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D6274-18 – Standard Test Method Technical Guide
📐 Introduction and Scope
ASTM D6274-18 covers the general procedures for conducting gamma logging in boreholes, wells, access tubes, caissons, or shafts for geotechnical investigations. A gamma log is defined as a record of gamma activity of the formation adjacent to a borehole with depth. Gamma logs are commonly used to delineate lithology, correlate measurements from different logging runs, and define stratigraphic correlation between boreholes.
This guide is restricted to gamma logging using nuclear counters with scintillation detectors, which are the most common gamma measurement devices in geotechnical applications. It is intended to be used in conjunction with Guide D5753.
⚙️ Equipment and Calibration
Gamma logging equipment includes scintillation detectors consisting of crystals coupled with photomultiplier tubes. The gamma log is typically recorded in units of counts per second (cps) or American Petroleum Institute (API) units. The API unit is defined as 1/200 of the difference between the count rate recorded in the middle of a radioactive bed and that in the nonradioactive bed, within the calibration pit at the University of Houston. However, the validity of this calibration pit has been called into question in recent years.
| 🟦 Parameter | 📐 Details |
|---|---|
| API Unit Definition | 1/200 of count rate difference between radioactive and nonradioactive beds |
| Calibration Facility | University of Houston |
| Common Recording Units | Counts per second (cps) or API units |
Gamma logs should be collected by an operator trained in geophysical logging procedures and interpreted by a professional experienced in log analysis.
Ensuring proper calibration and operator training is critical for accurate gamma log data.
📊 Data Interpretation and Units
Gamma logs are essential for lithology delineation, correlation of measurements, and stratigraphic correlation between boreholes. The values can be reported in SI or inch-pound units, but these systems should not be combined. Reporting in units other than SI does not constitute nonconformance with the standard.
| 🟦 Application | 📐 Description |
|---|---|
| Lithology Delineation | Identify formation types based on gamma activity |
| Correlation | Match measurements across different logging runs |
| Stratigraphic Correlation | Relate geological strata between boreholes |
This guide excludes spectral gamma logging, gamma-gamma density logs, and logging for minerals or petroleum applications. It offers a collection of information and does not recommend a specific course of action, emphasizing the use of professional judgment.
The standard does not address all safety concerns; users must establish appropriate safety, health, and environmental practices.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D6274-18?
The standard covers the general procedures for conducting gamma logging in boreholes for geotechnical investigations, excluding spectral gamma and nuclear source logging.
💡 How is the API unit defined in this standard?
The API unit is defined as 1/200 of the difference between the count rate recorded in the middle of a radioactive bed and that in the middle of a nonradioactive bed during calibration.
⚡ Who should collect and interpret gamma logs?
Gamma logs should be collected by an operator trained in geophysical logging procedures and interpreted by a professional experienced in log analysis.
📌 What types of logging are excluded from this guide?
Spectral gamma logging, gamma-gamma density logs, neutron activation logs, and logging for minerals or petroleum applications are excluded.