We have developed a new scheme with high accuracy and parsimonious memory requirements to reconstruct the source wavefield in reverse time migration (RTM). This scheme used a linear combination of the boundary wavefield in several layers to reconstruct the wavefield in the imaging domain. The value of the linear combination was stored in one buffer, making the computer memory requirement of our method equal to that of a method that only stored one layer of the boundary wavefield. The coefficients for the linear combination were determined by an optimization technique that minimized the finite difference error in the Fourier domain. The optimal coefficients were presented for finite-difference (FD) stencils of 5–15 grid points. The numerical error of our scheme was analyzed and compared with that of standard FD stencils, representing the conventional method that used multiple layers of boundary wavefield to back propagate the source wavefield. The accuracy of our method is only less accurate than the conventional method in theoretical analysis. However, the storage requirement of our method is merely 1/N of the conventional method if a 2N+1 grid-point FD stencil is used in the space. We have also extended the comparison to two other methods, a one-layer method and an extrapolation method, beyond the conventional method. The numerical results demonstrated that our method can accurately generate high-accuracy images in RTM.
The deposition and evolution of fine-grained sediments is a hot topic in fine-grained sedimentary rock studies and is important for accurately evaluating shale gas sweet spots. In this paper, the fine-grained deposition and evolution characteristics of the Wufeng-Longmaxi shales, major targets for Chinese shale gas exploration, were studied by using core observations, thin section analyses, scanning electron microscopy, geochemical analysis, and fossil identification. This work accurately identified six typical lithofacies; among them, the organic matter-rich siliceous shale facies (OMRSSF), the high-organic matter siliceous argillaceous shale facies (HOMSASF), and the medium-high organic matter low calcareous siliceous shale facies (M-HOMLCSASF) are favorable facies for shale gas exploration. The high-resolution isochronous unit in the shelf fine-grained sedimentary system was established, and the differential evolution of lithofacies in the system tract was discussed. The lithofacies deposition and differentiation in the transgressive system tract were controlled by the transgressive scale and tectonics under increasingly shallow water conditions. The lithofacies deposition and differentiation in the regressive system tract were controlled by tectonics and the preexisting lithofacies. The lithofacies in the regressive system tract had more frequent facies transitions and greater differentiation than those in the transgressive system tract, and they exhibited significant spatiotemporal inheritance. Sequential differential sedimentary sequences and symmetric differential sedimentary sequences were distinguished in the continental shelf sedimentary system. The lithofacies depocenters and subsidence centers were consistent in the transgressive system tract, while the tectonically active paleocontinent was important in the regression system tract. This study is of great significance for further high-resolution exploration of marine shale and improvement of the theory of shelf fine-grained sedimentary systems.
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