ASTM D6727/D6727M-16

Standard Guide for Conducting Borehole Geophysical Logging—Neutron

SIGNIFICANCE AND USE
5.1 An appropriately developed, documented, and executed guide is essential for the proper collection and application of neutron logs.  
5.2 The benefits of its use include improving selection of neutron logging methods and equipment; neutron log quality and reliability; usefulness of the neutron log data for subsequent display and interpretation.  
5.3 This guide applies to commonly used neutron logging methods for geotechnical applications.
SCOPE
1.1 This guide is focused on the general procedures necessary to conduct neutron or neutron porosity (hereafter referred to as neutron) logging of boreholes, wells, access tubes, caissons, or shafts (hereafter referred to as boreholes) as commonly applied to geologic, engineering, groundwater and environmental (hereafter referred to as geotechnical) explorations. Neutron soil moisture measurements made using neutron moisture gauges, are excluded. Neutron logging for minerals or petroleum applications is excluded, along with neutron activation logs where gamma spectral detectors are used to characterize the induced gamma activity of minerals exposed to neutron radiation.  
1.2 This guide defines a neutron log as a record of the rate at which thermal and epithermal neutrons are scattered back to one or more detectors located on a probe adjacent to a neutron source.  
1.2.1 Induction logs are treated quantitatively and should be interpreted with other logs and data whenever possible.  
1.2.2 Neutron logs are commonly used to: (1) delineate lithology, and (2) indicate the water-filled porosity of formations (see Fig. 1).   Nc  =  corrected neutron log,   N0  =  original neutron log,   Csh  =  computed shale fraction based upon the gamma log position between the endpoints of 10 and 120 cps, and   Φsh  =  estimate of shale non-effective porosity of about 40 % picked from intervals on the log where Φsh = 1.0.    
1.3 This guide is restricted to neutron logging with nuclear counters consisting of scintillation detectors (crystals coupled with photomultiplier tubes), or to He3-tube detectors with or without Cd foil covers or coatings to exclude thermalized neutrons.  
1.4 This guide provides an overview of neutron logging including: (1) general procedures; (2) specific documentation; (3 ) calibration and standardization, and (4) log quality and interpretation.  
1.5 To obtain additional information on neutron logs see References section in this guide.  
1.6 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This guide should not be used as a sole criterion for neutron logging and does not replace education, experience, and professional judgment. Neutron logging procedures should be adapted to meet the needs of a range of applications and stated in general terms so that flexibility or innovation are not suppressed. 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 without consideration of a project's many unique aspects. The word standard in the title of this document means that the document has been approved through the ASTM consensus process.  
1.7 Units—The values stated in either inch-pound units or SI units given in brackets are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be use independently of the other. Combining values from the two systems may result in non-conformance with the standard. Add if appropriate: “Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard.”  
1.8 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...

General Information

• Status: Published
• Publication Date: 31-Dec-2015
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.01 - Surface and Subsurface Investigation

Relations

• Replaces: ASTM D6727-01(2007) - Standard Guide for Conducting Borehole Geophysical Logging<char: emdash>Neutron
• Effective Date: 01-Jan-2016
• Refers: ASTM D5088-20 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-May-2020
• Refers: ASTM D5753-18 - Standard Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
• Effective Date: 01-Feb-2018
• Refers: ASTM D5608-16 - Standard Practices for Decontamination of Sampling and Non Sample Contacting Equipment Used at Low Level Radioactive Waste Sites
• Effective Date: 01-Feb-2016
• Refers: ASTM D5088-15a - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-Aug-2015
• Refers: ASTM D5088-15 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 15-Jan-2015
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM D5753-05(2010) - Standard Guide for Planning and Conducting Borehole Geophysical Logging
• Effective Date: 01-May-2010
• Refers: ASTM D5608-10 - Standard Practices for Decontamination of Field Equipment Used at Low Level Radioactive Waste Sites
• Effective Date: 01-May-2010
• Refers: ASTM D653-09 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Jan-2009
• Refers: ASTM D653-08a - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Dec-2008
• Refers: ASTM D653-08 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Nov-2008
• Refers: ASTM D5088-02(2008) - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 15-Sep-2008
• Refers: ASTM D653-07f - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 15-Dec-2007

Overview

ASTM D6727/D6727M-16 is the standard guide for conducting borehole geophysical logging using neutron methods for geotechnical applications. Developed by ASTM International, this guide outlines best practices for planning, executing, calibrating, and interpreting neutron logs in boreholes, wells, access tubes, caissons, or shafts. The standard is targeted at geoscientists, engineers, and environmental professionals involved in subsurface site characterization, groundwater investigations, and geotechnical engineering projects. It enhances data quality, reliability, and project outcomes through standardized procedures, ensuring clearer interpretation and better decision-making in a variety of subsurface investigations.

Key Topics

• Scope and Significance

  • Focuses exclusively on neutron or neutron porosity logging for geotechnical, engineering, environmental, and groundwater applications.
  • Excludes neutron soil moisture measurements and logging for mineral or petroleum exploration.

• Neutron Logging Fundamentals

  • Defines a neutron log as a record of thermal and epithermal neutron rates, captured by detectors on a probe adjacent to a neutron source.
  • Neutron logging equipment typically utilizes scintillation detectors or He3-tube detectors, sometimes with cadmium shielding.

• Calibration and Standardization

  • Emphasizes the importance of equipment calibration for accurate measurement of porosity and data comparability.
  • Offers guidance on establishing correlation between neutron counts and material properties like water-filled porosity.

• Procedure and Quality Assurance

  • Details steps for planning neutron logging programs, equipment selection, data collection, and logging speed.
  • Discusses critical documentation points: depth referencing, calibration, probe operation verification, and repeat logging for data validation.

• Interpretation and Reporting

  • Underlines the importance of integrating neutron logs with other geophysical and geological data.
  • Describes methods for correcting logs for clay content, borehole conditions, and lithologic variation.
  • Reporting requirements include equipment methodologies, calibration, performance verification, and software documentation.

Applications

• Geotechnical Site Characterization

  • Identifies subsurface lithology and assists in determining zones of water-filled porosity.
  • Provides data for evaluating the suitability of sites for engineering, construction, and infrastructure development.

• Groundwater Investigations

  • Supports hydrogeologists and environmental scientists in mapping aquifers, aquitards, and monitoring groundwater resources.
  • Facilitates the design of well completions, placement of screens, and assessment of formation permeability.

• Environmental Assessment

  • Plays a critical role in the characterization of subsurface conditions at contaminated sites, aiding in remediation planning.
  • Used to spot variations in formation saturation, assisting in understanding groundwater contamination pathways.

• Well Construction and Monitoring

  • Informs the installation of well screens, seals, and other completion materials.
  • Enables long-term monitoring of formation changes by comparing repeated neutron logs over time.

Related Standards

• ASTM D5753 - Planning and Conducting Borehole Geophysical Logging (companion to D6727/D6727M-16)
• ASTM D6274 - Guide for Conducting Borehole Geophysical Logging: Gamma
• ASTM D6167 - Guide for Mechanical Caliper Logging
• ASTM D5088 / D5608 - Practices for Decontamination of Field Equipment
• ASTM D653 - Terminology Relating to Soil, Rock, and Contained Fluids

Other referenced guides (some now withdrawn) such as ASTM D420 and D5730 provide complementary information on site characterization for engineering and environmental purposes.

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Keywords: borehole geophysics, neutron log, neutron porosity log, geotechnical logging, subsurface site characterization, well logging, ASTM D6727, calibration, groundwater investigation.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6727/D6727M − 16
Standard Guide for
Conducting Borehole Geophysical Logging—Neutron
This standard is issued under the fixed designation D6727/D6727M; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope* of action. This guide should not be used as a sole criterion for
neutron logging and does not replace education, experience,
1.1 This guide is focused on the general procedures neces-
and professional judgment. Neutron logging procedures should
sary to conduct neutron or neutron porosity (hereafter referred
be adapted to meet the needs of a range of applications and
to as neutron) logging of boreholes, wells, access tubes,
stated in general terms so that flexibility or innovation are not
caissons, or shafts (hereafter referred to as boreholes) as
suppressed. Not all aspects of this guide may be applicable in
commonly applied to geologic, engineering, groundwater and
all circumstances. This ASTM standard is not intended to
environmental (hereafter referred to as geotechnical) explora-
representorreplacethestandardofcarebywhichtheadequacy
tions. Neutron soil moisture measurements made using neutron
of a given professional service must be judged without
moisturegauges,areexcluded.Neutronloggingformineralsor
consideration of a project’s many unique aspects. The word
petroleum applications is excluded, along with neutron activa-
standard in the title of this document means that the document
tion logs where gamma spectral detectors are used to charac-
has been approved through the ASTM consensus process.
terize the induced gamma activity of minerals exposed to
neutron radiation. 1.7 Units—The values stated in either inch-pound units or
SI units given in brackets are to be regarded separately as
1.2 This guide defines a neutron log as a record of the rate
standard. The values stated in each system may not be exact
at which thermal and epithermal neutrons are scattered back to
equivalents; therefore, each system shall be use independently
one or more detectors located on a probe adjacent to a neutron
of the other. Combining values from the two systems may
source.
result in non-conformance with the standard. Add if appropri-
1.2.1 Induction logs are treated quantitatively and should be
ate: “Reporting of test results in units other than SI shall not be
interpreted with other logs and data whenever possible.
regarded as nonconformance with this standard.”
1.2.2 Neutron logs are commonly used to: (1) delineate
1.8 This standard does not purport to address all of the
lithology, and (2) indicate the water-filled porosity of forma-
safety concerns, if any, associated with its use. It is the
tions (see Fig. 1).
responsibility of the user of this standard to establish appro-
1.3 This guide is restricted to neutron logging with nuclear
priate safety and health practices and determine the applica-
counters consisting of scintillation detectors (crystals coupled
bility of regulatory requirements prior to use. The use of
with photomultiplier tubes), or to He -tube detectors with or
radioactive sources in neutron logging introduces significant
without Cd foil covers or coatings to exclude thermalized
safety issues related to the transportation and handling of
neutrons.
neutron sources, and in procedures to ensure that sources are
1.4 This guide provides an overview of neutron logging
not lost or damaged during logging. There are different
including: (1) general procedures; (2) specific documentation;
restrictions on the use of radioactive sources in logging in
(3 ) calibration and standardization, and (4) log quality and
different states, and the Nuclear Regulatory Agency (NRC)
interpretation.
maintains strict rules and regulations for the licensing of
personnel authorized to conduct nuclear source logging.
1.5 To obtain additional information on neutron logs see
References section in this guide.
2. Referenced Documents
1.6 This guide offers an organized collection of information
2.1 ASTM Standards:
oraseriesofoptionsanddoesnotrecommendaspecificcourse
D653 Terminology Relating to Soil, Rock, and Contained
Fluids
This guide is under the jurisdiction ofASTM Committee D18 on Soil and Rock
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Jan. 1, 2016. Published January 2016. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2001. Last previous edition approved in 2007 as D6727 – 01(2007). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D6727_D6727M-16. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6727/D6727M − 16
A–Single detector epithermal neutron log plotted in counts per second.
B–Dual-detector neutron log calibrated in limestone porosity units.
C–Gamma log showing maximum and minimum values used as endpoints for the gamma activity scale.
D–Dual detector neutron log plotted in porosity units corrected for the non-effective porosity of clay minerals using the equation:
N 5 N 2 C ·Φ
c 0 sh sh
where:
N = corrected neutron log,
c
N = original neutron log,
C = computed shale fraction based upon the gamma log position between the endpoints of 10 and 120 cps, and
sh
Φ = estimate of shale non-effective porosity of about 40 % picked from intervals on the log where Φ = 1.0.
sh sh
FIG. 1 Typical Neutron Logs for a Sedimentary Rock Environment
D5088 Practice for Decontamination of Field Equipment 3.2.5 thermalized neutron, n—neutron that has been slowed
Used at Waste Sites to a kinetic energy approximately equal to that of the thermal
D5608 Practices for Decontamination of Sampling and Non kinetic energy of the surrounding formation.
Sample Contacting Equipment Used at Low Level Radio-
3.2.6 volume of exploration, n—in geophysics, the volume,
active Waste Sites
which is non-spherical and has gradation boundaries, that
D5753 Guide for Planning and Conducting Borehole Geo-
contributes 90 percent of the measured response and it deter-
physical Logging
mined by a combination of theoretical and empirical modeling.
3. Terminology
4. Summary of Guide
3.1 Definitions:
4.1 This guide applies to borehole neutron logging.
3.1.1 For definitions of common technical terms in this
4.2 This guide briefly describes the significance and use,
standard, refer to Terminology D653.
apparatus, calibration and standardization, procedures, and
3.2 Definitions of Terms Specific to This Standard:
reports for conducting borehole neutron logging.
3.2.1 depth of exploration, n—in geophysics, the radial
distance from the measurement point to a point where the
5. Significance and Use
predominant measured response may be considered centered
5.1 An appropriately developed, documented, and executed
(not to be confused with depth below the surface).
guide is essential for the proper collection and application of
3.2.2 epithermal neutron, n—neutron with kinetic energy
neutron logs.
somewhat greater than the kinetic energy associated with
5.2 The benefits of its use include improving selection of
thermal lattice vibrations of the surrounding formation; such
neutron logging methods and equipment; neutron log quality
neutrons have been slowed enough by collisions with forma-
and reliability; usefulness of the neutron log data for subse-
tion minerals to interact with the detector, but the population of
quent display and interpretation.
epithermal neutrons is not strongly affected by absorption
cross-sections of trace minerals in the geologic environment. 5.3 This guide applies to commonly used neutron logging
methods for geotechnical applications.
3.2.3 neutron generator, n—a device which includes a
particle accelerator to generate a flux of high-energy neutrons,
6. Interferences
and which can be turned on and off through connection with an
6.1 Most extraneous effects on neutron logs are caused by
external power supply.
logging too fast, instrument problems, borehole conditions,
3.2.4 neutron slowing distance, n—the distance traveled by
partially saturated formations, and geologic conditions.
a neutron within a formation over the time required for the
neutron to be slowed to half of its original velocity by repeated 6.2 Logging too fast can significantly degrade the quality of
collisions with the atoms in the formation. neutronlogs,especiallywhenneutrondetectorsaredesignedto
D6727/D6727M − 16
excludethermalizedneutrons,resultinginrelativelylowcount- lectedusingdetectorsusinglithium-iodidescintillationcrystals
ing rates. Neutron counts measured at a given depth need to be coupled to photomultiplier tubes (Fig. 3).
averaged over a time interval such that the natural statistical 7.2.1 A neutron shield is needed for the storage of the
variation in the rate of neutron emission is negligible. neutron source during transport to and from the logging site.
7.2.2 A secure storage facility is needed for neutron source
6.3 Instrument problems include electrical leakage of cable
during the time between logging projects when the source
and grounding problems; degradation of detector efficiency
cannot be left in the shield in the logging truck.
attributed to loss of crystal transparency (fogging) or fractures
7.2.3 Radiation monitoring equipment is needed for check-
or breaks in the crystal; and mechanical damage causing
ing of radiation levels outside the neutron shield and in
separation of crystal and photomultiplier tube.
working areas during use of the neutron source to verify that
6.4 Borehole conditions include changes in borehole diam-
radiation hazards do not exist.
eter; borehole wall roughness whenever neutron logs are run
7.3 Neutron logging probes generate neutron fluxes using a
decentralized; and steel casing or cement in the annulus around
chemical radioactive source such as Ca or a combination of
casing, and thickness of the annulus.
Am and Be; or by using a neutron generator.
6.5 Geologic conditions include the presence of clay min-
7.4 Neutron probes generate nuclear counts as pulses of
erals with significant non- effective porosity (Fig. 2), and the
voltage that are amplified and clipped to a uniform amplitude.
presence of minerals such as chlorine with relatively large
7.4.1 Neutron probes used for geotechnical applications can
neutron absorption cross-sections.
be run centralized or decentralized (held against the side of the
6.6 Neutron log response is designed to measure water-
borehole); decentralized probes can be collimated (shielded on
filled pore spaces so that neutron logs do not measure unsatu-
the side away from the borehole wall to reduce the influence of
rated porosity.
the borehole fluid column). However, collimation requires an
impracticably heavy, large-diameter logging probe, and such
7. Apparatus
probes are rarely used in geotechnical logging (Fig. 3C).
7.1 Ageophysical logging system has been described in the
7.4.2 Neutron probes can have a single detector (Fig. 3A),
general guide (Section 6, Standard Guide D5753).
or a pair of detectors located at different separations from the
7.2 Neutron logs are collected with probes using He neutron source. When logging probes contain two detectors,
detectors,whichmaybecoatedwithCdtoexcludethermalized the far detector is significantly larger than the near detector to
neutrons, or may be un-coated to detect both thermal and compensate for the decreasing population of neutrons with
epithermal neutrons; neutron logs may occasionally be col- distance from the neutron source, as indicated in Fig. 3B.
FIG. 2 Comparison of Single Detector Epithermal Neutron Log with Clay Mineral Fraction Determined Form Core Samples for a Bore-
hole in Sedimentary Bedrock (from Keys, 1990)
D6727/D6727M − 16
A–Single detector run without centralization in a fluid-filled borehole.
B–Pair of detectors run with borehole centralization in a fluid-filled borehole.
C–Pair of detectors in a decentralized and collimated probe run in a fluid-filled borehole.
FIG. 3 Schematic Illustration of Neutron Logging Probes
7.4.3 Neutron probes are designed with source-to-receiver 8.1.3 Neutron calibration and standardization methods and
spacing such that measured neutron counts are proportional to
frequency shall be sufficient to meet project objectives.
theslowingdowndistanceoftheneutrons,whichisassumedto
8.1.3.1 Calibrationandstandardizationshouldbeperformed
be inversely proportional to the water-filled porosity of the
each time a neutron probe is suspected to be damaged,
formation.
modified, repaired, and at periodic intervals.
7.5 The Volume of Exploration and Depth of Exploration
8.2 Calibration is the process of establishing values for
are primarily determined by the moisture content of the
neutron response associated with specific values of water-filled
material near the probe which controls the average distance a
porosity in the sampled volume and is accomplished with a
neutron can travel before being absorbed.
representative physical model. Calibration data values related
7.5.1 The Volume of Exploration for neutron logs is gener-
to the physical properties (for example, formation porosity)
ally considered spherical with a radius of 1.5 to 2.5 ft [40 to 70
may be recorded in units (for example, cps), which can be
cm] from the midpoint between the neutron source and
converted to units of percent, water-saturated porosity.
detector(s) in typical geological formations.
8.2.1 Calibration is performed by recording neutron log
7.5.2 The Depth of Exploration for neutron logs is generally
response in cps recorded by one or a pair of detectors in
considered to be 1.5 to 2.5 ft [40 to 70 cm].
boreholes centered within volumes containing a uniform, fully
7.6 Vertical Resolution of neutron logs is determined by the
water-saturated medium of known porosity and mineral com-
size of the volume over which neutrons are scattered back
position.
towards the detector after being emitted by the source. In
8.2.2 Calibration volumes should be designed to contain
typical geological formations surrounding a fluid-filled
material as close as possible to that in the environment where
borehole, this is a roughly spherical volume about 1 to 2 ft [30
the logs are to be obtained to allow for effects such as borehole
to 60 cm] in diameter. Excessive logging speed can decrease
diameter, formation density, and formation chemical composi-
vertical resolution.
tion.
7.7 Measurement Resolution of neutron probes is deter-
8.2.3 Neutron log calibration is especially sensitive to
mined by the counting efficiency of the nuclear detector or
borehole diameter in water-filled boreholes because the neu-
detectors being used in the probe. Typical Measurement
tron flux from the logging probe interacts with water in the
Resolution is 1 cps.
borehole as well as that in pore spaces; therefore, neutron log
7.8 A variety of neutron logging equipment is available for
calibration is only accurate when applied to logs obtained in
geotechnical investigations. It is not practical to list all of the
boreholes of nearly the same diameter as that of the calibration
sources of potentially acceptable equipment.
environment.
8.2.4 Neutron log calibration procedures depend upon
8. Calibration and Standardization of Neutron Logs
whether single-detector or dual detector data are used.
8.1 General:
8.2.4.1 Neutron log calibration fails above the water level
8.1.1 National Institute of Standards and Technology
where neutrons streaming along the air-filled annulus around
(NIST) calibration and standardization procedures do not exist
the probe dominate the measured response of the equipment.
for neutron logging.
8.1.2 Neutronlogscanbeusedinaqualitative(forexample, 8.2.4.2 When counts from a single neutron detector are
comparative) or quantitative (for example, estimating water- used, the measured counts are assumed to be inversely propor-
filled porosity) manner depending upon the project objectives. tional to the logarithm of porosity.
D6727/D6727M − 16
8.2.4.3 When counts from a pair of detectors are used, the is 6 in. [15 cm] in diameter, and the known porosity values are
ratio of the counts from the near (short-spaced) to the far 1.88, 19.23, and 26.63 percent (Fig. 4).
(long-spaced) detector are assumed to be linearly proportional
8.3.3 For geotechnical applications, neutron logs can be
to porosity. presented in either cps or calibrated limestone porosity.
8.2.5 Neutronlogscanalsobecalibratedusingporositydata
8.3.4 Arepresentative borehole may be used to periodically
obtained from core, provided the following conditions are check neutron probe response providing the borehole and
satisfied:
surrounding environment does not change with time or their
8.2.5.1 Depth scales on log and core are adjusted by effects on neutron flux attenuation can be documented.
cross-correlation to ensure there are no depth offsets related to
8.3.5 An approximate neutron log field calibration can be
depth scale errors.
madeusingroughestimatesofthelargestandsmallestporosity
8.2.5.2 The core measurement technique either measures
values for the geological formations sampled by the log. This
total porosity equivalent to that inferred from neutron log
is done by identifying the maximum log value in cps with the
measurements, or calibration is only applied to intervals where
minimum porosity, and the minimum log value in cps with the
effective porosity is expected to equal total porosity (for
maximum porosity, and then extrapolating the porosity scale
example, clay-free sandstone).
between these endpoints using a logarithmic scale (Fig. 5).
8.2.5.3 Enough measurements are used to statistically over-
come the size mismatch between core plugs and the sample
9. Procedure
volume of the neutron log measurement.
9.1 See Section 8, Guide D5753 for planning a logging
8.3 Standardization is the process of checking logging
program, data formats, personnel qualifications, field
response to show evidence of repeatability and consistency,
documentation, and header documentation.
and to ensure that logging probes with different detector
9.1.1 Neutron specific information (source identity, detector
efficiencies measure the same neutron slowing flux attenuation
spacing, etc.) should be documented.
in the same formation. The response in cps of every neutron
9.2 Identify neutron logging objectives.
source/detector pair is different for the same environment.
9.3 Select appropriate equipment to meet objectives.
8.3.1 Calibration ensures standardization.
8.3.2 TheAmerican Petroleum Institute maintains a calibra- 9.3.1 Neutron logs may be run either centralized or free-
tion facility in Houston, Texas, where a borehole has been hanging where the probe lies against one side of the borehole
constructed to provide access to three fully-saturated blocks of (that is, as a mandrel), depending upon project objectives,
quarriedlimestoneofunusuallyconstantporosity;theborehole borehole diameter, and logging probe design.
FIG. 4 Construction Plan of the American Petroleum Institute Neutron Porosity Calibration Pit, Illustrating the Presence of Three Cali-
bration Environments Composed of Cylindrical Sections Quarried from Three Different Natural Geologic Formations (from Keys, 1990)
D6727/D6727M − 16
FIG. 5 Example of Single-Detector Epithermal Neutron Log Presented in Counts Per Second and Given an Approximate Field Calibra-
tion in Porosity Units Using Assumed Endpoints of 2 % for Anhydrite, a Dense, Low Porosity Mineral, and 40 % for a Shale Known
from Previous Experience to Contain Approximately that Amount of Non-Effective Porosity in the Form
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6727 − 01 (Reapproved 2007) D6727/D6727M − 16
Standard Guide for
Conducting Borehole Geophysical Logging—Neutron
This standard is issued under the fixed designation D6727;D6727/D6727M; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This guide is focused on the general procedures necessary to conduct neutron or neutron porosity (hereafter referred to as
neutron) logging of boreholes, wells, access tubes, caissons, or shafts (hereafter referred to as boreholes) as commonly applied to
geologic, engineering, groundwater and environmental (hereafter referred to as geotechnical) investigations.explorations. Neutron
soil moisture measurements made using neutron moisture gauges, are excluded. Neutron logging for minerals or petroleum
applications is excluded, along with neutron activation logs where gamma spectral detectors are used to characterize the induced
gamma activity of minerals exposed to neutron radiation.
1.2 This guide defines a neutron log as a record of the rate at which thermal and epithermal neutrons are scattered back to one
or more detectors located on a probe adjacent to a neutron source.
1.2.1 Induction logs are treated quantitatively and should be interpreted with other logs and data whenever possible.
1.2.2 Neutron logs are commonly used to: (1) delineate lithology, and (2) indicate the water-filled porosity of formations (see
Fig. 1).
1.3 This guide is restricted to neutron logging with nuclear counters consisting of scintillation detectors (crystals coupled with
photomultiplier tubes), or to He -tube detectors with or without Cd foil covers or coatings to exclude thermalized neutrons.
1.4 This guide provides an overview of neutron logging including: (1) general procedures; (2) specific documentation; (3 )
calibration and standardization, and (4) log quality and interpretation.
1.5 To obtain additional information on neutron logs see References section in this guide.
1.6 This guide is to be used in conjunction with Standard Guide D5753.
1.6 This guide offers an organized collection of information or a series of options and does not recommend a specific course
of action. This guide should not be used as a sole criterion for neutron logging and does not replace education, experience, and
professional judgment. Neutron logging procedures should be adapted to meet the needs of a range of applications and stated in
general terms so that flexibility or innovation are not suppressed. 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 without consideration of a project’s many unique aspects. The word standard in the title of
this document means that the document has been approved through the ASTM consensus process.
1.7 Units—The geotechnical industry uses English or SI units. The neutron log is typically recorded in units of counts per
second (cps) or in percent porosity.values stated in either inch-pound units or SI units given in brackets are to be regarded
separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be use
independently of the other. Combining values from the two systems may result in non-conformance with the standard. Add if
appropriate: “Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard.”
1.8 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
requirements prior to use. The use of radioactive sources in neutron logging introduces significant safety issues related to the
transportation and handling of neutron sources, and in procedures to insureensure that sources are not lost or damaged during
logging. There are different restrictions on the use of radioactive sources in logging in different states, and the Nuclear Regulatory
Agency (NRC) maintains strict rules and regulations for the licensing of personnel authorized to conduct nuclear source logging.
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved July 1, 2007Jan. 1, 2016. Published August 2007January 2016. Originally approved in 2001. Last previous edition approved in 20012007 as
D6727 – 01.D6727 – 01(2007). DOI: 10.1520/D6727-01R07.10.1520/D6727_D6727M-16.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6727/D6727M − 16
A–Single detector epithermal neutron log plotted in counts per second.
B–Dual-detector neutron log calibrated in limestone porosity units.
C–Gamma log showing maximum and minimum values used as endpoints for the gamma activity scale.
D–Dual detector neutron log plotted in porosity units corrected for the non-effective porosity of clay minerals using the equation:
N 5 N 2 C ·Φ
c 0 sh sh
where:
N = corrected neutron log,
c
N = original neutron log,
C = computed shale fraction based upon the gamma log position between the endpoints of 10 and 120 cps, and
sh
Φ = estimate of shale non-effective porosity of about 40 % picked from intervals on the log where Φ = 1.0.
sh sh
FIG. 1 Typical Neutron Logs for a Sedimentary Rock Environment
2. Referenced Documents
2.1 ASTM Standards:
D420 Guide to Site Characterization for Engineering Design and Construction Purposes (Withdrawn 2011)
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D5088 Practice for Decontamination of Field Equipment Used at Waste Sites
D5608 Practices for Decontamination of Field Equipment Used at Low Level Radioactive Waste Sites
D5730 Guide for Site Characterization for Environmental Purposes With Emphasis on Soil, Rock, the Vadose Zone and
Groundwater (Withdrawn 2013)
D5753 Guide for Planning and Conducting Borehole Geophysical Logging
D6167 Guide for Conducting Borehole Geophysical Logging: Mechanical Caliper
D6235 Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste
Contaminated Sites
D6274 Guide for Conducting Borehole Geophysical Logging - Gamma
D6429 Guide for Selecting Surface Geophysical Methods
3. Terminology
3.1 Definitions—Definitions: Definitions shall be in accordance with Terminology D653, Section 13 Ref 1, or as defined below.
3.1.1 For definitions of common technical terms in this standard, refer to Terminology D653.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 accuracy, n—how close measured log values approach true value. It is determined in a controlled environment. A
controlled environment represents a homogeneous sample volume with known properties.
3.2.1 depth of investigation,exploration, n—in geophysics, the radial distance from the measurement point to a point where the
predominant measured response may be considered centered, which is not centered (not to be confused with borehole depth (for
example, distance) measured from the surface.depth below the surface).
3.2.3 effective porosity, n—the volume percent of connected pore spaces within a formation that are capable of conducting
groundwater flow.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D6727/D6727M − 16
3.2.2 epithermal neutron, n—neutron with kinetic energy somewhat greater than the kinetic energy associated with thermal
lattice vibrations of the surrounding formation; such neutrons have been slowed enough by collisions with formation minerals to
interact with the detector, but the population of epithermal neutrons is not strongly affected by absorption cross-sections of trace
minerals in the geologic environment.
3.2.5 measurement resolution, n—the minimum change in measured value that can be detected.
3.2.3 neutron generator, n—a device which includes a particle accelerator to generate a flux of high-energy neutrons, and which
can be turned on and off through connection with an external power supply.
3.2.4 neutron slowing distance, n—the distance traveled by a neutron within a formation over the time required for the neutron
to be slowed to half of its original velocity by repeated collisions with the atoms in the formation.
3.2.8 repeatability, n—the difference in magnitude of two measurements with the same equipment and in the same environment.
3.2.5 thermalized neutron, n—neutron that has been slowed to a kinetic energy approximately equal to that of the thermal kinetic
energy of the surrounding formation.
3.2.10 total porosity, n—the total amount of pore space expressed as a volume fraction in percent in a formation; this total
consists of effective pore space which can conduct groundwater flow, and additional unconnected pores that will not conduct
groundwater.
3.2.11 vertical resolution, n—the minimum thickness that can be separated into distinct units.
3.2.6 volume of investigation,exploration, n—in geophysics, the volume volume, which is non-spherical and has gradation
boundaries, that contributes 90 percent of the measured response. It isresponse and it determined by a combination of theoretical
and empirical modeling. The volume of investigation is non-spherical and has gradational boundaries.
4. Summary of Guide
4.1 This guide applies to borehole neutron logging and is to be used in conjunction with Standard Guide logging.D5753.
4.2 This guide briefly describes the significance and use, apparatus, calibration and standardization, procedures, and reports for
conducting borehole neutron logging.
5. Significance and Use
5.1 An appropriately developed, documented, and executed guide is essential for the proper collection and application of
neutron logs. This guide is to be used in conjunction with Standard Guide D5753.
5.2 The benefits of its use include improving selection of neutron logging methods and equipment; neutron log quality and
reliability; usefulness of the neutron log data for subsequent display and interpretation.
5.3 This guide applies to commonly used neutron logging methods for geotechnical applications.
5.4 It is essential that personnel (see Section 8.3.2, Standard Guide D5753) consult up-to-date textbooks and reports on the
neutron technique, application, and interpretation methods.
6. Interferences
6.1 Most extraneous effects on neutron logs are caused by logging too fast, instrument problems, borehole conditions, partially
saturated formations, and geologic conditions.
6.2 Logging too fast can significantly degrade the quality of neutron logs, especially when neutron detectors are designed to
exclude thermalized neutrons, resulting in relatively low counting rates. Neutron counts measured at a given depth need to be
averaged over a time interval such that the natural statistical variation in the rate of neutron emission is negligible.
6.3 Instrument problems include electrical leakage of cable and grounding problems; degradation of detector efficiency
attributed to loss of crystal transparency (fogging) or fractures or breaks in the crystal; and mechanical damage causing separation
of crystal and photomultiplier tube.
6.4 Borehole conditions include changes in borehole diameter; borehole wall roughness whenever neutron logs are run
decentralized; and steel casing or cement in the annulus around casing, and thickness of the annulus.
6.5 Geologic conditions include the presence of clay minerals with significant non- effective porosity (Fig. 2), and the presence
of minerals such as chlorine with relatively large neutron absorption cross-sections.
6.6 Neutron log response is designed to measure water-filled pore spaces so that neutron logs do not measure unsaturated
porosity.
7. Apparatus
7.1 A geophysical logging system has been described in the general guide (Section 6, Standard Guide D5753).
D6727/D6727M − 16
FIG. 2 Comparison of Single Detector Epithermal Neutron Log with Clay Mineral Fraction Determined Form Core Samples for a Bore-
hole in Sedimentary Bedrock (from Keys, 1990)
7.2 Neutron logs are collected with probes using He detectors, which may be coated with Cd to exclude thermalized neutrons,
or may be un-coated to detect both thermal and epithermal neutrons; neutron logs may occasionally be collected using detectors
using lithium-iodide scintillation crystals coupled to photomultiplier tubes (Fig. 3).
7.2.1 A neutron shield is needed for the storage of the neutron source during transport to and from the logging site.
7.2.2 A secure storage facility is needed for neutron source during the time between logging projects when the source cannot
be left in the shield in the logging truck.
7.2.3 Radiation monitoring equipment is needed for checking of radiation levels outside the neutron shield and in working areas
during use of the neutron source to verify that radiation hazards do not exist.
7.3 Neutron logging probes generate neutron fluxes using a chemical radioactive source such as Ca or a combination of Am
and Be; or by using a neutron generator.
7.4 Neutron probes generate nuclear counts as pulses of voltage that are amplified and clipped to a uniform amplitude.
A–Single detector run without centralization in a fluid-filled borehole.
B–Pair of detectors run with borehole centralization in a fluid-filled borehole.
C–Pair of detectors in a decentralized and collimated probe run in a fluid-filled borehole.
FIG. 3 Schematic Illustration of Neutron Logging Probes
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7.4.1 Neutron probes used for geotechnical applications can be run centralized or decentralized (held against the side of the
borehole); decentralized probes can be collimated (shielded on the side away from the borehole wall to reduce the influence of the
borehole fluid column). However, collimation requires an impracticably heavy, large-diameter logging probe, and such probes are
rarely used in geotechnical logging (Fig. 3C).
7.4.2 Neutron probes can have a single detector (Fig. 3A), or a pair of detectors located at different separations from the neutron
source. When logging probes contain two detectors, the far detector is significantly larger than the near detector to compensate for
the decreasing population of neutrons with distance from the neutron source, as indicated in Fig. 3B.
7.4.3 Neutron probes are designed with source-to-receiver spacing such that measured neutron counts are proportional to the
slowing down distance of the neutrons, which is assumed to be inversely proportional to the water-filled porosity of the formation.
7.5 The Volume of InvestigationExploration and Depth of InvestigationExploration are primarily determined by the moisture
content of the material near the probe which controls the average distance a neutron can travel before being absorbed.
7.5.1 The Volume of InvestigationExploration for neutron logs is generally considered spherical with a radius of 1.5 to 2.5 ft
(40[40 to 70 cm)cm] from the midpoint between the neutron source and detector(s) in typical geological formations.
7.5.2 The Depth of InvestigationExploration for neutron logs is generally considered to be 1.5 to 2.5 ft (40[40 to 70 cm).cm].
7.6 Vertical Resolution of neutron logs is determined by the size of the volume over which neutrons are scattered back towards
the detector after being emitted by the source. In typical geological formations surrounding a fluid-filled borehole, this is a roughly
spherical volume about 1 to 2 ft (30[30 to 60 cm)cm] in diameter. Excessive logging speed can decrease vertical resolution.
7.7 Measurement Resolution of neutron probes is determined by the counting efficiency of the nuclear detector or detectors
being used in the probe. Typical Measurement Resolution is 1 cps.
7.8 A variety of neutron logging equipment is available for geotechnical investigations. It is not practical to list all of the sources
of potentially acceptable equipment.
8. Calibration and Standardization of Neutron Logs
8.1 General:
8.1.1 National Institute of Standards and Technology (NIST) calibration and standardization procedures do not exist for neutron
logging.
8.1.2 Neutron logs can be used in a qualitative (for example, comparative) or quantitative (for example, estimating water-filled
porosity) manner depending upon the project objectives.
8.1.3 Neutron calibration and standardization methods and frequency shall be sufficient to meet project objectives.
8.1.3.1 Calibration and standardization should be performed each time a neutron probe is suspected to be damaged, modified,
repaired, and at periodic intervals.
8.2 Calibration is the process of establishing values for neutron response associated with specific values of water-filled porosity
in the sampled volume and is accomplished with a representative physical model. Calibration data values related to the physical
properties (for example, formation porosity) may be recorded in units (for example, cps), which can be converted to units of
percent, water-saturated porosity.
8.2.1 Calibration is performed by recording neutron log response in cps recorded by one or a pair of detectors in boreholes
centered within volumes containing a uniform, fully water-saturated medium of known porosity and mineral composition.
8.2.2 Calibration volumes should be designed to contain material as close as possible to that in the environment where the logs
are to be obtained to allow for effects such as borehole diameter, formation density, and formation chemical composition.
8.2.3 Neutron log calibration is especially sensitive to borehole diameter in water-filled boreholes because the neutron flux from
the logging probe interacts with water in the borehole as well as that in pore spaces; therefore, neutron log calibration is only
accurate when applied to logs obtained in boreholes of nearly the same diameter as that of the calibration environment.
8.2.4 Neutron log calibration procedures depend upon whether single-detector or dual detector data are used.
8.2.4.1 Neutron log calibration fails above the water level where neutrons streaming along the air-filled annulus around the
probe dominate the measured response of the equipment.
8.2.4.2 When counts from a single neutron detector are used, the measured counts are assumed to be inversely proportional to
the logarithm of porosity.
8.2.4.3 When counts from a pair of detectors are used, the ratio of the counts from the near (short-spaced) to the far
(long-spaced) detector are assumed to be linearly proportional to porosity.
8.2.5 Neutron logs can also be calibrated using porosity data obtained from core, provided the following conditions are satisfied:
8.2.5.1 Depth scales on log and core are adjusted by cross-correlation to insureensure there are no depth offsets related to depth
scale errors.
8.2.5.2 The core measurement technique either measures total porosity equivalent to that inferred from neutron log
measurements, or calibration is only applied to intervals where effective porosity is expected to equal total porosity (for example,
clay-free sandstone).
8.2.5.3 Enough measurements are used to statistically overcome the size mismatch between core plugs and the sample volume
of the neutron log measurement.
D6727/D6727M − 16
8.3 Standardization is the process of checking logging response to show evidence of repeatability and consistency, and to
insureensure that logging probes with different detector efficiencies measure the same neutron slowing flux attenuation in the same
formation. The response in cps of every neutron source/detector pair is different for the same environment.
8.3.1 Calibration insuresensures standardization.
8.3.2 The American Petroleum Institute maintains a calibration facility in Houston, Texas, where a borehole has been
constructed to provide access to three fully-saturated blocks of quarried limestone of unusually constant porosity; the borehole is
6 in. (15 cm)[15 cm] in diameter, and the known porosity values are 1.88, 19.23, and 26.63 percent (Fig. 4).
8.3.3 For geotechnical applications, neutron logs can be presented in either cps or calibrated limestone porosity.
8.3.4 A representative borehole may be used to periodically check neutron probe response providing the borehole and
surrounding environment does not change with time or their effects on neutron flux attenuation can be documented.
8.3.5 An approximate neutron log field calibration can be made using rough estimates of the largest and smallest porosity values
for the geological formations sampled by the log. This is done by identifying the maximum log value in cps with the minimum
porosity, and the minimum log value in cps with the maximum porosity, and then extrapolating the porosity scale between these
endpoints using a logarithmic scale (Fig. 5).
9. Procedure
9.1 See Section 8, Guide D5753 for planning a logging program, data formats, personnel qualifications, field documentation,
and header documentation.
9.1.1 Neutron specific information (source identity, detector spacing, etc.) should be documented.
9.2 Identify neutron logging objectives.
9.3 Select appropriate equipment to meet objectives.
9.3.1 Neutron logs may be run eit
...