From an input VI vs Z (intervl3.1 velocity vs. depth) curve, RMS velocity spectra are generated and permafrost models are studied; these models demonstrate that depth computation is independent of the permafrost configuration. It is shown how the location of permafrost in the stratigraphic column affects RMS velocity curves. Also shown is a simple modeling method to obtain both the thickness and depth of burial of the permafrost from RMS velocity spectra.
Much of the world supply of sorely needed hydrocarbons lies buried beneath a near surface sheet of frozen rock called permafrost. This layer presents the explorationists with a difficult problem since the thickness as well as the velocity of the permafrost varies widely and sometimes abruptly, making it difficult to obtain a true structural picture of the subsurface. Even today, many explorationists will debate the proper technique to be used when converting seismic time data to depth. This is the first of a series of papers designed to clarify the correct relationships between seismic time and depth procedures when permafrost is present. It will be shown in this first paper that with sufficient seismic velocity information the permafrost configuration is irrevalent to depth computations.
The procedure used to demonstrate the irrevalence of permafrost presence in time to depth computations will be to use models. This will allow us to examine also how the characteristics of various configurations of near surface permafrost would appearto the interpreter on actual seismic velocity estimates.
Extensive study, both onshore and offshore in the Beaufort Sea/North Slope area of Alaska has indicated that two basic types of near surface permafrost configurations exist. Of course variations exist of each case but the variations do not affect this presentation, they should be explored when planning actual drill sites. The first basic case occurs in areas permanently frozen from the surface down. The second case is that of residual permafrost at depth overlain by unconsolidated sediments. Probably two rather indistinct zones exist in a complete permafrost column, attributable to the last two ice ages. The zones would explain the known presence of "lakes" of unfrozen sediments enclosed within the permafrost and the documented thinning of the permafrost as one moves northward offshore. (Hardy, 1965; Brown and Kupsch, 1974; Tsytovich, 1977; Hnatiuk and Randall, 1977).
Consider first the family of curves shown in Figure 1. These model curves are constructed simply by using a composite well function from the area as a source for VI vs Z values, and then by calculating, according to Dix (1955), RMS models with varying amounts of permafrost from the surface down. A velocity of 12500 ft/sec was arbitrarily chosen for permafrost. A known isopach from the model is shown by the dashed lines. Note the systematic decrease in time and increase in RMS velocity (when going from point B to point A) associated with an increase in the thickness of the high velocity permafrost surface layer.