Abstract：In the three-parameter velocity analysis,we first derive dip direction and dip angle of reflection interface from 3-D seismic data,then obtain mean square-root velocity.Only limited seismic data are used in velocity analysis,so that if the seismic data have low signal/noise ratio,we can hardly estlmate satisfactory dip direction,dip angle and mean square-root velocity,thus reducing the effect of three-parameter velocity analysis.Generally speaking,structural shape can be seen in stacked seismic section solong as the signal/noise ratio is not too low and the processing parameters are reasonalble on the whole.Stacked seismic section has big vertical and lateral sizes,sothe dip angles of seismic reflection legs in the stacked seismic section can be determined quite correctly.The dip angle of reflection leg in stacked seismic section is actually the time apparent dip angle of reflection interface.Having known the time dip angles in inlineand crossline directions,and the corresponding average velocity,we can compute thetrue dip direction and true dip angle of reflection interface.The dip direction and dipangles we obtained in this way can show the general structural shape in quite largeextent.Although the dip directions and dip angles slightly deviate from those atpoints where velocity is analsyed,they can be the reference and restriction of thelatters.After the three-parameter velocity analysis is restricted by using the dip directions and dip angles of wide reflection interfaces,big errors of dip direction and dipangle can be avoided desirably,thus improving the velocity analysis of the seismicdata whose signal/noise ratio is low.It is practically and theroretically shown thatthe method is feasible and improves the processing of 3-D seismic data.
