It is well known that an average-velocity-depth function can be computed directly from reflection times and normal moveouts, but, in current practice, results of such computations yield only a single function for an area as a whole, without disclosing the presence of a velocity gradient if it should exist.Normal moveout is a function of velocity; therefore, if moveout changes for a selected reflection are posted on a base map and contoured, contour values will be indicative of concomitant changes in velocity. To be of use quantitatively, raw normal moveouts must be adjusted for effects of known variables and a judgment must be made of the likelihood that changes in normal moveout are due to significant velocity changes and not to random timing errors. A field example is presented to illustrate application of the concept.

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