A new method of modeling zero offset reflection data accommodates both vertical and lateral velocity variations and allows continuous changes in reflectivity throughout the section. Each point in the subsurface is considered a point scatterer, and the energy at each point is distributed over the diffraction curve that would be observed if it were the only scatterer in the subsurface. When the velocity varies only with depth, the diffraction curves are relatively simple to determine. Computing the diffraction curves when there are lateral velocity variations, however, requires extensive ray tracing. Our method avoids the extensive ray trace by approximating the exact diffraction curves with perturbed diffraction curves computed for a model in which the velocity varies only with depth. The slowness (reciprocal velocity) function is written as the sum of two functions: the first is large and depends only on depth and the other is small and varies both with depth and position along the line. Ray paths are traced for the first slowness function and used to calculate diffraction curves. For each depth point these same ray paths are used to calculate travel time perturbations due to the laterally varying part of the slowness. The travel time perturbations are added to the diffraction curve to obtain an approximation to the exact diffraction curve.