Manual seismic reflection tomography

Prestack depth migration needs a good velocity model to produce a good imag e; in fact, finding the velocity model is one of the goals of prestack dept h migration. Migration velocity analysis uses information produced by the m igration to update the current velocity model for use in the next migration iteration. Several techniques are currently used to estimate migration vel ocities, ranging from trial and error to automatic methods like reflection tomography. Here, we present a method that combines aspects of some of the more accurate methods into an interactive procedure for viewing the effects of residual normal moveout corrections on migrated common reflection point (CRP) gathers. The residual corrections are performed by computing travelt imes along raypaths through both the current velocity model and the velocit y model plus suggested model perturbations. The differences between those s ets of traveltimes are related to differences in depth, allowing the user t o preview the approximate effects of a velocity change on the CRP gathers w ithout remigrating the data. As with automatic tomography, the computed dep th differences are essentially backprojected along raypaths through the mod el, yielding a velocity update that flattens the gathers. Unlike automatic tomography, in which an algebraic inverse problem is solved by the computer for all geologic layers simultaneously, our method estimates shallow veloc ities before proceeding deeper and requires substantial user intervention, both in flattening individual CRP gathers and in deciding the appropriatene ss of the suggested velocity updates in individual geologic units.