ASTM D6274-98
(Guide)Standard Guide for Conducting Borehole Geophysical Logging-Gamma
Standard Guide for Conducting Borehole Geophysical Logging-Gamma
SCOPE
1.1 This guide covers the general procedures necessary to conduct gamma, natural gamma, total count gamma, or gamma ray (hereafter referred to as gamma) logging of boreholes, wells, access tubes, caissons, or shafts (hereafter referred to as boreholes) as commonly applied to geologic, engineering, ground-water and environmental (hereafter referred to as geotechnical) investigations. Spectral gamma and logging where gamma measurements are made in conjunction with a nuclear source are excluded (for example, neutron activation and gamma-gamma density logs). Gamma logging for mineral or petroleum applications are excluded.
1.2 This guide defines a gamma log as a record of gamma activity of the formation adjacent to a borehole with depth (See Fig.1).
1.2.1 Gamma logs are commonly used to: delineate lithology, correlate measurements made on different logging runs, and define stratigraphic correlation between boreholes (See Fig. 2).
1.3 This guide is restricted to gamma logging with nuclear counters consisting of scintillation detectors (crystals coupled with photomultiplier tubes), which are the most common gamma measurement devices used in geotechnical application.
1.4 This guide provides an overview of gamma logging including: general procedures; specific documentation; calibration and standardization, and log quality and interpretation.
1.5 To obtain additional information on gamma logs see Section 13.
1.6 This guide is to be used in conjunction with Guide D 5753.
1.7 Gamma logs should be collected by an operator that is trained in geophysical logging procedures. Gamma logs should be interpreted by a professional experienced in log analysis.
1.8 The geotechnical industry uses English or SI units. The gamma log is typically recorded in units of counts per second (cps) or American Petroleum Institute (API) units.
This guide does not purport to address all of the safety and liability problems (for example, lost or lodged probes and equipment decontamination) associated with its use.
1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1.11 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word "Standard" in the title of this document means only that the document has been approved through the ASTM consensus process.
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Designation: D 6274 – 98
Standard Guide for
Conducting Borehole Geophysical Logging - Gamma
This standard is issued under the fixed designation D 6274; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 1.9 This guide does not purport to address all of the safety
and liability problems (for example, lost or lodged probes and
1.1 This guide covers the general procedures necessary to
equipment decontamination) associated with its use.
conduct gamma, natural gamma, total count gamma, or gamma
1.10 This standard does not purport to address all of the
ray (hereafter referred to as gamma) logging of boreholes,
safety concerns, if any, associated with its use. It is the
wells, access tubes, caissons, or shafts (hereafter referred to as
responsibility of the user of this standard to establish appro-
boreholes) as commonly applied to geologic, engineering,
priate safety and health practices and determine the applica-
ground-water and environmental (hereafter referred to as
bility of regulatory limitations prior to use.
geotechnical) investigations. Spectral gamma and logging
1.11 This guide offers an organized collection of informa-
where gamma measurements are made in conjunction with a
tion or a series of options and does not recommend a specific
nuclear source are excluded (for example, neutron activation
course of action. This document cannot replace education or
and gamma-gamma density logs). Gamma logging for minerals
experience and should be used in conjunction with professional
or petroleum applications are excluded.
judgment. Not all aspects of this guide may be applicable in all
1.2 This guide defines a gamma log as a record of gamma
circumstances. This ASTM standard is not intended to repre-
activity of the formation adjacent to a borehole with depth (See
sent or replace the standard of care by which the adequacy of
Fig. 1).
a given professional service must be judged, nor should this
1.2.1 Gamma logs are commonly used to: delineate lithol-
document be applied without consideration of a project’s many
ogy, correlate measurements made on different logging runs,
unique aspects. The word “Standard” in the title of this
and define stratigraphic correlation between boreholes (See
document means only that the document has been approved
Fig. 2).
through the ASTM consensus process.
1.3 This guide is restricted to gamma logging with nuclear
counters consisting of scintillation detectors (crystals coupled
2. Referenced Documents
with photomultiplier tubes), which are the most common
2.1 ASTM Standards:
gamma measurement devices used in geotechnical applica-
D 653 Terminology Relating to Soil, Rock and Contained
tions.
Fluids
1.4 This guide provides an overview of gamma logging
D 5088 Practice for Decontamination of Field Equipment at
including: general procedures; specific documentation; calibra-
Non-Radioactive Waste Sites
tion and standardization, and log quality and interpretation.
D 5608 Practice for Decontamination of Field Equipment
1.5 To obtain additional information on gamma logs see
Used at Low Level Radioactive Waste Sites
Section 13.
D 5753 Guide for Planning and Conducting Borehole Geo-
1.6 This guide is to be used in conjunction with Guide
physical Logging
D 5753.
D 6167 Guide for Conducting Borehole Geophysical Log-
1.7 Gamma logs should be collected by an operator that is
ging - Mechanical Caliper
trained in geophysical logging procedures. Gamma logs should
be interpreted by a professional experienced in log analysis.
3. Terminology
1.8 The geotechnical industry uses English or SI units. The
3.1 Definitions:
gamma log is typically recorded in units of counts per second
3.1.1 Definitions shall be in accordance with Terms and
(cps) or American Petroleum Institute (API) units.
Symbols D 653 Section 13 Ref (1), or as defined below.
3.2 Definitions of Terms Specific to This Standard:
This guide is under the jurisdiction of ASTM Committee D-18 on and is the
direct responsibility of Subcommittee D8.01 on Site Characterization for Environ-
mental Purposes. Annual Book of ASTM Standards, Vol 04.08.
Current edition approved May 10, 1998. Published January 1999. Annual Book of ASTM Standards, Vol 04.09.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6274–98
NOTE 1—This figure demonstrates, how the log can be used to identify specific formations, illustrating scale wrap-around for a local gamma peak, and
showing how the contact between two formations is picked to coincide with the half-way point of the transition between the gamma activities of the two
formations.
FIG. 1 Example of a Gamma Log From Near the South Rim of the Grand Canyon
3.2.1 accuracy, n—how close measured log values ap- response may be considered centered, which is not to be
proach true value. It is determined in a controlled environment. confused with borehole depth (for example, distance) mea-
A controlled environment represents a homogeneous sample sured from the surface.
volume with known properties. 3.2.5 measurement resolution, n—the minimum change in
3.2.2 dead time, n—the time after each pulse when a second measured value that can be detected.
pulse cannot be detected. 3.2.6 repeatability, n—the difference in magnitude of two
3.2.3 dead time effect, n—the inability to distinguish measurements with the same equipment and in the same
closely-spaced nuclear counts leads to a significant underesti- environment.
mation of gamma activity in high radiation environments and 3.2.7 vertical resolution, n—the minimum thickness that
is known as the “dead time effect”. can be separated into distinct units.
3.2.4 depth of investigation, n—the radial distance from the 3.2.8 volume of investigation, n—the volume that contrib-
measurement point to a point where the predominant measured utes 90 percent of the measured response. It is determined by
D6274–98
NOTE 1—From a study site showing how the gamma logs can be used to identify where beds intersect each of the individual boreholes, demonstrating
lateral continuity of the subsurface geology.
FIG. 2 Example of Gamma Logs From Two Boreholes
a combination of theoretical and empirical modeling. The 5. Significance and Use
volume of investigation is non-spherical and has gradational
5.1 An appropriately developed, documented, and executed
boundaries.
guide is essential for the proper collection and application of
gamma logs. This guide is to be used in conjunction with Guide
4. Summary of Guide
D 5753.
4.1 This guide applies to borehole gamma logging and is to
5.2 The benefits of its use include improving selection of
be used in conjunction with Guide D 5753.
gamma logging methods and equipment; gamma log quality
4.2 This guide briefly describes the significance and use,
and reliability; usefulness of the gamma log data for subse-
apparatus, calibration and standardization, procedures, and
reports for conducting borehole gamma logging. quent display and interpretation.
D6274–98
5.3 This guide applies to commonly used gamma logging to be averaged over a time interval such that the natural
methods for geotechnical applications. statistical variation in the rate of gamma photon emission is
5.4 It is essential that personnel (see 8.3.2 of Guide D 5753)
negligible (see Fig. 3).
consult up-to-date textbooks and reports on the gamma tech-
6.3 Instrument problems include electrical leakage of cable
nique, application, and interpretation methods.
and grounding problems; degradation of detector efficiency
attributed to loss of crystal transparency (fogging) or fractures
6. Interferences
or breaks in the crystal; and mechanical damage causing
6.1 Most extraneous effects on gamma logs are caused by
separation of crystal and photomultiplier tube.
logging too fast, instrument problems, borehole conditions, and
6.4 Borehole conditions include changes in borehole diam-
geologic conditions.
eter (especially in the fluid-filled portion); casing type and
6.2 Logging too fast can significantly degrade the quality of
number; radioactive elements in drilling fluid in the borehole,
gamma logs. Gamma counts originating at a given depth need
NOTE 1—The fluctuations in gamma activity in courts per second is shown to vary by progressively smaller amounts as the averaging period (time
constant) is increased from 1 to 20 s.
FIG. 3 Example of Natural Statistical Fluctuation of Gamma Counts From a Test Source of Given Strength
D6274–98
or in cement or slurry behind casing; and steel casing or cement 8.2 Calibration is the process of establishing values for
in the annulus around casing, and thickness of the annulus. gamma response associated with specific levels of radioisotope
6.5 Geologic conditions include high levels of radiation concentration in the sampled volume and is accomplished with
which can degrade the efficiency of gamma counting through a representative physical model. Calibration data values related
the dead time effect, energy level of emitted gammas, forma- to the physical properties (for example, radioisotope concen-
tion density, and lithologic bed geometry. tration) may be recorded in units (for example, cps), that can be
converted to units of radioactive element concentration (for
7. Apparatus
example, ppm Radium-226 or percent Uranium-238 equiva-
7.1 A geophysical logging system has been described in the
lents).
general guide (Section 6 of Guide D 5753).
8.2.1 Calibration is performed by recording gamma log
7.2 Gamma logs are collected with probes using scintilla-
response in cps in boreholes centered within volumes contain-
tion detectors.
ing known homogenous concentrations of radioactivity ele-
7.2.1 The most common gamma detectors are sodium io-
ments.
dide (NaI).
8.2.2 Calibration volumes should be designed to contain
7.2.2 Other gamma detectors include cesium iodide (CsI)
material as close as possible to that in the environment where
and bismuth germanate (BGO).
the logs are to be obtained to allow for effects such as gamma
7.3 Gamma probes generate nuclear counts as pulses of
energy level, formation density, and activity of daughter
voltage that are amplified and clipped to a uniform amplitude.
isotopes on the calibration process.
7.3.1 Gamma probes used for geotechnical applications
8.3 Standardization is the process of checking logging
typically can be logged inside of a 2 in. (5 cm) diameter
response to show evidence of repeatability and consistency,
monitoring well.
and to insure that logging probes with different detector
7.4 The volume of investigation and depth of investigation
efficiencies measure the same amount of gamma activity in the
are determined by the density of the material near the probe,
same formation. The response in cps of every gamma detector
which controls the average distance a gamma photon can travel
is different for the same radioactive environment.
before being absorbed.
8.3.1 Calibration ensures standardization.
7.4.1 The volume of investigation for gamma logs is gen-
8.3.2 The American Petroleum Institute maintains a bore-
erally considered spherical with a radius of 0.5 to 1.0 ft (15 to
hole in Houston, Texas, where two formations have been
30 cm) from the center of the detector in typical geological
fabricated to provide homogeneous levels of gamma activity so
formations. The volume becomes elongated when detector
that probes can be standardized on the basis of the response in
length exceeds approximately 0.5 ft (15 cm).
th
these boreholes. 1 API gamma unit is 1/200 of the full scale
7.4.2 The depth of investigation for gamma logs is generally
response in the representative shale model in this borehole (see
considered to be 0.5 to 1.0 ft (15 to 30 cm).
Guide D 5753).
7.5 Vertical resolution of gamma logs is determined by the
8.3.3 For geotechnical applications, gamma logs should be
size of the volume from which gammas can reach a nuclear
presented in API units for standardization.
detector suspended in the borehole. In typical geological
8.3.4 A representative borehole may be used to periodically
formations surrounding a fluid-filled borehole, this is a roughly
check gamma probe response providing the borehole and
spherical volume about 1 to 2 ft (30 to 60 cm) in diameter.
surrounding environment does not change with time or their
Excessive logging speed can decrease vertical resolution.
effects on gamma response can be documented.
7.6 Measurement resolution of gamma probes is determined
8.3.5 A small radioactive source(s) (thorium-treated lantern
by the counting efficiency of the nuclear detector being used in
mantles, small bottles of potassium chloride, laboratory radio-
the probe. Typical measurement resolution is 1 cps.
active test sources, or sleeves containing natural radioisotopes
7.7 A variety of gamma logging equipment is available for
(phosphate sands, etc.) placed over the gamma detector can be
geotechnical investigations. It is not practical to list all of the
used to check calibration if the sources have been related to a
sources of potentially acceptable equipment.
calibration facility.
8. Calibration and Standardization of Gamma Logs
8.4 Gamma log output needs to be corrected for dead time
when logging in formations with unusually large count rates,
8.1 General:
such as uranium rich pegmatites or phosphatic sands, and areas
8.1.1 National Institute of Standards and Technology
contaminated with radioactive waste.
(NIST) calibration and standardization procedures do not exist
8.4.1 Dead time corrections are usually negligible under
for gamma logging.
typical logging conditions when measured gamma counts are
8.1.2 Gamma logs can be used in a qualitative (for example,
less than a few hundred counts per second.
comparative) or quantitative (for example, estimating radioiso-
tope concentration) manner depending upon the project objec- 8.4.2 Dead time corrections are estimated by comparing the
tives. gamma log response under the influence of two similar
8.1.3 Gamma calibration and standardization methods and radioactive sources. The measured count rate would approxi-
frequency shall be sufficient to meet project objectives. mately double over that with one source when both sources are
8.1.3.1 Calibration and standardization should be performed placed in the sample volume of the logging tool. The dead time
each time a gamma probe is suspected to be damaged, cau
...
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