- Email: [email protected]
Some notes on the importance of airborne gamma-ray spectrometry in International Geochemical Mapping
Journal of GeochemicalExploration, 49 (1993) 201-212
201
Elsevier Science Publishers B.V., Amsterdam
Some notes on the importance of airborne gamma-ray spectrometry in International Geochemical Mapping Arthur G. Darnley Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8, Canada (Received 6 January 1993; accepted after revision 19 July 1993)
ABSTRACT Airborne gamma-ray spectrometry (AGRS) is an important component of the International Geochemical Mapping (IGM) project. It permits geochemical mapping of radioactive elements (naturally occurring and man-made) by remote sensing. Data can be reported in terms of radioelement abundance and nuclide dose-rate. The technique offers continent-wide coherency because it may be applied across any land surface, with 90% of the response coming from the top 25 cm (termed the A2~ horizon). Quantitative correlation of ground and airborne data can be assured within definable limits because airborne equipment is calibrated by relating airborne to upper regolith ground measurements, under controlled conditions, over test ranges several km in length. Evidence ranging from local to continental scales suggests that the reverse of the calibration process enables AGRS to provide an important supplementary function for global mapping. In compiling geochemical maps for large regions, based on conventional surficial geochemical sampling methods, significant differences are commonly observed in element base levels for individual survey blocks, especially where methods have not been standardized. Uncertainty then exists as to the extent to which apparent changes in base level are due to real geochemical differences, or to the methods employed. Continuous profiles of surface Th and K abundance, derived from airborne data, provide an independent reference level against which estimates of Th and K, derived from various direct sampling techniques, can be compared. This comparison can identify inconsistencies in Th and K data, which may in turn raise questions concerning the consistency of other data. This is important in the context of establishing reliable global geochemical baselines for all elements.
1. I N T R O D U C T I O N
Phase 1 of the International Geochemical Mapping (IGM)project* commenced in 1988 to plan the process of obtaining globally consistent, comprehensive, geochemical base-line data to serve a broad range of environmental and economic applications (Darnley, 1990). Phase 2, the implementation of *The International Geochemical Mapping project began as Project 259 of the International Geological Correlation Program, sponsored jointly by UNESCO and IUGS. Phase 2, launched in 1993, is identified as Project 360.
0375-6742/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
202
A.G.DARNLEY
International Geochemical Mapping, with the subtitle of Global Geochemical Baselines, will begin with the collection of very wide-spaced drainage, regolith, and water samples based on a 160 X 160 km grid, involving approximately 5000 site-clusters worldwide (Darnley, 1993 ). This work is intended to establish a network of reference materials to control later, more detailed, mapping. The sites from which the reference materials are taken can also be used for subsequent periodic monitoring to detect changes in the environment. The reference collection will have the secondary purpose of delineating large geochemical features. The resulting data compilation will overlap blocks of existing data, many of which, because of contrasting methodologies and hence inconsistencies in abundance base levels, will appear discordant within regional patterns. Airborne gamma-ray spectrometry (AGRS) is a major component of the International Geochemical Mapping project because it provides: (a) unique information concerning the spatial distribution of radioactive elements, which are an integral part of the geochemical environment; (b) the opportunity to link regions and continents through continuous standardized sampling along each flight line. 1.1. Airborne gamma ray spectrometry and the naturally occurring radioelements Airborne gamma ray spectrometry (AGRS) was originally developed to explore for uranium. Subsequently it was recognised that it had wider geological applications because of the ubiquity of the naturally radioactive elements and their usefulness as lithological indicators and markers of economically interesting geological processes, such as metasomatic and hydrothermal alteration. The use of AGRS for quantitative mapping of surface radioelement distribution was established during the early 1970s. By measuring the gamma flux from 4°K, 2~4Bi and 2°8T1, the surface abundance of K, U, and Th can be determined, expressed as % K, ppm equivalent uranium (eU) and ppm equivalent thorium (eTh) respectively. Potassium, U and Th, which directly or indirectly (via daughter products ) are responsible for the natural radiation background, are found in measurable amounts in almost all rocks and soils, K as a major element, U and Th as trace elements. Thorium occurs principally in resistate minerals and in most surficial environments is an immobile element; K is widely distributed in potash feldspars, micas and clay minerals. The primary potassic silicate minerals, particularly biotite, are susceptible to chemical weathering but in the surface environment much of the K released is relocated in clay minerals so that in regions of low relief K can be considered a relatively immobile element. The relative stability of Th and K in the surface environment is demonstrated by the fact that maps of eTh: K ratiovalues are often seen to correlate closely with lithological boundaries.
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONALGEOCHEMICAL MAPPING
203
In contrast to Th and K, U is a mobile element because under wet oxidising conditions it can be readily dissolved. The relative and absolute abundances of K, eU and eTh, determined by AGRS, can be used to characterise many rock types and zones of mineralization, principally via the derivative regolith. This is because AGRS measurements relate to the radioelement content of the upper part of the land surface. Gamma radiation is attenuated by matter, attenuation increasing with electron density, and there is an exponential fall-off in intensity with distance as radiation travels through rock, soil, water, vegetation or air. Approximately 90% of the non-atmospheric natural gamma radiation measured at the surface of the ground originates in the top 25 cm (Gregory and Horwood, 1961 ). It is convenient to refer to this surficial regolith source material, 0 to 25 cm, as the A25 horizon, analogous to (and including) the Ap cultivated horizon. Thus, AGRS measures the radioelements in (or, in the case of recent fall-out, on) the A25 horizon, whatever its composition. It includes residual or transported soils, wind-blown sand, in situ or transported rock, vegetation and organic matter. In geochemical terms AGRS is a remote sensing technique, but sensitivity and resolution diminish with distance and results become increasingly less definitive at more than 150 m above the ground. Most regional AGRS surveys are standardized at a mean terrain clearance of 125 m. Typical specifications for an AGRS survey result in a measurement of K, eU, and eTh at approximately 50 m intervals, i.e. 20 overlapping samples/line km. Each AGRS sample is very large, of the order of several thousands of cubic meters; this is a major difference from conventional geochemical samples. Standardized data collection, calibration and data reduction procedures, as described in International Atomic Energy Agency reports (IAEA, 1976, 1991 ), allow the calculation of mean surface abundances for any radioelement present, with definable uncertainties.
1.2. AGRS and anthropogenic radiation Radioelement data form part of any comprehensive description of the geochemical environment. Radioactive elements are of great interest to the general public and health authorities. Following the commencement of nuclear weapons tests in the late 1940s, and accidental nuclear emissions of various types beginning in the 1950s, there has been growing concern amongst the general public about the distribution of anthropogenic fall-out products, which raise the background radiation level and may contaminate the food chain. About 10 gamma emitters have been widely identified (IAEA, 1991 ). Most have relatively short half-lives and disappear within a few months or years, but 137Cs is more persistent. The most recent example of extensive contami-
204
A.G.DARNLEY
nation pertains to 137Cs (half-life 30 y) from the Chernobyl nuclear accident. In 1986 measurable amounts were distributed over a large area of NW Europe. AGRS provides an effective method of monitoring the distribution and amount of man-made radiation present in any region. Within 8 days of the Chernobyl accident a wide-spaced AGRS reconnaissance survey had been flown over all of Sweden and a dose-rate map produced. Thus, AGRS can be used specifically for monitoring fall-out in a sudden emergency such as Chernobyl, or in less urgent situations measurements of fall-out can be made as part of routine AGRS mapping for earth science purposes. Results can be expressed in various ways, e.g., radiation dose rates or element concentrations. In North America and Europe, AGRS data have been used as part of investigations into natural radon emissions to identify areas with a high uranium background, where high radon flux might present a health hazard. AGRS is a versatile tool for mapping the spatial distribution of all sources of gamma radiation enclosing the h u m a n environment. The largest single compilation of AGRS natural radioelement data covers the conterminous USA (Duval, 1990). The Swedish experience in using AGRS to map fall-out from the Chernobyl reactor accident is described by Mellander (1989). For recent reviews with extensive bibliographies of the applications of AGRS and the evolution of the method, see Darnley and Ford (1989) and Darnley ( 1991 ). 2. APPLICATIONOF AGRS TO INTERNATIONALGEOCHEMICALMAPPING
2.1. Radioelement baseline data The plan to obtain international reference sample data, taken from a widespaced network of sites has already been mentioned. Radioelement data, natural and anthropogenic, are components of such a database. They can be derived either from gamma ray measurements made with calibrated portable ground equipment at each of these sites when samples are collected, or by subsequent gamma ray measurements in a laboratory. Since these scattered ground measurements will form part of a primary global database it is highly desirable to connect them with continuous cross-country AGRS profiles; this linkage can provide important additional benefits, discussed in the following sections. The AGRS equivalent to wide-spaced surface sampling consists of widespaced profiles. Profiles of continuous AGRS data at a m a x i m u m 160 km line spacing (preferably 80 k m ) will provide a skeletal framework of radioelement baseline data to which existing (or future) detailed AGRS surveys can be tied. It will complement multi-element, wide-spaced ground sampling.
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONAL GEOCHEMICAL MAPPING
205
2.2. Level problems in geochemical data AGRS is able to provide an important additional function beyond that of mapping the distribution of radioactive elements. It relates to the problem of levelling. Since regional geochemical mapping by surface methods began in the 1960s, this activity has entailed the use of different sample media in different geographic regions; e.g., soils, laterites, and desert wash; stream and lake sediments; heavy minerals; glacial till. For examples of the complex effects of different sample media and different analytical treatments upon spatial patterns and quantitative values, see Bolviken et al. ( 1986 ). Even within regions where a single sample medium and a nominally similar analytical approach have been used, level discrepancies between adjacent data blocks are often apparent, due to work being carried out at different times or by different organizations using non-identical procedures. Problems of this type become obvious when compilations are attempted. Major discontinuities are present where changes in methodology have taken place. These are most commonly but not exclusively observed at administrative boundaries. These geochemical "faults" constitute a major obstacle to the compilation of geochemical atlases from existing data. Plant and Ridgway (1990) have reviewed the situation within Western Europe. A method of normalizing broadly similar adjoining or overlapping sets of lake sediment data has been demonstrated by Garrett et al. ( 1991 ). The problems of relating overlapping lake and stream sediment data have been discussed by Davenport (1990) and Garrett et al. (1990). In their examples the data sets under consideration have involved relatively close spaced sampling. The problems increase as sample density decreases. The levelling of scattered "islands" of dissimilar data presents additional difficulties. In order to compile a world geochemical map or establish reliable baselines, there is clearly a need for a worldwide geochemical reference datum (a geochemical "mean sea-level") against which all surveys can be compared. Ideally this would be a datum for all elements, all types of media and all analytical methods. The wide-spaced drainage sampling which is the starting point for multi-element baselines can only go part way towards meeting the levelling requirement. It provides a series of geochemical control points or the equivalent of "spot-heights". These drainage samples are selected to be representative of large areas, but they are necessarily few in number. This is deliberate, in order to restrict the number of samples to a quantity that can be processed by a small number of internationally recognised laboratories and thereby ensure consistent quality. The reference sample coverage is of necessity non-overlapping and discontinuous. The resulting spatial patterns cannot define the position of geochemical boundaries except in a very generalised way.
206
A.G. DARNLEY
2.3. Relevance of AGRS profiles to geochemical levelling One of the first demonstrations of the relationship of airborne to ground gamma ray data was made in 1967 over a test area in Bancroft, Ontario, which was chosen because it was a glaciated area with some outcrop, typical of much of the Canadian shield (Fig. 1, from Darnley and Fleet, 1968). Later, for calibration purposes, test lines several km in length were delineated in several countries, using areas of relatively uniform radioelement content such as an alluvial plain (e.g., Charbonneau and Darnley, 1970), or a dry lake bed, where the K, eU, eTh content had been determined by systematic ground measurements using calibrated portable spectrometers. Using these calibration lines, a linear correlation can be established between ground and airborne count rates for each aircraft installation, thus providing the basis for quantitative surveys wherever they are required (subject to a number of conditions and corrections; for details and additional references see IAEA, 1979, 1990, 1991 ). Such surveys have now been undertaken on all continents with the exception of Antarctica. To demonstrate the applicability of the method in an equatorial high-rainfall environment, Fig. 2 (from Grasty et al., 1992) shows the correlation obtained between ground and airborne gamma ray measurements, along a 14 km line in an area of rubber plantations in NE Malaysia. BANCROFT RADIOMETRIC TEST STRIP COMPARISON OF GROUND AND AIR DATA 92m CLEARANCE 1300m
15oo 1 |
A T 10 m / s ,*,\
_/~x
/ K /t''~i'-a
~-"
i./T
,oo-t
._jio
500-~
,300m I I
I
OO
,x
200"
~"
~1
.-
h ....
Ax'
.~,.... ^ /\.
"
/ X. .. K~/
/',
\
//'\..",
/ " ~\
x
k-J
i~\ "',.
/
, /--k,,,--u
100'
PERCENTAGE 4
o .
.
. S
.
.
.
. M
.
.
. A
OUTCROP
.
.
.
.
. M
,,,,I,,=
.
. S
~
......
III,I
i ti + + ,. ~ 1%.. . . . . . . . S .( G P SPS p/ f f
Ilil
,~-.-, G
.-~
.. N
Fig. 1. AGRS as a mapping tool in glaciated terrain. A comparison of ground and air gamma ray spectrometry profiles over Bancroft, Ontario, radiometric test strip, using early experimental equipment. Geological symbols: A = amphibolite; G = granite; M = marble; N G = nepheline gneiss; P = paragneiss; S = syenite. (From Darnley and Fleet, 1968. )
AIRBORNE
GAMMA-RAY
SPECTROMETRY
IN INTERNATIONAL
GEOCHEMICAL
MAPPING
207
50
0Th
o ~o
t~i
,.
I
A.J W ~.~, I~' " ~ll/~,l.
I--
0 0
0 "iI--
7 • 'I~I_~'W, FI A'~I " .vl(
i V~/~,,,,AA/V
10
v
20
eU
-g Z
o
I
I---
u./ 0 :7 0
5
,A,,A
I llrl
3,' n,-
-5
K Z 0 I,Z uJ 0 Z 0 C.)
.h.i
f j.~v
¢n c,3
" '
I-
2 0
,,,
NVIlh
'i"VV 1 I v
VlV IV"
~(l~,[IFl v II~I ~,,
i
,
,
.
.
,
,
2
4
6
8
10
12
14
DISTANCE (km) - -
AIRBORNE PROFIL;E
•
GROUND M E A S U R E M E N T
Fig. 2. AGRS as a mapping tool in tropical terrain. A comparison of spot ground measurements of K, eU and eTh, obtained with a calibrated portable gamma ray spectrometer, with airborne K, eU and eTh profiles, along a 14 km flight line in NE Malaysia. (From Grasty et al., 1992. )
208
A.G. DARNLEY
Note that spot, not continuous, measurements were taken on the ground because of difficult access. The conterminous United States provides the largest area over which data are currently available to allow a comparison to be made (for K only) between AGRS and conventional surface sampling. During the 1960s Shacklette and co-workers sampled regolith material, principally soil, from a depth of about 20 cm from locations approximately 80 km apart over the whole country (863 sites). The - 2 0 0 mesh fraction was analyzed for K by flame photometry and the results plotted as sample-point symbols on a map (Shacklette et al., 1971, pp. D52-53 ). Shacklette reported the geometric mean for the whole US as 1.2% K. The western half of the country (west of Longitude 97°W) has a geometric mean of 1.7% K; the eastern half, 0.74% K. AGRS data collected as part of the US National U r a n i u m Resource Evaluation program enabled Duval (1990) to compile and publish, in colour, a coherent K map of the USA, based on a flight line spacing which was generally between 5 and 10 km. For the purpose of comparing the two sets of K regolith data, the author has hand-contoured Shacklette's map. Given the much wider spacing of the ground-data sample sites, the spatial patterns and the abundances produced from the two totally independent data sets are surprisingly similar. High areas and low regions match. From visual inspection of Duvars colour map, the median K value from AGRS is between 1.8 and 2.1% for the US west of Longitude 97 °W and between 0.6 and 0.9% for the US east of 97 ° W. These values are close to Shacklette's geometric means quoted above. It is important to note that each data set was acquired and processed as a single project, with internal quality control, resulting in quantitative consistency across the continent and hence the concordance of the two maps. This result could not have been achieved if either of the data sets had contained any large blocks of unstandardized information.
2.4. Application of AGRS profiles to geochemical levelling The concordance between ground and airborne gamma ray data which can be observed at small and continental scales provides strong evidence for believing that AGRS can assist in geochemical levelling. This follows from the fact that AGRS provides continuous data along each flight line with equipment which is calibrated to provide results in terms of the surface radioelement concentration of the regolith. The equipment gathers data in a uniform way whatever the nature of the surface. Thus it can link and compare separate data blocks. In 1969 a 3500 km AGRS profile was flown across the Canadian shield from Ottawa to Yellowknife NWT, which, although count rates were not at that time converted to radioelement concentrations, shows that natural radioelement base-line levels for wide blocks (xoo k m ) of country can differ by a factor o f t e n (Darnley et al., 1971 ).
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONALGEOCHEMICAL MAPPING
209
In climates where chemical weathering is negligible, the correlation between AGRS and conventional ground sampling data is generally satisfactory for Th, K and U. Correlation between airborne eTh and ground eTh (or Th) in the A25 horizon, shows consistency in all environments. Potassium correlation tends to be somewhat inferior to that observed for Th for a number of technical reasons which are discussed in the IAEA reports. The experience with U data has been more variable. The correlation observed between airborne and ground eU determinations (i.e. both by gamma ray) is good provided both measurements are obtained when a similar soil moisture profile applies (Grasty, pers.com. ). Where a direct method of U analysis (e.g., XRF or NAA) has been employed for the determination of U in ground samples, discrepancies are present where chemical weathering has caused parent and/ or daughter element separation over distances of more than a few cm. The effect of chemical weathering depends in part on the mineralogical species involved. Because 214Bi is used as a proxy for U (hence the need for the term eU) in gamma ray mapping, there is the potential for gross discrepancies between chemically determined U and eU, in part because of gross differences in sample size. In conventional geochemistry the sample weight is measured in grams; in AGRS, tonnes. Thus, because each AGRS sample is very large, migration of constituents can take place and still remain within the sample. However, for the reasons indicated, (and also because of corrections applied for atmospheric Rn) quantitative estimation of U by AGRS is inherently more uncertain than for Th and K. To summarise this section, extensive surveys over the past 20 years in regions ranging from the arctic to equatorial rainforest have demonstrated that properly corrected and calibrated airborne eTh and K data indicate the Th and K content of the A25 horizon. This is to be expected because it is the reverse of the calibration process. Median Th and K abundances determined by AGRS and conventional soil sampling along common profiles will normally agree within close limits provided that the soil is not abnormally wet or dry at the time of the airborne measurements and provided geochemical analyses of surface soil samples are based upon natural gamma-ray spectrometry, neutron activation, XRF or other total estimation methods. Regrettably, Th determinations have rarely been included in past analytical work on soils. Geochemical levelling is a basic requirement of the IGM project. AGRS offers this by means of cross-country profiles. Equivalent Th and K profiles may be used as reference levels against which Th and K data obtained by sampling and analysis of A25 material can be compared. Equivalent Th is the preferred geochemical reference datum. Potassium provides a supplementary datum, as does the ratio of Th: K. Approximate normalization factors can be derived for blocks of discordant data. The use of AGRS to compare resistate element levels in survey blocks where sample media other than A25 horizon,
210
A.G. DARNLEY
or regolith derived stream sediment, have been employed for conventional geochemical mapping may prove to be a useful extension of the technique. The use of AGRS to "flag" the existence of suspect data blocks is well within the current state of the art. It involves flying AGRS profiles at a line-spacing determined by the dimensions and complexity of the data blocks to be checked. For example, AGRS profiles at 5 km line-spacing provide continuous sampiing over a 250 m strip along each line, representing 5% of a block's total area. A 10 km line-spacing would sample 2.5%. Where there is a substantial discrepancy (say > 20%) in the median values of airborne and regolith eTh and K data along one or more profiles, or where there is a noticeable change in the Th: K ratio of one data set and not in the other, an explanation is required. Where there is a discontinuity or step in Th and K ground survey data which is not reflected in coincident airborne data then it is probable that there are methodological inconsistencies with respect to the ground data, involving a change in sampling, sample preparation or analytical techniques. If this is so, it is probable that these inconsistencies will apply to other elements in addition to Th and K, depending upon the details of the work. This is important information with respect to establishing global geochemical baselines, because as previously indicated, the lack of consistency in past data collection methods can make it difficult to distinguish between real and apparent regional differences. 3. S U M M A R Y
Subject to proper calibration and use, AGRS is a unique tool for mapping the distribution of all radioactive elements, whatever their origin, in the surface of the regolith (termed the A25 horizon ); the data which are acquired are an essential part of the description of the geochemical environment. In addition to the need to establish baseline data for radioelements, AGRS offers a partial solution to a potential problem associated with the implementation of international geochemical mapping, namely, that of levelling many discrete datasets. The measurement of eTh and K by AGRS along continuous profiles offers a standardised global reference datum, independent of the type of land surface, which can identify arbitrary level changes in blocks of conventional geochemical data, provided Th and K are included in every analytical suite and they are analysed by total decomposition or direct instrumental methods. A comparison of two published K maps for the whole USA, both relating to the regolith, one produced from conventional sampling by Shacklette et al. ( 1971 ), the other by AGRS by Duval (1990), provides prima facie evidence in support of this concept. Concordance of ground and airborne Th and K data offers some assurance of, but cannot guarantee, the consistency of data for other relatively immobile elements. Discordance indicates problems which
AIRBORNEGAMMA-RAYSPECTROMETRYIN INTERNATIONALGEOCHEMICALMAPPING
211
require investigation before data should be included in regional or global map compilations. ACKNOWLEDGMENTS
The writer is grateful to Peter H. Davenport of the Newfoundland Department of Mines and Energy and Agnete Steenfelt of the Geological Survey of Greenland for providing data and critical comments relating to earlier versions of this paper which have assisted in the evolution of the author's ideas concerning the validity of using AGRS for geochemical levelling. Also to Roberr Garrett and Robert Grasty of Geological Survey of Canada for helpful comments on some of the problems involved.
REFERENCES Bolviken, B., Bergstrom, J., Bjorklund, A., Kontio M., Lehmuspelto, P., Lindholm, T., Magnusson, J., Ottesen, R.T., Steenfelt, A. and Volden, T., 1986. Geochemical Atlas of Northern Fennoscandia, Scale 1:4,000,000, Nordkalott Project, Geological Survey of Sweden, 170 pp. Charbonneau, B.W. and Darnley, A.G., 1970. A test strip for calibration of airborne gamma ray spectrometers. Geological Survey of Canada, Paper 70-1 B, pp. 27-32. Darnley, A.G., 1990. International geochemical mapping: a new global project. J. Geochem. Explor., 39: 1-13. Darnley, A.G., 1991. The development of airborne gamma-ray spectrometry: a case study in technological innovation and acceptance. J. Nucl. Geophys., 5:377-402. Darnley, A.G., 1993. News item: International Geochemical Mapping. J. Geochem. Explor., 48: 97-104. Darnley, A.G. and Fleet, M., 1968. Evaluation of airborne gamma ray spectrometry in the Bancroft and Elliot Lake areas of Ontario, Canada. In: Proc. 5th Symp. Remote Sensing of the Environment, University of Michigan, Ann Arbor, MI, pp. 833-853. Darnley, A.G., and Ford, K.L., 1989. Regional airborne gamma-ray surveys: a review. In: G.D. Garland (Editor), Proceedings of Exploration '87; Third Decennial International Conference on Geophysical and Geochemical Exploration for Minerals and Groundwater. Ontario Geological Survey, Special Volume 3, pp. 229-240. Darnley, A.G., Grasty, R.L. and Charbonneau, B.W., 1971. A radiometric profile across part of the Canadian Shield. Geol. Surv. Can., Paper 70-46, 42 pp. Davenport, P.H., 1990. A comparison of regional geochemical data from lakes and streams in northern Labrador: implications for mixed-media geochemical mapping. J. Geochem. Explor., 39:117-151. Duval, J.S., 1990. Modern aerial gamma-ray spectrometry and regional potassium map of the conterminous United States. J. Geochem. Explor., 39: 249-253. Garrett, R.G., Banville, R.M.P. and Adcock, S.W., 1990. Regional geochemical data compilation and map preparation, Labrador, Canada. J. Geochem. Explor., 39:91-116. Garrett, R.G., Beaumier, M. and Davenport, P.H., 199 I. Quebec-Labrador geochemical transect, James Bay to the Labrador Sea 53-55 °N. In: Current Activities Forum, Program with Abstracts. Geological Survey of Canada, p. 8. Grasty, R.L., Whitton, R.M. and Duffy, A., 1992. Back calibration and reprocessing an airborne
212
A.G.DARNLEY
gamma ray survey, Malaysia. In: Expanded Abstracts with Biographies, 1992 Technical Program, 62nd Annual International SEG Meeting, New Orleans, LA, pp. 550-551. Gregory, A.F.and Horwood, J.L., 1961. A laboratory study of gamma-ray spectra at the surface of rocks. Mines Branch Research Report R 85, Department of Mines and Technical Surveys, Ottawa, 52 pp. IAEA, 1976. Radiometric Reporting Methods and Calibration in Uranium Exploration. Technical Report Series 174, STI/DOC/10/174, IAEA, Vienna, 57 pp. IAEA, 1979. Gamma-ray surveys in uranium exploration. Technical Report Series 186, STI/ DOC/10/186, IAEA, Vienna, 90 pp. IAEA, 1990. The use of gamma ray data to define the natural radiation environment, IAEATECDOC-566, IAEA, Vienna, 48 pp. IAEA, 1991. Airborne gamma ray spectrometer surveying. Technical Report Series 323, STI/ DOC! 10/323, IAEA, Vienna, 97 pp. Mellander, H., 1989. Airborne gamma spectrometric measurements of the fall-out over Sweden after the nuclear reactor accident at Chernobyl, USSR. Internal report IAEA/NENF/NM89-1, IAEA, Vienna, 41 pp. Plant, J.A. and Ridgway, J., 1990. Inventory of geochemical surveys in Western Europe. In: A.Demetriades, J.Locutura, R.T. Ottesen (Compilers), Geochemical Mapping of Western Europe Towards the Year 2000. Pilot Project Report, Appendix 10, NGU Report 90-105, Geol. Surv. Norway, 400 pp. Shacklette, H.T., Hamilton, J.C., Boerngen, J.G. and Bowles, J.M., 1971. Elemental composition of surficial materials in the conterminous United States. U.S. Geol. Surv., Prof. Pap. 574-D, 71 pp.
201
Elsevier Science Publishers B.V., Amsterdam
Some notes on the importance of airborne gamma-ray spectrometry in International Geochemical Mapping Arthur G. Darnley Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8, Canada (Received 6 January 1993; accepted after revision 19 July 1993)
ABSTRACT Airborne gamma-ray spectrometry (AGRS) is an important component of the International Geochemical Mapping (IGM) project. It permits geochemical mapping of radioactive elements (naturally occurring and man-made) by remote sensing. Data can be reported in terms of radioelement abundance and nuclide dose-rate. The technique offers continent-wide coherency because it may be applied across any land surface, with 90% of the response coming from the top 25 cm (termed the A2~ horizon). Quantitative correlation of ground and airborne data can be assured within definable limits because airborne equipment is calibrated by relating airborne to upper regolith ground measurements, under controlled conditions, over test ranges several km in length. Evidence ranging from local to continental scales suggests that the reverse of the calibration process enables AGRS to provide an important supplementary function for global mapping. In compiling geochemical maps for large regions, based on conventional surficial geochemical sampling methods, significant differences are commonly observed in element base levels for individual survey blocks, especially where methods have not been standardized. Uncertainty then exists as to the extent to which apparent changes in base level are due to real geochemical differences, or to the methods employed. Continuous profiles of surface Th and K abundance, derived from airborne data, provide an independent reference level against which estimates of Th and K, derived from various direct sampling techniques, can be compared. This comparison can identify inconsistencies in Th and K data, which may in turn raise questions concerning the consistency of other data. This is important in the context of establishing reliable global geochemical baselines for all elements.
1. I N T R O D U C T I O N
Phase 1 of the International Geochemical Mapping (IGM)project* commenced in 1988 to plan the process of obtaining globally consistent, comprehensive, geochemical base-line data to serve a broad range of environmental and economic applications (Darnley, 1990). Phase 2, the implementation of *The International Geochemical Mapping project began as Project 259 of the International Geological Correlation Program, sponsored jointly by UNESCO and IUGS. Phase 2, launched in 1993, is identified as Project 360.
0375-6742/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
202
A.G.DARNLEY
International Geochemical Mapping, with the subtitle of Global Geochemical Baselines, will begin with the collection of very wide-spaced drainage, regolith, and water samples based on a 160 X 160 km grid, involving approximately 5000 site-clusters worldwide (Darnley, 1993 ). This work is intended to establish a network of reference materials to control later, more detailed, mapping. The sites from which the reference materials are taken can also be used for subsequent periodic monitoring to detect changes in the environment. The reference collection will have the secondary purpose of delineating large geochemical features. The resulting data compilation will overlap blocks of existing data, many of which, because of contrasting methodologies and hence inconsistencies in abundance base levels, will appear discordant within regional patterns. Airborne gamma-ray spectrometry (AGRS) is a major component of the International Geochemical Mapping project because it provides: (a) unique information concerning the spatial distribution of radioactive elements, which are an integral part of the geochemical environment; (b) the opportunity to link regions and continents through continuous standardized sampling along each flight line. 1.1. Airborne gamma ray spectrometry and the naturally occurring radioelements Airborne gamma ray spectrometry (AGRS) was originally developed to explore for uranium. Subsequently it was recognised that it had wider geological applications because of the ubiquity of the naturally radioactive elements and their usefulness as lithological indicators and markers of economically interesting geological processes, such as metasomatic and hydrothermal alteration. The use of AGRS for quantitative mapping of surface radioelement distribution was established during the early 1970s. By measuring the gamma flux from 4°K, 2~4Bi and 2°8T1, the surface abundance of K, U, and Th can be determined, expressed as % K, ppm equivalent uranium (eU) and ppm equivalent thorium (eTh) respectively. Potassium, U and Th, which directly or indirectly (via daughter products ) are responsible for the natural radiation background, are found in measurable amounts in almost all rocks and soils, K as a major element, U and Th as trace elements. Thorium occurs principally in resistate minerals and in most surficial environments is an immobile element; K is widely distributed in potash feldspars, micas and clay minerals. The primary potassic silicate minerals, particularly biotite, are susceptible to chemical weathering but in the surface environment much of the K released is relocated in clay minerals so that in regions of low relief K can be considered a relatively immobile element. The relative stability of Th and K in the surface environment is demonstrated by the fact that maps of eTh: K ratiovalues are often seen to correlate closely with lithological boundaries.
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONALGEOCHEMICAL MAPPING
203
In contrast to Th and K, U is a mobile element because under wet oxidising conditions it can be readily dissolved. The relative and absolute abundances of K, eU and eTh, determined by AGRS, can be used to characterise many rock types and zones of mineralization, principally via the derivative regolith. This is because AGRS measurements relate to the radioelement content of the upper part of the land surface. Gamma radiation is attenuated by matter, attenuation increasing with electron density, and there is an exponential fall-off in intensity with distance as radiation travels through rock, soil, water, vegetation or air. Approximately 90% of the non-atmospheric natural gamma radiation measured at the surface of the ground originates in the top 25 cm (Gregory and Horwood, 1961 ). It is convenient to refer to this surficial regolith source material, 0 to 25 cm, as the A25 horizon, analogous to (and including) the Ap cultivated horizon. Thus, AGRS measures the radioelements in (or, in the case of recent fall-out, on) the A25 horizon, whatever its composition. It includes residual or transported soils, wind-blown sand, in situ or transported rock, vegetation and organic matter. In geochemical terms AGRS is a remote sensing technique, but sensitivity and resolution diminish with distance and results become increasingly less definitive at more than 150 m above the ground. Most regional AGRS surveys are standardized at a mean terrain clearance of 125 m. Typical specifications for an AGRS survey result in a measurement of K, eU, and eTh at approximately 50 m intervals, i.e. 20 overlapping samples/line km. Each AGRS sample is very large, of the order of several thousands of cubic meters; this is a major difference from conventional geochemical samples. Standardized data collection, calibration and data reduction procedures, as described in International Atomic Energy Agency reports (IAEA, 1976, 1991 ), allow the calculation of mean surface abundances for any radioelement present, with definable uncertainties.
1.2. AGRS and anthropogenic radiation Radioelement data form part of any comprehensive description of the geochemical environment. Radioactive elements are of great interest to the general public and health authorities. Following the commencement of nuclear weapons tests in the late 1940s, and accidental nuclear emissions of various types beginning in the 1950s, there has been growing concern amongst the general public about the distribution of anthropogenic fall-out products, which raise the background radiation level and may contaminate the food chain. About 10 gamma emitters have been widely identified (IAEA, 1991 ). Most have relatively short half-lives and disappear within a few months or years, but 137Cs is more persistent. The most recent example of extensive contami-
204
A.G.DARNLEY
nation pertains to 137Cs (half-life 30 y) from the Chernobyl nuclear accident. In 1986 measurable amounts were distributed over a large area of NW Europe. AGRS provides an effective method of monitoring the distribution and amount of man-made radiation present in any region. Within 8 days of the Chernobyl accident a wide-spaced AGRS reconnaissance survey had been flown over all of Sweden and a dose-rate map produced. Thus, AGRS can be used specifically for monitoring fall-out in a sudden emergency such as Chernobyl, or in less urgent situations measurements of fall-out can be made as part of routine AGRS mapping for earth science purposes. Results can be expressed in various ways, e.g., radiation dose rates or element concentrations. In North America and Europe, AGRS data have been used as part of investigations into natural radon emissions to identify areas with a high uranium background, where high radon flux might present a health hazard. AGRS is a versatile tool for mapping the spatial distribution of all sources of gamma radiation enclosing the h u m a n environment. The largest single compilation of AGRS natural radioelement data covers the conterminous USA (Duval, 1990). The Swedish experience in using AGRS to map fall-out from the Chernobyl reactor accident is described by Mellander (1989). For recent reviews with extensive bibliographies of the applications of AGRS and the evolution of the method, see Darnley and Ford (1989) and Darnley ( 1991 ). 2. APPLICATIONOF AGRS TO INTERNATIONALGEOCHEMICALMAPPING
2.1. Radioelement baseline data The plan to obtain international reference sample data, taken from a widespaced network of sites has already been mentioned. Radioelement data, natural and anthropogenic, are components of such a database. They can be derived either from gamma ray measurements made with calibrated portable ground equipment at each of these sites when samples are collected, or by subsequent gamma ray measurements in a laboratory. Since these scattered ground measurements will form part of a primary global database it is highly desirable to connect them with continuous cross-country AGRS profiles; this linkage can provide important additional benefits, discussed in the following sections. The AGRS equivalent to wide-spaced surface sampling consists of widespaced profiles. Profiles of continuous AGRS data at a m a x i m u m 160 km line spacing (preferably 80 k m ) will provide a skeletal framework of radioelement baseline data to which existing (or future) detailed AGRS surveys can be tied. It will complement multi-element, wide-spaced ground sampling.
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONAL GEOCHEMICAL MAPPING
205
2.2. Level problems in geochemical data AGRS is able to provide an important additional function beyond that of mapping the distribution of radioactive elements. It relates to the problem of levelling. Since regional geochemical mapping by surface methods began in the 1960s, this activity has entailed the use of different sample media in different geographic regions; e.g., soils, laterites, and desert wash; stream and lake sediments; heavy minerals; glacial till. For examples of the complex effects of different sample media and different analytical treatments upon spatial patterns and quantitative values, see Bolviken et al. ( 1986 ). Even within regions where a single sample medium and a nominally similar analytical approach have been used, level discrepancies between adjacent data blocks are often apparent, due to work being carried out at different times or by different organizations using non-identical procedures. Problems of this type become obvious when compilations are attempted. Major discontinuities are present where changes in methodology have taken place. These are most commonly but not exclusively observed at administrative boundaries. These geochemical "faults" constitute a major obstacle to the compilation of geochemical atlases from existing data. Plant and Ridgway (1990) have reviewed the situation within Western Europe. A method of normalizing broadly similar adjoining or overlapping sets of lake sediment data has been demonstrated by Garrett et al. ( 1991 ). The problems of relating overlapping lake and stream sediment data have been discussed by Davenport (1990) and Garrett et al. (1990). In their examples the data sets under consideration have involved relatively close spaced sampling. The problems increase as sample density decreases. The levelling of scattered "islands" of dissimilar data presents additional difficulties. In order to compile a world geochemical map or establish reliable baselines, there is clearly a need for a worldwide geochemical reference datum (a geochemical "mean sea-level") against which all surveys can be compared. Ideally this would be a datum for all elements, all types of media and all analytical methods. The wide-spaced drainage sampling which is the starting point for multi-element baselines can only go part way towards meeting the levelling requirement. It provides a series of geochemical control points or the equivalent of "spot-heights". These drainage samples are selected to be representative of large areas, but they are necessarily few in number. This is deliberate, in order to restrict the number of samples to a quantity that can be processed by a small number of internationally recognised laboratories and thereby ensure consistent quality. The reference sample coverage is of necessity non-overlapping and discontinuous. The resulting spatial patterns cannot define the position of geochemical boundaries except in a very generalised way.
206
A.G. DARNLEY
2.3. Relevance of AGRS profiles to geochemical levelling One of the first demonstrations of the relationship of airborne to ground gamma ray data was made in 1967 over a test area in Bancroft, Ontario, which was chosen because it was a glaciated area with some outcrop, typical of much of the Canadian shield (Fig. 1, from Darnley and Fleet, 1968). Later, for calibration purposes, test lines several km in length were delineated in several countries, using areas of relatively uniform radioelement content such as an alluvial plain (e.g., Charbonneau and Darnley, 1970), or a dry lake bed, where the K, eU, eTh content had been determined by systematic ground measurements using calibrated portable spectrometers. Using these calibration lines, a linear correlation can be established between ground and airborne count rates for each aircraft installation, thus providing the basis for quantitative surveys wherever they are required (subject to a number of conditions and corrections; for details and additional references see IAEA, 1979, 1990, 1991 ). Such surveys have now been undertaken on all continents with the exception of Antarctica. To demonstrate the applicability of the method in an equatorial high-rainfall environment, Fig. 2 (from Grasty et al., 1992) shows the correlation obtained between ground and airborne gamma ray measurements, along a 14 km line in an area of rubber plantations in NE Malaysia. BANCROFT RADIOMETRIC TEST STRIP COMPARISON OF GROUND AND AIR DATA 92m CLEARANCE 1300m
15oo 1 |
A T 10 m / s ,*,\
_/~x
/ K /t''~i'-a
~-"
i./T
,oo-t
._jio
500-~
,300m I I
I
OO
,x
200"
~"
~1
.-
h ....
Ax'
.~,.... ^ /\.
"
/ X. .. K~/
/',
\
//'\..",
/ " ~\
x
k-J
i~\ "',.
/
, /--k,,,--u
100'
PERCENTAGE 4
o .
.
. S
.
.
.
. M
.
.
. A
OUTCROP
.
.
.
.
. M
,,,,I,,=
.
. S
~
......
III,I
i ti + + ,. ~ 1%.. . . . . . . . S .( G P SPS p/ f f
Ilil
,~-.-, G
.-~
.. N
Fig. 1. AGRS as a mapping tool in glaciated terrain. A comparison of ground and air gamma ray spectrometry profiles over Bancroft, Ontario, radiometric test strip, using early experimental equipment. Geological symbols: A = amphibolite; G = granite; M = marble; N G = nepheline gneiss; P = paragneiss; S = syenite. (From Darnley and Fleet, 1968. )
AIRBORNE
GAMMA-RAY
SPECTROMETRY
IN INTERNATIONAL
GEOCHEMICAL
MAPPING
207
50
0Th
o ~o
t~i
,.
I
A.J W ~.~, I~' " ~ll/~,l.
I--
0 0
0 "iI--
7 • 'I~I_~'W, FI A'~I " .vl(
i V~/~,,,,AA/V
10
v
20
eU
-g Z
o
I
I---
u./ 0 :7 0
5
,A,,A
I llrl
3,' n,-
-5
K Z 0 I,Z uJ 0 Z 0 C.)
.h.i
f j.~v
¢n c,3
" '
I-
2 0
,,,
NVIlh
'i"VV 1 I v
VlV IV"
~(l~,[IFl v II~I ~,,
i
,
,
.
.
,
,
2
4
6
8
10
12
14
DISTANCE (km) - -
AIRBORNE PROFIL;E
•
GROUND M E A S U R E M E N T
Fig. 2. AGRS as a mapping tool in tropical terrain. A comparison of spot ground measurements of K, eU and eTh, obtained with a calibrated portable gamma ray spectrometer, with airborne K, eU and eTh profiles, along a 14 km flight line in NE Malaysia. (From Grasty et al., 1992. )
208
A.G. DARNLEY
Note that spot, not continuous, measurements were taken on the ground because of difficult access. The conterminous United States provides the largest area over which data are currently available to allow a comparison to be made (for K only) between AGRS and conventional surface sampling. During the 1960s Shacklette and co-workers sampled regolith material, principally soil, from a depth of about 20 cm from locations approximately 80 km apart over the whole country (863 sites). The - 2 0 0 mesh fraction was analyzed for K by flame photometry and the results plotted as sample-point symbols on a map (Shacklette et al., 1971, pp. D52-53 ). Shacklette reported the geometric mean for the whole US as 1.2% K. The western half of the country (west of Longitude 97°W) has a geometric mean of 1.7% K; the eastern half, 0.74% K. AGRS data collected as part of the US National U r a n i u m Resource Evaluation program enabled Duval (1990) to compile and publish, in colour, a coherent K map of the USA, based on a flight line spacing which was generally between 5 and 10 km. For the purpose of comparing the two sets of K regolith data, the author has hand-contoured Shacklette's map. Given the much wider spacing of the ground-data sample sites, the spatial patterns and the abundances produced from the two totally independent data sets are surprisingly similar. High areas and low regions match. From visual inspection of Duvars colour map, the median K value from AGRS is between 1.8 and 2.1% for the US west of Longitude 97 °W and between 0.6 and 0.9% for the US east of 97 ° W. These values are close to Shacklette's geometric means quoted above. It is important to note that each data set was acquired and processed as a single project, with internal quality control, resulting in quantitative consistency across the continent and hence the concordance of the two maps. This result could not have been achieved if either of the data sets had contained any large blocks of unstandardized information.
2.4. Application of AGRS profiles to geochemical levelling The concordance between ground and airborne gamma ray data which can be observed at small and continental scales provides strong evidence for believing that AGRS can assist in geochemical levelling. This follows from the fact that AGRS provides continuous data along each flight line with equipment which is calibrated to provide results in terms of the surface radioelement concentration of the regolith. The equipment gathers data in a uniform way whatever the nature of the surface. Thus it can link and compare separate data blocks. In 1969 a 3500 km AGRS profile was flown across the Canadian shield from Ottawa to Yellowknife NWT, which, although count rates were not at that time converted to radioelement concentrations, shows that natural radioelement base-line levels for wide blocks (xoo k m ) of country can differ by a factor o f t e n (Darnley et al., 1971 ).
AIRBORNE GAMMA-RAY SPECTROMETRY IN INTERNATIONALGEOCHEMICAL MAPPING
209
In climates where chemical weathering is negligible, the correlation between AGRS and conventional ground sampling data is generally satisfactory for Th, K and U. Correlation between airborne eTh and ground eTh (or Th) in the A25 horizon, shows consistency in all environments. Potassium correlation tends to be somewhat inferior to that observed for Th for a number of technical reasons which are discussed in the IAEA reports. The experience with U data has been more variable. The correlation observed between airborne and ground eU determinations (i.e. both by gamma ray) is good provided both measurements are obtained when a similar soil moisture profile applies (Grasty, pers.com. ). Where a direct method of U analysis (e.g., XRF or NAA) has been employed for the determination of U in ground samples, discrepancies are present where chemical weathering has caused parent and/ or daughter element separation over distances of more than a few cm. The effect of chemical weathering depends in part on the mineralogical species involved. Because 214Bi is used as a proxy for U (hence the need for the term eU) in gamma ray mapping, there is the potential for gross discrepancies between chemically determined U and eU, in part because of gross differences in sample size. In conventional geochemistry the sample weight is measured in grams; in AGRS, tonnes. Thus, because each AGRS sample is very large, migration of constituents can take place and still remain within the sample. However, for the reasons indicated, (and also because of corrections applied for atmospheric Rn) quantitative estimation of U by AGRS is inherently more uncertain than for Th and K. To summarise this section, extensive surveys over the past 20 years in regions ranging from the arctic to equatorial rainforest have demonstrated that properly corrected and calibrated airborne eTh and K data indicate the Th and K content of the A25 horizon. This is to be expected because it is the reverse of the calibration process. Median Th and K abundances determined by AGRS and conventional soil sampling along common profiles will normally agree within close limits provided that the soil is not abnormally wet or dry at the time of the airborne measurements and provided geochemical analyses of surface soil samples are based upon natural gamma-ray spectrometry, neutron activation, XRF or other total estimation methods. Regrettably, Th determinations have rarely been included in past analytical work on soils. Geochemical levelling is a basic requirement of the IGM project. AGRS offers this by means of cross-country profiles. Equivalent Th and K profiles may be used as reference levels against which Th and K data obtained by sampling and analysis of A25 material can be compared. Equivalent Th is the preferred geochemical reference datum. Potassium provides a supplementary datum, as does the ratio of Th: K. Approximate normalization factors can be derived for blocks of discordant data. The use of AGRS to compare resistate element levels in survey blocks where sample media other than A25 horizon,
210
A.G. DARNLEY
or regolith derived stream sediment, have been employed for conventional geochemical mapping may prove to be a useful extension of the technique. The use of AGRS to "flag" the existence of suspect data blocks is well within the current state of the art. It involves flying AGRS profiles at a line-spacing determined by the dimensions and complexity of the data blocks to be checked. For example, AGRS profiles at 5 km line-spacing provide continuous sampiing over a 250 m strip along each line, representing 5% of a block's total area. A 10 km line-spacing would sample 2.5%. Where there is a substantial discrepancy (say > 20%) in the median values of airborne and regolith eTh and K data along one or more profiles, or where there is a noticeable change in the Th: K ratio of one data set and not in the other, an explanation is required. Where there is a discontinuity or step in Th and K ground survey data which is not reflected in coincident airborne data then it is probable that there are methodological inconsistencies with respect to the ground data, involving a change in sampling, sample preparation or analytical techniques. If this is so, it is probable that these inconsistencies will apply to other elements in addition to Th and K, depending upon the details of the work. This is important information with respect to establishing global geochemical baselines, because as previously indicated, the lack of consistency in past data collection methods can make it difficult to distinguish between real and apparent regional differences. 3. S U M M A R Y
Subject to proper calibration and use, AGRS is a unique tool for mapping the distribution of all radioactive elements, whatever their origin, in the surface of the regolith (termed the A25 horizon ); the data which are acquired are an essential part of the description of the geochemical environment. In addition to the need to establish baseline data for radioelements, AGRS offers a partial solution to a potential problem associated with the implementation of international geochemical mapping, namely, that of levelling many discrete datasets. The measurement of eTh and K by AGRS along continuous profiles offers a standardised global reference datum, independent of the type of land surface, which can identify arbitrary level changes in blocks of conventional geochemical data, provided Th and K are included in every analytical suite and they are analysed by total decomposition or direct instrumental methods. A comparison of two published K maps for the whole USA, both relating to the regolith, one produced from conventional sampling by Shacklette et al. ( 1971 ), the other by AGRS by Duval (1990), provides prima facie evidence in support of this concept. Concordance of ground and airborne Th and K data offers some assurance of, but cannot guarantee, the consistency of data for other relatively immobile elements. Discordance indicates problems which
AIRBORNEGAMMA-RAYSPECTROMETRYIN INTERNATIONALGEOCHEMICALMAPPING
211
require investigation before data should be included in regional or global map compilations. ACKNOWLEDGMENTS
The writer is grateful to Peter H. Davenport of the Newfoundland Department of Mines and Energy and Agnete Steenfelt of the Geological Survey of Greenland for providing data and critical comments relating to earlier versions of this paper which have assisted in the evolution of the author's ideas concerning the validity of using AGRS for geochemical levelling. Also to Roberr Garrett and Robert Grasty of Geological Survey of Canada for helpful comments on some of the problems involved.
REFERENCES Bolviken, B., Bergstrom, J., Bjorklund, A., Kontio M., Lehmuspelto, P., Lindholm, T., Magnusson, J., Ottesen, R.T., Steenfelt, A. and Volden, T., 1986. Geochemical Atlas of Northern Fennoscandia, Scale 1:4,000,000, Nordkalott Project, Geological Survey of Sweden, 170 pp. Charbonneau, B.W. and Darnley, A.G., 1970. A test strip for calibration of airborne gamma ray spectrometers. Geological Survey of Canada, Paper 70-1 B, pp. 27-32. Darnley, A.G., 1990. International geochemical mapping: a new global project. J. Geochem. Explor., 39: 1-13. Darnley, A.G., 1991. The development of airborne gamma-ray spectrometry: a case study in technological innovation and acceptance. J. Nucl. Geophys., 5:377-402. Darnley, A.G., 1993. News item: International Geochemical Mapping. J. Geochem. Explor., 48: 97-104. Darnley, A.G. and Fleet, M., 1968. Evaluation of airborne gamma ray spectrometry in the Bancroft and Elliot Lake areas of Ontario, Canada. In: Proc. 5th Symp. Remote Sensing of the Environment, University of Michigan, Ann Arbor, MI, pp. 833-853. Darnley, A.G., and Ford, K.L., 1989. Regional airborne gamma-ray surveys: a review. In: G.D. Garland (Editor), Proceedings of Exploration '87; Third Decennial International Conference on Geophysical and Geochemical Exploration for Minerals and Groundwater. Ontario Geological Survey, Special Volume 3, pp. 229-240. Darnley, A.G., Grasty, R.L. and Charbonneau, B.W., 1971. A radiometric profile across part of the Canadian Shield. Geol. Surv. Can., Paper 70-46, 42 pp. Davenport, P.H., 1990. A comparison of regional geochemical data from lakes and streams in northern Labrador: implications for mixed-media geochemical mapping. J. Geochem. Explor., 39:117-151. Duval, J.S., 1990. Modern aerial gamma-ray spectrometry and regional potassium map of the conterminous United States. J. Geochem. Explor., 39: 249-253. Garrett, R.G., Banville, R.M.P. and Adcock, S.W., 1990. Regional geochemical data compilation and map preparation, Labrador, Canada. J. Geochem. Explor., 39:91-116. Garrett, R.G., Beaumier, M. and Davenport, P.H., 199 I. Quebec-Labrador geochemical transect, James Bay to the Labrador Sea 53-55 °N. In: Current Activities Forum, Program with Abstracts. Geological Survey of Canada, p. 8. Grasty, R.L., Whitton, R.M. and Duffy, A., 1992. Back calibration and reprocessing an airborne
212
A.G.DARNLEY
gamma ray survey, Malaysia. In: Expanded Abstracts with Biographies, 1992 Technical Program, 62nd Annual International SEG Meeting, New Orleans, LA, pp. 550-551. Gregory, A.F.and Horwood, J.L., 1961. A laboratory study of gamma-ray spectra at the surface of rocks. Mines Branch Research Report R 85, Department of Mines and Technical Surveys, Ottawa, 52 pp. IAEA, 1976. Radiometric Reporting Methods and Calibration in Uranium Exploration. Technical Report Series 174, STI/DOC/10/174, IAEA, Vienna, 57 pp. IAEA, 1979. Gamma-ray surveys in uranium exploration. Technical Report Series 186, STI/ DOC/10/186, IAEA, Vienna, 90 pp. IAEA, 1990. The use of gamma ray data to define the natural radiation environment, IAEATECDOC-566, IAEA, Vienna, 48 pp. IAEA, 1991. Airborne gamma ray spectrometer surveying. Technical Report Series 323, STI/ DOC! 10/323, IAEA, Vienna, 97 pp. Mellander, H., 1989. Airborne gamma spectrometric measurements of the fall-out over Sweden after the nuclear reactor accident at Chernobyl, USSR. Internal report IAEA/NENF/NM89-1, IAEA, Vienna, 41 pp. Plant, J.A. and Ridgway, J., 1990. Inventory of geochemical surveys in Western Europe. In: A.Demetriades, J.Locutura, R.T. Ottesen (Compilers), Geochemical Mapping of Western Europe Towards the Year 2000. Pilot Project Report, Appendix 10, NGU Report 90-105, Geol. Surv. Norway, 400 pp. Shacklette, H.T., Hamilton, J.C., Boerngen, J.G. and Bowles, J.M., 1971. Elemental composition of surficial materials in the conterminous United States. U.S. Geol. Surv., Prof. Pap. 574-D, 71 pp.