geophysics · depth conversion
N-Layer Depth Conversion (Cumulative Stacking)
click formula to derive ↑
Inputs
Paste one row per line — separate columns with a comma or tab: Interval Velocity (ft/s), Interval TWT Thickness (ms)
⚠ Needs at least 2 valid rows to calculate — 0 so far.
Description
Stacks a sequence of already-known interval velocities and their own TWT thicknesses — one row per layer, typically obtained one interval at a time from Checkshot Average & Interval Velocity or Interval Velocity from RMS Velocities (Dix) — into a single cumulative depth, the way a full multi-layer checkshot or velocity survey is actually converted to depth rather than relying on one section-wide average. Each layer's contribution to depth is computed independently (Vint,i × one-way time of that layer alone); the calculator simply runs the cumulative sum down the stack, in contrast to the single-interval methods it is meant to be chained after.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Z | ft | Cumulative Depth |
| Vavg | ft/s | Effective Average Velocity |
| Vint,i, Δti | — | One row per layer, top to bottom, in stacking order: that layer's own interval velocity (from Checkshot Average & Interval Velocity, Interval Velocity from RMS Velocities (Dix), or offset-well control) and that layer's own two-way time thickness (NOT cumulative TWT — the TWT this layer alone occupies). Each layer's depth contribution is independent of every other layer; only the running total, not any layer's own arithmetic, depends on row order. Paste at least 2 layers. |
Assumptions
- Each row's interval velocity and TWT thickness genuinely belong to that layer alone (not cumulative from the datum) and are supplied in top-to-bottom stacking order starting at the datum
- No gaps or overlaps between successive layers' TWT thicknesses — the layers stack to form one continuous section from the datum to the base of the deepest layer
- Each layer's interval velocity is already well-determined (e.g. from a two-station checkshot pair, a Dix interval, or reliable offset-well control) — this calculator performs no independent velocity estimation of its own
Limitations
- Errors in any single layer's interval velocity or TWT thickness propagate directly into every depth at or below that layer — an error in a shallow layer biases the entire stack beneath it, not just that one interval
- Returns only the final cumulative depth and effective average velocity at the base of the deepest layer, not a full depth at every intermediate horizon — for a depth at one specific intermediate layer, run the stack only as far as that layer
- Provides no geological QC of its own on whether the pasted interval velocities are physically reasonable for the rock types involved beyond a simple sedimentary-range flag — cross-check against sonic or checkshot control wherever available
Use Cases
- → Multi-layer prognosis away from well control: Stack a sequence of Dix-derived interval velocities (one per mapped horizon pair) into a cumulative pre-drill depth estimate for a prospect with no nearby checkshot control.
- → Full checkshot survey reduction: Chain successive station-pair results from Checkshot Average & Interval Velocity down a well's full checkshot table into one cumulative depth-time relationship, instead of only ever computing one two-station interval at a time.
- → QC against single-Vavg and V0-k methods: Compare the layered stack's cumulative depth and effective Vavg against the single-average-velocity and linear-gradient depth-conversion methods to see how much the full layered detail changes the converted depth relative to either simplification.
Related Calculations
Region Notes
Gulf of Mexico
Strong compaction-driven velocity increase through the Miocene-Pliocene section makes single-average methods increasingly unreliable below about 3,000m TVD — this is exactly the setting a full layered stack is meant for, chaining several Dix or checkshot intervals rather than one basin-wide Vavg.
Permian Basin
Layering Delaware Mountain Group clastics separately from the underlying Wolfcamp shale (rather than averaging across both) captures the real velocity contrast between them, which a single-Vavg conversion to Wolfcamp top would otherwise blur.
Global
Always paste layers in genuine top-to-bottom stacking order with no gaps — a skipped or duplicated interval silently shifts every depth below it, and this calculator has no way to detect a missing layer on its own.
References
Need geoscience support? BauerCalc is just one of the tools we build. If you're looking for independent expertise in well planning, geosteering, reservoir characterization, or opportunity evaluation, visit BauerSubsurface.com.