Visco-acoustic Resverse-Time Migration In Anisotropic Medium

In recent years,with the development of seismic exploration,the targets become more and more complex,which leads to a huge challenge to the imaging techniques.So the imaging techniques must be developed so that they can be adapted to the irregular surface,highly steep structures,violent velocity variations and complex reservoir,etc.As we all know,the underground medium is anisotropy and viscosity universally,the acoustic assumption does not describe seismic wave propagation property accurately,which making the seismic response of the actual geological structure be distorted,or even the effective information is ignored.In order to obtain high precision amplitude preserved migration,we need correct the influence of anisotropy and viscosity to the image.And this requires a viscoacoustic wave equation which can describe the seismic wave propagation property in this medium accurately,and a set of complete reverse time migration imaging technology.As to these issues,the dissertation mainly do the following researches:(1)Based on the theory of elastic waves,we derived a pseudo-acoustic equation in VTI medium which expressed by velocity and stress.Based on this equation,a robust prestack reverse time migration strategy is proposed.We also popularized it into TTI media by using coordinate transformation.The energy conservation proved that the velocity-stress equations,derived from elastic theory,are more stable than equations that derived from the dispersion relation.For anisotropic parameters with,the issue about residual shear instability can be solved by introducing a shear correction.Numerical simulation shows that the new equation can not only describe the kinematic characteristics of qP wave,and also adapt well to TTI media.Furthermore,the numerical examples verified the stability of the equations.(2)Neglecting anisotropy and attenuation in seismic wave propagation will result in inaccuracy image,such as problems of diffracted wave convergence and seismic wave attenuation.So it is urgent to take anisotropy and viscosity into account.So it is necessary to consider both of the characteristics in practical production.In this paper,starting from the basic theory of elastic wave in TI media,and introducing the GSLS theory of isotropic medium into anisotropic,we derive anisotropic attenuation of pseudo acoustic equation based on the GSLS model by the acoustic approximation method.The numerical results show that the VTI viscoacoustic wave equation can not only describe the propagation of wave in anisotropic media accurately,but also reflect the effect of absorption and attenuation.Reverse time migration of HESS model on VTI medium shows that the attenuation pseudo acoustic equations can image more clearly,such as the high steep dip angle complex structure,make deep amplitude distribution more balanced,and obtain more accurate reliable amplitude imaging profile.(3)In order to solve the problem that the data is short of memory,it can cause instability of wave field back propagation.In this chapter,based on a SLS model and second-order quasi-differential equations,we derive a new viscoacoustic wave equation in VTI media.On the other hand,to address the high frequency instability problem caused by the backward propagation,we introduce a regularization operator to eliminate this problem,and implement reverse time migration with amplitude compensation.