Two-Way Travel Time: Seismic Time-to-Depth Conversion, Zero-Offset Reflections, and WCSB Mapping

Two-way travel time (TWT) is the elapsed time for a seismic wave to travel from its source down to a given subsurface reflector and back up to a receiver at the Earth's surface, and it is the fundamental measurement axis of reflection seismology because raw seismic data is recorded and displayed in time, not depth. Every horizon picked on a seismic section, every fault, every amplitude anomaly is located first by its two-way travel time, usually expressed in milliseconds, and only later converted to a depth in metres or feet through a velocity model. The minimum two-way travel time to a reflector is that of a normal-incidence wave with zero offset, meaning the idealized case where source and receiver are coincident and the ray travels straight down and straight back up; any real source-receiver separation, called offset, adds extra path length and therefore extra time, the increase known as normal moveout (NMO) that processing removes during stacking. Because TWT is a round trip, it is roughly twice the one-way time used in a check-shot or vertical seismic profile, and converting between the seismic time domain and the geologist's depth domain is the single most consequential and error-prone step in the interpretation workflow. The conversion requires a velocity field: interval velocities from sonic logs, stacking velocities from the seismic processing, or check-shot and VSP measurements that directly tie a depth in a wellbore to a one-way time. A reflector at 600 ms TWT might be 900 m deep in a slow young clastic section or 1,400 m deep where fast carbonates speed the wave, so the same time can map to very different depths depending on the overburden velocity. In the Western Canadian Sedimentary Basin, where the layered foreland-basin stratigraphy ranges from shallow Cretaceous Mannville and Viking clastics down through the Devonian carbonates of the Leduc, Nisku, and Wabamun, interpreters build velocity models from dense well control and then convert key horizons from TWT to depth to plan wells, compute reservoir volumes, and tie seismic to existing penetrations. Errors in the velocity model propagate directly into depth errors, so a structural closure that looks robust in time can shrink or vanish in depth if the overburden velocity varies laterally, a phenomenon called velocity pull-up or push-down. Two-way travel time is also the basis for vertical resolution arguments: because seismic measures time, the ability to separate two closely spaced reflectors depends on the dominant period of the wavelet in time, which only becomes a thickness once velocity is applied. Mastery of the time-to-depth relationship, and honest treatment of its uncertainty, separates a reliable prospect map from a misleading one.

From Time to Depth with a Velocity Model

Depth conversion turns a picked TWT horizon into a depth map using a velocity field. The simplest method applies a single average velocity from surface to the horizon; more rigorous workflows use layer-cake interval velocities, each layer bounded by a picked horizon and assigned a velocity from sonic logs or check-shots, so the wave is tracked through realistic overburden. In the WCSB, dense well control lets interpreters grid interval velocities laterally and capture changes such as a thickening shale or a high-velocity carbonate that would otherwise distort the depth. The quality of the final depth map is only as good as the velocity model, which is why check-shot surveys tying a wellbore depth directly to one-way time are run on important wells.

Velocity Pull-Up Beneath Fast Rock

When a high-velocity body such as a Leduc reef or a salt sits in the overburden, waves travelling beneath it arrive earlier than waves through slower surrounding rock, pulling the underlying reflector up in time and creating an apparent structural high that does not exist in depth. This velocity pull-up has condemned and promoted prospects across Alberta. The cure is depth conversion with an interval-velocity model that honours the fast body, after which the false time structure flattens out. Interpreters cross-check by mapping a deeper, regionally flat marker and watching whether it mirrors the shallow anomaly, a classic diagnostic for a velocity artifact rather than real relief.

Related Terms

Two-way travel time is the time axis of Seismic Data, and converting it to depth requires velocity information often derived alongside impedance from well logs. The shape of each reflection in time is governed by the Wavelet, whose dominant period sets vertical resolution, while the strength of the reflection is its Amplitude. The zero-offset assumption that defines minimum TWT is the same idealization corrected by normal moveout before stacking.

Real-World WCSB Scenario: A Phantom High over a Leduc Reef

An interpreter mapping a Devonian clastic target near Drayton Valley, Alberta sees a clean structural high in two-way travel time directly beneath a known Leduc reef build-up and recommends a CAD 4.5 million well on the crest. A senior reviewer flags the geometry as a textbook velocity pull-up: the fast carbonate reef speeds waves to the deeper marker, lifting it in time without any real depth relief. Depth conversion with an interval-velocity model built from three offset sonic logs is run on the horizon.

After conversion the apparent high collapses to a gentle, non-closing dip, confirming the time structure was an artifact of the overlying reef velocity. The prospect is dropped before drilling, saving the well cost, and the team reallocates the budget to a separate lead that holds closure in both time and depth.