Minty Base Line Gamma

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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.


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.



Australian radioelement baseline


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.





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).


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.





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





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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

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