geophysics · depth conversion
Time-to-Depth Conversion (Average Velocity)
Z = Vavg × (TWT/2)
click formula to derive ↑
Inputs
ft/s
ms
Description
Converts a seismic two-way travel time pick to depth using a single representative average velocity — the simplest and most widely used depth-conversion method, and the appropriate choice wherever a reliable Vavg is already known (e.g. from a checkshot survey) at or near the location being converted.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Z | ft | Depth |
| Vavg | ft/s | Representative average velocity from the datum to the target depth — ideally checkshot-calibrated (see Checkshot Average & Interval Velocity) rather than a generic regional value. |
| TWT | ms | Two-way, zero-offset (stacked/migrated) travel time pick for the horizon being converted to depth. |
Assumptions
- A single Vavg value adequately represents the velocity field from the datum down to the depth of interest — no significant lateral velocity variation across the area being converted
- TWT is a genuine two-way, zero-offset (stacked/migrated) time pick, not a raw offset-gather time
- Vavg is representative at the specific location being converted, not extrapolated far from where it was measured or calibrated
Limitations
- Uses one velocity value for the entire section above the target — cannot represent velocity variation with depth; use the V0-k linear gradient method or a full layered model where velocity changes significantly with depth
- Errors in Vavg propagate directly and linearly into depth error — a 5% Vavg error produces roughly a 5% depth error
- Least reliable far from well control (checkshot/VSP) or in areas of lateral velocity variation (facies change, overpressure, salt/carbonate bodies)
Use Cases
- → Well tie / prospect depth estimate: Convert a mapped seismic TWT horizon to an estimated depth using a checkshot-calibrated Vavg from a nearby well.
- → Quick-look depth conversion: Provide a fast, defensible first-pass depth estimate for a prospect before a full velocity model is built.
- → QC against layered models: Cross-check a full layered depth-conversion result against this single-Vavg estimate as a sanity check.
Related Calculations
Region Notes
Permian Basin
Checkshot-calibrated Vavg to top Wolfcamp typically runs 3,600-4,000 m/s in the Midland Basin — always tie to the nearest well rather than using a basin-wide average, since Vavg varies measurably across sub-basins.
Gulf of Mexico
Strong velocity increase with depth in unconsolidated Miocene-Pliocene sediments makes a single Vavg increasingly unreliable below about 3,000m TVD — prefer a layered or V0-k approach for deeper targets.
Global
This method's accuracy is fundamentally limited by how representative the single Vavg value is — always state the depth range over which Vavg was calibrated when reporting a converted depth.
References
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