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《Chinese Journal of Geophysics》 2015-09
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Full-wavefield VSP reverse-time migration based on the adaptive optimal finite-difference scheme

CAI Xiao-Hui;LIU Yang;WANG Jian-Min;WANG Wei-Hong;REN Zhi-Ming;State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum;CNPC Key Laboratory of Geophysical Prospecting,China University of Petroleum;Exploration and Development Research Institute of Daqing Oilfield Company Limited;  
Vertical seismic profiling(VSP)data provides higher resolution information,lower environmental noise and better information beside wells than surface seismic data.Meanwhile,reverse-time migration(RTM)based on two-way wave equation has no dip limitations on the image.It is significant to implement the RTM on VSP data.The four key factors of RTM are wavefield extrapolation,absorbing boundary conditions(ABC),imaging condition and noise suppressing.For the first factor,the paper compares the finite-difference(FD)method based on optimal scheme(optimal-based FD method)with the FD method based on space Taylor series expansion(space TE-based FD method)and the FD method based on time-space Taylor series expansion(time-space TE-based FD method)to indicate the accuracy of the optimal-based FD method.Moreover,to improve the efficiency,the adaptive variable-length spatial operators method is introduced into the optimal-based FD method(adaptive optimal-based FD method).The adaptive optimal-based FD method is adopted to solve the acoustic wave equation and thus implements the wavefield extrapolation of RTM efficiently and accurately.Second,the hybrid ABC is used to suppress artificial reflections from the model boundaries caused by the limited computational boundaries.Then the paper compares normalized cross-correlation imaging condition with deconvolution imaging condition for imaging,which indicates the prior is more stable and efficient.Finally,the Laplace operator filtering is applied to remove the low frequency noises.The conventional migration from VSP data using only up-going primary wavefield often suffers from poor illumination and limited migration scope.To enhance the illumination,enlarge the migration scope and improve the accuracy,the analysis of VSP RTM imaging with direct wave,primary wave,first-order multiple,second-order multiple and higher-order multiples implies that the full-wavefield(complete acoustic wavefield data without direct wave)can be treated as effective wave to implement prestack RTM.RTM test on VSP model data shows that the adaptive optimal-based FD method is more efficient and accurate than the fixed-length TE-based FD method,and suitable to VSP RTM.VSP RTM result using full-wavefield is clearer and images a larger area than VSP RTM only using up-going field.Imaging test on VSP field data demonstrates the efficiency and accuracy of our method.
【Fund】: 国家自然科学基金项目(41074100 41474110);; 教育部新世纪优秀人才支持计划(NCET-10-0812)联合资助
【CateGory Index】: P631.44
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