Abstract：Lateral velocity variation causes error in stack velocity analysis. Applying forward modeling method for velocity variant medium,we simulate field multifold lay-out,and calculate CDP time-distance curve and stack velocity for the model having different lateral velocity gradients. The Stack velocity is quantitatively compared with mean square root velocity in real medium to discover the cause and regularity of the velocity error. It is shown that in the medium where velocity varies laterally, conventional stack velocity is related to not only velocity and travel time at CMP but also the second derivative of velocity in lateral direction. The error sign of stack velocity depends on the direction of lateral velocity gradient in a spread length,and the error value depends on variation rate of the lateral velocity gradient in the length. The error variation rate helps people to well know the analysis results of conventional stack velocity and to make necessary error correction using effective method.
