1、Designation: D 6274 98 (Reapproved 2004)Standard Guide forConducting Borehole Geophysical Logging - Gamma1This standard is issued under the fixed designation D 6274; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the general procedures necessary toconduct gamma, natural gamma, total count gamma, or gammaray (hereaft
3、er referred to as gamma) logging of boreholes,wells, access tubes, caissons, or shafts (hereafter referred to asboreholes) as commonly applied to geologic, engineering,ground-water, and environmental (hereafter referred to asgeotechnical) investigations. Spectral gamma and loggingwhere gamma measure
4、ments are made in conjunction with anuclear source are excluded (for example, neutron activationand gamma-gamma density logs). Gamma logging for mineralsor petroleum applications are excluded.1.2 This guide defines a gamma log as a record of gammaactivity of the formation adjacent to a borehole with
5、 depth (SeeFig. 1).1.2.1 Gamma logs are commonly used to delineate lithol-ogy, correlate measurements made on different logging runs,and define stratigraphic correlation between boreholes (SeeFig. 2).1.3 This guide is restricted to gamma logging with nuclearcounters consisting of scintillation detec
6、tors (crystals coupledwith photomultiplier tubes), which are the most commongamma measurement devices used in geotechnical applica-tions.1.4 This guide provides an overview of gamma loggingincluding general procedures, specific documentation, calibra-tion and standardization, and log quality and int
7、erpretation.1.5 To obtain additional information on gamma logs, seeSection 13.1.6 This guide is to be used in conjunction with GuideD 5753.1.7 Gamma logs should be collected by an operator that istrained in geophysical logging procedures. Gamma logs shouldbe interpreted by a professional experienced
8、 in log analysis.1.8 The geotechnical industry uses English or SI units. Thegamma log is typically recorded in units of counts per second(cps) or American Petroleum Institute (API) units.1.9 This guide does not purport to address all of the safetyand liability problems (for example, lost or lodged p
9、robes andequipment decontamination) associated with its use.1.10 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-
10、bility of regulatory limitations prior to use.1.11 This guide offers an organized collection of informa-tion or a series of options and does not recommend a specificcourse of action. This document cannot replace education orexperience and should be used in conjunction with professionaljudgment. Not
11、all aspects of this guide may be applicable in allcircumstances. This ASTM standard is not intended to repre-sent or replace the standard of care by which the adequacy ofa given professional service must be judged, nor should thisdocument be applied without consideration of a projects manyunique asp
12、ects. The word “Standard” in the title of thisdocument means only that the document has been approvedthrough the ASTM consensus process.2. Referenced Documents2.1 ASTM Standards:2D 653 Terminology Relating to Soil, Rock and ContainedFluidsD 5088 Practice for Decontamination of Field EquipmentUsed at
13、 Nonradioactive Waste SitesD 5608 Practice for Decontamination of Field EquipmentUsed at Low Level Radioactive Waste SitesD 5753 Guide for Planning and Conducting Borehole Geo-physical LoggingD 6167 Guide for Conducting Borehole Geophysical Log-ging: Mechanical Caliper3. Terminology3.1 Definitions:3
14、.1.1 Definitions shall be in accordance with TerminologyD 653, Section 13, Ref (1), or as defined below.1This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rockand is the direct responsibility of Subcommittee D18.01 on Surface and SubsurfaceCharacterization.Current edition approv
15、ed July 1, 2004. Published August 2004. Originallyapproved in 1998. Last previous edition approved in 1998 as D 6274 - 98.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, ref
16、er to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2 Definitions of Terms Specific to This Standard:3.2.1 accuracy, nhow close measured log values ap-proach true value. It i
17、s determined in a controlled environment.A controlled environment represents a homogeneous samplevolume with known properties.3.2.2 dead time, nthe time after each pulse when a secondpulse cannot be detected.3.2.3 dead time effect, nthe inability to distinguishclosely-spaced nuclear counts leads to
18、a significant underesti-mation of gamma activity in high radiation environments andis known as the “dead time effect”.3.2.4 depth of investigation, nthe radial distance from themeasurement point to a point where the predominant measuredresponse may be considered centered, which is not to beconfused
19、with borehole depth (for example, distance) mea-sured from the surface.3.2.5 measurement resolution, nthe minimum change inmeasured value that can be detected.3.2.6 repeatability, nthe difference in magnitude of twomeasurements with the same equipment and in the sameenvironment.NOTE 1This figure dem
20、onstrates how the log can be used to identify specific formations, illustrating scale wrap-around for a local gamma peak, andshowing how the contact between two formations is picked to coincide with the half-way point of the transition between the gamma activities of the twoformations.FIG. 1 Example
21、 of a Gamma Log From Near the South Rim of the Grand CanyonD 6274 98 (2004)23.2.7 vertical resolution, nthe minimum thickness thatcan be separated into distinct units.3.2.8 volume of investigation, nthe volume that contrib-utes 90 % of the measured response. It is determined by acombination of theor
22、etical and empirical modeling. The vol-ume of investigation is non-spherical and has gradationalboundaries.4. Summary of Guide4.1 This guide applies to borehole gamma logging and is tobe used in conjunction with Guide D 5753.4.2 This guide briefly describes the significance and use,apparatus, calibr
23、ation and standardization, procedures, andreports for conducting borehole gamma logging.5. Significance and Use5.1 An appropriately developed, documented, and executedguide is essential for the proper collection and application ofgamma logs. This guide is to be used in conjunction with GuideD 5753.5
24、.2 The benefits of its use include improving selection ofgamma logging methods and equipment, gamma log qualityNOTE 1From a study site showing how the gamma logs can be used to identify where beds intersect each of the individual boreholes, demonstratinglateral continuity of the subsurface geology.F
25、IG. 2 Example of Gamma Logs From Two BoreholesD 6274 98 (2004)3and reliability, and usefulness of the gamma log data forsubsequent display and interpretation.5.3 This guide applies to commonly used gamma loggingmethods for geotechnical applications.5.4 It is essential that personnel (see the Personn
26、el sectionof Guide D 5753) consult up-to-date textbooks and reports onthe gamma technique, application, and interpretation methods.6. Interferences6.1 Most extraneous effects on gamma logs are caused bylogging too fast, instrument problems, borehole conditions, andgeologic conditions.6.2 Logging too
27、 fast can significantly degrade the quality ofgamma logs. Gamma counts originating at a given depth needto be averaged over a time interval such that the naturalstatistical variation in the rate of gamma photon emission isnegligible (see Fig. 3).6.3 Instrument problems include electrical leakage of
28、cableand grounding problems, degradation of detector efficiencyattributed to loss of crystal transparency (fogging) or fracturesor breaks in the crystal, and mechanical damage causingseparation of crystal and photomultiplier tube.6.4 Borehole conditions include changes in borehole diam-eter (especia
29、lly in the fluid-filled portion); casing type andnumber; radioactive elements in drilling fluid in the borehole,or in cement or slurry behind casing; and steel casing or cementin the annulus around casing, and thickness of the annulus.NOTE 1The fluctuations in gamma activity in counts per second is
30、shown to vary by progressively smaller amounts as the averaging period (timeconstant) is increased from 1 to 20 s.FIG. 3 Example of Natural Statistical Fluctuation of Gamma Counts From a Test Source of Given StrengthD 6274 98 (2004)46.5 Geologic conditions include high levels of radiationwhich can d
31、egrade the efficiency of gamma counting throughthe dead time effect, energy level of emitted gammas, forma-tion density, and lithologic bed geometry.7. Apparatus7.1 A geophysical logging system has been described in thegeneral guide (the Apparatus section of Guide D 5753).7.2 Gamma logs are collecte
32、d with probes using scintilla-tion detectors.7.2.1 The most common gamma detectors are sodium io-dide (NaI).7.2.2 Other gamma detectors include cesium iodide (CsI)and bismuth germanate (BGO).7.3 Gamma probes generate nuclear counts as pulses ofvoltage that are amplified and clipped to a uniform ampl
33、itude.7.3.1 Gamma probes used for geotechnical applicationstypically can be logged inside of a 2-in. (5-cm) diametermonitoring well.7.4 The volume of investigation and depth of investigationare determined by the density of the material near the probe,which controls the average distance a gamma photo
34、n can travelbefore being absorbed.7.4.1 The volume of investigation for gamma logs is gen-erally considered spherical with a radius of 0.5 to 1.0 ft (15 to30 cm) from the center of the detector in typical geologicalformations. The volume becomes elongated when detectorlength exceeds approximately 0.
35、5 ft (15 cm).7.4.2 The depth of investigation for gamma logs is generallyconsidered to be 0.5 to 1.0 ft (15 to 30 cm).7.5 Vertical resolution of gamma logs is determined by thesize of the volume from which gammas can reach a nucleardetector suspended in the borehole. In typical geologicalformations
36、surrounding a fluid-filled borehole, this is a roughlyspherical volume about 1 to 2 ft (30 to 60 cm) in diameter.Excessive logging speed can decrease vertical resolution.7.6 Measurement resolution of gamma probes is determinedby the counting efficiency of the nuclear detector being used inthe probe.
37、 Typical measurement resolution is 1 cps.7.7 A variety of gamma logging equipment is available forgeotechnical investigations. It is not practical to list all of thesources of potentially acceptable equipment.8. Calibration and Standardization of Gamma Logs8.1 General:8.1.1 National Institute of Sta
38、ndards and Technology(NIST) calibration and standardization procedures do not existfor gamma logging.8.1.2 Gamma logs can be used in a qualitative (for example,comparative) or quantitative (for example, estimating radioiso-tope concentration) manner depending upon the project objec-tives.8.1.3 Gamma
39、 calibration and standardization methods andfrequency shall be sufficient to meet project objectives.8.1.3.1 Calibration and standardization should be performedeach time a gamma probe is suspected to be damaged,modified, repaired, and at periodic intervals.8.2 Calibration is the process of establish
40、ing values forgamma response associated with specific levels of radioisotopeconcentration in the sampled volume and is accomplished witha representative physical model. Calibration data values relatedto the physical properties (for example, radioisotope concen-tration) may be recorded in units (for
41、example, cps), that can beconverted to units of radioactive element concentration (forexample, ppm Radium-226 or percent Uranium-238 equiva-lents).8.2.1 Calibration is performed by recording gamma logresponse in cps in boreholes centered within volumes contain-ing known homogenous concentrations of
42、radioactivity ele-ments.8.2.2 Calibration volumes should be designed to containmaterial as close as possible to that in the environment wherethe logs are to be obtained to allow for effects such as gammaenergy level, formation density, and activity of daughterisotopes on the calibration process.8.3
43、Standardization is the process of checking loggingresponse to show evidence of repeatability and consistency,and to ensure that logging probes with different detectorefficiencies measure the same amount of gamma activity in thesame formation. The response in cps of every gamma detectoris different f
44、or the same radioactive environment.8.3.1 Calibration ensures standardization.8.3.2 The American Petroleum Institute maintains a bore-hole in Houston, Texas, where two formations have beenfabricated to provide homogeneous levels of gamma activity sothat probes can be standardized on the basis of the
45、 response inthese boreholes. 1 API gamma unit is 1/200thof the full scaleresponse in the representative shale model in this borehole (seeGuide D 5753).8.3.3 For geotechnical applications, gamma logs should bepresented in API units for standardization.8.3.4 A representative borehole may be used to pe
46、riodicallycheck gamma probe response providing the borehole andsurrounding environment does not change with time or theireffects on gamma response can be documented.8.3.5 A small radioactive source(s) (thorium-treated lanternmantles, small bottles of potassium chloride, laboratory radio-active test
47、sources, or sleeves containing natural radioisotopes(phosphate sands, etc.) placed over the gamma detector can beused to check calibration if the sources have been related to acalibration facility.8.4 Gamma log output needs to be corrected for dead timewhen logging in formations with unusually large
48、 count rates,such as uranium-rich pegmatites or phosphatic sands, and areascontaminated with radioactive waste.8.4.1 Dead time corrections are usually negligible undertypical logging conditions when measured gamma counts areless than a few hundred counts per second.8.4.2 Dead time corrections are es
49、timated by comparing thegamma log response under the influence of two similarradioactive sources. The measured count rate would approxi-mately double over that with one source when both sources areplaced in the sample volume of the logging tool. The dead timecauses the count rates to be slightly less than double. Dead timeis given by the formula:Dead Time 5 t05 2N11 N2 N12!/N12N11 N2! (1)Corrected count rate 5 N* 5 N/1Nt0!D 6274 98 (2004)5where:N1,N2= the count rates measured using each of the twosimilar sources,N12= the count rate obtained using both of the s
