Investigation On Prestack Imaging By Reverse Time Migration For VSP Data

With the continuous development of oil and gas exploration,the exploration and development of oil and gas has brought new challenges to the traditional seismic exploration technology.In particular,it puts forward higher requirements for the accuracy of seismic exploration,the improvement of seismic resolution can identify large-angle structures and high velocity rock structure.Hence,VSP is a kind of observation system different from the ground surface.The receiving point of VSP data is distributed inside the medium which is closer to the target layer,so it can reduce the absorption of high frequency components in the signal by the low velocity zone.In the large-angle structure of the imaging,it can collect the seismic waves which the ground cannot collect.Therefore,compared with the traditional surface seismic exploration,VSP seismic data often have higher imaging accuracy and wider bandwidth.In this paper,according to the characteristics of VSP observation system,some research has been done in numerical simulation of VSP data and the reverse time migration imaging of VSP data.Firstly,in the part of numerical simulation of VSP data,we study two methods of the wave equation dissociation,such as conventional grid and staggered grid.Through the comparison and analysis,it is found that the staggered grid has higher calculation accuracy under the same parameters.This paper proves that the positive and inverse VSP observation system satisfy the principle of travel time reciprocal,the common detector gathers in the positive VSP observation system can be regarded as the common shot gathers of the inverse VSP observation system.Therefore,in order to improve the computational efficiency and imaging accuracy,the inverse VSP observation system can be used to calculate the reverse time migration in the data processing.Secondly,ricker wavelet as the excitation source in the reverse time migration process,the selection of its parameters will affect the imaging accuracy of reverse time migration.On the basis of the analysis of the wavelet spectrum,some useful conclusions are obtained,which are helpful to improve the accuracy of the reverse time migration imaging of VSP data.Lastly,in the model test,in the aspect of imaging condition,this paper selects the cross correlation imaging condition to keep the imaging accuracy.For suppressing the low frequency noise interference,the Laplacian noise suppression method is optimized to improve the accuracy in imaging profile.For the boundary condition and storage strategies,the random boundary condition and PML boundary with efficient boundary storage strategies are researched and analyzed.The latter greatly reduce the storage volume of wave field and have higher imaging accuracy.