二阶标量声波差分有限差分正演模拟，加PML吸收边界（完全匹配层），实现边界无反射效果，可以添加各种模型，进行正演模拟。(2D -acoustic equation finite-difference forward modelling,which include perfect match layer.it can achieve the result that have no reflection,and it also can achieve other the result of mode