geophysics · velocity
Interval Velocity from RMS Velocities (Dix)
Vint = √[(Vrms₂²·TWT₂ − Vrms₁²·TWT₁) / (TWT₂ − TWT₁)]
click formula to derive ↑
Inputs
ft/s
ms
ft/s
ms
Description
Applies the Dix (1955) equation to derive interval velocity from stacking (RMS) velocities picked at two horizons during seismic velocity analysis. This is the standard method for converting a seismic processing velocity field into interval velocities away from well control, and is the seismic-processing counterpart to the well-based checkshot interval velocity.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vint | ft/s | Interval Velocity |
| Vrms₁ | ft/s | Stacking (RMS) velocity picked from NMO velocity analysis at the shallower horizon. |
| TWT₁ | ms | Two-way, zero-offset travel time to the shallower horizon, at the same location the Vrms₁ pick was made. |
| Vrms₂ | ft/s | Stacking (RMS) velocity picked from NMO velocity analysis at the deeper horizon. |
| TWT₂ | ms | Two-way, zero-offset travel time to the deeper horizon, at the same location the Vrms₂ pick was made. |
Assumptions
- Subsurface is treated as horizontally layered (or the interval velocity is being computed along a genuinely flat reflector pair)
- Stacking velocities were picked from small-spread, hyperbolic NMO analysis — not corrected for anisotropy or long-offset non-hyperbolic moveout
- TWT₁ and TWT₂ are two-way, zero-offset times to the same two horizons the Vrms values were picked at
Limitations
- Highly sensitive to picking error — small errors in Vrms or TWT are amplified by the subtraction and division, especially for thin intervals (small TWT₂−TWT₁)
- Breaks down under dipping reflectors, strong velocity anisotropy, or significant lateral velocity variation — a full velocity inversion or tomography is needed in these settings
- Provides interval velocity, not depth directly — pair with a time-to-depth calculator to convert TWT horizons to depth once interval velocities are known
Use Cases
- → Velocity model building without well control: Derive a layered interval-velocity model directly from seismic processing velocities in areas with sparse or no checkshot/well data.
- → QC against checkshot interval velocity: Cross-check Dix-derived interval velocity against checkshot-derived interval velocity (Checkshot Average & Interval Velocity calculator) at a well location to validate the seismic velocity field.
- → Depth conversion input: Feed Dix interval velocities into a layer-cake time-to-depth conversion workflow away from well control.
Related Calculations
Region Notes
Gulf of Mexico
Deepwater Miocene sections commonly show Dix-derived interval velocities of 2,200-2,800 m/s in shale-dominated intervals — reasonably consistent with checkshot control where available, but salt proximity severely degrades Dix reliability due to steep dip and velocity pull-up/push-down.
Permian Basin
Onshore, relatively flat-lying Permian stratigraphy is a favorable setting for the Dix equation's horizontal-layering assumption; Dix-derived Wolfcamp interval velocities typically fall within 5-10% of checkshot-derived values where both exist.
Global
Dix interval velocities become progressively less reliable below strong velocity inversions (e.g. below a fast carbonate or salt layer) — always sanity-check against any available well control rather than trusting the seismic-only value in isolation.
References
Primary source
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