Browsing by Author "Niu, Feng-Lin"

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    Helmholtz decomposition with a scalar Poisson equation in elastic anisotropic media
    (Elsevier, 2024) Fang, Xin-Yu; Yao, Gang; Zheng, Qing-Qing; Zhang, Ping-Min; Wu, Di; Niu, Feng-Lin
    P- and S-wave separation plays an important role in elastic reverse-time migration. It can reduce the artifacts caused by crosstalk between different modes and improve image quality. In addition, P- and S-wave separation can also be used to better understand and distinguish wave types in complex media. At present, the methods for separating wave modes in anisotropic media mainly include spatial non-stationary filtering, low-rank approximation, and vector Poisson equation. Most of these methods require multiple Fourier transforms or the calculation of large matrices, which require high computational costs for problems with large scale. In this paper, an efficient method is proposed to separate the wave mode for anisotropic media by using a scalar anisotropic Poisson operator in the spatial domain. For 2D problems, the computational complexity required by this method is 1/2 of the methods based on solving a vector Poisson equation. Therefore, compared with existing methods based on pseudo-Helmholtz decomposition operators, this method can significantly reduce the computational cost. Numerical examples also show that the P and S waves decomposed by this method not only have the correct amplitude and phase relative to the input wavefield but also can reduce the computational complexity significantly.
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    Local events-based fast RTM surface-offset gathers via dip-guided interpolation
    (Springer Nature, 2021) Zhao, Yang; Niu, Feng-Lin; Fu, Lei; Cheng, Cheng; Chen, Jin-Hong; Huo, Shou-Dong
    Reverse Time Migration (RTM) Surface Offset Gathers (SOGs) are demonstrated to deliver more superior residual dip information than ray-based approaches. It appears more powerful in complex geological settings, such as salt areas. Still, the computational cost of constructing RTM SOGs is a big challenge in applying it to 3D field data. To tackle this challenge, we propose a novel method using dips of local events as a guide for RTM gather interpolation. The residual-dip information of the SOGs is created by connecting local events from depth-domain to time-domain via ray tracing. The proposed method is validated by a synthetic experiment and a field example. It mitigates the computational cost by an order of magnitude while producing comparable results as fully computed RTM SOGs.
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    Master event based backazimuth estimation and its application to downhole microseismic monitoring
    (Elsevier, 2022) Meng, Xiao-Bo; Chen, Hai-Chao; Niu, Feng-Lin; Du, Yi-Jing
    Microseismic monitoring provides a valuable tool for evaluating the effectiveness of hydraulic fracturing operations. However, robust detection and accurate location of microseismic events are challenging due to the low signal to noise ratio (SNR) of their signals on seismograms. In a downhole monitoring setting, P-wave polarization direction measured from 3-component records is usually considered as the backazimuth of the microseismic event, i.e., the direction of the event. The direction and arrival time difference between the P and S waves is used to locate the seismic event. When SNR is low, an accurate estimate of event backazimuth becomes very challenging with the traditional covariance matrix method. Here we propose to employ a master event and use a grid search method to find the backazimuth of a target event that maximizes the dot product of the two backazimuthal vectors of the master and target events. We compared the backazimuths measured with the proposed grid-search and the conventional covariance-matrix methods using a large synthetic dataset. We found that the grid-search method yields more accurate backazimuth estimates from low SNR records when measurements are made at single geophone level. When array data are combined, the proposed method also has some advantage over the covariance-matrix method, especially when the number of geophones is low. We also applied the method to a microseismic dataset acquired by a hydraulic fracturing project at a shale play site in southwestern China and found that the relocated microseismic events tend to align along existing faults more tightly than those in the original catalog.