Study On Seismic Modeling Based On Lowrank Decomposition And Reverse Time Migration

Seismic exploration of complex hydrocarbon reservoirs needs to develop new seismic modeling and seismic imaging methods, which can simulate the seismic wavefield propagated in a complex medium and obtain a seismic imaging with both kinematically and dynamically correct wavefield characteristics. For this purpose, based on acoustic wave equation, this paper, focusing on the issues of seismic wavefield extrapolation and true-amplitude image with reverse time migration(RTM), carries out the theoretical analysis and studies the methods.Seismic extrapolation is an essential part of seismic modeling and imaging. Spectral method and finite-difference method are the two most popular and straightforward ways of implementing wave extrapolation in time domain. To improve the computational efficiency of spectral method, this paper uses lowrank decomposition to deal with the space-wavenumber domain wavefield extrapolator and propose staggered grid lowrank(SGL) method, which can significantly improve the computational efficiency without decreasing its accuracy. This paper also proposes staggered grid lowrank finite-difference(SGLFD) method by combining the staggered lowrank method and finite-difference to avoid numerical dispersion. The SGLFD method obtains the optimal finite-difference coefficients by matching the spectral response in the mixed space-wavenumber domain for a wide range of spatial wavenumbers. This method has high accuracy and efficiency, and is free of dispersion. This paper studies the general principle to calculate the optimal coefficients from the recursive integral time-extrapolation(RITE) operators. To improve the stability, this paper uses taper and weighted least-square to design an unconditionally stable finite-difference method. The numerical tests demonstrate that the proposed methods have high accuracy and can suppressnumerical dispersion error, which is valuable for seismic modeling and seismic imaging in complex mediums.The wavefield propagation affects the amplitude of the imaging results. Based on the Kirchhoff inversion/migration theory, this paper analyzes the geometric spreading and transmission losses in RTM. After applying high-frequency asymptotic analysis to the amplitude behavior of the wave propagation operators used in RTM, this paper proves that the geometric spreading losses and their compensation are automatically included in RTM and derive formulas to compensate for transmission losses. The derivations and proof provide a theoretical basis for the compensation of transmission losses in true-amplitude RTM. The results from the numerical experiments are consistent with the theoretical analysis and demonstrate that compensating of transmission loss helps to improve the amplitude accuracy of RTM and enhance the imaging events of weak seismic signal. In order to improve the accuracy of RTM, this paper uses the SGLFD method for RTM’s forward and backward wavefield propagations and develops SGLFD RTM. To improve the efficiency of RTM, this paper parallelizes the SGLFD RTM with MPI and OpenMP. The numerical tests with synthetic models and real field data demonstrate its validity and feasibility.