An Improved Sonic Transit Time-To-Porosity Transform

Over the 20 years since acoustic velocity well logging was introduced, deficiencies have been noted in the transit time, t, to porosity, transform (popularly referred to as the Wyllie time average equation). In particular, at the porosity extremes, O and 10O%, the transit time seems to vary less than predicted. Furthermore, for unconsolidated sands, the transform must be modified using a so-called "lack of compaction" correction--an inconvenient, complicating factor. Thus, a new empirical transform, based on extensive field observations of transit time versus porosity, is proposed. It provides superior transit time - porosity correlation over the entire porosity range, it suggests more consistent matrix velocities for given rock lithology, and it permits the determination of porosity in unconsolidated, low velocity sandstones without the need to determine a "lack of compaction," or similar, correction factor. The proposed equation enables both the novice and experienced log analyst to utilize more easily acoustic velocity measurements for porosity evaluation over a wide range of rock situations; and it permits the inclusion of these velocity measurements into computer programs in a much more straight-forward manner.