Abstract:By Fourier transform, scalar wave equation can be transformed into single square root equation, which is a basic equation for post-stack migration.The radical expression in the equation may be approximated in different ways; consequently, there are different migration methods,The finite difference depth migration equation in space-frequency domain can be derived when the radical expression is approximated by continued fraction.In practice, this equation is solved by split-ting into two ones (called thin lens and diffraction term equations),which are solved alternately,Crank-Nicolson scheme is used in second-order difference so that both the high accuracy and the stability of wave field extrapolation can be ensured.Furthermore, "one sixth knack"of approximation is taken for higher accuracy. In summary, this migration process is composed of the following steps; first, by Fourier transfes orm, a seismic section is transformed from time domain to frequency domain; then, for each frequency,the wave field is extrapolated from known depth Z to unknown depth Z+ΔZ by making use of the thin lens term equation and the diffraction term equation; finally, the component extrapolated results for alI frequencies are summed up to form a resultant migration result at depthZ+ΔZ.summation process is also migration image process.Two theoretical models illustrated in the paper all bring good migration result.