The effects of migration velocity errors on traveltime accuracy in prestack Kirchhoff time migration and the image of PS converted waves

摘要

We analyze prestack PS migration images and their focusing sensitivity to errors in the computed PS traveltimes. The key analysis tool is a formula that defines PS traveltimes errors as explicit functions of velocity model errors. The most important factors in this formula are the PS velocity and the P-to-S velocity ratio. Analysis shows that the error in PS traveltime for shallow events is usually larger than that for deep events for a given error in the velocity model. Also the PS traveltime is affected more severely by errors in the PS-velocity model than in the P-to-S velocity ratio. The effect of traveltime errors increases with dip angle of reflectors. Numerical analysis shows that, for a fixed scatterpoint, the effect of the PS-wave velocity error is several times larger than the effect of the error in the P-to-S velocity ratio. Examples from field data show that the PS-wave velocity must be estimated accurately with errors less than 1% in order perfectly flatten the events in common-image-point (CIP) gathers. In contrast, an error in the PS-velocity ratio of Lip to several percent is allowed. This suggests that for acceptable PS-wave migration, only the PS-velocity model and a rough estimate of the P-to-S velocity ratio is needed. This finding is useful for processing PS-wave data because it is difficult and time consuming to estimate the velocity ratio accurately from the real data. This finding is also useful for our understanding of PS-wave behavior and for PS-wave imaging.