Shear Velocity Estimator

Estimate shear velocity (Vs) from P-wave velocity (Vp) using empirical relationships. Essential for AVO analysis, rock physics modeling, and elastic property estimation when Vs logs are unavailable. Choose the appropriate method based on lithology.

Rock Physics AVO Elastic Properties Castagna

Input Parameters

Estimation Method

For shales and shaly formations

P-wave Velocity

P-wave velocity from log or seismic

Quick Presets

Results

Theory & References

Mudrock Line (Castagna et al., 1985)

Vs = 0.8621 × Vp - 1.1724 (km/s)

Application:

  • Derived for shales and shaly formations
  • Most widely used empirical relationship
  • Valid for brine-saturated shales
  • Vp range: 1.5-6.0 km/s

Reference: Castagna, J.P., Batzle, M.L., and Eastwood, R.L., 1985, Relationships between compressional-wave and shear-wave velocities in clastic silicate rocks: Geophysics, v. 50, p. 571-581.

Greenberg-Castagna (1992)

Vs = 0.8042 × Vp - 0.8559 (km/s)

Application:

  • For clean sandstones
  • Water-saturated conditions
  • Better accuracy than mudrock line for sands
  • Considers different mineral composition

Reference: Greenberg, M.L., and Castagna, J.P., 1992, Shear-wave velocity estimation in porous rocks: Theoretical formulation, preliminary verification and applications: Geophysical Prospecting, v. 40, p. 195-209.

Carbonate Relationship (Pickett)

Vs = Vp / 1.9

Application:

  • Simplified relationship for limestones
  • Fixed Vp/Vs ratio of 1.9
  • Good first approximation for carbonates
  • Consider porosity effects for accuracy

Note: Carbonates show wider Vp/Vs variability (1.7-2.0) depending on porosity and mineralogy. This is a typical average.

Custom Linear Relationship

Vs = a × Vp + b (km/s)

Application:

  • User-defined coefficients for local calibration
  • Calibrate using well data (dipole sonic logs)
  • Account for regional variations
  • Most accurate when calibrated to field data

Calibration: Use crossplots of Vp vs Vs from wells with full sonic logs to derive local coefficients via linear regression.

Vp/Vs Ratio Method

Vs = Vp / (Vp/Vs ratio)

Typical Vp/Vs Values:

  • 1.5-1.7: Consolidated sandstones, tight sands
  • 1.7-1.9: Shales, typical sandstones
  • 1.9-2.0: Carbonates, unconsolidated sands
  • 2.0+: High fluid content, gas sands

Note: Vp/Vs is sensitive to fluid content and lithology, making it valuable for AVO analysis and fluid discrimination.

Poisson's Ratio

ν = (Vp²/Vs² - 2) / (2 × Vp²/Vs² - 2)

Interpretation:

  • 0.05-0.15: Very consolidated rocks
  • 0.15-0.25: Typical sandstones
  • 0.25-0.35: Shales, unconsolidated sands
  • 0.35-0.45: High fluid content, gas effect

Applications: Poisson's ratio is used in AVO analysis, fluid substitution modeling (Gassmann), and mechanical property estimation for geomechanics.

Usage Guidelines

Method Selection:

  • Shales: Use Mudrock Line (Castagna)
  • Clean Sandstones: Use Greenberg-Castagna
  • Carbonates: Use Carbonate relationship (Vp/Vs = 1.9)
  • Mixed Lithology: Consider weighted average
  • With Well Control: Use Custom Linear calibrated to local data

Quality Check:

  • Verify Vp/Vs is in reasonable range (1.5-2.2 for most rocks)
  • Compare with regional trends if available
  • Check Poisson's ratio for lithology consistency
  • Validate against nearby wells with Vs logs

Applications

Common Uses:

  • AVO Modeling: Generate synthetic gather when Vs log missing
  • Rock Physics: Calculate elastic moduli (μ, λ, E, K)
  • Gassmann Substitution: Input for fluid effects modeling
  • Seismic Inversion: Constraint for Vs inversion
  • 4D Seismic: Baseline rock property estimation
  • Geomechanics: Mechanical properties for wellbore stability