected ﬂuid. The coupled mechanism acts on two diﬀerent length scales and is known as a thermoporoelastic mechanism. The matched-ﬁlter detection of microseismic events allows interpretation of extent of injection well stimulation and the relationship between injection and seismicity.