The Study On Prestack Depth Migration Imaging Method Of Ocean Bottom Node

Since the new century,due to the continuous development of new marine energy by various countries,various exploration technologies applied to marine energy have developed rapidly.Among them,the Ocean Bottom Node(OBN)seismic exploration technology has the advantages of high flexibility,systematic layout and convenient recovery,and has solved the problem of traditional marine seismic exploration affected by sea water and towline.Therefore,it has been widely used in the exploration of natural gas hydrates in the seabed.However,in the data acquisition process of OBN seismic exploration,the observation mode of sea surface excitation and seabed reception is used,which results in a huge difference in the elevation between the shot point and the detection point.Therefore,when processing the OBN seismic data,the correction of the fluctuation of the shot point is a problem that must be solved.The usual datum level correction method usually adopts elevation static correction,which has the advantage of simplicity and rapidity.However,in the areas where the altitude difference between the shot point and the detection point is large,this correction method will cause great errors,resulting in large deviations in the velocity field.Although it does not affect the superposition imaging effect,this deviation will cause great loss of accuracy in the migration processing based on the wave field continuation algorithm.In OBN seismic exploration,the altitude difference between shot point and geophone point is much larger than that of common land fluctuation,and this error will be more obvious.Therefore,in view of the unique layout of OBN seismic exploration,it is of great practical significance to seek a prestack depth migration imaging technology that can accurately correct the elevation difference between shot point and detection point.Firstly,aiming at the problem of elevation difference between shot point and detection point,this paper studies the method of datum level correction for OBN seismic exploration.On the basis of previous studies at home and abroad,the advantages and disadvantages of various methods of datum level correction are systematically analyzed and demonstrated,including static correction method,step-by-step accumulation method,wave field extension method and direct downward extension method.Based on the common migration method of excitation and reception on the horizontal surface,the acoustic wave equation is deduced and decomposed into one-way wave equation,and the one-way wave phase-shifted wave field continuation operator suitable for pre-stack depth migration of common-shot gathers is given.The imaging rate and effect of wave equation migration are closely related to the advantages and disadvantages of the operator.Based on the phase-shifting operator,the splitting-step Fourier(SSF)and Fourier finite difference(FFD)wave field continuation operators are derived according to the different mathematical approximations of the vertical wavenumbers in the operator.According to the relative errors of the two operators,the advantages and disadvantages of the two operators are analyzed,and two wave field extension operators are used to complete prestack depth migration imaging.The imaging effect and operation efficiency of the two operators are compared and analyzed with the help of imaging results and calculation speed.On the basis of the complete general wave equation migration method,the paper chooses the wave field extension datum correction method suitable for wave equation migration imaging,designs the wave field pre-stack depth migration imaging method suitable for OBN seismic exploration,and tests the theoretical model of OBN,realizes the complete migration imaging process,verifies the validity and accuracy of the migration imaging process deduced in this paper.The research shows that the prestack depth migration method based on wavefield extension datum correction method can accurately image seismic data under OBN observation mode.Compared with static correction method,the extended path of wavefield extension method coincides with the actual propagation path of seismic wave,which truly reflects the propagation path of seismic wave in medium,and can obtain accurate results with less error.