Abstract

Quantifying uncertainties of surface seismic velocity models is a difficult mission. The flat common image gathers from depth migration provide only non-unique solutions assessing the quality of a velocity model. The use of vertical seismic profiling (VSP) data provides quantification of the velocity model uncertainty. First method is done with ray traced first arrival times (FAT) through a surface seismic velocity model, compared with measured FAT from the down-hole receivers. This method provides a depth profile of time differences indicating velocity uncertainty as a function of depth. Second approach makes use of first arrival angles (FAA) derived from three component analysis of VSP data. Model calculated first arrival angles of seismic rays through a velocity model are compared with measured FAA to quantify velocity model uncertainties and to provide clues for further updating velocity model. Seven seismic velocity models were diagnosed with six sets of VSP data recorded at four wells. Case studies from the Gulf of Mexico are presented to demonstrate this methodology.

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