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High-Frequency Resonances in Borehole Geophones Bias Source Parameters of Induced Seismicity at Preston New Road, UK

The high-frequency content of seismic events contains important information about their rupture process and source properties. However, instrument limitations at high frequencies can sometimes interfere with the signal, producing biased and misleading results. The smaller the event, the higher the frequency band of interest required. Thus, for microseismic events (magnitude < 2), accurate source analysis hinges on the instrument’s ability to record high-frequency signal without interference. During hydraulic fracturing operations, microseismic monitoring is commonly conducted by an array of geophones deployed in a nearby monitoring borehole. While close proximity of the geophones to injection provides the unparalleled low signal-to-noise ratio (SNR) required to detect microseismic events, geophones are also known for issues with resonances due to spurious frequencies, tube waves, and clamping (e.g., Faber and Maxwell, 1997; St-Onge and Eaton, 2011). Moment magnitude (Mw) and corner frequency (fc), two important earthquake source parameters, depend on the shape of the frequency spectrum and are thus easily affected by geophone resonances. Accurate Mw estimates relay the amount of energy released by earthquakes and are necessary for reliable b-values, a parameter used by hydraulic fracturing operators to evaluate real-time seismic hazard. Corner frequency is equally valuable as it is used to compute stress drop (Δσ). The stress drop contains information about the size of the rupture, which is used both by geomechanical modelers to evaluate the evolution of fracture growth and in seismic hazard analysis to evaluate the ground motions expected at the surface.