Errors in stacking velocity—True velocity conversion over complex geologic situations

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Abstract
The problems inherent in the conversion of stacking velocities to true vertical velocities have been studied by nonzero offset raypath tracing of primary P‐waves through a suite of varied geologic sections. It is shown that conversion errors can be caused by timing errors as a result of migration and also by raypath distortions due to complex overburdens. The raypath modeling shows that (1) the customary assumption that stacking velocities approximate root‐mean‐square velocities is erroneous in areas of geologic complexity. (2) Essential velocity information from horizons overlain by geologic irregularities is often difficult to recognize and is apt to be ignored or misidentified. (3) Standard fitting of hyperbolic curves to produce normal moveout traces is erroneous in areas of geologic complexity. Consequently there is a need for correct migration, which takes into account all overburden complexities, of all traces in a common‐depth‐point gather prior to velocity determination so that realistic velocities can be obtained.

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