Abstract：In the case of 2-D steep formations,phase-shift continuation method brings high image accuracy,good stability,fast operation;but it can hardly be adapted to lateral velocity variation. Finite difference method in frequency-space domain causes convenient operation,good stability and satisfactory adaptability to vertical and lateral velocity variations;however it results in poor image accuracy and slow operanon.Combining fast 45°finite difference method with phase-shift continuation method adapts whole continuation to vertical and lateral velocity variations,and achieves accurate migration of steep formations.The mixed continuation concept is extended to 3-D phase shift plus finite-difference mixed continuation method,and then 3-D one-step depth migration technique is put forward which applies to the medium where velocity vertically and laterally,and where geologic structures are arbitrarily complex.Numerical modeling result says that this techniqrue is characterized by fast migration and high accuracy.
