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Vp/Vs Ratio Calculator

The ratio of P-wave velocity to S-wave velocity.

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Vp/Vs Ratio

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Overview

The Vp/Vs ratio is a dimensionless parameter that expresses the relationship between compressional (P-wave) and shear (S-wave) velocities in a medium. In seismology, it is a critical diagnostic tool used to determine rock lithology, porosity, and the presence of pore fluids such as gas or water.

Symbols

Variables

R = Vp/Vs Ratio, = P-Wave Vel, = S-Wave Vel

Vp/Vs Ratio
Variable
P-Wave Vel
m/s
S-Wave Vel
m/s

Apply it well

When To Use

When to use: This ratio is used when interpreting seismic reflection or refraction data to differentiate between lithologies like sandstone and shale. It is particularly effective in hydrocarbon exploration to identify fluid-saturated zones, as gas saturation significantly decreases the P-wave velocity while having minimal effect on the S-wave velocity.

Why it matters: It allows geoscientists to estimate Poisson's ratio, a fundamental elastic property of the Earth's crust, without needing direct core samples. In engineering and hazard assessment, it helps identify unconsolidated or fluid-saturated sediments that may be prone to liquefaction during an earthquake.

Avoid these traps

Common Mistakes

  • Assuming a constant ratio across all rock types.
  • Convert units and scales before substituting, especially when the inputs mix m/s.
  • Interpret the answer with its unit and context; a percentage, rate, ratio, and physical quantity do not mean the same thing.

One free problem

Practice Problem

A seismic survey in a limestone formation records a P-wave velocity of 6000 m/s and an S-wave velocity of 3200 m/s. Calculate the Vp/Vs ratio for this formation.

P-Wave Vel6000 m/s
S-Wave Vel3200 m/s

Solve for: ratio

Hint: Divide the P-wave velocity (vp) by the S-wave velocity (vs).

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Wikipedia: P-wave
  2. Wikipedia: S-wave
  3. Wikipedia: Poisson's ratio
  4. Stein, S., & Wysession, M. (2003). An Introduction to Seismology, Earthquakes, and Earth Structure. Blackwell Publishing.
  5. Halliday, Resnick, and Walker, Fundamentals of Physics
  6. Wikipedia: Vp/Vs ratio
  7. Introduction to Seismology (Peter M. Shearer)
  8. The Rock Physics Handbook (Mavko, Mukerji, and Dvorkin)