Velocity Layering: Interval Velocity, Seismic Time-to-Depth, and Why Velocity Layers Differ From Bedding

Velocity layering refers to the division of the subsurface into intervals of rock or sediment that share a common seismic velocity, as distinct from the geological layering or bedding defined by depositional and lithologic boundaries. The two often coincide, but not always, and the distinction matters because a seismic survey records travel time, not depth, and converting that time to depth requires a velocity model built from velocity layers rather than from stratigraphic beds. A thick, uniform shale may behave as a single velocity layer even where it contains many thin bedding planes, while a lithologically continuous formation can split into two velocity layers if compaction, porosity, or fluid content changes its acoustic velocity with depth. Geophysicists characterize velocity layering through several related velocity types: interval velocity, the average velocity through one specific layer measured between its top and base reflections; average velocity, from surface to a given reflector; root-mean-square or RMS velocity, used in normal-moveout corrections; and stacking velocity, derived from the curvature of reflection hyperbolas in a common-midpoint gather. Interval velocity is the quantity most directly tied to a velocity layer, computed classically with the Dix equation from the RMS velocities and travel times of the layer's bounding reflectors. Velocity layering controls nearly every step of seismic interpretation: the normal-moveout correction that flattens reflections before stacking, the migration that moves dipping events to their true position, the time-to-depth conversion that turns a seismic time horizon into a drillable depth target, and the prediction of pore pressure from velocity anomalies. A low-velocity layer, where velocity decreases with depth beneath a faster layer, is particularly important because it can hide from refraction surveys and can signal overpressure, gas charge, or unconsolidated sediment. In the Western Canadian Sedimentary Basin, velocity layering is fundamental to mapping the Montney, Duvernay, Cardium, Viking, and deeper Leduc and Nisku carbonate targets, where a few percent error in interval velocity over thousands of metres can shift a predicted reservoir depth by tens of metres and cause a horizontal well to be landed above or below the target zone. Velocity models for WCSB seismic are built and calibrated against sonic logs, vertical seismic profiles, and check-shot surveys from existing wells, then interpolated across the seismic volume so that every interpreted horizon can be tied to a reliable depth. Because acoustic velocity responds to lithology, porosity, pore fluid, compaction, and pressure, velocity layering also carries direct geological information: a sharp interval-velocity contrast can flag a porosity sweet spot, a tight cemented streak, a gas-charged sand, or an overpressured shale long before a well is drilled.

Building A Calibrated Velocity Model

A WCSB interpreter starts with stacking velocities picked across the seismic volume, converts them to interval velocities through the Dix equation, then ties the model to hard data at well control: sonic logs, check-shot surveys, and vertical seismic profiles. Where the seismic-derived interval velocity disagrees with the well sonic, the geophysicist adjusts the model layer by layer until the synthetic seismogram matches the recorded trace at the wellbore. Only then is the velocity field interpolated across the survey to depth-convert interpreted horizons. For a Duvernay play, this calibration routinely reconciles a several-percent mismatch that, left uncorrected, would misplace the brittle shale target.

Pore Pressure From Velocity Anomalies

Velocity layering also drives pre-drill pore-pressure prediction. In a normally compacting shale, velocity rises smoothly with depth; a layer where velocity reverses or falls below the compaction trend signals undercompaction and overpressure. WCSB drillers use this to anticipate kicks in deep Foothills and Deep Basin wells, setting casing under AER Directive 010 above the predicted high-pressure interval. A missed low-velocity overpressure layer can cause a well-control event, so the velocity model feeds directly into the casing-point and mud-weight program.

Related Terms

Velocity layering underlies Interval Velocity, the per-layer velocity computed from a velocity model, and feeds Normal Moveout, the correction that uses velocity to flatten reflections before stacking. It is the input to Seismic Migration, which repositions dipping reflectors using the velocity field, and ultimately to Time-to-Depth Conversion, where the layered velocity model turns interpreted travel times into drillable depths. Each step inherits any error in the underlying velocity layers.

Real-World WCSB Scenario: Landing A Duvernay Lateral On Depth

A Duvernay operator in the Kaybob area plans a horizontal landing in a brittle shale interval only 30 m (about 98 ft) thick. The initial seismic time-to-depth model, built from stacking velocities alone, predicts the target top at 3,420 m (11,220 ft). After calibrating the interval velocities to three offset sonic and check-shot surveys, the model corrects upward by 24 m (79 ft), placing the true target at 3,396 m. The velocity re-modelling effort costs roughly CAD 60,000 in processing and interpretation time.

The corrected depth let the geosteering team land the lateral inside the brittle window on the first attempt, avoiding a costly sidetrack. Mis-landing by 24 m would have placed much of the 2,800 m lateral in non-prospective rock, cutting recoverable reserves and frac effectiveness across a CAD 9 million well.