///Reverse-time
    migration (RTM) based on two-way wave equation, has many advantages. Comparing
    with the Kirchhoff migration and one-way wave equation migration which are
    widely used in practice, RTM has a simple principle, it obtains imaging
    accurately with no angle constraints, and it can be adapted to arbitrary
    velocity model and able to handle imaging point with multi travel time etc. In
    this paper, acoustic wave equation pre-stack RTM based on high-order finite
    difference algorithm is implemented. Perfectly matched layer(PML) absorbing
    boundary condition is used to absorb boundary reflections. According to
    analyses of the wavefield decomposition cross-correlation imaging condition and
    the source-normalized cross-correlation imaging condition, the authors attempt
    to improve the imaging condition by combining with the advantages of both.At
    last, theoretical and real data validate that this method can eliminate noises
    better and provide more relatively accurate information of reflection
    coefficients.