interval velocity, Oil & Gas Glossary

Interval velocity describes the propagation speed of a seismic wave, typically a compressional or P-wave, through a discrete rock layer or stratigraphic interval, and it is symbolized as vint in the geophysics literature. The parameter is fundamental to depth conversion, the process that transforms time-domain seismic images into the depth-domain geological models drilling engineers and reservoir geoscientists use to plan wells in the Western Canadian Sedimentary Basin. Interval velocity is commonly derived from one of two sources: sonic logs, which measure acoustic travel time through formations at the borehole scale and produce direct interval velocity readings in units of m/s or ft/s, or from the changes in stacking velocity between successive reflection events on a common midpoint gather, where the Dix equation converts root-mean-square velocities into layered interval velocities. The Dix equation, formulated by C. Hewitt Dix in 1955, states vint squared equals (vrms2 squared times t2 minus vrms1 squared times t1) divided by (t2 minus t1), where t1 and t2 are zero-offset two-way times to the top and base of the interval, and vrms1 and vrms2 are the corresponding root-mean-square velocities. In the WCSB, interval velocities are routinely used to image the Montney siltstone in northeastern British Columbia and northwestern Alberta, where velocities typically range from 3,800 to 4,500 m/s (12,470 to 14,765 ft/s), and the Duvernay Shale in the Kaybob, Edson, and Willesden Green fairways with similar values. Cardium sandstone intervals in the Pembina area show velocities of 4,000 to 4,800 m/s (13,123 to 15,748 ft/s), while underlying Mannville sands typically run 3,500 to 4,200 m/s (11,483 to 13,780 ft/s). Operators including Canadian Natural Resources Limited, Cenovus Energy Inc, and Tourmaline rely on accurate interval velocity models to position horizontal wells within thin Montney or Duvernay zones, particularly where structural dip and lateral lithology changes make time-domain interpretation ambiguous. The discipline intersects with seismic velocity analysis, time-depth tie generation through checkshot surveys, and anisotropy corrections in unconventional resource plays where horizontal-to-vertical velocity ratios can exceed 1.20.

Key Takeaways

Dix Equation Derivation: The Dix equation transforms stacking or root-mean-square velocities into interval velocities for individual subsurface layers, using two-way travel times to upper and lower interval boundaries. Although elegant in principle, it assumes flat, isotropic, horizontally layered media and breaks down where dip exceeds 10 degrees or where strong anisotropy (common in WCSB shales) introduces errors of 5 to 15 percent that must be corrected with VTI or HTI tomographic processing.
Sonic Log Calibration: Sonic logs measure compressional slowness in microseconds per foot or per metre and provide direct interval velocity at the borehole. A slowness of 100 us/ft converts to 3,048 m/s (10,000 ft/s), and integrating sonic logs against checkshot surveys ties seismic time-depth functions to the wellbore. Operators routinely use Schlumberger DSI or Halliburton XMAC logs to derive vint for Montney and Duvernay velocity models, with sample rates of 0.5 to 1.0 ft and log costs of CAD 25,000 to CAD 60,000 per well.
Depth Conversion Application: Interval velocity is the engine of depth conversion, where time horizons multiplied by interval velocity yield true vertical depth maps. In the WCSB, accurate vint reduces top-of-pay prediction error from plus or minus 15 metres to under plus or minus 5 metres in a Duvernay horizontal well, which prevents geosteering excursions out of the target zone and avoids costly side-tracks that can run CAD 800,000 or more per occurrence on a 3,000 m TVD pad well.
Anisotropy and AVO Effects: Interval velocities differ between vertical (well-derived) and horizontal (seismic-derived) propagation in shale-rich plays. Thomsen anisotropy parameters epsilon and delta describe this difference, and Montney epsilon values of 0.10 to 0.25 are typical. Ignoring anisotropy mispositions seismic events by 30 to 90 metres at 3,000 m depth, distorts AVO attributes, and degrades amplitude-versus-offset interpretations used for fluid and lithology discrimination during prospect ranking.
Regulatory and Reserve Context: Interval velocity models underpin volumetric reserve estimates submitted to the Alberta Energy Regulator under Directive 058 and the BC Energy Regulator under section 5 reporting. Reservoir thickness errors of 10 percent driven by inaccurate vint propagate directly into proved and probable reserves disclosed in NI 51-101 statements, with material implications for capital allocation, ARO assessment under Directive 011, and lender reserve-based loan covenants.

Dix Equation Pitfalls in Structurally Complex WCSB Plays

The Dix equation assumes laterally homogeneous, isotropic, flat-layered geology, but Foothills thrust belts west of Calgary and the structurally deformed Front Ranges violate every one of those assumptions. In the Turner Valley anticline and Moose Mountain triangle zone, stacking velocities are perturbed by dipping reflectors and out-of-plane energy, so naive Dix conversion can mis-place a Mississippian carbonate top by 80 to 150 metres. Pre-stack depth migration and tomographic velocity inversion are required to correct these errors, and the resulting interval velocity volumes (in m/s on a 25 by 25 m grid) honour both seismic and well control. A single Foothills 3D velocity rebuild typically runs CAD 350,000 to CAD 600,000 over a 200 square kilometre survey area.

Sonic Log Integration and Synthetic Seismograms

Tying interval velocity from sonic logs to seismic time begins with a checkshot or vertical seismic profile that establishes the time-depth relationship at the wellbore. The sonic log is drift-corrected to match checkshot times, then convolved with a wavelet to produce a synthetic seismogram. A good Montney synthetic in the Dawson Creek area will correlate at 80 percent or better against surface seismic, confirming the velocity model. Discrepancies of more than 10 milliseconds in two-way time at the Doig phosphate marker, a common Montney top, trigger reprocessing of the sonic log for cycle skips or borehole-rugosity-induced errors before the model is accepted for depth conversion.

Fast Facts

C. Hewitt Dix published the Dix equation in 1955 while working for Magnolia Petroleum (later ExxonMobil), and it remains one of the most-cited papers in exploration geophysics seven decades later. Before Dix, geophysicists relied on shooting refraction profiles and laboriously matching arrival times to bedding planes, a process that could take weeks for a single line. The Dix transform reduced velocity analysis from days to hours and unlocked the routine generation of depth-converted seismic that drove the Permian Basin oil boom of the late 1950s and, by extension, Alberta's Devonian carbonate plays.

Interval velocity is the building block of depth conversion and connects directly to several related glossary concepts. Seismic velocity is the broader category that includes interval, stacking, average, and root-mean-square velocity flavours, each used for distinct geophysical workflows. Common midpoint gathers supply the offset traces from which stacking and ultimately interval velocities are picked. Acoustic impedance, the product of velocity and density, is computed from interval velocity in inversion workflows that predict reservoir quality. The sonic log is the direct borehole measurement of interval velocity used to calibrate seismic velocity models throughout the WCSB.

WCSB Field Scenario: Montney Horizontal Geosteering Near Dawson Creek

An operator drilling a 2,800 m TVD, 3,000 m lateral Upper Montney well northeast of Dawson Creek used a 3D seismic-derived interval velocity cube to predict the top of the C-zone target at 2,815 m. The initial velocity model used naive Dix conversion of stacking velocities, which placed the C-zone top within plus or minus 20 m of seismic prediction. After tying eight offset sonic logs and applying anisotropy corrections with epsilon of 0.18 and delta of 0.07, the model tightened to plus or minus 5 m. The well landed within the 12 m C-zone at 2,818 m TVD on first attempt, avoiding a CAD 1.2 million side-track that the previous lateral on the same pad had required. Total velocity reprocessing cost CAD 180,000 across 80 square kilometres of 3D data.

Subsequent wells on the pad benefited from the calibrated model and were drilled with reduced logging-while-drilling gamma redundancy, saving an additional CAD 40,000 per well. The pad-wide IRR lifted by approximately 3 percent versus the pre-velocity-update plan, and the operator extended the same anisotropy framework to a neighbouring 240-section Duvernay block under AER Directive 083 land tenure.