Upload
gerardo-fanton
View
232
Download
0
Embed Size (px)
Citation preview
8/8/2019 Minty Base Line Gamma
http://slidepdf.com/reader/full/minty-base-line-gamma 1/5
AESC2006, Melbourne, Australia. 1
Towards an Australian radioelement baseline database
Brian MintyGeoscience AustraliaGPO Box 378
Canberra, ACT, [email protected]
INTRODUCTION
Global prosperity depends almost entirely on the
Earth’s natural resources. Mineral and hydrocarbon
resources, building materials, and soil and water underpin our standard of living. The sustainable
management of these natural resources is thus essential.
Airborne geophysics is one of the most cost-effective
tools that provide information for the mapping and
management of the earth’s resources. Radioelement
mapping by airborne gamma-ray spectrometry, in
particular, has been extensively used in the earth and
environmental sciences. These data are widely used in
geological mapping, mineral exploration, environmental
studies, and land management – particularly in areas
where public health is an issue. In Australia,
radioelement data are also increasingly being used for
soil mapping, and as an essential ingredient in land
degradation and salinity studies.
Government agencies in Australia have funded the
systematic mapping of the continent by airborne
geophysical methods over the past 50 years.
Approximately 75% of the continent is now covered by
airborne gamma-ray spectrometric surveys (Figure 1).
These data are publicly available, and comprise a
valuable national resource. The quality of the data from
recent surveys is far better than that of older surveys.
Data acquisition systems, as well as calibration and data
processing procedures, have improved with time. Early
surveys measured total count (TC) only. These give a
measure of total radioactivity. Later surveys used
differential and multichannel spectrometers, and give
estimates of the gamma radiation due to each of
potassium (K), uranium (U) and thorium (Th). Current
survey data conform to International Atomic Energy
Agency (IAEA) standards, and are reported in absolute
concentration units for K, U and Th, and in air-absorbed
dose rate for total count. However, up until the early
1990’s, survey data were still reported in units of counts
per second. The older survey data values thus depend
on the instrumentation and survey parameters, and the
data values on adjacent surveys are not necessarily
comparable. These problems limit the usefulness of the
national gamma-ray spectrometric database.
The main advantage of having all surveys registered to
the same datum is that they are easily merged into larger
regional compilations. These facilitate the recognition
and interpretation of broad-scale regional features in the
data. Lessons learnt in one area are more easily applied
to other areas. Calibrated radioelement data are alsomore easily interpreted in terms of public health issues
– specifically radiation risk.
Geoscience Australia and State Geological Surveys are
working towards establishing a national baseline
database of Australian gamma-ray spectrometric data
that is consistent with the global radioelement baseline.
If new survey data are acquired and processed
according to IAEA standards, then they will be
registered to the global radioelement baseline, and
SUMMARY
Approximately 75% of Australia is covered by public-
domain, airborne gamma-ray spectrometric surveys.
However, all the older surveys are in units of c/s and
their data values depend on the survey instrumentation
and acquisition parameters. Also, many of the newer
surveys were inadequately calibrated with the result that
data values on adjacent surveys are not necessarily
comparable. This limits the usefulness of these data.
Geoscience Australia and State Geological Surveys are
working towards establishing a national baseline database
of Australian gamma-ray spectrometric data that is
consistent with the global radioelement baseline
established by the International Atomic Energy Agency.
This will be achieved by: (a) ensuring consistency in the
calibration and processing of new gamma-ray
spectrometric data through the use of standard processing
procedures and calibration facilities that are tied to the
global datum, and; (b) adjusting older surveys to the
global datum through back-calibration and automatic grid
merging.
Surveys that are registered to the same datum are easily
merged into regional compilations which facilitate the
recognition and interpretation of broad-scale regional
features, and allow lessons learnt in one area to be more
easily applied to other areas.
Key words: gamma-ray spectrometery, airborne gamma-
ray spectrometry, back-calibration, grid merging,
radioelement baseline.
8/8/2019 Minty Base Line Gamma
http://slidepdf.com/reader/full/minty-base-line-gamma 2/5
Australian radioelement baseline
AESC2006, Melbourne, Australia. 2
hence the national baseline. Older data can be adjusted
to this international datum through suitable back-
calibration
This paper describes the international datum, and the
procedures to be followed to ensure that surveys are
registered to this datum.
A GLOBAL RADIOELEMENT BASELINE
A radioelement baseline is a set of radioelement
measurements that conform to a particular standard. The
international radioelement baseline for gamma-ray
spectrometric data is that of the IAEA (IAEA, 2006).
The IAEA global standards
The IAEA (IAEA, 2006) has recommended a set of
procedures for the establishment of a global
radioelement baseline for gamma-ray spectrometric data
based on the reference standards used for laboratory
gamma-ray spectrometry issued by the IAEA
Seibersdorf Laboratory (IAEA, 1987) in Austria (Figure
2).
These reference materials are the primary reference
standard for gamma-ray spectrometric applications.
Registering new surveys to the IAEA baseline
The primary requirement for the calibration of portable
and airborne gamma-ray spectrometers is access to
calibration pads with known concentrations of the
radioelements.
Figure 2. IAEA geological reference materials for K, U
and Th that are used for calibrating laboratory gamma-
ray spectrometers. The reference materials are referred
to as the RG-set – RGK-1, RGU-1 and RGTh-1 (Figure
courtesy M. Matolin)
A typical set of calibration pads will comprise a
minimum of 4 pads – three of which are enriched in
each of K, U and Th, with the fourth serving as a
background pad.
The pads are used for estimating stripping ratios for
airborne gamma-ray spectrometers, and both stripping
ratios and window sensitivities for portablespectrometers. Sensitivities for airborne spectrometers
are estimated from calibration flights over a calibration
range that is surveyed with a portable spectrometer at
the same time as calibration flights are made. Thus, if
the portable spectrometer is calibrated on pads that are
tied to the IAEA global datum, then the airborne
spectrometer system will be calibrated to this datum as
well.
The assigned grades of spectrometer calibration pads
must therefore be consistent with the IAEA Seibersdorf
Laboratory standards. When calibration pads are
constructed, the source material is systematically
sampled and analysed using a laboratory gamma-rayspectrometer. If the laboratory spectrometer is
calibrated using the IAEA RG set of reference materials,
then the new calibration pads will be registered to the
IAEA standard.
Alternatively, new calibration pads can have their
grades assigned through a cross-calibration exercise
using a portable spectrometer that has been calibrated
using a set of pads that are tied to the IAEA standard. A
full description of how to do this inter-pad calibration is
described in IAEA (2006).
There are 3 types of calibration pad facilities in
Australia:
(a) Fixed pads for calibrating portable spectrometers
(Dickson and Lovborg, 1984). These pads were
built by the CSIRO in North Ryde, Sydney, in
1983, but have been subsequently moved to
Penrith. There are 5 pads, each 2 m in diameter.
The pads were included in an inter-pad calibration
exercise by Lovborg (1984), who showed that a
well-calibrated portable spectrometer can be used
to re-assign grades to calibration pads to make
them internationally consistent. The fixed CSIRO
pads are thus tied to the global datum.
(b) Transportable calibration pads for calibrating both portable and airborne spectrometers (Grasty et al.,
1991). These pads have dimensions 1×1×0.3 m,
and because they are transportable, are easily used
for estimating stripping ratios for airborne gamma-
ray spectrometers, and both stripping ratios and
sensitivities for portable spectrometers. The pads
were manufactured in Canada and are tied to the
global datum.
8/8/2019 Minty Base Line Gamma
http://slidepdf.com/reader/full/minty-base-line-gamma 3/5
Australian radioelement baseline
AESC2006, Melbourne, Australia. 3
Figure 3. Calibration of a portable gamma-ray
spectrometer on transportable concrete calibration pads
(c) Transportable calibration pads constructed by the
CSIRO at North Ryde in the early 1990s (Figure 3).
These have the same dimensions, and similar
concentrations, to the pads of Grasty et al , andwere tied to the fixed CSIRO (2m-diameter) pads
during a cross-calibration exercise in 1995. These
pads are thus also tied to the global datum.
IAEA (1991) and IAEA (1993) describe the calibration
and data processing requirements for portable and
airborne gamma-ray spectrometers. If these guidelines
are followed using pads that are tied to the global
datum, then radioelement estimates from these systems
will be consistent with the global radioelement baseline.
Registering old surveys to the IAEA baseline
Two complementary methods are being used to adjustolder Australian survey data to conform to the
international baseline:
(a) Back-calibration of older airborne gamma-ray
surveys (Grasty et al., 1992): A portable gamma-
ray spectrometer, calibrated according to IAEA
standards, is used to estimate the concentrations of
the radioelements beneath carefully selected
portions of some of the airborne survey flight
lines. For each of TC, K, U and Th, the ratios of
the average airborne count rates (in the vicinity of
the ground readings) and the average ground
concentrations are used to estimate correction
factors that can be used to convert the airbornedata to standard concentration units (nG/hr for TC,
% K, and ppm eU and eTh).
(b) Automatic merging of airborne gamma-ray
spectrometric surveys (Minty, B.R.S., 2000): Where
calibrated surveys (which conform to the
international baseline) overlap older un-calibrated
surveys, the differences in the data values in the
overlap areas can sometimes be used to correct the
un-calibrated surveys to the international datum
(Figure 4). This technology is being widely used
to minimise the amount of back-calibration
required to adjust surveys to the international
baseline.
CONCLUSIONS
A national radioelement baseline for gamma-rayspectrometric data in Australia requires that all gamma-
ray data be acquired and processed in a consistent way.
The appropriate standard is that of the IAEA (2006).
Geoscience Australia and State Geological Surveys are
developing a national baseline database of Australian
gamma-ray spectrometric data that is tied to the IAEA
global datum.
Australian calibration facilities are tied to the global
datum. If new surveys are flown with equipment
calibrated using these facilities, and use calibration and
processing procedures recommended by the IAEA, then
the resulting estimates of radioelement concentrations
will be consistent with the global datum. Older surveyscan also be adjusted to the global datum through back-
calibration and automatic grid merging.
There are many advantages of having a consistent
national radioelement database. Surveys that are
registered to the same datum are easily merged into
regional compilations which facilitate the recognition
and interpretation of broad-scale regional features, and
allow lessons learnt in one area to be more easily
applied to other areas.
ACKNOWLEDGMENTS
This paper is published with the permission of the CEO,Geoscience Australia. .
REFERENCES
Dickson, B.L. and Lovborg, L., 1984. An Australian facility
for the calibration of portable gamma-ray spectrometers.
Exploration Geophysics, 15, 260-262.
Grasty, R.L., Holman, P.B., and Blanchard, Y.B., 1991.
Transportable calibration pads for ground and airborne
gamma-ray spectrometers. GSC Paper 90-23, Geological
Survey of Canada, Ottawa.
Grasty, R.L., Whitton, R.M., and Duffy, A., 1992. Back
calibration and reprocessing of an airborne gamma-ray survey,
Malaysia. Expanded Abstracts, 62nd Annual Meeting, Societyof Exploration Geophysicists, 550-551.
IAEA, 1987. Preparation and Certification of IAEA Gamma-
ray Spectrometry Reference Materials RGU-1, RGTh-1 and
RGK-1. Techn. Report-IAEA/RL/148, International Atomic
Energy Agency, Vienna.
IAEA, 1991. Airborne gamma-ray spectrometer surveying.
Technical Report Series, No. 323. International Atomic
Energy Agency, Vienna.
8/8/2019 Minty Base Line Gamma
http://slidepdf.com/reader/full/minty-base-line-gamma 4/5
Australian radioelement baseline
AESC2006, Melbourne, Australia. 4
IAEA, 2003. Guidelines for radioelement mapping using
gamma-ray spectrometry data. IAEA-TECDOC-1363,
International Atomic Energy Agency, Vienna.
IAEA, 2006. Status of radioelement mapping – towards a
global radioelement baseline. IAEA-TECDOC-(in prep),
International Atomic Energy Agency, Vienna.
Løvborg, L., 1984. The calibration of portable and airborne
gamma-ray spectrometers – theory, problems and facilities.
Report Risø-M-2456, Roskilde.
Minty, B.R.S., 2000. Automatic merging of gridded airborne
gamma-ray spectrometric surveys. Exploration Geophysics, v.
31, 47-51.
Figure 1. Airborne gamma-ray spectrometric data acquisition by Geoscience Australia and State Geological Surveys
8/8/2019 Minty Base Line Gamma
http://slidepdf.com/reader/full/minty-base-line-gamma 5/5
Australian radioelement baseline
AESC2006, Melbourne, Australia. 5
Minty
-17.5
141.0 145.5
-17.5
-19.5
145.5141.0
-19.5
(b)
-17.5
141.0 145.5
-17.5
-19.5
145.5141.0
-19.5
(a)
Figure 4. Automatic grid levelling of total count airborne gamma-ray spectrometric data from North Queensland, Australia:
(a) composite grid before levelling, and (b) after levelling