ASTM D6727-01(2007)
(Guide)Standard Guide for Conducting Borehole Geophysical LoggingNeutron
Standard Guide for Conducting Borehole Geophysical Logging<char: emdash>Neutron
SIGNIFICANCE AND USE
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.
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.
This guide applies to commonly used neutron logging methods for geotechnical applications.
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.
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. 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 D5753.
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 d...
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Designation: D6727 − 01(Reapproved 2007)
Standard Guide for
Conducting Borehole Geophysical Logging—Neutron
This standard is issued under the fixed designation D6727; 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 1.7 This guide offers an organized collection of information
oraseriesofoptionsanddoesnotrecommendaspecificcourse
1.1 This guide is focused on the general procedures neces-
of action. This guide should not be used as a sole criterion for
sary to conduct neutron or neutron porosity (hereafter referred
neutron logging and does not replace education, experience,
to as neutron) logging of boreholes, wells, access tubes,
and professional judgment. Neutron logging procedures should
caissons, or shafts (hereafter referred to as boreholes) as
be adapted to meet the needs of a range of applications and
commonly applied to geologic, engineering, groundwater and
stated in general terms so that flexibility or innovation are not
environmental (hereafter referred to as geotechnical) investi-
suppressed. Not all aspects of this guide may be applicable in
gations. Neutron soil moisture measurements made using
all circumstances. This ASTM standard is not intended to
neutron moisture gauges, are excluded. Neutron logging for
representorreplacethestandardofcarebywhichtheadequacy
minerals or petroleum applications is excluded, along with
of a given professional service must be judged without
neutron activation logs where gamma spectral detectors are
consideration of a project’s many unique aspects. The word
used to characterize the induced gamma activity of minerals
standard in the title of this document means that the document
exposed to neutron radiation.
has been approved through the ASTM consensus process.
1.2 This guide defines a neutron log as a record of the rate
1.8 The geotechnical industry uses English or SI units. The
at which thermal and epithermal neutrons are scattered back to
neutron log is typically recorded in units of counts per second
one or more detectors located on a probe adjacent to a neutron
(cps) or in percent porosity.
source.
1.9 This standard does not purport to address all of the
1.2.1 Induction logs are treated quantitatively and should be
safety concerns, if any, associated with its use. It is the
interpreted with other logs and data whenever possible.
responsibility of the user of this standard to establish appro-
1.2.2 Neutron logs are commonly used to: (1) delineate
priate safety and health practices and determine the applica-
lithology, and (2) indicate the water-filled porosity of forma-
bility of regulatory requirements prior to use. The use of
tions (see Fig. 1).
radioactive sources in neutron logging introduces significant
1.3 This guide is restricted to neutron logging with nuclear
safety issues related to the transportation and handling of
counters consisting of scintillation detectors (crystals coupled
neutron sources, and in procedures to insure that sources are
with photomultiplier tubes), or to He -tube detectors with or
not lost or damaged during logging. There are different
without Cd foil covers or coatings to exclude thermalized
restrictions on the use of radioactive sources in logging in
neutrons.
different states, and the Nuclear Regulatory Agency (NRC)
1.4 This guide provides an overview of neutron logging
maintains strict rules and regulations for the licensing of
including: (1) general procedures; (2) specific documentation;
personnel authorized to conduct nuclear source logging.
(3 ) calibration and standardization, and (4) log quality and
interpretation. 2. Referenced Documents
1.5 To obtain additional information on neutron logs see 2.1 ASTM Standards:
References section in this guide. D420 Guide to Site Characterization for Engineering Design
and Construction Purposes (Withdrawn 2011)
1.6 This guide is to be used in conjunction with Standard
D653 Terminology Relating to Soil, Rock, and Contained
Guide D5753.
Fluids
1 2
This guide is under the jurisdiction ofASTM Committee D18 on Soil and Rock For referenced ASTM standards, visit the ASTM website, www.astm.org, or
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Characterization. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved July 1, 2007. Published August 2007. Originally the ASTM website.
approved in 2001. Last previous edition approved in 2001 as D6727 – 01. DOI: The last approved version of this historical standard is referenced on
10.1520/D6727-01R07. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6727 − 01 (2007)
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 3.2.3 effective porosity, n—the volume percent of connected
Used at Waste Sites pore spaces within a formation that are capable of conducting
D5608 Practices for Decontamination of Field Equipment groundwater flow.
Used at Low Level Radioactive Waste Sites
3.2.4 epithermal neutron, n—neutron with kinetic energy
D5730 Guide for Site Characterization for Environmental
somewhat greater than the kinetic energy associated with
Purposes With Emphasis on Soil, Rock, the Vadose Zone
thermal lattice vibrations of the surrounding formation; such
and Groundwater (Withdrawn 2013)
neutrons have been slowed enough by collisions with forma-
D5753 Guide for Planning and Conducting Borehole Geo-
tionmineralstointeractwiththedetector,butthepopulationof
physical Logging
epithermal neutrons is not strongly affected by absorption
D6167 Guide for Conducting Borehole Geophysical Log-
cross-sections of trace minerals in the geologic environment.
ging: Mechanical Caliper
3.2.5 measurement resolution, n—the minimum change in
D6235 Practice for Expedited Site Characterization of Va-
measured value that can be detected.
dose Zone and Groundwater Contamination at Hazardous
Waste Contaminated Sites
3.2.6 neutron generator, n—a device which includes a
D6274 Guide for Conducting Borehole Geophysical Log-
particle accelerator to generate a flux of high-energy neutrons,
ging - Gamma
and which can be turned on and off through connection with an
D6429 Guide for Selecting Surface Geophysical Methods
external power supply.
3.2.7 neutron slowing distance, n—the distance traveled by
3. Terminology
a neutron within a formation over the time required for the
3.1 Definitions—Definitions shall be in accordance with
neutron to be slowed to half of its original velocity by repeated
Terminology D653, Section 13 Ref 1, or as defined below.
collisions with the atoms in the formation.
3.2 Definitions of Terms Specific to This Standard:
3.2.8 repeatability, n—the difference in magnitude of two
3.2.1 accuracy, n—howclosemeasuredlogvaluesapproach
measurements with the same equipment and in the same
true value. It is determined in a controlled environment. A
environment.
controlled environment represents a homogeneous sample
3.2.9 thermalized neutron, n—neutron that has been slowed
volume with known properties.
to a kinetic energy approximately equal to that of the thermal
3.2.2 depth of investigation, n—the radial distance from the
kinetic energy of the surrounding formation.
measurement point to a point where the predominant measured
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
D6727 − 01 (2007)
groundwater flow, and additional unconnected pores that will 6. Interferences
not conduct groundwater.
6.1 Most extraneous effects on neutron logs are caused by
3.2.11 vertical resolution, n—the minimum thickness that
logging too fast, instrument problems, borehole conditions,
can be separated into distinct units.
partially saturated formations, and geologic conditions.
3.2.12 volume of investigation, n—the volume that contrib-
6.2 Logging too fast can significantly degrade the quality of
utes 90 percent of the measured response. It is determined by
neutronlogs,especiallywhenneutrondetectorsaredesignedto
a combination of theoretical and empirical modeling. The
excludethermalizedneutrons,resultinginrelativelylowcount-
volume of investigation is non-spherical and has gradational
ing rates. Neutron counts measured at a given depth need to be
boundaries.
averaged over a time interval such that the natural statistical
variation in the rate of neutron emission is negligible.
4. Summary of Guide
6.3 Instrument problems include electrical leakage of cable
4.1 This guide applies to borehole neutron logging and is to
and grounding problems; degradation of detector efficiency
be used in conjunction with Standard Guide D5753.
attributed to loss of crystal transparency (fogging) or fractures
4.2 This guide briefly describes the significance and use,
or breaks in the crystal; and mechanical damage causing
apparatus, calibration and standardization, procedures, and
separation of crystal and photomultiplier tube.
reports for conducting borehole neutron logging.
6.4 Borehole conditions include changes in borehole diam-
5. Significance and Use
eter; borehole wall roughness whenever neutron logs are run
decentralized;andsteelcasingorcementintheannulusaround
5.1 An appropriately developed, documented, and executed
casing, and thickness of the annulus.
guide is essential for the proper collection and application of
neutron logs. This guide is to be used in conjunction with
6.5 Geologic conditions include the presence of clay min-
Standard Guide D5753.
erals with significant non- effective porosity (Fig. 2), and the
presence of minerals such as chlorine with relatively large
5.2 The benefits of its use include improving selection of
neutron absorption cross-sections.
neutron logging methods and equipment; neutron log quality
and reliability; usefulness of the neutron log data for subse-
6.6 Neutron log response is designed to measure water-
quent display and interpretation.
filled pore spaces so that neutron logs do not measure unsatu-
rated 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
Guide D5753) consult up-to-date textbooks and reports on the 7.1 Ageophysical logging system has been described in the
neutron technique, application, and interpretation methods. general guide (Section 6, Standard Guide D5753).
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 − 01 (2007)
7.2 Neutron logs are collected with probes using He 7.5 The Volume of Investigation and Depth of Investigation
detectors,whichmaybecoatedwithCdtoexcludethermalized are primarily determined by the moisture content of the
neutrons, or may be un-coated to detect both thermal and material near the probe which controls the average distance a
epithermal neutrons; neutron logs may occasionally be col- neutron can travel before being absorbed.
lectedusingdetectorsusinglithium-iodidescintillationcrystals 7.5.1 The Volume of Investigation for neutron logs is
coupled to photomultiplier tubes (Fig. 3). generally considered spherical with a radius of 1.5 to 2.5 ft (40
7.2.1 A neutron shield is needed for the storage of the to 70 cm) from the midpoint between the neutron source and
neutron source during transport to and from the logging site. detector(s) in typical geological formations.
7.2.2 A secure storage facility is needed for neutron source 7.5.2 The Depth of Investigation for neutron logs is gener-
during the time between logging projects when the source ally considered to be 1.5 to 2.5 ft (40 to 70 cm).
cannot be left in the shield in the logging truck.
7.6 Vertical Resolution of neutron logs is determined by the
7.2.3 Radiation monitoring equipment is needed for check-
size of the volume over which neutrons are scattered back
ing of radiation levels outside the neutron shield and in
towards the detector after being emitted by the source. In
working areas during use of the neutron source to verify that
typical geological formations surrounding a fluid-filled
radiation hazards do not exist.
borehole, this is a roughly spherical volume about 1 to 2 ft (30
7.3 Neutron logging probes generate neutron fluxes using a to 60 cm) in diameter. Excessive logging speed can decrease
vertical resolution.
chemical radioactive source such as Ca or a combination of
Am and Be; or by using a neutron generator.
7.7 Measurement Resolution of neutron probes is deter-
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
be run centralized or decentralized (held against the side of the
7.8 A variety of neutron logging equipment is available for
borehole); decentralized probes can be collimated (shielded on
geotechnical investigations. It is not practical to list all of the
the side away from the borehole wall to reduce the influence of
sources of potentially acceptable equipment.
the borehole fluid column). However, collimation requires an
8. Calibration and Standardization of Neutron Logs
impracticably heavy, large-diameter logging probe, and such
probe
...
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