1. A method of estimating lateral positioning uncertainties of a seismic image, the method comprising: at a computer system having one or more processors and memory storing programs to be executed by the one or more processors: receiving a velocity model, the velocity model including a plurality of velocity profiles, a velocity profile comprising a lateral velocity gradient (g(x i ,z j )) in a direction orthogonal to a raypath, a vertical velocity gradient (G(x i ,z j )) in a direction coincident with the raypath, a base velocity value (V M ) used by the velocity model for generating the seismic image, and a velocity range including the base velocity, wherein the velocity range extends between a low limit (V LO ) and a high limit (V HI ) and the difference between the base velocity and either the low limit or the high limit represents an uncertainty in the base velocity; deriving a plurality of lateral velocity gradient uncertainties as functions of the lateral velocity gradients; generating multiple lateral velocity gradient profiles, each lateral velocity gradient profile including a random realization of the plurality of lateral velocity gradient uncertainties; calculating perturbation raypaths originating from a surface point of the seismic image based on the velocity model and the multiple lateral velocity gradient profiles; and estimating a lateral positioning uncertainty for a target location at a predefined depth of the seismic image based on lateral distributions of the perturbation raypaths at the predefined depth.