Research And Application Of Prestack Depth Migration Method Based On VTI Medium

The complex and steep structural belt is usually the zone of oil and gas accumulation,also is the key target zone in today's oil and gas exploration and development.The imaging precision of the conventional migration imaging method in this structure does not meet the requirement of seismic data processing because of the change of surface topography,tectonic complex and its severe changes happen on transverse velocity,which leads to the uncertainty of the structural interpretation.The migration section is obtained based on the theory of the wave equation prestack depth migration,which is rich in seismic wave amplitude information and it can effectively solve this problem that the imaging precision is not high in this zone of high-steep complex structure.The migration imaging process of the actual seismic data generally chooses the vertical direction as the dominant direction of the seismic wave propagation,which ignores the change of the seismic wave propagation in the transverse direction,thus limiting the migration imaging precision of geological media and resulting in many possible interpretations of migration section artifacts,lithology,structure and reservoir identification.To solve this problem,a prestack depth migration method based on the stack of bidirectional wavefield in vertical transversely isotropic(VTI)media is proposed in this paper.This method first divides the wave equation in the F-K domain into one-way wave equations in vertical and horizontal directions,uses a wide wave field extrapolation step and linear interpolation respectively to calculate the values of one-way wave field in the vertical and horizontal directions,then calculates the total value of the wavefield by the proportions of the values of wavefield in the vertical and horizontal directions,and finally achieves the migration imaging of the seismic data.This prestack depth migration method takes into consideration the information of seismic wave not only in the vertical direction but also in the horizontal direction.It can effectively solve the stratigraphic dip limit and the numerical dispersion,and achieve a higher imaging precision of the complex surfaces and steeply-dipping structure.Finally the effectiveness of this prestack depth migration method is verified by analyzing the theory of migration imaging and the simulation of salt dome model.The wave equation prestack depth migration algorithm is mainly through the migration operator to calculation.The precision of the migration operator directly restricts the migration imaging quality,and the method of calculation of the migration operator is mainly using the method of numerical calculation to the gradual approximation of migration operator.But the approximation degree of migration operator is not high because of using the existing incremental algorithm method to calculate the migration operator.It has certain error compared with the accurate value of the migration operator.In order to better solve the above problems,this paper puts forward a method which based on coefficient optimization two-way superposition of wave field of prestack depth migration method in VTI media.This method using the pade approximation rational function to expand wavefield extrapolation operators,and then using the chebyshev polynomial to optimize the expansion coefficient,which getting a new wavefield extrapolation operator eventually.This paper validates the new migration operator imaging effect by comparing and analyzing the migration operator error and migration section imaging effect which obtained by using the method of mathematical simulation and model simulation.In conclusion,the prestack depth migration algorithm is to improve the imaging accuracy effectively in the complex tectonic zone.