Wyllie versus Raymer-Hunt-Gardner
Learning objectives
- Contrast the linear Wyllie and the curved Raymer-Hunt-Gardner transforms
- Explain why Wyllie over-reads at high porosity
- State when each transform is appropriate
- Read both sonic porosities for a given DT
A Line and a Curve
Wyllie's time average is a straight line, fitted in the mid-porosity, well-compacted band. It is simple and exact for that range, but it has no built-in ceiling. Raymer, Hunt, and Gardner found that real rocks bend away from the straight line, and proposed an empirical curve
It needs no separate compaction factor and tracks measured velocities across a far wider range of porosity.
Where They Part
Plot both for a sandstone and the story is clear. Through the low and middle porosities they run almost together, so in that band it scarcely matters which you use. But as the transit time climbs, the straight Wyllie line keeps rising and runs above the RHG curve: at high porosity Wyllie over-reads, sometimes badly, while the RHG curve stays close to the rocks. That over-read is the same flaw the compaction correction patched, expressed a different way.
Which to Use
For clean, consolidated rock at moderate porosity, the Wyllie time average is the classic and is perfectly good. For high porosity, soft rock, or a wide interval where you want one transform that behaves everywhere, Raymer-Hunt-Gardner is the safer default. Either way, the sonic porosity is then ready to be read alongside the density and neutron, and any gap between them is itself a clue, the subject of the next section.
References
- Raymer, L. L., Hunt, E. R., and Gardner, J. S. (1980). An improved sonic transit time-to-porosity transform. SPWLA 21st Annual Logging Symposium.
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.