Acquisition Log
An acquisition log is the wireline or logging-while-drilling (LWD) record that is physically recorded in the field at the time the measurement is made, as opposed to the final processed log delivered to the operator after quality-control checks, environmental corrections, and depth editing have been applied. The acquisition log contains the raw sensor output at each depth sample, together with the ancillary data recorded at the time of logging: cable tension, logging speed, tool temperature, borehole temperature, caliper reading, and tool configuration parameters. These ancillary channels are essential for evaluating the quality of the primary measurement curves and for applying the corrections that convert raw sensor output into calibrated formation properties. In practice, the term is also used more loosely to refer to the log curve as it appears on the monitor screen at the wellsite in real time, before any post-acquisition processing is applied, distinguishing it from the corrected and resampled version delivered days or weeks later. Understanding what the acquisition log represents and what corrections are still needed to make it interpretable is a prerequisite for log quality control.
Key Takeaways
- Logging speed (the rate at which the wireline tool is pulled upward through the borehole) directly controls the vertical resolution and the signal-to-noise ratio of the acquisition log. Most wireline logging tools record data at a fixed sample rate in time (typically 10 to 120 samples per second depending on tool type) and rely on the depth encoder on the surface cable drum to convert time samples to depth samples. At a logging speed of 300 metres per hour (5 m/min, typical for compensated neutron tools), a tool sampling at 60 Hz produces one sample every 8.3 centimetres, adequate for detecting beds thicker than about 25 centimetres. At 600 m/hr the same sampling produces one sample every 16.7 centimetres, halving the vertical resolution. If the logging speed is too high for the tool's integration time, thin beds are smeared and their true peak values are not captured in the acquisition log; a tight pay interval of 30 centimetres thickness can be completely missed by a tool logging at 900 metres per hour because the sample interval may exceed the bed thickness. Standard practice is to log at speed appropriate for the measurement type and to flag intervals where the logging speed exceeded the recommended maximum on the header of the acquisition log.
- The repeat section is the standard quality control procedure for acquisition logs: after the primary upward log pass, the tool is lowered to approximately 150 to 300 metres below the zone of interest and pulled back up again, repeating the measurement over a portion of the borehole. The two passes should overlay closely; significant differences between the main pass and the repeat pass indicate tool malfunction, sensor drift, changes in borehole conditions (caving, swelling, fluid invasion changing), or errors in the depth tracking system. Overlap tolerance for most measurement types is plus or minus 2 percent of the scale range for resistivity, plus or minus 2 API units for gamma ray, and plus or minus 0.015 g/cc for density. If the repeat section does not meet these tolerances, the log cannot be considered reliable and must be relogged or used with documented quality caveats. In the WCSB, the repeat section is required by most operators as a condition of log acceptance and is specified in the logging program before the tool string is run.
- LWD acquisition logs differ from wireline acquisition logs in how depth and time references are established. Wireline logging uses a cable with an encoder wheel at the surface measuring cable length to assign depth; LWD tools record measurements as a function of time and use the driller's depth (bit depth from the surface) interpolated to the same timestamps to assign formation depth. The driller's depth is affected by drill string stretch (which is significant in deep or deviated wells: a 5,000-metre drill string can stretch 3 to 5 metres under its own weight), heave on floating vessels, and bit weight variability, introducing depth uncertainties of 1 to 5 metres in the LWD acquisition log relative to wireline depth. Gamma ray correlations between LWD acquisition logs and subsequent wireline logs, run after the bit has advanced further, are used to quantify and correct the LWD depth offset so that the two log datasets align for combined interpretation.
- Environmental corrections applied to the acquisition log convert raw sensor output to measurements that represent the true formation property rather than a combination of formation and borehole effects. The density acquisition log, for example, records photoelectric effect count rates from two detectors at different spacings that are sensitive to formation density but also affected by the density of the drilling mud, the standoff between the tool pad and the borehole wall (measured by the caliper), and the borehole diameter. The correction algorithm uses the caliper reading and the mud density from the acquisition header to compute a spine-and-ribs correction to the density curve; the corrected density is the value used in interpretation, not the raw count-rate ratio. For the gamma ray, a barite correction must be applied if the mud contains barite (a barium sulfate weighting material) because barium attenuates gamma rays from the formation, making the recorded GR appear artificially low; the acquisition header must record the barite concentration for the correction to be valid.
- Tool motion artifacts in acquisition logs occur when the logging tool moves faster or slower than its nominal speed, or when it vibrates or sticks and slips in the borehole, causing the depth assignment to be inconsistent with the actual position of the tool. In LWD, the acoustic imager and resistivity imager acquisition logs are particularly sensitive to tool rotation rate: if the drillstring rotates slowly through a sticky section (stick-slip), the image data samples are compressed on one side of the borehole and stretched on the other, creating a warped image that may not show true bedding dip or fracture orientation correctly. In wireline, sticking of the tool in tight sections creates a gap in the depth log where the cable continues to pay out but the tool is not moving, followed by a jump when the tool breaks free and the log compresses a section of borehole into a shorter depth interval. Both types of tool motion artifacts are logged in the quality control ancillary channels and must be identified and documented before the acquisition log is used for quantitative interpretation.
Wireline Versus LWD Acquisition: Operational Differences
Wireline acquisition logs are run on a conductor cable lowered into the wellbore after drilling has reached total depth (or a casing shoe), with the tool pulled upward past the zone of interest at controlled speed. The cable provides both mechanical support (tension) and an electrical conductor for power and data transmission. The surface system uses the encoder wheel on the cable winch drum to record depth continuously, and the depth reference is typically tied to a casing collar locator (CCL) run in the tool string: the CCL detects magnetic anomalies at casing collar joints whose depths are measured at surface when the casing was run, providing a series of precisely known depth reference points that can be used to check the cable depth throughout the logged interval.
LWD acquisition logs are recorded while drilling, with the tool sensors located 1 to 15 metres above the drill bit in the bottom-hole assembly. The tool records data continuously to onboard memory while drilling, with a subset of real-time data transmitted upward through the drillstring to surface via mud pulse or electromagnetic telemetry. The real-time acquisition data has coarser depth sampling (every 30 centimetres to 1 metre, controlled by telemetry bandwidth) and may have missing sections when the telemetry link was broken by pump interruptions or tool faults. The full-resolution memory acquisition log is downloaded from the tool when it is pulled to surface after the bit trip, providing complete data at the highest sample rate the tool can record (typically 2.5 to 10 centimetres depth sampling). The memory-quality LWD acquisition log is the primary data used in interpretation, while the real-time acquisition data serves for drilling decisions and geosteering.
Depth Shifting and Merging Multiple Acquisition Logs
Most wells are logged with multiple tool strings run at different times on different trips: a resistivity-gamma ray string while drilling, a density-neutron-gamma ray string on wireline, and possibly a dipole sonic tool on a third run. Each acquisition log is on a different depth reference, and systematic offsets of 0.5 to 3 metres between the different runs are common due to cable stretch differences, wireline calibration differences, and LWD-to-wireline depth offsets. Before multi-log interpretation (crossplot analysis, volumetric calculations, or geomechanical modelling), the acquisition logs must be depth-shifted to a common reference, typically the wireline gamma ray from the shallowest run or a detailed LWD gamma ray with many distinctive marker beds. Gamma ray shifts are applied first because the GR is present in nearly all runs and provides the most distinctive depth markers in the section.
Fast Facts
The earliest wireline acquisition logs were recorded on paper strip charts, with the logging engineer manually annotating the header information (wellname, depth references, tool type, calibration date) while the paper chart scrolled beneath the recording galvanometer pen. Digital acquisition logging began in the 1970s, when Schlumberger introduced the CSU (Cyber Service Unit) field computers that digitised and stored log data on magnetic tape; the digital acquisition log was post-processed in a computing centre to produce the printed log copy delivered to the operator. The log data format standard most widely used today for digital acquisition logs is LAS (Log ASCII Standard), originally developed by the Canadian Well Logging Society in 1989 to provide a simple, universal format for exchanging single-well log data; LAS 2.0 (1992) and LAS 3.0 (1999) added multi-channel support. The WITSML (Wellsite Information Transfer Standard Markup Language) format is used for real-time LWD acquisition data transmitted from the rig to the operator's office over the internet, enabling remote geosteering decisions without travelling to the wellsite. The Canadian Well Logging Society (CWLS) publishes recommended practice documents for wireline log acquisition in the WCSB, including minimum tool specifications, logging speed limits, repeat section requirements, and calibration standards.
Synonyms and Related Terminology
The acquisition log is sometimes called the field log, raw log, or wellsite log to distinguish it from the final processed delivery. In LWD, the memory log (downloaded after the bit trip) is the highest-quality version of the acquisition log. Related terms include logging-while-drilling (LWD, the acquisition of formation evaluation measurements by sensors located in the bottom-hole assembly while the drill bit advances; the LWD acquisition log is recorded in tool memory and in partial form via telemetry in real time, providing formation evaluation data without a separate wireline trip), wireline log (the acquisition of formation evaluation measurements by instruments lowered on a wireline cable after drilling; the wireline acquisition log is recorded at the surface system and provides the definitive depth reference and the highest vertical resolution formation evaluation data for the well), logging speed (the rate at which the wireline tool or the drill bit advances past the formation, expressed in metres per hour; too-high a logging speed reduces the vertical resolution of the acquisition log and can cause thin-bed measurements to be smeared or missed; each tool type has a recommended maximum logging speed for adequate data quality), repeat section (the standard quality control procedure in wireline logging in which the tool is repositioned below a logged interval and pulled back up through the same depth range a second time; the close agreement of the two passes over the same interval confirms tool stability, depth consistency, and measurement repeatability), and environmental corrections (adjustments applied to the raw acquisition log data to remove the effects of borehole conditions, including mud properties, borehole diameter, tool standoff, temperature, and pressure, that would otherwise cause the sensor output to reflect borehole rather than formation properties; corrections are applied using calibration coefficients and borehole condition measurements recorded with the acquisition log).