Abstract：In processing the seismic data taken from complicated-surface area.， terrible topographic undulation brings many troubles in static correction，velocity analysis，time-depth conversion，depth migration and so on. Some schemes for solving the datum problem are put forward here. According to present data processing tech-nique，any horizontal plane near ground surface may be taken as datum for static correction，and it has no influence on velocity analysis and stacked migration;however，in general circumstances，it s$ould not be the datum for time-depth conversion and depth migration because time-depth conversion and depth migration must start from ground surface or smoothed ground surface. A method for deriving thesmoothed ground surface from CMP datum of static correction is given to form a common datum for velocity spectrum，time-depth conversion and depth migration in depth domain.
