Velocity Anomaly: Seismic Pull-Up, Push-Down, and Depth-Conversion Error in WCSB Carbonate Plays
A velocity anomaly is a distortion in seismic data caused by lateral or vertical changes in the speed at which sound travels through the rock, and it is one of the most persistent traps in subsurface interpretation. Seismic reflection sections are recorded in two-way travel time, not depth, so the position of every reflector depends on the cumulative velocity of all the rock above it. When an overlying interval has an unusually high or low velocity compared to its surroundings, it bends the time image of everything beneath it. The two classic expressions are velocity pull-up and velocity push-down. A pull-up occurs when a fast body, such as a tight carbonate reef, a salt pillar, or a cemented channel, sits above a deeper reflector; the fast rock shortens the travel time, so the deeper horizon appears to arch upward into a false structural high directly beneath the fast feature. A push-down, sometimes called a velocity sag, is the opposite: a slow body such as a gas-charged sand, a shale-filled channel, or a porous zone delays the wavefront, dragging the underlying reflector down into a false low. Neither feature is real structure; both are artifacts of the velocity field, and confusing them with genuine relief has led to dry holes across the Western Canadian Sedimentary Basin for decades. The classic WCSB example is the Devonian reef play. A Leduc or Nisku buildup is a tight, fast carbonate encased in slower shale; on a time section the reflectors below the reef pull up into an apparent anticline, which early interpreters sometimes mistook for a deeper drilling target when it was only the shadow of the reef above. Gas effect is the mirror image: a shallow gas-charged Mannville or Belly River sand slows the section and pushes down the events beneath it, creating a sag that can be misread as a syncline or that masks a real underlying high. Resolving velocity anomalies is the job of velocity analysis and, ultimately, depth conversion. Interpreters build a velocity model from checkshot and sonic-log control, tie it to seismic interpretation, and convert time horizons to depth so that the geometric distortions are removed and only true structure remains. The presence of a velocity anomaly is also a valuable direct hydrocarbon indicator: a localized push-down combined with an amplitude bright spot is one of the more reliable seismic signatures of shallow gas. Detecting and correctly diagnosing these features separates a sound prospect from an expensive geometric illusion, and modern prestack depth migration is designed specifically to handle laterally varying velocity that simpler time processing cannot.
Key Takeaways
- Pull-Up Is A False High From Fast Rock: When a high-velocity body such as a Leduc or Nisku reef, a salt pillar, or a tight cemented channel overlies a deeper reflector, it shortens travel time and arches the underlying horizon into an apparent structural high. The high is not real; it is the velocity shadow of the fast feature above, and drilling it as a target risks a dry hole.
- Push-Down Is A False Low From Slow Rock: A low-velocity body such as a gas-charged sand or a shale-filled channel delays the wavefront and drags the underlying reflector downward into a velocity sag. A shallow gas Mannville sand can push down deeper events by tens of milliseconds, mimicking a syncline or masking a genuine high beneath.
- Time Versus Depth Is The Root Cause: Because seismic is recorded in two-way travel time, every reflector's apparent position depends on the integrated velocity above it. Velocity anomalies vanish only after proper depth conversion using a velocity model built from checkshot and sonic control tied to well markers.
- Push-Down Can Be A Gas Indicator: A localized push-down paired with an amplitude bright spot and an AVO anomaly is one of the more reliable direct hydrocarbon indicators for shallow gas in WCSB Cretaceous sands. The same physics that distorts the image also flags the reservoir, turning the artifact into useful evidence when correctly interpreted.
- Depth Migration Handles Lateral Velocity: Simple time migration assumes mild velocity variation and fails over strong lateral contrasts like reef edges or salt. Prestack depth migration uses a laterally varying velocity model to position reflectors correctly, the modern tool for imaging beneath WCSB carbonate buildups and dissolution-collapse features.
Reef Pull-Up Beneath A Devonian Leduc Buildup
Consider a Leduc reef encased in Ireton shale near the Rimbey-Meadowbrook trend. The reef carbonate carries a velocity near 6,000 m/s while the surrounding shale runs closer to 4,000 m/s. On the time section, the Cooking Lake platform reflector beneath the reef pulls up by 30 to 50 milliseconds directly under the buildup, forming a convincing apparent anticline. An interpreter who picks that as a deeper target is chasing geometry, not rock. Depth conversion with a reef-aware velocity model flattens the false high and confirms the platform is conformable, redirecting the prospect to the reef itself, which is the genuine reservoir.
Gas Push-Down As A Shallow Hazard And An Opportunity
A shallow gas-charged Belly River sand near 600 m can slow the section enough to push down underlying Mannville reflectors by 20 to 40 milliseconds. For a deep target this sag is a nuisance that must be corrected before mapping. For a shallow gas prospect it is a gift: the push-down, combined with a bright amplitude and a class III AVO response, gives a high-confidence shallow gas signature that operators have drilled successfully across the Medicine Hat and Hatton fields. The same feature is also a drilling hazard flag, warning of overpressured shallow gas that demands a diverter and careful surface casing design under AER Directive 008.
Fast Facts
Velocity pull-up beneath Devonian reefs was responsible for some of the earliest dry holes in the WCSB carbonate boom of the 1950s, when interpreters drilled deep apparent highs that turned out to be pure velocity artifacts of the fast reef overhead. The hard-won recognition that a reef casts a velocity shadow eventually flipped the exploration logic: the artifact itself became a reef-detection tool. Today a clean pull-up signature is read as positive evidence that a tight carbonate buildup, and therefore a potential reservoir, exists above the distorted reflector.
Related Terms
A velocity anomaly is diagnosed and removed through velocity analysis and depth conversion, which rely on direct velocity control from a checkshot survey to anchor the model to true depth. The geometric distortions are part of the broader practice of seismic interpretation, where separating real structure from artifact is the core skill. Velocity anomalies also interact with spherical divergence correction, since both depend on an accurate subsurface velocity field, and errors in one can propagate into the other during processing.
Real-World WCSB Scenario: A False High Near Swan Hills
An operator evaluating a Swan Hills carbonate prospect near Judy Creek maps an apparent Beaverhill Lake high on a time-migrated 3D and prepares to drill a CAD 7 million vertical well. A geophysicist flags that the high sits directly beneath a thick, fast platform interval and could be a velocity pull-up rather than real structure. The team builds a depth model from offset well sonic and checkshot data and re-images with prestack depth migration, at a processing cost near CAD 90,000.
The depth section shows the apparent high collapsing to a gentle, non-prospective monocline; the structure was an artifact. The CAD 90,000 reprocessing saved a CAD 7 million dry hole, and the team redirected capital to a genuine flank target confirmed in true depth.