Modeling seismic wave propagation in TTI media using multiaxial complex frequency shifted nearly perfectly matched layer method

The nearly perfectly matched layer (NPML) is a non-split perfectly matched layer that directly transforms the wave field. Compared to other non-split perfectly matched layers, NPML is computationally efficient and has advantages such as not changing the form of the equation and being easy to implement. However, in TTI medium simulations it is found that the NPML is very unstable and cannot absorb near-grazing incident waves. This explains why the CPML is currently used more frequently. In this study, the complex frequency shifted transform is used to enhance the absorption of near-grazing incident waves with NPML and to avoid the generation of low-frequency singular values. At the same time, the double damping profile is used to improve the stability of the boundary we called MCFS-NPML. Subsequently, this method is also applied to seismic wave equations in poroelastic media. In order to further improve the absorption capacity of the boundary and weaken the deviation caused by the discrete difference, a new attenuation function is proposed.