Application of the nearly perfectly matched layer to the propagation of low-frequency acoustic waves

Seismic modelling is a very effective method to help us understand the characteristics of the seismic wave propagation in the Earth's interior. However, one tough problem is the presence of spurious reflections from the boundaries of the truncated computational domain, especially the reflections of low-frequency energy. In this paper, we apply the nearly perfectly matched layer (NPML) technique to suppress the artificial reflections from the edges of the 2D model in the case of acoustic waves of a low-frequency source (5 Hz). For the implementation of the finite-difference operator, we employ fourth-order accuracy methods in space and second-order accuracy methods in time. Through the numerical comparison between the NPML and the technique known as the convolutional perfectly matched layer (CPML), which is considered the boundary condition of optimum absorbing, we demonstrate that the NPML has better absorbing performance than the CPML. We suggest that a greater amount of analysis will be needed to study how NPML works facing the wide variety of complex media that are commonly used in seismic modelling