Microseismicity in the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand: Determination and Application of a Matched-Filter Threshold

Abstract:

The high-temperature, fluid-dominated Ngatamariki geothermal field is located in the central Taupo Volcanic Zone (TVZ), 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. 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 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. As in matched-filter detection studies elsewhere, we require that the sum of correlation coefficients across the network exceed a noise-based threshold in order to recognise a detection. The threshold is defined by R×MAD, where R is a scalar coefficient and MAD is the median absolute deviation of the correlation coefficient sum throughout the day. The R coefficient is a function of the recording environment, network geometry and time-varying noise characteristics, and we estimate it empirically using synthetic testing. By seeding the continuous data with scaled versions of representative templates, we examine how the amplitudes of detectable events are compared with correlation and detection efficiency. Synthetic testing yields a threshold for this geothermal region of R=7.1. Using this threshold, events with amplitudes as small as ~2% of the original templates’ amplitudes are detected with 95% confidence. The matched filter detection of microseismic events allows further interpretation of geologic structure at Ngatamariki and of the relationship between production, injection and seismicity.