Analytic and numerical considerations for velocity grid smoothing in ray based modelling and migration

Marcus R. Wilson, Hassan Khaniani, John C. Bancroft

Ray tracing is a fast and effective general purpose method for propagating seismic waves through a geological model. According to Snell's law, rays bend relative to the gradient vector that is produced by the local velocity contrast. Velocity model smoothing allows for a more accurate computation of the gradient directions close to discontinuities in the velocity model. However, smoothing also changes the velocity gradient at points farther away from these boundaries. Care must be taken when smoothing a velocity model to ensure the rays refract in the right direction without changing the position of the reflector interface. We describe possible strategies to compute accurate gradient directions so as to minimize the need for velocity model smoothing. A simple, special purpose method for computing more accurate gradient vectors at a dipping interface while removing the artifacts produced by smoothing is presented.