CONCLUSIONS

On the basis of the idea of migration dephasing, the effect of the parabolic approximation of the exact one-way wave equation is explained as undermigration and the effect of the Crank-Nicolson numerical method that is used to solve the diffraction equation is explained as overmigration. The modified wave equation that the numerical solution actually satisfies for the parabolic diffraction wave equation shows that the trace spacing interval plays a larger role than the depth step size of downward continuation in determining the accuracy of the numerical solution.