Focal Mechanism Determination of Induced Microearthquakes in an Oil Field Using Full Waveforms from Shallow and Deep Seismic Networks
Name
Junlun_Li_focal_mechanism.pdf
Size
3.95 MB
Format
Adobe PDF
Checksum (MD5)
1281222966bb4ecd3ef06b9bf494b9eb
Author(s)
Li, Junlun
Kuleli, Huseyin Sadi
Zhang, Haijiang
Toksoz, M. Nafi
Date Issued
May 9, 2011
Publisher
Massachusetts Institute of Technology. Earth Resources Laboratory
Series/Report no.
Earth Resources Laboratory Industry Consortia Annual Report;2011-15
Abstract
A new, high frequency, full waveform matching method is used to study the focal
mechanisms of small, local earthquakes induced in an oil field, which are monitored by a sparse near-surface network and a deep borehole network. The determined source properties are helpful for understanding the local stress regime in this field. During the waveform inversion, we maximize both the phase and amplitude matching between the observed and modeled waveforms. We also use the polarities of the first P-wave arrivals and the average S/P amplitude ratios to better constrain the matching. An objective function is constructed to include all four criteria. For different hypocenters and source types, comprehensive synthetic tests show that our method is robust to determine the focal mechanisms under the current array geometries, even when there is considerable velocity inaccuracy. The application to several tens of induced microseismic events showed satisfactory waveform matching between modeled and observed seismograms. The majority of the events have a strike direction parallel with the major NE-SW faults in the region, and some events trend parallel with the NW-SE conjugate faults. The results are consistent with the in-situ well breakout measurements and the current knowledge on the stress direction of this region. The source mechanisms of the studied events together with the hypocenter distribution indicate that the microearthquakes are caused by the reactivation of preexisting faults. We observed that the faulting mechanism varies with depth, from strike-slip dominance at shallower depth to normal faulting dominance at greater depth.
Subjects
Microseismic
Inversion
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