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Calculation of the Focal Mechanism for Composite Microseismic Events
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 79th EAGE Conference and Exhibition 2017, Jun 2017, Volume 2017, p.1 - 5
Abstract
It is generally difficult to obtain a reliable single-event source mechanism with a sparse surface array, mainly due to the typically low signal/noise ratio and poor azimuthal coverage. In this study, we propose an inversion procedure to estimate the focal mechanism of composite microseismic events - i.e., a set of events interpreted to share a common focal mechanism - recorded using a sparse surface network. Our method uses polarities of P-wave first motion together with Sh/P amplitude ratios. Sensitivity analysis using synthetic data indicates that reliable focal solutions can be obtained if both the amplification factor on Sh/P ratio is within the range of 0.25 ∼ 1.0, and > 50% polarities are correctly picked. We apply our approach to a set of 13 microseismic events recorded during hydraulic-fracture stimulation of the Marcellus Shale formation in West Virginia and Pennsylvania, USA. Similar to previous studies of this area, we obtain a focal mechanism comprised of northwest or northeast trending strike-slip faulting accompanied by a minor thrust-faulting component.