Gas Column Height from Velocity

Estimate gas column height from velocity anomaly using Gassmann fluid substitution and rock physics. Relates observed velocity changes to gas saturation and predicts gas-water contact depth from velocity profiles.

Gassmann Rock Physics Gas Column Velocity Anomaly

Input Parameters

Velocity Data

m/s
Velocity measured in gas-bearing zone
m/s
Expected velocity with 100% brine saturation

Rock Properties

fraction
Valid range: 0.05-0.45
GPa
Quartz: 37, Calcite: 77, Dolomite: 95
GPa
Must match Baseline Vp: G = ρ·Vs² (e.g. ρ 2.25 g/cm³, Vs 1630 m/s → 6 GPa)
g/cm³
Quartz: 2.65, Calcite: 2.71

Fluid Properties

g/cm³
Fresh water: 1.0, Seawater: 1.03, Brine: 1.05-1.15
GPa
Typical range: 2.0-3.0 GPa
g/cm³
Low pressure: 0.05-0.1, High pressure: 0.2-0.3
GPa
Typical range: 0.01-0.2 GPa (pressure-dependent)

Column Height Parameters

m
Depth where gas saturation is highest
m
Thickness of gas-water transition zone (0 = sharp contact)

Quick Presets

Results

Theory & References

Gassmann Fluid Substitution

Relates changes in rock velocity to pore fluid composition:

K_sat = K_dry + (1 - K_dry/K_min)² / (φ/K_fl + (1-φ)/K_min - K_dry/K_min²)

  • K_sat: Saturated rock bulk modulus
  • K_dry: Dry rock frame modulus
  • K_min: Mineral grain modulus
  • K_fl: Pore fluid modulus
  • φ: Porosity

Velocity: Vp = √((K + 4G/3) / ρ)

Wood's Equation (Mixed Fluids)

For mixed gas-brine fluids in the pore space:

1/K_mix = Sg/K_gas + (1-Sg)/K_brine

ρ_mix = Sg·ρ_gas + (1-Sg)·ρ_brine

Where Sg is gas saturation (fraction). This assumes:

  • Uniform fluid mixing (Wood's law, lower bound)
  • No patch saturation effects
  • Low frequency (seismic) regime

Applications & Limitations

Best for:

  • Gas column height estimation from velocity anomalies
  • Detecting gas-water contacts in seismic data
  • Predicting velocity changes from saturation logs
  • QC of DHI (Direct Hydrocarbon Indicator) analysis

Limitations:

  • Assumes known rock frame properties (K_dry, G)
  • Sensitive to porosity and mineral moduli
  • Low frequency assumption (< 100 Hz typical)
  • Homogeneous saturation within seismic scale