Lithology From Vp/Vs Velocity Ratios
dipole shear sonic log provides compressional and shear travel
time values that can be used for quicklook lithology estimation, based on the
compressional to shera velocity ratio (Vp/Vs) -- see graph shown below.
It is probably not accurate enough for quantitative log analysis purpses. Vp/Vs
is relatively independent of porosity but may have some gas effect
When recorded especially for the purpose, seismic data can be inverted to
provide compressional, shear, Vp/Vs ratio, and Poisson's ratio seismic sections.
The Vp/Vs ratio inversion can be interpreted for litholgy using the same
quicklook rules shown below. Values lower than those for the expected
lithology may indicate gas or inadequate inversion processing.
1: Vp/Vs = DTS / DTC
Sandstone Vp/Vs = 1.55 – 1.70, normally near 1.60
Dolostone Vp/Vs = 1.65 – 1.80, normally near 1.70
Limestone Vp/Vs = 1.80 – 1.90, , normally near 1.80
Anhydrite and Shale Vp/Vs = 1.90 - 2.00, , normally near 1.95