Magnitude and Peak Amplitude Relationship for Microseismicity Induced by Hydraulic Fracture Experiment

Trevor Smith, Adam Arce, & Chen Ji

Published August 15, 2016, SCEC Contribution #7038, 2016 SCEC Annual Meeting Poster #258

Waveform cross-correlation technique is widely used to improve the detection of small magnitude events induced by hydraulic fracturing. However, when events are detected, assigning a reliable magnitude is a challenging task, especially considering their small signal amplitude and high background noise during injections. In this study, we adopt the Match & Locate algorithm (M&L, Zhang and Wen, 2015) to analyze seven hours of continuous seismic observations from a hydraulic fracturing experiment in Central California. The site of the stimulated region is only 300-400m away from a 16-receiver vertical-borehole array which spans 230 m. The sampling rate is 4000 Hz. Both the injection sites and borehole array are more than 1.7 km below the surface. This dataset has previously been studied by an industry group, producing a catalog of 1134 events with moment magnitudes (Mw) ranging from -3.1 to -0.9. In this study, we select 202 events from this catalog with high signal to noise ratios to use as templates. Our M&L analysis produces a new catalog that contains 2119 events, which is ~10 times more detections than the number of templates and about two times the original catalog. Using these two catalogs, we investigate the relationship of moment magnitude difference (ΔMW) and local magnitude difference (ΔML) between the detected event and corresponding template event. ΔML is computed using the peak amplitude ratio between the detected and template event for each channel. Our analysis yields an empirical relationship of ΔMW=0.64-0.65ΔML with R2 of 0.99. The coefficient of ~2/3 suggests that the information of the event’s corner frequency could be lost (Hanks and Boore, 1984). The cause might not be unique, which implies that Earth’s attenuation at this depth range (>1.7 km) is significant; or the 4000 Hz sampling rate is not sufficient. This relationship is crucial to estimate the b-value of the microseismicity induced by hydraulic fracture experiments. The analysis using the M&L catalog with the relation ΔMW = 2/3ΔML results in a normal b-value of 1.1, while a smaller b value of 0.88 would be obtained if ΔMW = ΔML instead.

Key Words
Induced Microseismicity

Smith, T., Arce, A., & Ji, C. (2016, 08). Magnitude and Peak Amplitude Relationship for Microseismicity Induced by Hydraulic Fracture Experiment. Poster Presentation at 2016 SCEC Annual Meeting.

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