此程序是二维分数阶粘弹性声波有限差分法的正演模拟，时间导数二阶精度，空间分数阶导数2阶精度，含波长快照，未加边界，确保可运行。(This program is a two-dimensional fractional viscoelastic acoustic forward modeling of finite difference method, the time derivative second-order accuracy,