geophysics · velocity
Vp/Vs Ratio
Vp/Vs = DTs / DTc
click formula to derive ↑
Inputs
μs/ft
μs/ft
Description
Computes the ratio of P-wave to S-wave slowness (equivalently the inverse ratio of velocities) from sonic log measurements. Vp/Vs is one of the most diagnostic rock physics parameters for distinguishing fluid type and lithology.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vp/Vs | — | Vp/Vs Ratio |
| DTs | μs/ft | Shear-wave slowness from a dipole or multipole sonic log — the travel time of a shear wave per unit distance. |
| DTc | μs/ft | Compressional-wave slowness from a sonic log — the travel time of a P-wave per unit distance. |
Assumptions
- Both DTc and DTs are measured at the same depth and represent the same rock volume (no significant depth shift between curves)
- The formation is acoustically isotropic, so a single Vp/Vs value adequately characterizes the interval
- Sonic measurements are free of cycle skipping or excessive noise, particularly on the shear curve
Limitations
- Shear sonic logs are less commonly available and noisier than compressional logs, increasing uncertainty in Vp/Vs
- Vp/Vs alone cannot uniquely separate lithology from fluid effects — combine with density (Gassmann) or crossplots for robust interpretation
- Thin beds and washouts can bias slowness readings, especially for the shear curve in soft formations
Use Cases
- → Gas sand identification: Flag intervals with low Vp/Vs as candidates for gas-charged reservoir, pending confirmation with density and resistivity.
- → AVO class determination: Use Vp/Vs together with acoustic impedance to predict the expected AVO response class at a reservoir top.
- → Fluid substitution input: Provide the shear velocity needed as a fixed parameter in Gassmann fluid substitution modeling.
Related Calculations
Region Notes
Permian Basin
Wolfcamp shale typically shows Vp/Vs of 1.6-1.7, while overlying anhydrite/carbonate can exceed 1.9 — useful for picking formation tops on logs.
Gulf of Mexico
Pliocene-Pleistocene gas sands commonly show Vp/Vs below 1.6, contrasting sharply with surrounding shale (Vp/Vs ~2.0-2.2), driving strong Class III AVO anomalies.
North Sea
Chalk reservoirs (e.g., Ekofisk) often show Vp/Vs near 1.9-2.0; departures below this can indicate fracturing or hydrocarbon effects.
References
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