Forward Modeling And Reverse Time Migration Of QP-wave Equation In TI Media

There is wide range of anisotropy in underground media.Ignoring the existence of anisotropy,it is impossible to accurately describe the propagation characteristics of seismic waves in actual media.Thus,it affects the accuracy of imaging results.The transversely isotropic(TI)media is the most common and widespread anisotropic media.Therefore,it is quite necessary to study the migration imaging method of TI media.At present,reverse time migration is the most accurate imaging method.It is based on the theory of the two-way wave equation and makes full use of the abundant information of the wave field.It breaks through the limitation of dip angle and can deal with the media model of strong lateral speed variation,which greatly improves the imaging effect of complex structure such as salt dome flank,salt dome,vertical fault and so on.The forward modeling of seismic wave field is an effective way to study the propagation characteristics of seismic waves in underground media.The research on numerical simulation methods of seismic wave will lay the foundation for reverse time migration in TI media.The kinematic characteristics of the quasi-P(qP)wave under acoustic approximation are in good agreement with the P wave in elastic wave.Moreover,the qP wave equation avoids the problems of large calculation,low efficiency and difficult separation of longitudinal and transverse waves when we solve the elastic wave equation directly.Therefore,it is more practical to study the numerical simulation and reverse time migration methods of the qP-wave equation in TI media.In this paper,the key problems of numerical simulation and reverse time migration in TI media are studied.We derived the qP-wave equation in TI media in detail.We use the improved perfectly matched layer(PML)boundary control equation applicable to the TI media qP-wave equation to deal with the wave field boundary and achieve good results.Meanwhile,the corresponding higher order finite difference scheme is also given.Then,we analyze the pseudo shear wave suppression strategy and the stability problem in the numerical simulation of the qP-wave equation.In order to improve the precision of numerical simulation,the difference coefficient is optimized based on the theory of least squares,and the frequency dispersion can be suppressed better.The reverse time migration methods have the problem of low frequency noise,large amount of storage and large amount of computation,which restrict the actual application of it.We analyzed the imaging condition and denoising method of reverse time migration.The normalized wave field separation cross-correlation imaging condition based on Poynting vector is applied to reverse time migration,which can effectively suppress low frequency noise.In addition,we studied the storage strategy of reverse time migration.By improving the storage mode of wave field,the effective boundary storage strategy is applied to implement reverse time migration,which greatly reduces the amount of storage in the migration.In order to solve the problem of large computation in reverse time migration,we optimized the code in programming.Moreover,we use MPI parallel algorithm to achieve parallel computation of reverse time migration,which greatly improves the computation efficiency.Finally,the validity and applicability of the above algorithms are verified by model tests.