check-shot survey

A check shot survey is a borehole seismic measurement in which a seismic source at surface fires and a downhole geophone clamped to the borehole wall records the first-arrival travel time of the compressional (P-wave) seismic pulse at multiple discrete depth stations from total depth upward, providing a series of one-way travel time measurements that calibrate the velocity of the rock column above each receiver position and allow the interpreter to convert seismic reflection times (measured in two-way travel time, TWT, in milliseconds) to true depths in metres with accuracy that is not achievable from sonic logs alone because sonic tools measure short-interval acoustic velocity that is susceptible to cycle skipping, borehole rugosity effects, and mud invasion alteration of the near-wellbore rock properties; in Western Canada Sedimentary Basin seismic interpretation projects, the check shot survey is the mandatory calibration step before constructing a synthetic seismogram, building a depth conversion model for prospect volumetrics, or tying a wireline log formation top to a specific seismic reflection event on 2D or 3D seismic data. In WCSB Montney and Duvernay horizontal play fairways where well spacing is 200 to 800 m and depth to target ranges from 1,800 to 3,200 m, check shot velocity calibration reduces depth uncertainty at the landing zone from plus or minus 30 to 60 m (sonic-only extrapolation) to plus or minus 5 to 15 m, directly controlling the accuracy of horizontal well kick-off depths and the length of the lateral within the target formation; an uncalibrated depth conversion that places the landing zone 25 m structurally high of the target can land the lateral in the overlying non-reservoir shale sequence, causing a loss of an entire 2 to 3 km horizontal section from productive pay. WCSB exploration programs in Devonian carbonate plays (Leduc, Nisku, Wabamun) similarly depend on check shot calibration to convert seismic time maps to structural depth maps with sufficient accuracy to test the integrity of reef-flank four-way closures that may have only 20 to 50 m of structural relief above the hydrocarbon-water contact.

• Check shot survey acquisition procedure and geophone station selection in WCSB exploration wells: A standard WCSB check shot survey is acquired after the well is drilled and before or immediately after wireline logging, with the borehole in an open-hole condition or through casing if an open-hole run is not possible. The surface seismic source (dynamite shot hole in land surveys, air gun array in offshore surveys) fires a controlled pulse while a single or multi-level downhole geophone string (typically 1 to 4 levels with 15 to 100 m spacing) is clamped to the borehole wall at each station depth; the downhole geophone records the full wavefield including the direct arrival (the first break), the first multiple, and any formation reflections. Station spacing in WCSB check shot surveys is typically 50 to 150 m in the overburden and 15 to 30 m across key reservoir intervals (Viking, Cardium, Montney top, Duvernay) where fine-scale velocity variation is needed for accurate synthetic seismogram construction; a standard WCSB check shot survey has 20 to 60 stations from surface to total depth, with the shallowest station at 50 to 200 m below surface and the deepest station at or within 50 m of total depth.
• Check shot velocity analysis: interval velocity calculation and sonic log drift correction in WCSB depth conversion workflows: The one-way travel time to each geophone station is picked from the first break on the downhole geophone record; the interval velocity between consecutive stations is calculated as the depth difference divided by the travel time difference, giving a station-to-station average velocity that represents the bulk acoustic velocity of the rock interval between those stations. The check shot interval velocity is compared to the continuous sonic log integrated travel time (ITT) over the same interval; if the sonic ITT consistently under-reads the check shot time (as is common in WCSB wells where borehole washout accelerates the sonic reading), a drift correction is applied to the sonic log by scaling or shifting the sonic ITT to match the check shot time at each station, producing a calibrated sonic log that is subsequently used to construct the synthetic seismogram. In WCSB Cretaceous exploration wells, sonic drift corrections of 2 to 8 percent are routine in the Upper Cretaceous shale-dominated Colorado Group (2,000 to 3,000 m depth range) due to extensive borehole enlargement in smectite-rich shales; uncorrected synthetic seismograms in this depth range show time misties of 8 to 20 ms TWT at key Cardium and Viking reflectors.
• Well-to-seismic tie and synthetic seismogram calibration using check shot velocity in WCSB 3D seismic projects: The check shot-calibrated synthetic seismogram is the link between the geological column logged at the well and the seismic reflection dataset; the synthetic trace (computed by convolving the acoustic impedance log with the estimated seismic wavelet) is overlaid on the actual seismic traces extracted around the wellbore and shifted in time until the character of the synthetic matches the actual seismic within accepted tolerance of 3 to 8 ms TWT. In WCSB 3D seismic interpretation projects targeting Viking, Cardium, or Montney formations, the check shot tie quality is reported as the cross-correlation coefficient between synthetic and actual seismic at the well; a cross-correlation above 0.85 at the target level indicates a reliable tie that supports extending horizon picks away from the well into undrilled parts of the survey. A poor tie (cross-correlation below 0.7) usually indicates either cycle skipping in the sonic log over a specific interval, an erroneous check shot first-break pick at a washout-affected station, or a wavelet phase mismatch between the estimated and actual seismic wavelet; each of these is diagnosed and corrected before the interpretation is extended.
• Depth conversion accuracy from check shot velocity and its impact on WCSB Montney horizontal well landing: In WCSB Montney unconventional play development, the horizontal well landing zone is a specific bench within the Montney Formation (Upper, Middle, or Lower Montney) that may be only 10 to 25 m thick at depths of 1,800 to 3,200 m; depth uncertainty from seismic time-to-depth conversion must be smaller than the bench thickness to ensure the planned well trajectory stays within the target bench throughout the 2 to 3 km horizontal section. Check shot velocity calibration at a pilot hole or vertical offset well constrains the average velocity to target to within 1 to 2 percent accuracy, reducing depth uncertainty to plus or minus 20 to 45 m at Montney depths; further refinement using a detailed interval velocity model built from multiple check shot wells across the pad location reduces uncertainty to plus or minus 8 to 15 m, which is adequate to define the landing depth for horizontal wells within the 15 to 25 m target bench thickness. WCSB operators in the Dawson Creek and Groundbirch Montney fairways routinely acquire check shot surveys at one in four to one in six wells on development pads to maintain a calibrated velocity model as development progresses into previously undrilled areas.
• VSP versus check shot: choosing the right borehole seismic program for WCSB exploration objectives: The check shot survey and vertical seismic profile (VSP) are related borehole seismic measurements but serve different purposes in WCSB exploration programs. A check shot survey records only the first-break direct arrival at each station and is processed for travel time picks to build a velocity function; it is fast (4 to 8 hours acquisition and processing for a 40-station survey), inexpensive ($15,000 to $40,000 for a land survey), and adequate for depth conversion and synthetic seismogram tie work. A VSP records the full wavefield at each station (direct wave, upgoing reflected waves, and multiples) and can be processed to produce an image of the formation around the borehole at higher frequency than surface seismic, detect formation reflections below total depth, and provide corridor stacks for well-to-seismic tie at higher bandwidth than the surface seismic data. WCSB operators choose a VSP over a check shot survey when: the synthetic seismogram tie is poor and the cause is uncertain (VSP provides a direct reflection record for comparison); when a target formation is within 200 to 400 m below total depth of a nearby well and a VSP look-ahead is needed to confirm the target before committing to additional drilling; or when offset VSP is needed to image a fault or reef edge 100 to 500 m from the borehole.

Check Shot Calibration Correcting Montney Landing Depth Error in Northeast BC Pad Development

A northeast British Columbia Montney pad operator planned five horizontal wells targeting the Middle Montney bench at an interpreted seismic depth of 2,285 m TVD based on a sonic-log velocity function extrapolated from a vertical calibration well 4 km to the northwest. A check shot survey acquired at the first pad pilot hole (40 stations from 200 to 2,420 m depth) measured a check shot interval velocity in the Upper Cretaceous shale section 5.3 percent higher than the integrated sonic log velocity, attributable to caliper-confirmed borehole washout in the Sully and Bad Heart shale intervals; the corrected depth-to-top Middle Montney was 2,308 m TVD, 23 m deeper than the uncalibrated sonic prediction. Without the check shot correction, the planned kick-off depths and build-up rates for the five horizontal wells would have landed the laterals in the Upper Montney bench rather than the Middle Montney target, missing the highest-deliverability section of the formation. The corrected landing depth, applied to all five wells, resulted in lateral placement within 3 m of the planned Middle Montney target across an 8 km lateral total length, with average initial production rates of 6.2 MMcf/d per well consistent with best-offset Middle Montney type curves in the Groundbirch area.

Related Terms

Synthetic seismogram is the primary product calibrated by check shot velocity; the check shot-corrected sonic log is convolved with the seismic wavelet to produce the synthetic trace used for well-to-seismic horizon tie in WCSB 3D seismic interpretation projects. Vertical seismic profile (VSP) is the full-wavefield extension of the check shot survey; VSP records upgoing reflections at each station and produces a borehole image used when the check shot synthetic seismogram tie alone is insufficient for WCSB exploration objectives. Depth conversion is the seismic interpretation workflow that check shot velocity calibration supports; converting WCSB time-structure maps to depth-structure maps for volumetric calculations and horizontal well planning requires check shot-derived velocity functions at calibration wells. Sonic log provides the continuous interval velocity measurement that is drift-corrected against check shot times; the calibrated sonic log is the input to synthetic seismogram generation and acoustic impedance inversion in WCSB formation evaluation workflows. Seismic character is confirmed and calibrated by the synthetic seismogram that the check shot enables; polarity, amplitude, and waveshape of the synthetic at each geological boundary must match the actual seismic character before the interpreter extends horizon correlations into undrilled areas of a WCSB 3D survey.