Velocity Survey: Check-Shot, VSP, and Sonic Calibration for Time-Depth Conversion
A velocity survey is a set of borehole measurements that establish how fast seismic energy travels through subsurface rock as a function of depth, producing the average velocity versus depth function that geophysicists need to tie surface seismic reflection data to real well geology. Because surface seismic records arrival times in two-way travel time, measured in seconds, while a wellbore is logged in true vertical depth, measured in metres or feet, a velocity survey supplies the conversion that links the two domains. The most common form is the check-shot survey, in which a geophone or accelerometer is clamped against the borehole wall at a series of known depths while a surface source, often an air gun suspended in a mud pit or a vibroseis truck on the lease, fires a controlled pulse. The one-way travel time from source to receiver at each station gives a directly measured time-depth pair. Acquiring this data set is known in the field as "shooting a well." A closely related method is the vertical seismic profile, or VSP, which records the full seismic wavefield at many closely spaced depths and resolves both the downgoing direct arrival and upgoing reflections, allowing a much finer calibration than sparse check-shots alone. Velocity surveys also draw on the sonic or acoustic log, which measures interval transit time over short intervals; because the sonic responds at ultrasonic frequencies and can drift due to borehole rugosity, dispersion, and invasion, its integrated travel time is routinely corrected to match the check-shot drift curve before it is trusted for depth conversion. In the Western Canadian Sedimentary Basin, accurate velocity control matters most where thick, fast carbonates such as the Leduc and Nisku overlie slower clastic sequences, and where Montney and Duvernay resource plays require precise depth ties to land horizontal wells within a few metres of a target sub-unit. A velocity error of even two percent can mis-position a seismic horizon by 20 m to 40 m at 2,000 m depth, enough to land a lateral in the wrong interval. Operators therefore treat the velocity survey as the foundation of seismic interpretation, depth conversion, pore pressure prediction, and amplitude analysis.
Key Takeaways
- Time-to-depth backbone: Surface seismic is recorded in two-way travel time while wells are measured in depth. A check-shot delivers physically measured one-way times at known depths, so it converts seismic horizons into true vertical depth without relying on assumed velocity laws. This is the single most reliable depth-tie an interpreter has.
- Check-shot versus VSP: A check-shot survey uses sparse stations, typically 15 to 30 levels, recording only the first-break direct arrival. A VSP records the full wavefield at closely spaced depths and resolves upgoing reflections, enabling a corridor stack that overlays directly on surface seismic for a much tighter calibration.
- Sonic drift correction: The integrated sonic transit time accumulates error from borehole washout, frequency dispersion, and mud-filtrate invasion. The difference between integrated sonic time and check-shot time is the drift curve, and the sonic is bulk-shifted and blocked to honour the check-shot before it is used for synthetic seismograms.
- WCSB depth precision: In Montney and Duvernay development, a horizontal lateral must stay within a target window only a few metres thick. A two percent velocity miscalibration translates to a 20 m to 40 m horizon error at 2,000 m, large enough to place the wellbore in the wrong sub-zone and degrade completion results.
- Shooting a well: The field term for acquiring a velocity survey. Sources include air guns and vibroseis; the downhole tool is a clamped geophone. Average velocities in a single WCSB well range from roughly 1,800 m/s (5,900 ft/s) in shallow unconsolidated clastics to over 6,000 m/s (19,700 ft/s) in tight carbonate.
How a Check-Shot Survey Is Acquired Downhole
During a check-shot survey, the wireline tool carrying a clamping geophone is run to total depth, then stationed at selected levels on the way up, commonly at formation tops and at regular intervals between them. At each station the arm clamps the receiver firmly to the borehole wall to suppress tool-string noise, and the surface source fires several shots that are stacked to improve signal-to-noise. The first-break arrival time is picked, corrected to a vertical path and to a common seismic reference datum, and paired with the station depth. The result is a table of time-depth pairs and the interval velocities derived between them, both reported in metric and imperial units so they can be cross-checked against the sonic and against regional velocity trends.
Velocity Surveys in WCSB Depth Conversion
Across the deep basin in west-central Alberta and northeast British Columbia, depth conversion is complicated by strong lateral velocity change between the Cretaceous clastic section and the underlying Devonian carbonates. A check-shot or VSP in a key well anchors the velocity model, and operators such as Tourmaline and ARC Resources use that anchor to build a layer-cake or gridded velocity field for the whole survey area. Without it, a structural high on a time map can be a velocity artifact rather than real relief. The survey also feeds pore pressure work in the overpressured Montney, where velocity inversion against compaction trends flags zones that demand higher mud weight before drilling proceeds.
Fast Facts
A check-shot survey is among the cheapest insurance in the entire seismic workflow. A slickline or wireline check-shot in a typical WCSB well runs 15 to 30 levels and adds only a few hours of rig time, yet it underpins the depth conversion of a 3D survey that may have cost several million dollars to acquire and process. The velocity span captured in one borehole is striking: energy can travel more than three times faster through a tight Nisku carbonate than through the shallow, water-saturated, unconsolidated section near surface.
Related Terms
A velocity survey is closely tied to the Sonic Log, whose interval transit times are calibrated against the check-shot drift curve before use. It feeds the Vertical Seismic Profile, a richer borehole-seismic method that resolves reflections as well as direct arrivals. The output is expressed in Two-Way Time, the domain of surface seismic, and it yields Interval Velocity between stations, the building block of every depth-conversion velocity model.
Real-World WCSB Scenario: Montney Depth-Tie Near Karr
An operator developing the Montney near Karr in northwest Alberta drilled a vertical pilot before a multi-well horizontal pad. A wireline check-shot survey was run over 22 levels from surface to a total depth of 3,250 m at a cost of roughly CAD 35,000, including the geophysical crew and air-gun source. The first-break times tied the Montney top on the 3D volume 18 m shallower than the pre-survey time-depth function had predicted, correcting a velocity pull-up caused by the overlying fast carbonate.
The corrected depth surface let the geosteering team reposition the landing point so all four laterals stayed inside the upper Montney target. Completion engineers reported more consistent stage breakdown pressures across the pad, and the operator credited the small check-shot spend with avoiding a misplaced bench that could have cost millions in lost recovery.