velocity-shot measurement, Oil & Gas Glossary

What Is a Velocity-Shot Measurement?

A velocity-shot measurement is a borehole seismic technique that fires a seismic source at the surface while a geophone receiver clamped inside the wellbore records the direct arrival travel time of the first compressional wave, providing interval velocity and time-depth relationship calibration points that tie wireline log depths to seismic two-way time at discrete formation boundaries.

Key Takeaways

Velocity shots provide the definitive time-to-depth conversion needed to correlate wireline log interpretations with surface seismic data.
Interval velocities between successive velocity-shot depths calibrate the sonic log and identify cycle-skips or tool errors in sonic data.
The technique is also called a check-shot survey when used for quality control of the sonic log velocity integration.
Multiple receiver depths provide a time-depth curve used to tie stratigraphic markers from the well to reflectors on the seismic section.
Velocity shots combined with a downgoing wavelet measurement enable synthetic seismogram generation for seismic-to-well tie.

How Velocity-Shot Measurements Work

A velocity-shot survey is conducted with the drill string out of the hole, typically as part of the wireline logging programme at the end of the well. A geophone tool, also called a seismic receiver or wall-lock geophone, is clamped against the borehole wall at a series of depths corresponding to key formation boundaries or at regular depth intervals. At the surface, a seismic source, typically an airgun array offshore or a weight-drop or vibroseis source onshore, fires and the surface electronics record the firing time precisely.

The direct P-wave (compressional wave) travels from the source down through the earth to the geophone, arriving after a travel time equal to the source-to-receiver distance divided by the average velocity along that raypath. The geophone records this arrival time, and the travel time from source firing to first arrival is measured precisely, typically to 0.25-1 millisecond accuracy. The process is repeated at multiple receiver depths throughout the well. The result is a series of one-way travel time, depth pairs. By differencing travel times between successive depth stations, interval velocities between stations are calculated. These interval velocities, converted to two-way times by doubling, provide calibration anchors for the seismic time scale.

Velocity-Shot Measurement Applications Across International Jurisdictions

In Canada, check-shot surveys are standard wireline programme components for WCSB exploration wells and appraisal wells where seismic-to-well tie quality is important. AER well data submissions include sonic log data used in seismic interpretation; check-shot calibration of the sonic log ensures that the integrated sonic log time-depth curve matches the direct-arrival time-depth relationship measured by the check-shot, providing confidence in the velocity model used for seismic event correlation. Montney tight gas appraisal programmes in the WCSB Deep Basin rely on check-shot calibrated velocity models to reliably predict formation depths at infill well locations from surface seismic data.

In the United States, velocity-shot surveys are required deliverables for all Gulf of Mexico exploration wells by BSEE permit conditions; the time-depth curve is a fundamental input to the seismic interpretation workflows that justify reserve assessments and field development proposals in deepwater projects. In Norway, Sodir's well data retention requirements for the NCS include check-shot data as a mandatory wireline deliverable from exploration wells; Equinor's seismic interpretation of Johan Sverdrup used extensively calibrated check-shot surveys from appraisal wells to ensure that the Jurassic reservoir seismic mapping was accurately tied to depths at which the oil-water contact and sand quality had been confirmed by drilling. In Australia, NOPSEMA-regulated exploration wells in the Carnarvon and Browse basins are required to acquire check-shot surveys as part of the minimum wireline programme to enable regulatory-compliant seismic interpretation supporting resource estimates.

Fast Facts

The typical precision of a check-shot first-arrival time pick is approximately 0.5 milliseconds. Over a depth range of 3,000 metres with average velocity of 2,500 m/s, the two-way time is 2,400 milliseconds. A 0.5 ms timing error in the check-shot corresponds to a 1.25-metre depth error at that point. Across multiple check-shot stations, timing errors can accumulate or partially cancel. For deepwater seismic interpretation where well landing positions must be accurate to within 5-10 metres, multiple check-shot stations with sub-millisecond timing precision are essential for velocity model calibration.

Velocity Shots and Synthetic Seismogram Generation

Velocity-shot measurements are a prerequisite for generating a high-quality synthetic seismogram, which is the modelled seismic trace computed from the well logs and used to tie log-based geological interpretations to reflectors on the seismic section. The synthetic seismogram is computed as a convolution of the earth's reflection coefficient series (derived from density and sonic log impedance contrasts) with the seismic wavelet extracted from the data near the well. The time-depth conversion used to position the reflection coefficient series in time must match the actual seismic time-depth relationship, and only the check-shot survey provides this independent calibration. A synthetic seismogram computed from an uncalibrated sonic log may tie to the seismic section at a time offset of 10-50 milliseconds from the true formation depth, causing misidentification of stratigraphic markers and erroneous correlation of formation tops with seismic reflectors.

Tip: When running a check-shot survey, include at least one station in a known, geologically simple formation (a thick, laterally uniform limestone or evaporite) to serve as a quality control point for the first-arrival time pick. In complex formations with thin beds or high-angle incidence from the surface source, the first arrival may not be a simple P-wave and the time pick may be ambiguous. A quality control station in a simple formation with a predictable velocity provides a cross-check that confirms the time pick methodology is correct before the data is used for velocity model calibration.

Velocity-shot measurement is also known as:

Check-shot survey — the most common operational name; emphasises the quality-control function of verifying the sonic log velocity by comparison with direct travel time measurements
Vertical seismic profile (VSP) — the more comprehensive survey type that uses the same downhole geophone technology but records full wavefields (direct, upgoing, and downgoing waves) at many depth stations; velocity shots are the simplest subset of VSP data acquisition
Time-depth survey — the functional description used when the primary deliverable is the time-depth curve rather than interval velocities

Frequently Asked Questions

What is the difference between a check-shot survey and a VSP?

A check-shot survey acquires only the direct first-arrival time at each receiver station, providing a time-depth relationship but not the full wavefield. A vertical seismic profile (VSP) records the complete wavefield at each depth station, including the downgoing direct wave, the upgoing reflected waves from below, and multiple reflections. VSP data provides not only the time-depth calibration of a check-shot but also a one-dimensional seismic image of the subsurface below the well that can be used for ahead-of-bit prediction, fault identification, and detailed seismic-to-well tie at much higher vertical resolution than surface seismic data. VSP surveys require significantly more acquisition time and processing cost than simple check-shot surveys, but provide proportionally more information.

Why is check-shot calibration of the sonic log important?

The sonic log measures compressional wave travel time in microseconds per foot or microseconds per metre by reading the direct wave travel time between two receivers in the borehole wall at centimetre spacings. Errors in the sonic measurement — cycle skips where the tool picks the wrong part of the waveform, gas effect in gas-saturated pore space, altered zone effects from invasion, or bad hole conditions — accumulate through depth integration and cause the integrated sonic time-depth curve to drift away from the true seismic time-depth relationship. Check-shots identify these errors by comparing the directly measured first-arrival time at each check-shot station with the sonic log integral at the same depth. Drift correction is applied to force agreement at check-shot stations, producing a calibrated time-depth curve that can be reliably used for synthetic seismogram generation and seismic-to-well tie.

Why Velocity-Shot Measurements Matter in Oil and Gas

Seismic-to-well tie is the bridge between the two primary data types used in subsurface evaluation: the high-vertical-resolution but areal-point-measurement of wireline logs, and the low-vertical-resolution but spatially continuous coverage of surface seismic reflection data. Every exploration well drilled in a new area, every appraisal well in a development campaign, and every infill well in a producing field depends on the ability to accurately correlate the geological interpretations made at the wellbore with the seismic events visible across the field. Velocity-shot measurements provide the calibration data that makes this correlation possible with confidence. Without check-shots, seismic-to-well ties rely on integrated sonic logs that may carry uncorrected errors of tens of milliseconds, corresponding to depth mismatches of tens of metres at typical basin velocities — sufficient to misidentify the reservoir interval, miss the productive zone, or incorrectly predict the location of the oil-water contact across the field.

Velocity-Shot Measurement: Definition, Check-Shot Survey, and Seismic-to-Well Tie