Semi-exact local absorbing boundary condition for seismic wave simulation

An absorbing boundary condition is necessary in seismic wave simulation for eliminating the unwanted artificial reflections from model boundaries. Existing boundary condition methods often have a trade-off between numerical accuracy and computational efficiency. We proposed a local absorbing boundary condition for frequency-domain finite-difference modelling. The proposed method benefits from exact local plane-wave solution of the acoustic wave equation along predefined directions that effectively reduces the dispersion in other directions. This method has three features: simplicity, accuracy and efficiency. Numerical simulation demonstrated that the proposed method has higher efficiency than the conventional methods such as the second-order absorbing boundary condition and the perfectly matched layer (PML) method. Meanwhile, the proposed method shared the same low-cost feature as the first-order absorbing boundary condition method

Post-Stack 4D Seismic Inversion for Improved Monitoring at Sleipner Field

More than 20 million tons of CO2 has been injected into the Utsira formation at Sleipner field since 1996. The injected CO2 in the subsurface is characterized on the post-stack seismic images as a stack of high-amplitude bright spots which caused interference effects in the seismic response of CO2-saturated thin beds. In this study, a dedicated time-lapse (4D) inversion workflow is proposed, where the estimated time-strain will play the role of the low-frequency initial model for the model-based inversion as there are no additional independent measurements or repeated well logs available in the provided dataset for the Sleipner area. The purpose of the workflow is to improve the confidence in 4D post-stack inversion without using any additional data to the seismic traces. Application of the proposed workflow proves to be effective for what it is designed for i.e. for time lapse inversion of the Sleipner data. The use of time-strain was particularly useful for the construction of the low frequency initial model for the model-based inversion. It helped the model-based inversion to drastically enhance the results in terms of impedance change values of the CO2 plume and to eliminate most of the incorrect signals