Visco-acoustic VTI Resverse-time Migration Based On Interferometric Imaging Condition

With the increasing complexity of the object of seismic exploration,the seismic imaging technique called Reverse Time Migration(RTM),which is suitable for the irregular topography,high and steep conformation,complicated velocity distribution and complex reservoir,has been continuously improved and developed.The reverse time migration technology based on two-way wave equation has a low frequency inherent imaging noise and limited resolution.So it can not provide accurate amplitude information,and its deep imaging amplitude energy is weak and uneven distribution,which restricts the development of the complex structure of deep reservoir in high precision imaging technology.In addition,viscosity and anisotropy are prevail in the earth medium.The viscosity is reflected that the energy of the seismic wave will be absorbed and attenuated to some extent in the process of underground propagation.Anisotropy exists widely in most sedimentary rocks,especially the shale in obvious stratification structure,thin interbeds in limestone and the directional arrangement of vertical fractures in large pieces of rocks.If the underground medium is just simply regarded as isotropic media,it's likely to ignore the effective seismic information,resulting in inaccurate seismic imaging,which will affect the subsequent seismic interpretation.Therefore,in the fine exploration and development stage at present,how to solve the more complex underground geological structures and the widespread viscosity,anisotropic,heterogeneous are of significance.This paper aims at the problems above,I have done the following work:First,start with the general acoustic wave equation,I deduced the visco-acoustic wave equation based on the viscosity theory of standard linear solid model(SLS).When proceeding the reverse time migration imaging,it's required to change the attenuation term from the plus sign to the minus sign to compensate the attenuation of the underground medium.According to the elastic wave equation in the anisotropic media,I deduced quasi-acoustic equation in the VTI medium based on Alkhalifah's "acoustic approximation" theory,which can be used to describe the kinematic characteristics of quasi P wave in anisotropic medium.Taking viscosity and anisotropy into account,I deduced the quasi differential equation in the visco-acoustic VTI medium.The pseudo spectral method is used to construct the forward and inverse wave field operator,which have improved the computational efficiency and reduced memory consumption.Then,I used the perfectly matched layer(PML)boundary condition to absorb the boundary reflection.At last,the final effect of reverse time migration imaging is determined by the imaging condition.When the underground medium has obvious small scale velocity changes,the underground seismic response will not be described completely by the traditional imaging condition.In this paper,a new imaging condition,called the interference imaging condition,is introduced to improve the seismic artifacts due to inaccurate velocity model.Compared with the traditional cross-correlation imaging condition,the Wigner distribution function(WDF)filter is used to attenuate the random noise in the wavefield.I used the interferometric imaging condition based on the WDF filter function to the visco-acoustic VTI medium,which makes the imaging result more accurate and clearer.