The use of stacking velocities for time-depth conversion usually requires that very laborious corrections be made to compensate for distortions inherent to their measurement. The most readily available velocities are those that result from Seismic data processing. In this paper, a procedure is described in which all available velocities are used to create a set of reliable time-depth functions which ties known check-shot control. The errors from using raw stacking velocities for time-depth conversion average 5% to 10%. Results of this study show that depth errors can be reduced to an average of 1% to 3% when ever there is check-shot control within the same geological province if the calibration method is used. The principle value of this procedure is that it requires only minimal human involvement being, primarily, a computer data-base manipulation.
The use of Seismic Stacking Velocities for the determination of depths and pressures is a popular subject in the literature. Faust (1950) described interval velocity as an exponential function of depth with proportional factors to account for lithologic and age differences. Pennebaker (1968) described how these relationships could be extended to the prediction of Pore Pressures and used Seismic Stacking velocities for that purpose. Since Pennebaker's paper in 1968, it has been common for engineers to ask the geophysical departments to provide estimated depths to pressure and to estimate Pore Pressures ahead of the bit. Geophysicists have been somewhat wary of such strict application of stacking velocities being painfully aware of the many distortions inherent to that parameter. As a result, many a geophysicist has spent long hours with calculator in hand removing distortions and making educated guesses as to the correct depths and interval velocities pertinent to a well site.
The advent of massive amounts of digital processing has led to the development of databases in which the stacking velocities are stored for use in subsequent processing. These data-bases can be augmented by the purchase of data-bases from some of the seismic contractors. Thus, there is currently a readily available source of redundant seismic velocity data.
This paper will show how such a velocity data- base may be converted into a reasonably accurate set time-depth functions.
Due to the noisy nature of Seismic Stacking Velocities, either extensive editing or averaging must be done before they are sufficiently accurate for depth conversion. Blackburn (1980) has shown that Seismic Stacking Velocities commonly have pseudocyclical errors due to interval velocity changes that occur abruptly within the spread length. These variations are due to structural complexities, rapid lithology changes, and hydrocarbon accumulations. These may thought of as velocity inhomogeneities. In order to minimize the effects of such cyclical errors, averaging over distances of the order of the spread length must be done prior to depth conversion.
The method of spatial averaging that is used is a common one. The area to be covered is grided and circles are drawn centered on each grid point. The procedure is shown in figures 1.and 2.
