波场正演模拟中Sarma边界条件的改进

Improvement of Sarma boundary condition in wavefield forward modeling.

在有限元地震波正演模拟中,采用Sarma吸收边界条件会在介质与衰减层之间的界面上产生人为反射,这是由于在衰减层中引入阻尼矩阵,使衰减层与介质之间出现明显物性差异所致。为此,对Sarma吸收边界条件进行了改进,将衰减层内所采用的恒定比例系数改为变比例系数,即在衰减层厚度相等的条件下,在衰减层内设置一个过渡带,在过渡带比例系数由小变大,在衰减层内比例系数取恒定值。这样,使介质与衰减层之间的物性参数由突变改变为渐变,不仅压制了人为反射,而且保证了反射波在衰减层中被充分吸收。数值模拟表明,改进的Sarma边界条件方法有效实用。

When Sarma absorbing boundary condition is adopted in finite element forward modeling,spurious reflections will arise on the in- terfaces between the medium region and the damping region.The reason is that the damping matrix in the damping region makes the medium region and the damping region different in properties.So, the proportion coefficient in the damping region is changed from constant to variable.Setting the damping region as the same thick- ness,the transition region is located in the damping region.In the transition region,the proportion coefficient increases and reaches a constant in the end.By this means,the properties between the damping region and the medium are altered from abrupt changes to gradual changes;which suppress the spurious reflections and ensur the reflections can be sufficiently absorbed.The validity of the method is demonstrated by numerical simulation.