Junlun Li

Focal Mechanism Determination using High Frequency, Full Waveform Information

In this research, we use high frequency waveform information to determine the focal mechanisms of small local earthquakes at an oil reservoir. During the waveform inversion, we maximize both the phase and amplitude matching between the observed and synthetic waveforms. In addition, we use the polarities of the first P-wave arrivals and the S/P amplitude ratios to better constrain the matching between the synthetic and observed waveforms. The objective function is constructed to include all four criteria. Due to the complexity in the objective function, it is almost impossible to directly perform an inversion with derivative techniques. Instead, an optimized grid search method is used to search over all possible ranges of fault strike, dip and rake, as well as a predetermined range of earthquake locations. To speed up the algorithm, a library of Green‟s functions is pre-calculated for each of the moment tensor components and possible earthquake locations. Careful optimizations in filtering and crosscorrelation are performed to further improve the grid search algorithm, such that no filtering and cross correlations are performed in searching through the parameter space of strike, dip, and rake. Consequently, speed is boosted tenfold by these optimizations in filtering and cross correlation. We apply the new method to induced seismic events in an oil reservoir. Satisfactory matching between synthetic and observed seismograms is obtained, as well as reasonable focal mechanisms, considering the local geological structure and possible causes for induced seismicity.

Frequency-Domain Finite-Difference Acoustic Modeling With Free Surface Topography Using Embedded Boundary Method

In this paper, we present a method to model acoustic wave propagation in the frequency-domain in the presence of free surface topography using the embedded boundary method. The advantage of this method is to solve for the pressure field at each frequency on regular finite difference grids but with sub-cell resolution (up to 2 order accuracy) for the irregular free surface. The topographic free surface condition is implemented in 2 order accuracy, as in the regular domain and that global 2 order accuracy is guaranteed. We use the level set method to obtain the projection points and normal directions corresponding to the ghost points in the scheme in the irregular domain. The computational cost for solving the modified sparse matrix for the pressure field increases very little compared to that for a flat surface. We have benchmarked our solver with a 2-D Gaussian hill model and simulated wave propagating in a modified Canadian foothills model. This solver can be used as the forward engine in the full waveform inversion, and we are working underway to perform full waveform inversions with real land survey data with considerable presence of surface topography.

Microseimic Monitoring of the Yibal Oil Field

Yibal Oil and Gas field has a great deal of induced microseismic activity, which has been monitored extensively for more than 9 years through cooperation between the Petroleum Development of Oman (PDO), Massachusetts Institute of Technology (MIT) and Sultan Qaboos University (SQU). In this study, we investigate the location of the microseimic events using three earthquake location methods: (1) classical linearized hypocenter method (hypoinverse), (2) probabilistic earthquake location (NonLinLoc) and (3) full waveform inversion method. The data set consists of approximately 1500 selected good quality seismic events recorded by a near-surface seismic network over a 9-year period.

A Hybrid Method For Modeling SH Wave Scattering From Fractures In 2D Heterogeneous Medium: Coupling of Boundary Element And Finite Difference Methods

We present a hybrid method to model the SH wave scattering from 2D fractures embedded in a heterogeneous medium by coupling Boundary Element Method (BEM) and Finite Different Method (FDM) in the frequency domain. We use FDM to propagate SH wave from a source through heterogeneities to localized homogeneous domains where fractures are embedded within artificial boundaries. According to Huygens’ Principle, the boundary points can be regarded as “secondary” point sources and their values are determined by FDM. Given the incident fields from these point sources, we apply BEM to model the scattering from fractures and propagate them back to the artificial boundaries. FDM then takes the boundaries as secondary sources and continues propagating the scattered field into the heterogeneous medium. We also present a numerical iterative scheme that can account for the multiple scattering between different sets of fractures or between fractures and heterogeneities. We first compare the results calculated from this hybrid method with pure BEM method to show the accuracy of the hybrid approach and the iterative scheme. This method is then applied to calculate the wave scattered from fractures embedded in a horizontally layered medium and a more complicated Marmousi model.