Network Performance And Seismic Noise Characterisation In The Wairakei Geothermal Field

Abstract

Characterising microseismicity in developed geothermal fields can be useful for understanding deep reservoir structure and the response to field development and operations. The ability to detect microearthquakes of interest depends on the design of the network and prevailing background seismic noise levels. Developed geothermal fields can be affected by a range of anthropogenic noise sources, particularly those associated with geothermal power plant infrastructure (e.g. power house, steamfield, drilling, traffic, civil works etc.), as well as natural background noise sources (e.g. ocean and meteorological activity) including those specific to a geothermal environment (subsurface fluid movement). How extensively seismic monitoring instrumentation is deployed throughout the field is usually a trade-off between site availability, access, cost and desired monitoring objectives (coverage and detection). Incorporation of borehole seismometers can result in significant improvements in signal to noise ratios and greater number of detected microseismic events relative to surface seismic station networks. Subsequently, borehole networks have the potential to provide more useful information about the reservoir. We investigate noise characteristics and array performance for a network of borehole seismometers deployed at the Wairakei geothermal field. An assessment is made of noise levels arising from various sources. Borehole sensors at different depth levels are compared to study the relative improvement in noise isolation with depth. Implications for magnitude and detection thresholds for station depths across the network are discussed. We review the network coverage and discuss general guidelines for planning seismic instrumentation of a geothermal field to mitigate the effect of high and varying noise.