one-way time, Oil & Gas Glossary

One-way time is the seismic travel time for acoustic energy to pass once between the surface of the Earth and a receiver at a known depth in a borehole, measured directly during a check-shot survey or a vertical seismic profile (VSP). It is the foundational measurement that ties the depth domain, in which wells are drilled and logged, to the time domain, in which surface seismic data are recorded and interpreted. A surface energy source, typically a vibrator truck or an air gun in a mud pit, fires a pulse; a geophone clamped at a measured depth in the wellbore records the first arrival; and the elapsed time is the one-way time to that depth. Repeating this at a series of depths builds a time-depth function, the calibrated relationship that says, for example, that the top of a target reaches 1.42 seconds one-way at 3,150 m. Surface reflection seismic, by contrast, records energy that travels down to a reflector and back up, so it is displayed in two-way time, which is essentially twice the one-way time to a given interface for a vertical path. The distinction matters because interpreters pick horizons in two-way time on a seismic section, but drillers, geologists, and reservoir engineers need those picks converted to true vertical depth, and the one-way time measured in the well is what makes that conversion accurate rather than a guess from regional velocity averages. A check-shot survey records just enough points, often hundreds of metres apart, to define the time-depth function, while a VSP places receivers much closer together, on the order of 15 to 25 m, so it produces both a detailed time-depth relationship and an actual seismic image at the borehole that can be tied directly to the surface volume. From the spacing of one-way times an interpreter derives interval velocities, the speed of sound through each layer, which feed velocity models for depth conversion, amplitude analysis, and pore-pressure prediction. In WCSB practice, a check-shot or VSP run in a Montney or Duvernay delineation well calibrates the seismic so that a horizon mapped across a township ties the actual formation tops in the well, removing the depth error that uncalibrated time-to-depth would carry. One-way time therefore links two-way time displays to interval velocity models and underpins every reliable seismic depth prediction.

Key Takeaways

Single pass surface to receiver: One-way time is the travel time for energy to go once from a surface source down to a borehole geophone at known depth, measured in a check-shot survey or VSP. It is the direct, in-situ tie between the depth domain where wells are drilled and the time domain where surface seismic is recorded, and it anchors all subsequent depth conversion.
Two-way time is the reflection counterpart: Surface reflection seismic energy travels down and back, so sections are displayed in two-way time, roughly double the one-way time for a vertical path to a reflector. Interpreters pick horizons in two-way time but must convert to depth, and the borehole one-way measurement is what calibrates that conversion to real formation tops.
Check-shot versus VSP detail: A check-shot survey records sparse points, often hundreds of metres apart, sufficient only for a time-depth function. A VSP uses tightly spaced receivers near 15 to 25 m, yielding both the time-depth relationship and an actual seismic image at the well that ties directly into the surface 3D volume for confident horizon correlation.
Interval velocities come from the slope: Dividing depth increments by the corresponding one-way time increments gives interval velocity, the sound speed through each layer. These velocities build the model used for time-to-depth conversion, seismic amplitude calibration, and pore-pressure prediction, so the accuracy of one-way time controls the accuracy of the entire velocity framework.
WCSB calibration removes depth error: A check-shot or VSP in a Montney or Duvernay well calibrates the seismic so a horizon mapped across a survey ties the true formation top at the wellbore. Without it, depth conversion relies on regional velocity averages that can misplace a target by tens of metres, costing accuracy in landing horizontal laterals.

From One-Way Time to a Velocity Model

The raw deliverable of a check-shot is a table of depth versus one-way time, but its value emerges when those points are differenced into interval velocities. Between two receiver depths, the change in depth divided by the change in one-way time gives the average speed of sound through that interval, which typically increases with burial as rocks compact and lithify. A WCSB geophysicist building a depth-conversion model for a Duvernay project layers these well-derived interval velocities into a regional velocity volume, blending the precise borehole control with the spatial coverage of surface seismic. The result converts two-way-time horizon picks into depth maps that honor the actual tops drilled, which is what lets a team confidently predict the depth of a target several kilometres from the nearest control well.

Why Horizontal Wells Demand Accurate Time-Depth Ties

In WCSB unconventional development the economic target is a thin, high-quality interval that a horizontal lateral must stay inside for two kilometres or more. If the seismic depth conversion is off by even 15 m because it relied on uncalibrated velocities, the geosteering team starts the lateral in the wrong zone and burns expensive drilling and completion budget correcting it. A one-way-time survey in a vertical pilot hole calibrates the velocity model to that exact location, so the depth-converted target surface the drillers steer to reflects reality. For Montney and Duvernay programs, the modest cost of a check-shot or VSP in the pilot is routinely justified by the avoided cost of a mis-landed multi-million dollar horizontal well.

Fast Facts

The factor-of-two relationship between one-way and two-way time is only exactly true for a vertical ray; for any dipping reflector or offset source the geometry stretches the path, which is why interpreters cannot simply halve a seismic time and read off depth. The borehole check-shot sidesteps that complication entirely by measuring the genuine vertical one-way time at the well, and this is precisely why a single calibrated well can correct a regional velocity model that, left to surface data alone, would systematically misplace deep targets by amounts that grow with depth and structural dip.

One-way time is the building block of two-way time, the domain in which surface reflection sections are displayed and horizons are interpreted. Successive one-way times yield interval velocity, the layer-by-layer sound speed that drives every velocity model. The measurement is acquired in a vertical seismic profile or its sparser cousin the check-shot survey, and together these tools deliver the calibrated time-depth relationship that makes seismic depth conversion trustworthy at the wellbore.

Calibrating a Duvernay Delineation Well

An operator delineating a Duvernay shale lease near Fox Creek, Alberta, drilled a vertical pilot to roughly 3,400 m before kicking off the planned horizontal. The team ran a VSP with receivers spaced about 20 m apart to record one-way times throughout the section, because the surface 3D depth conversion across the township had been built on regional velocities and showed a 20 m uncertainty band at the target. Landing the upcoming two-kilometre lateral inside the thin organic-rich interval depended on shrinking that uncertainty at this exact location.

The VSP one-way times tightened the interval velocity model and tied the seismic target horizon to the actual Duvernay top within a few metres. The survey added roughly 120,000 CAD to the pilot-hole program, a small fraction of the lateral and completion cost, and the well was landed and held in zone, protecting the multi-million dollar horizontal investment from a costly mis-landing.