Browsing by Author "Gilbert, Hersh J."
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- ItemOpen AccessComparisons and Implementations of Least-squares Reverse Time Migration and Full Waveform Inversion in Acoustic Media(2019-08-12) Yang, Lei; Trad, Daniel O.; Lines, Larry R.; Gilbert, Hersh J.Least-squares reverse time migration (LSRTM) and full waveform inversion (FWI) can both provide an image of the subsurface. In this thesis, I investigate the connections and differences between LSRTM and FWI, both in time and frequency domains in acoustic media. Generally, LSRTM can be treated as the inner loop of the FWI algorithm and solved in the image domain. Different from FWI, LSRTM uses reflection data and requires an accurate operator since linearization implies the operator (Born modeling) is independent of the model (reflectivity). FWI uses diving waves and reflection data and is more robust to velocity errors in the initial model because the operator is itself and changed during the optimization. Comparing implementations domains, LSRTM in the frequency domain is relatively easier to formulate than in the time domain. It has the advantage that the wavefield is solved simultaneously for many shots. Unfortunately, the frequency domain formulation becomes too expensive in 3D surveys in terms of memory requirements. Modern implementations of time domain LSRTM require less computer memory and are more efficient than the frequency domain for the 3D case. A similar situation happens for the FWI problem. One advantage of the frequency domain FWI is the easy formulation for the multigrid method. For this approach, updates of the velocity model start from the low frequency components, which can correct the model in the first several iterations and make the result more accurate.
- ItemOpen AccessMicroseismic Monitoring of a Duvernay Hydraulic-Fracturing Stimulation, Alberta Canada: Processing and Interpretation assisted by Finite-Difference Synthetic Seismograms(2019-12) Rodríguez-Pradilla, Germán; Eaton, David W.; Lawton, Don C.; Gilbert, Hersh J.; Chen, Shengnan; Brudzinski, MikeThe increase in the development of unconventional oil and gas reservoirs in the past years has triggered anomalously high seismic activity in several sedimentary basins around the world, particularly in North American basins located in central and eastern United States and in western Canada. The Duvernay shale play, located in central Alberta, Canada, is an example of the seismicity-triggering effect by hydraulic fracturing stimulations required to produce hydrocarbons from low-permeability formations. To better understand the seismic mechanisms associated with hydraulic fracturing in this area, a local seismic monitoring array comprised by short-period and broadband sensors, was temporally installed to monitor a multi-stage hydraulic-fracturing (MSHF) stimulation on four horizontal wells drilled in the Duvernay Formation near the town of Fox Creek, Alberta, where multiple earthquakes associated with HF have been reported in recent years. This thesis presents a robust workflow for modelling and processing passive seismic data acquired with this local monitoring array, in order to automatically detect and characterize the microseismicity associated with the monitored MSHF stimulation. This characterization includes the epicentre location, depth, magnitude, fault size, and radiated energy of all the detected microseismic events, and the focal mechanisms of the seismic events with the largest magnitudes (up to ML 3.77 in this study). The obtained microseismicity is then integrated with other datasets from the monitored reservoir (well logs and production from the monitored and nearby wells, and 3D seismic) to identify reactivated faults that triggered the largest-magnitude seismic events detected during this monitoring program, and to characterize the unconventional reservoir to forecast the hydrocarbon production after the stimulation. A magnitude scale based on the duration of coda waves was also calibrated for the study area in central Alberta, which can be implemented similar seismic monitoring programs for magnitude estimation as it does not require the installation of broadband sensors. Finally, the generated ground motions of the seismic events with the largest magnitudes detected in this dataset, were determined using the local and regional seismic monitoring arrays to assess the uncertainty of a set of Ground Motion Prediction Equations (GMPE) recently developed for the Fox Creek Area at close and distant hypocentral distances. These GMPEs are fundamental to quantify the seismic hazard of induced earthquakes to nearby communities and infrastructure.
- ItemOpen AccessMoment-Tensor Analysis of Seismicity Related to Hydraulic Fracturing in North America(2019-01-30) Zhang, Hongliang; Eaton, David W. S.; Innanen, Kristopher A.; Gilbert, Hersh J.; Lines, Laurence R.; Zhang, HaijiangVarious types of seismic arrays are being used to monitor seismicity associated with oil and gas operations. These different types of recording systems exhibit significantly different characteristics in azimuthal coverage, magnitude detection threshold, signal-to-noise ratio and waveform frequency content. Taking these factors into account, this thesis focuses on implementing source-mechanism inversions for three typical monitoring geometries including the regional seismic network, sparse surface array and dense shallow borehole array. With the aim of investigating potential indicators for the discrimination between fluid-injection induced seismicity and natural earthquakes in the Western Canada Sedimentary Basin, the waveform-fitting-based moment-tensor inversion is performed for eight induced earthquakes with M > 3 and a nearby M 5.3 inferred natural earthquake, all of which were recorded by regional seismic networks. Based on the inverted parameters, the focal depth is found to be the most robust for parameter to distinguish the induced seismicity from natural earthquakes since the induced events considered here are significantly shallower than one observed nature event and most intraplate earthquakes in the Canadian Shield. Moreover, in addition to a dominant double-couple (DC) mechanism this is common to nearly all events, non-negligible non-DC components (typically > 25%) are observed within the moment-tensor solutions for most of the induced events. To overcome limitations of typically low signal-to-noise ratio and poor azimuthal coverage for the sparse surface array, a novel regularized approach is developed to estimate a composite focal mechanism from a set of microearthquakes recorded by this type of array. It operates by minimizing the weighted misfits of both SH/P amplitude ratios (in absolute sense and logarithmic scale) and P-wave polarities, using a regularization parameter determined from the trade-off curve for these values. The regularized approach reduces the multiplicity of solutions and avoids the use of signed amplitude ratios, which may be ambiguous for data with low signal-to-noise ratio. For the hydraulic-fracturing induced event sequences recorded by a dense shallow borehole array near Fox Creek area, a least-squares inversion based on 3C P-wave amplitudes has been implemented, which avoids fitting the relatively high-frequency waveforms and the picking of S-wave amplitudes that are contaminated by P-wave coda. The recovered source mechanisms are dominantly strike-slip with sub-vertical nodal planes, although a distinct cluster of events is characterized by more complex mechanisms with slip on a shallow-dipping plane accompanied by significant (> 30%) non-DC components. The non-DC components may be due to tensile crack opening and/or co-slipping on several fault strands that have been mapped using 3D seismic data. In addition, in the absence of direct stress measurements, moment-tensor solutions of the hydraulic-fracturing induced event sequences are used to estimate the local stress field near the Fox Creek area. The estimated orientation of SHmax differs from the median regional SHmax direction by ~ 15º, but it agrees with the nearest available borehole measurement from the World Stress Map. Mohr circle analysis indicates that N-S trending faults, which hosted the largest events (MW > 1.5), are mis-oriented for slip and required a relatively large increase in pore pressure (12 ± 4 MPa) in order to be brought to a state of incipient failure.
- ItemOpen AccessProcessing of Multicomponent Seismic Data from West-Central Alberta(2019-11-15) Aldhaw, Hussain; Lawton, Don Caleb; Gilbert, Hersh J.; Trad, Daniel O.Bigstone 3C3D was acquired in 2014 near Fox Creek, Alberta. The total volume acquired is 200 sq. km. A portion of approximately 44 sq. km. of the total volume is selected and processed for PP and PS data from geometry to post stack time migration, including registering the events of P-P and P-S data. However, near surface is generally challenging for the P-S data as it is hard to pick first breaks. That is also related to velocity analysis as statics and velocity are dependent to each other. In order to overcome this obstacle, we did further investigation to the near surface and used well data with P-wave velocities to give an optimum processed P-S data. In addition, we investigated the binning of P-P and P-S synthetic data based on a nearby well to test the binning effect on some seismic attributes such as fold, azimuth distribution, offset distribution, etc.
- ItemOpen AccessSource Parameters and Tectonic Setting of the 2017 St. Elias Earthquake Sequence near the Southern Terminus of the Eastern Denali Fault, Northwestern Canada(2019-09-18) Choi, Minhee; Eaton, David W. S.; Enkelmann, Eva; Dettmer, Jan; Gilbert, Hersh J.On May 1, 2017 two earthquakes of magnitude 6.2 and 6.3, respectively, occurred in proximity to the eastern Denali fault (EDF). These double mainshock events were followed by more than 2,700 aftershocks. Moment-tensor inversion of the mainshock signals shows that the initial event produced reverse slip on a steeply dipping fault with a NW-SE strike direction, while the second produced left-lateral strike-slip on a near-vertical fault with an E-W strike direction. A double-difference relocation method, coupled with clustering analysis, was applied to the aftershock distribution, confirming that seismicity was localized along two previously unmapped fault structures. Stress inversion indicates that the maximum principal stress axis is oriented almost perpendicular to the EDF, suggesting that the fault system is not well oriented for strike-slip in the contemporary stress field. Coulomb stress analysis indicates that the second event was likely triggered by the first one (static stress triggering), with a delay of about two hours. A generalized model is developed to explain the observations, wherein gravitational potential from the >4,000 m high Mount Fairweather, as well as strain partitioning along the plate boundary, produce a stress regime that extends inboard towards the EDF.
- ItemOpen AccessSTRUCTURE IMAGING IN THE AREA OF MT. MEAGER(2019-09-20) Su, Hongyi; Gilbert, Hersh J.According to Natural Resources Canada, South Mt. Meager project, which taken place in this area, is the most advanced geothermal power project in all Canada (Natural Resources Canada, 2017). We mainly focus on studying a background reference model in this study. Passive Seismic Tomography (PST) is the geophysical technique that I applied in this study.
- ItemOpen AccessWell-log validated waveform inversion of reflection seismic data(2019-08-22) Romahn Reynoso, Sergio Jorge; Innanen, Kristopher A.; Gray, Samuel H.; Lines, Larry R.; Lawton, Don C.; Trad, Daniel O.; Gilbert, Hersh J.This thesis develops, examines and refines the basic waveform inversion procedures of reflection seismic data under an alternative perspective that makes an effort to assign to each element of full-waveform inversion (FWI) workflow an element of standard seismic processing. The basic procedures can be summarized as an iterative cycle of modelling, migration and inversion. The modelling part was carried out with a fourth order finite-difference scalar acoustic algorithm, assuming a 2D medium governed by the constant density acoustic wave equation. The two-way wave operators (analogous to reverse-time migration), used as standard engine in FWI to produce the gradient, is replaced with one-way wave operators. The role of the pseudo-Hessian as a gradient preconditioner is substituted with a deconvolution imaging condition. A key element of this methodology is the incorporation of log information to calibrate the update direction (i.e., a well-log validation approach), rather than determining a scalar step-length with a line search (i.e., a data-validation approach). This inversion perspective leads to remarkably accurate results and significant computational savings. Once a well-log validated waveform procedure was set up, I evaluated the scope of well-log validation under several factors (such as geological complexity, well location, log interval, and well-data uncertainty) that may influence the performance of this technique due to the well-log limitation of providing punctual information. I also analyzed the repercussions that the seismic acquisition parameters, the random noise and a deficient initial model would have on the inversion. Having established pros and cons of well-log validated waveform inversion with one-way wave equation migration, I addressed the unknown-wavelet issue. A process of updating both the amplitude and phase of an initial wavelet estimate is set up to produce stable inversions in both synthetic and field environments. In the latter case, profiles extracted from a log-validated waveform inversion of the Hussar land data set, when compared to two blind validation wells, are observed to accurately reproduce structures in zones producing significant reflection energy.