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Detection and Location of Microseismic Events Using The Subspace Detector
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- Author / Creator
- Bestmann, Ibinabo
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Seismic events or earthquakes occur due to stress perturbations in the sub-surface and are related to the generation of new faults or reactivation of pre-existing faults. Completeness of an earthquake catalogue is critical for gaining a better understanding of sub-surface processes and this can be achieved through detection and location of seismic events. They are especially important in applications such as monitoring reservoir changes during hydraulic fracturing treatments, as it can aid in revealing the extent of fracture growth. However, there are challenges involved in both processes. The low magnitude of most seismic events and the presence of noise may reduce the accuracy of most event detection methods. Furthermore, most event detection algorithms produce initial estimates of arrival times which sometimes contain large errors, degrading the accuracy of earthquake location procedures. The uncertainty associated with the estimated hypocenter locations can sometimes be larger than the seismic source dimensions as a result, restricting the resolution of the seismicity image.
In this work, a statistical approach known as the subspace detector is investigated and used to detect weak and variable events embedded in noise. It involves the construction of a vector space comprised of signals to be detected from seismic sources of interest. Waveforms are grouped together based on similarity to increase sensitivity of the detector to events of a particular seismic source, and waveform alignment is applied to reduce waveform variability in the subspace representation. The vector representation of the signals representing the seismic sources is shown to contain hypocenter information on the signals represented and have improved signal-to-noise ratios relative to the templates forming the subspace. Detected events are relocated relative to these vector representations, and further improvements in location accuracy can be achieved by correcting arrival time inconsistencies and reducing variation in back-azimuths via cross correlation.
The detection capabilities of the subspace detector are compared to the conventional matched filter and short-time average/long-time average (STA/LTA) detectors via tests on synthetic and real data examples. The results show that the subspace detector produces more detections than the matched filter at a reasonable false alarm rate. It can also be made general to accommodate a variety of waveforms and offers more detections relative to the STA/LTA detector with fewer false alarms. Event locations obtained by relocation relative to the vector representations are also compared to those obtained by a standard relocation technique in the form of the double difference algorithm. It is found that the relative locations obtained using both the subspace detector and the double difference algorithm are fairly similar. There is also a significant reduction in the spatial extent of event locations after relocation with the subspace detector compared to routine absolute location techniques.
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- Subjects / Keywords
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- Graduation date
- Spring 2020
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.