geophysics · velocity
Checkshot Average & Interval Velocity
Vavg = Z₂/T₂ | Vint = (Z₂−Z₁)/(T₂−T₁)
click formula to derive ↑
Inputs
ft
ms
ft
ms
Description
Computes both average velocity (cumulative depth over one-way time from the datum) and interval velocity (layer thickness over layer one-way time) from a two-station checkshot or zero-offset VSP survey. Average velocity characterizes the overburden as a single value; interval velocity isolates the true velocity of the layer between the two stations, and the two are frequently confused despite answering different questions.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Vavg | ft/s | Average Velocity |
| Vint | ft/s | Interval Velocity |
| Z₁ | ft | True vertical depth (TVD) of the shallower checkshot/VSP station. Often the surface or near-surface datum for a simple two-station read. |
| T₁ | ms | One-way first-break travel time recorded at the shallow station. Checkshot/VSP times are inherently one-way — do not confuse with two-way (TWT) picks from surface seismic. |
| Z₂ | ft | True vertical depth (TVD) of the deeper checkshot/VSP station, e.g. a target reservoir depth. |
| T₂ | ms | One-way first-break travel time recorded at the deep station. |
Assumptions
- Checkshot/VSP times represent direct downgoing first-break picks, not two-way reflection times
- Z₁ and Z₂ are true vertical depths (TVD) at the geophone stations, not measured depth (MD) in a deviated well
- No significant lateral source-to-well offset, so the raypath can be treated as effectively vertical
Limitations
- A two-station read cannot detect velocity variation within the interval — use a full multi-station checkshot survey and its layer-by-layer interval velocities for detailed velocity modeling
- Sonic-log-derived velocity (see Vp/Vs, Acoustic Impedance) and checkshot-derived velocity commonly disagree (sonic drift) — checkshot/VSP values are the calibration standard, not the sonic log
- Deviated wells require a TVD correction before Z₁/Z₂ are used here; measured depth (MD) will bias both Vavg and Vint
Use Cases
- → Sonic log drift correction: Compare checkshot-derived Vavg/Vint against integrated sonic velocity to build a drift-correction (check-shot correction) curve for well-to-seismic ties.
- → Time-depth conversion input: Feed Vavg directly into the Time-to-Depth Conversion calculator to convert a seismic TWT pick to depth at the well.
- → Velocity model calibration: Use Vint values from successive station pairs to calibrate interval velocities in a layered velocity model, cross-checked against Dix-derived interval velocities from surface seismic.
Related Calculations
Region Notes
Permian Basin
Wolfcamp shale intervals typically show checkshot-derived Vint of 3,600-4,200 m/s; overlying Delaware Mountain Group clastics run slower (2,800-3,400 m/s), producing a visible velocity contrast useful for QC against sonic-derived Vp.
Gulf of Mexico
Unconsolidated Miocene-Pliocene shale sections show strongly increasing Vavg with depth (commonly 1,800 m/s near seafloor, rising past 3,000 m/s by 3,000m TVD) — a single Vavg value is a poor stand-in for interval velocity here; prefer the layer-by-layer Vint.
Global
Always confirm whether a vendor-supplied checkshot table already reports one-way time or two-way time before using T₁/T₂ here — reporting conventions vary, and using TWT directly will halve the computed velocities.
References
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