ASTM D6727-01
(Guide)Standard Guide for Conducting Borehole Geophysical Logging-Neutron
Standard Guide for Conducting Borehole Geophysical Logging-Neutron
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, ground-water and environmental (hereafter referred to as geotechnical) investigations. 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 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 is to be used in conjunction with Standard Guide D 5753.
1.7 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.
1.8 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.
1.9 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 insure 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.
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Designation: D 6727 – 01
Standard Guide for
Conducting Borehole Geophysical Logging—Neutron
This standard is issued under the fixed designation D 6727; 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 and professional judgment. Neutron logging procedures should
be adapted to meet the needs of a range of applications and
1.1 This guide is focused on the general procedures neces-
stated in general terms so that flexibility or innovation are not
sary to conduct neutron or neutron porosity (hereafter referred
suppressed. Not all aspects of this guide may be applicable in
to as neutron) logging of boreholes, wells, access tubes,
all circumstances. This ASTM standard is not intended to
caissons, or shafts (hereafter referred to as boreholes) as
representorreplacethestandardofcarebywhichtheadequacy
commonly applied to geologic, engineering, ground-water and
of a given professional service must be judged without
environmental (hereafter referred to as geotechnical) investi-
consideration of a project’s many unique aspects. The word
gations. Neutron soil moisture measurements made using
standard in the title of this document means that the document
neutron moisture gauges, are excluded. Neutron logging for
has been approved through the ASTM consensus process.
minerals or petroleum applications is excluded, along with
1.8 The geotechnical industry uses English or SI units. The
neutron activation logs where gamma spectral detectors are
neutron log is typically recorded in units of counts per second
used to characterize the induced gamma activity of minerals
(cps) or in percent porosity.
exposed to neutron radiation.
1.9 This standard does not purport to address all of the
1.2 This guide defines a neutron log as a record of the rate
safety concerns, if any, associated with its use. It is the
at which thermal and epithermal neutrons are scattered back to
responsibility of the user of this standard to establish appro-
one or more detectors located on a probe adjacent to a neutron
priate safety and health practices and determine the applica-
source.
bility of regulatory requirements prior to use. The use of
1.2.1 Induction logs are treated quantitatively and should be
radioactive sources in neutron logging introduces significant
interpreted with other logs and data whenever possible.
safety issues related to the transportation and handling of
1.2.2 Neutron logs are commonly used to: (1) delineate
neutron sources, and in procedures to insure that sources are
lithology, and (2) indicate the water-filled porosity of forma-
not lost or damaged during logging. There are different
tions (see Fig. 1).
restrictions on the use of radioactive sources in logging in
1.3 This guide is restricted to neutron logging with nuclear
different states, and the Nuclear Regulatory Agency (NRC)
counters consisting of scintillation detectors (crystals coupled
maintains strict rules and regulations for the licensing of
with photomultiplier tubes), or to He -tube detectors with or
personnel authorized to conduct nuclear source logging.
without Cd foil covers or coatings to exclude thermalized
neutrons.
2. Referenced Documents
1.4 This guide provides an overview of neutron logging
2.1 ASTM Standards:
including: (1) general procedures; (2) specific documentation;
D 420 Guide to Site Characterization for Engineering, De-
(3) calibration and standardization, and (4) log quality and
sign, and Construction Purposes
interpretation.
D 653 Terminology Relating to Soil, Rock and Contained
1.5 To obtain additional information on neutron logs see
Fluids
References section in this guide.
D 5088 Practice for Decontamination of Field Equipment at
1.6 This guide is to be used in conjunction with Standard
Non-Radioactive Waste Sites
Guide D 5753.
D 5608 Practice for Decontamination of Field Equipment
1.7 This guide offers an organized collection of information
Used at Low Level Radioactive Waste Sites
oraseriesofoptionsanddoesnotrecommendaspecificcourse
D 5730 Guide for Site Characterization for Environmental
of action. This guide should not be used as a sole criterion for
Purposes with Emphasis on Soil, Rock, Vadose Zone, and
neutron logging and does not replace education, experience,
Ground Water
D 5753 Guide for Planning and Conducting Borehole Geo-
physical Logging
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved Nov. 10, 2001. Published January 2002.
Annual Book of ASTM Standards, Vol 04.08.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6727–01
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 – C · F
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
F = estimate of shale non-effective porosity of about 40 % picked from intervals on the log where F = 1.0.
sh sh
FIG. 1 Typical Neutron Logs for a Sedimentary Rock Environment
D 6167 Guide for Conducting Borehole Geophysical Log- tionmineralstointeractwiththedetector,butthepopulationof
ging: Mechanical Caliper epithermal neutrons is not strongly affected by absorption
D 6235 Practice for Expediated Site Characterization of cross-sections of trace minerals in the geologic environment.
Vadose Zone and GroundWater Contamination at Hazard-
3.2.5 measurement resolution, n—the minimum change in
ous Waste Contaminated Sites
measured value that can be detected.
D 6274 Guide for Conducting Borehole Geophysical
3.2.6 neutron generator, n—a device which includes a
Logging—Gamma
particle accelerator to generate a flux of high-energy neutrons,
D 6429 Guide for Selecting Surface Geophysical Methods
andwhichcanbeturnedonandoffthroughconnectionwithan
external power supply.
3. Terminology
3.2.7 neutron slowing distance, n—the distance traveled by
3.1 Definitions—Definitions shall be in accordance with
a neutron within a formation over the time required for the
Terminology D 653, Section 13 Ref 1, or as defined below.
neutron to be slowed to half of its original velocity by repeated
3.2 Definitions of Terms Specific to This Standard:
collisions with the atoms in the formation.
3.2.1 accuracy, n—how close measured log values ap-
3.2.8 repeatability, n—the difference in magnitude of two
proachtruevalue.Itisdeterminedinacontrolledenvironment.
measurements with the same equipment and in the same
A controlled environment represents a homogeneous sample
environment.
volume with known properties.
3.2.9 thermalized neutron, n—neutron that has been slowed
3.2.2 depth of investigation, n—the radial distance from the
to a kinetic energy approximately equal to that of the thermal
measurement point to a point where the predominant measured
kinetic energy of the surrounding formation.
response may be considered centered, which is not to be
3.2.10 total porosity, n—the total amount of pore space
confused with borehole depth (for example, distance) mea-
expressed as a volume fraction in percent in a formation; this
sured from the surface.
total consists of effective pore space which can conduct
3.2.3 effective porosity, n—thevolumepercentofconnected
ground-water flow, and additional unconnected pores that will
pore spaces within a formation that are capable of conducting
not conduct ground-water.
ground-water flow.
3.2.11 vertical resolution, n—the minimum thickness that
3.2.4 epithermal neutron, n—neutron with kinetic energy
can be separated into distinct units.
somewhat greater than the kinetic energy associated with
3.2.12 volume of investigation, n—the volume that contrib-
thermal lattice vibrations of the surrounding formation; such
utes 90 percent of the measured response. It is determined by
neutrons have been slowed enough by collisions with forma-
a combination of theoretical and empirical modeling. The
volume of investigation is non-spherical and has gradational
Annual Book of ASTM Standards, Vol 04.09. boundaries.
D6727–01
4. Summary of Guide 6.3 Instrument problems include electrical leakage of cable
and grounding problems; degradation of detector efficiency
4.1 This guide applies to borehole neutron logging and is to
attributed to loss of crystal transparency (fogging) or fractures
be used in conjunction with Standard Guide D 5753.
or breaks in the crystal; and mechanical damage causing
4.2 This guide briefly describes the significance and use,
separation of crystal and photomultiplier tube.
apparatus, calibration and standardization, procedures, and
6.4 Borehole conditions include changes in borehole diam-
reports for conducting borehole neutron logging.
eter; borehole wall roughness whenever neutron logs are run
5. Significance and Use
decentralized;andsteelcasingorcementintheannulusaround
casing, and thickness of the annulus.
5.1 An appropriately developed, documented, and executed
6.5 Geologic conditions include the presence of clay min-
guide is essential for the proper collection and application of
neutron logs. This guide is to be used in conjunction with erals with significant non- effective porosity (Fig. 2), and the
presence of minerals such as chlorine with relatively large
Standard Guide D 5753.
5.2 The benefits of its use include improving selection of neutron absorption cross-sections.
neutron logging methods and equipment; neutron log quality
6.6 Neutron log response is designed to measure water-
and reliability; usefulness of the neutron log data for subse-
filled pore spaces so that neutron logs do not measure unsat-
quent display and interpretation.
urated porosity.
5.3 This guide applies to commonly used neutron logging
methods for geotechnical applications.
7. Apparatus
5.4 It is essential that personnel (see Section 8.3.2, Standard
7.1 Ageophysical logging system has been described in the
Guide D 5753) consult up-to-date textbooks and reports on the
general guide (Section 6, Standard Guide D 5753).
neutron technique, application, and interpretation methods.
7.2 Neutron logs are collected with probes using He
detectors,whichmaybecoatedwithCdtoexcludethermalized
6. Interferences
neutrons, or may be un-coated to detect both thermal and
6.1 Most extraneous effects on neutron logs are caused by
epithermal neutrons; neutron logs may occasionally be col-
logging too fast, instrument problems, borehole conditions,
lectedusingdetectorsusinglithium-iodidescintillationcrystals
partially saturated formations, and geologic conditions.
coupled to photomultiplier tubes (Fig. 3).
6.2 Logging too fast can significantly degrade the quality of
7.2.1 A neutron shield is needed for the storage of the
neutronlogs,especiallywhenneutrondetectorsaredesignedto
neutron source during transport to and from the logging site.
excludethermalizedneutrons,resultinginrelativelylowcount-
ing rates. Neutron counts measured at a given depth need to be 7.2.2 A secure storage facility is needed for neutron source
averaged over a time interval such that the natural statistical during the time between logging projects when the source
variation in the rate of neutron emission is negligible. cannot be left in the shield in the logging truck.
FIG. 2 Comparison of Single Detector Epithermal Neutron Log with Clay Mineral Fraction Determined Form Core Samples for a
Borehole in Sedimentary Bedrock (from Keys, 1990)
D6727–01
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
7.2.3 Radiation monitoring equipment is needed for check- towards the detector after being emitted by the source. In
ing of radiation levels outside the neutron shield and in typical geological formations surrounding a fluid-filled bore-
working areas during use of the neutron source to verify that hole, this is a roughly spherical volume about 1 to 2 ft (30 to
radiation hazards do not exist. 60 cm) in diameter. Excessive logging speed can decrease
7.3 Neutron logging probes generate neutron fluxes using a vertical resolution.
chemical radioactive source such as Ca or a combination of
7.7 Measurement Resolution of neutron probes is deter-
Am and Be; or by using a neutron generator.
mined by the counting efficiency of the nuclear detector or
7.4 Neutron probes generate nuclear counts as pulses of
detectors being used in the probe. Typical Measurement
voltage that are amplified and clipped to a uniform amplitude.
Resolution is 1 cps.
7.4.1 Neutron probes used for geotechnical applications can
7.8 A variety of neutron logging equipment is available for
be run centralized or decentralized (held against the side of the
geotechnical investigations. It is not practical to list all of the
borehole); decentralized probes can be collimated (shielded on
sources of potentially acceptable equipment.
the side away from the borehole wall to reduce the influence of
the borehole fluid column). However, collimation requires an
8. Calibration and Standardization of Neutron Logs
impracticably heavy, large-diameter logging probe, and such
8.1 General:
probes are rarely used in geotechnical logging (Fig. 3C).
8.1.1 National Institute of Standards and Technology
7.4.2 Neutron probes can have a single detector (Fig. 3A),
(NIST) calibration and standardization procedures do not exist
or a pair of detectors located at different separations from the
for neutron logging.
neutron source. When logging probes contain two detectors,
8.1.2 Neutronlogscanbeusedinaqualitative(forexample,
the far detector is significantly larger than the near detector to
comparative) or quantitative (for example, estimating water-
compensate for the decreasing population of neutrons with
filled porosity) manner depending upon the project objectives.
distance from the neutron source, as indicated in Fig. 3B.
8.1.3 Neutron calibration and standardization m
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