# Dictionary:VS/VP

The ratio of velocities of S-wave to P-wave. This ratio is sensitive to lithology or interstitial fluid. The ratio is about 1/2 for most rocks, is larger for gas saturation, and zero for fluids. Measurements of Poisson's ratio ${\displaystyle \sigma }$ give equivalent information.
${\displaystyle {\frac {V_{s}}{V_{p}}}=\left[{\frac {0.5-\sigma }{1-\sigma }}\right]^{1/2},\;\sigma ={\frac {0.5-\left({\frac {V_{s}}{V_{p}}}\right)^{2}}{1-\left({\frac {V_{s}}{V_{p}}}\right)^{2}}}}$.