Velocity Analysis by Iterative Prestack Depth Migration: A Proposal , by John T. Etgen

In areas of complex structure, velocity analysis and imaging are interrelated. Knowledge of the interval velocities is required to image the subsurface properly. However, the interval velocities are not usually known in advance. An estimate of the subsurface structure can improve the estimate of the interval velocities and lead to a better image of the geologic structure. This suggests an iterative approach to the imaging and velocity analysis problem in complicated structure. If the problem is posed in an optimization framework, iterative prestack depth migration can provide an interval velocity model that best images the subsurface. I propose an algorithm to do this. Prestack migration (like CMP stacking), is more affected by the low wavenumbers of the interval velocity model than high wavenumbers. Low wavenumber components of the interval velocity field control gross timing and positioning of events. When forming an image of the subsurface, it is most important to estimate the lowest wavenumber components of the interval velocity first. This will give a properly focused and positioned image. With this in mind, I propose approximating interval velocity models with a set of smooth basis functions. This results in far fewer parameters to be optimized than if the velocity were allowed to be independent at many thousands of points. Most importantly, the interval velocity model will be in terms of more easily resolved parameters.


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