Migration Velocity Analysis Based On Common-Shot-Depth Migration Applied to the Seismic Data of the Caribbean Oceanic Plateau

Abstract

SUMMARY In areas where the geology is characterized by strong vertical and lateral velocity gradients, pre-stack depth migration will produce the optimum subsurface image. an accurate velocity model is required for an accurate pre-stack depth migration. We developed a velocity analysis based on recursive pre-stack depth migration that gives better results than the traditional CMP velocity analysis when applied to data over the Caribbean oceanic plateau, First. the shot records are migrated with a series of constant velocities for each identified geological layer. the velocity that best flattens the event in common-receiver gathers (CRG) is picked. A large number of velocity analyses are run along the seismic line in order to define the reflector geometry. For each layer we obtain the depth and interval velocity in order to make a model down to that depth. This model is then used as the input for the next layer velocity analysis. Finally, when all the layers have been picked, the entire line is depth migrated with the derived model. the migration algorithm used is a common-shot Kirchhoff technique. which uses the ray-tracing method of Vidale (1988) to compute the traveltimes needed to determine the operator shape. We applied this migration velocity analysis technique to seismic profiles of the CASIS cruise to define the deep structure of the Caribbean oceanic plateau. We imaged a 2500 m deep reflection (sub-B″) beneath the top of the plateau (B″ horizon) which defines the top of a 4500–4700 m s−1 layer. the images obtained with this method will help to define ODP sites that will provide information about geochemical and petrological variations of the volcanic basement.