Using beam forming to maximise event detection using small broadband seismometer arrays

In response to cases of injection-induced seismicity, regulators have responded by applying traffic-light scheme type monitoring requirements. Such schemes may require operators to detect small-magnitude events. While this can be done with dense arrays of surface geophones, or by deploying instruments downhole, the cost of doing so may be prohibitive for every stage of every well. Instead, we seek to maximise what can be detected using small (10-20) arrays of surface stations. We do so by developing a beamforming/stacking algorithm where STA/LTA functions derived from the P, SV and SH components are combined to reveal events with low signal-to-noise ratio. We apply our method to a case study from North America where induced events were recorded on a network of 17 broadband seismometers placed around a hydraulic fracturing site. Where conventional event detection algorithms reveals 4 induced events during the monitoring period, our method reveals 20 such events, an improvement in event detection of 500%.