Abstract：The paper only analyzed the following coneepdons concerning to relation between depth migration and time migration，relation between prestack depth migration and poststack depth migration，normal ray and imaging ray,ray migration and coherent inversion etc; made clear that the all migration formula are defined in depth domain,the velocities and the depths can be known as approaching to true values when the velocities of the each layer above current layer are correct and the events on common-image-gathers are aligned after updating velocity model;expounded the importance of ray migration in a process of building velocity-depth model and the methods reducing the disadvan-tages. Only deeply understanding these conceptions can the ability of solving the problems by using depth migration method be improved.
