Tutorial: the seismic response to strong vertical velocity change

Conventional seismic data processing, whether pre-stack data conditioning or migration, is designed with the theory of P-wave reflected energy in mind for travel paths involving only a single reflection. Hence, any energy propagating with other modes or travel paths is not dealt with appropriately during conventional processing. It is primarily for this reason that we spend so much time pre-conditioning seismic data to meet the assumptions of the subsequent processes such as migration. Here I consider a typical North Sea environment where there are both strong vertical compaction gradients and high velocity contrasts in the layered Chalk Formation. I assess the behaviour of refracted and mode-converted seismic arrivals when subjected to conventional processing in order to explain how they give rise to anomalous events that contaminate migrated common reflection point gathers and images.