traveltime, Oil & Gas Glossary

Traveltime in reflection seismology is the elapsed time it takes a seismic signal to leave a source, propagate down through the Earth, reflect off a subsurface boundary where acoustic impedance changes, and return to a receiver. Because the surveyed quantity is time rather than distance, the vertical axis of a conventional seismic section is measured in seconds or milliseconds of two-way traveltime, abbreviated TWT, meaning the full down-and-back journey of the wave. This single fact governs almost everything about how geoscientists read seismic data: a reflector that appears at 1.5 seconds is not at 1.5 of anything spatial until a velocity model converts that time into a depth. The relationship is straightforward in principle, since depth equals one half of the two-way traveltime multiplied by the velocity of the rock the wave passed through, but the difficulty is that velocity is neither constant nor directly visible on the time section. Sound travels slowly through shallow, poorly compacted shale and much faster through deep, dense carbonate or salt, so the same time interval can represent very different thicknesses depending on lithology and burial. Interpreters therefore build a velocity model from several sources: processing velocities derived from the moveout of reflections across source-receiver offsets, checkshot surveys that drop a geophone down a borehole and time a surface shot to known depths, and vertical seismic profiles that record first arrivals along the length of the well to give both average and interval velocity. Interval velocity, the speed across a specific layer, is commonly extracted from stacking velocities using the Dix equation, and the resulting layered velocity field is what turns a time-domain picture into a depth structure that can be tied to well tops. Traveltime also carries information beyond simple structure: the difference in arrival time between a flat reflector and a structurally high one reveals folds and faults, the way traveltime changes with offset, called moveout, constrains velocity and can flag gas-charged sands through a velocity pull-down, and apparent time-structure can be distorted by overlying velocity anomalies, the classic velocity pull-up beneath a fast salt or carbonate body. In the Western Canadian Sedimentary Basin, where targets range from shallow Mannville channels to deep Montney and Duvernay shale, accurate time-to-depth conversion is essential because a horizontal well must be geosteered to a target that may be only a few metres thick, and an error of even 10 to 15 milliseconds in picking a reflector or a few percent in velocity can place a wellbore above or below the intended zone. Traveltime is thus the raw currency of seismic interpretation, and the discipline of converting it faithfully into depth, accounting for anisotropy, compaction, and lateral velocity change, is one of the central skills of basin-scale exploration and development geophysics.

Key Takeaways

Two-Way Time Is the Vertical Axis: A conventional seismic section plots two-way traveltime, the full source-to-reflector-to-receiver journey, in seconds or milliseconds rather than depth. Every structural feature an interpreter sees is initially a time feature, and nothing on the section corresponds to a true spatial position until a velocity model converts time into depth.
Depth Needs Velocity: The core conversion is depth equals one half of the two-way traveltime multiplied by velocity. Because velocity rises with compaction and varies by lithology, from slow shallow shale to fast carbonate or salt, an identical time interval can represent very different thicknesses, so a credible velocity model is mandatory for any reliable depth structure.
Velocity From Wells and Moveout: Velocity models draw on processing or stacking velocities from reflection moveout, checkshot surveys that time surface shots to a borehole geophone, and vertical seismic profiles that record first arrivals along the well. Interval velocities across individual layers are typically derived from stacking velocities using the Dix equation.
Traveltime Encodes Structure and Fluids: Differences in arrival time map folds, faults, and dip, while the variation of traveltime with offset, called moveout, constrains velocity and can indicate gas through a velocity pull-down in charged sands. Velocity anomalies above a target can distort apparent time-structure, producing pull-up or pull-down artifacts that must be corrected.
Conversion Errors Move Wellbores: In thin WCSB targets such as the Montney or Duvernay, a few milliseconds of mispick or a few percent of velocity error can land a horizontal well above or below a zone only metres thick. Accurate time-to-depth conversion, including anisotropy correction, directly governs geosteering success and completion results.

Time-to-Depth Conversion Workflow

A practical depth conversion begins with picking key reflectors in time, then assigning each interval a velocity from checkshots, VSPs, or a calibrated stacking-velocity field. The interpreter ties seismic time picks to formation tops in wells, adjusts the velocity model until synthetic ties match, and applies the layered velocities to convert the full time volume to depth. Residual mismatches between converted depth and well tops are resolved with a velocity correction grid. In the Duvernay, where the target shale may be 30 to 50 m thick at 3,000 m, this workflow is what keeps a horizontal landing point within the high-quality interval.

Velocity Anomalies and Apparent Structure

Lateral velocity change is the most common source of false structure on a time section. A fast carbonate buildup or a Prairie Evaporite salt body speeds up the waves passing through it, pulling deeper reflectors up in time and creating an apparent high that vanishes once depth conversion is applied. Conversely, a gas-charged or undercompacted shale slows the waves and pushes reflectors down, mimicking a low. Recognizing these pull-up and pull-down effects prevents operators from drilling a velocity artifact instead of a genuine trap, a mistake that has cost dry-hole budgets across many basins.

Fast Facts

The Dix equation, published by C. Hewitt Dix in 1955, remains the workhorse for extracting interval velocity from stacking velocities seven decades later, despite being a simple root-mean-square relationship derived for flat, horizontal layers. Its enduring use is striking given how much seismic processing has otherwise advanced into prestack depth migration and full-waveform inversion. The persistence of a 1955 formula in modern interpretation underlines that traveltime analysis is ultimately built on a small set of robust physical relationships between time, velocity, and depth.

Traveltime is woven into the wider toolkit of seismic interpretation. It is the measured product of a seismic reflection, the bounce of energy off an impedance contrast, and it cannot be turned into structure without a model of interval velocity, the speed across each layer. The variation of traveltime with source-receiver distance is described by moveout, and the entire effort of relating time to spatial position is captured by depth conversion, the step that ties seismic time to drillable depth.

Real-World WCSB Scenario: Montney Geosteering From Time Data

An operator planning a Montney horizontal near Dawson Creek shoots 3D seismic and maps the target reflector at roughly 1.65 seconds two-way traveltime. Checkshot data from two offset wells give an average velocity that converts this to about 2,450 m, but the interval velocity through the overlying Doig phosphate is higher than the regional trend, and an uncorrected conversion would place the landing point 9 m too shallow, above the best Montney rock. The geophysics team rebuilds the velocity model with the local checkshots and reties to well tops.

With the corrected depth model the well is landed inside the target interval and geosteered using real-time gamma and the seismic depth surface, holding the lateral in zone across 2,500 m. The episode shows how a modest velocity error, translated from traveltime, can be the difference between a productive completion and one that straddles a barrier, and why disciplined time-to-depth work pays for itself on every horizontal well.