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Poisson's Ratio vs. Compressional To Shear Velocity Ratio (VPVS)

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poissons-ratio-vs-compressional-to-shear-velocity-ratio

Poisson's Ratio vs. Compressional To Shear Velocity Ratio (VPVS)

INPUTS

VPVS ratio should generally be between 1.60 and 2.20. A VPVS ratio outside this range may indicate light hydrocarbon effect or bad sonic data.

OUTPUTS
Instructions

This crossplot displays the relationship between sonic velocities and Poisson's ratio.

Enter sonic compressional and shear slowness values in us/m to see the associated Poisson's ratio. Note that shear slowness should generally be at least 1.60 times greater than compressional slowness. Also note that us/m equals us/ft multiplied by 3.28 ft/m, and that velocity is the reciprocal of slowness.

Velocity ratios can be used as an indicator of lithology. Laboratory analysis of water saturated rocks yields VPVS=1.9 for limestone, VPVS=1.8 for dolostone, VPVS=1.6 in low porosity shaly sandstones, and VPVS=1.8 for clean high porosity sands.

Inputs are highlighted in yellow and outputs are in green. The data point associated with sonic inputs is plotted on the crossplot.

The following lithology based values for Poisson's ratio may be useful:

hard sandstone0.18 to 0.22
soft sandstone0.22 to 0.40
siltstone0.20 to 0.28
shale0.30 to 0.40
dolostone0.28
limestone0.31
anhydrite0.32
anthracite coal0.38
bituminous coal0.35
lignite coal0.38

Light hydrocarbon decreases the velocity of both compressional and shear sonic waves. However, compressional velocity is affected more than shear velocity, and this means Poisson's ratio will be unrealistically low, if raw data are used to calculate. Data suggest that a Poisson's ratio less than 0.18, or a VPVS ratio less than 1.60 reflects light hydrocarbon effect.

Reference

Murphy, W.F., Schwartz, L.M., and Hornby, B., "Interpretation Physics of Vp and Vs in Sedimentary Rocks." SPWLA Thirty-Second Annual Logging Symposium, June 16-19, 1991.

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