Lithology From Vp/Vs Velocity Ratios
The
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
RECOMMENDED
PARAMETERS:
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
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