Towards a systematic understanding of the effects of velocity model errors on depth and time migration of seismic data

The analysis and interpretation of seismic data takes place in three fundamental seismic domains. These are unmigrated time, which corresponds to normal incidence or stacked seismic data, migrated depth, which corresponds to the physical structure of the earth, and finally migrated time, a non-physical intermediate domain. Simplified models may be derived from the seismic data and transformed between the domains using ray theoretical techniques, specifically image rays and normal rays. This ray-theoretical modelling is extremely cheap and may be used to define the velocity zones for depth or time wave-theoretical migration of the seismic data itself. Furthermore, techniques can be developed to analyse the effects of velocity errors on time and depth migration. The positional error zones that result from velocity uncertainty are complex and often quite surprising, in that they relate to the local geology in a way that is often difficult to imagine intuitively. These velocity uncertainty analyses should be an invaluable aid to the interpreter in determining migration velocity sensitivity and, in particular, in placing reliable confidence limits on the spatial predictions made from both time- and depth-migrated seismic data.