A measure that indicates that the pore fluid is gas rather than liquid. On a crossplot of S-wave velocity (VS) versus P-wave velocity (VP), the mudrock line for water-filled sandstones has water velocity as its intercept with the VP-axis but gas-filled sandstones have a different slope and the intercept zero. The fluid factor, for the normal reflection coefficients for P- and S- waves RP and RS is:
- Castagna, J. P. and Backus, M. M., Eds., 1993, Offset-dependent reflectivity–Theory and practice of AVO analysis: Soc. Expl. Geophys.