Abstract:The dip direction and dip angle of a subsurface interface must be first determined in three-parameter velocity analysis that needs predetermined dip angle. Accurate dip direction and dip angle can not be calculated without exact velocity, so that correct result can't be obtained once. The use of the iteration algorithm makes the three parameters gradually approach their accurate values. The iteration converges when dip angles are lower than 45 degrees. When there are noises, they distort the event times in the stacked gathers and cause erroneous dip direction; therefore, dip direction must be estimated in each iteration. Theoretical and real data trials show that this iteration algorithm is feasible in calculating the parameters of subsurface interface, and makes the three parameter velocity analysis result much better than conventional ones.