All rocks and soils are naturally radioactive, containing various proportions of a variety of radioactive elements. The natural decay of these elements produces a variety of radiation types (alpha, beta, and gamma) at specific energy levels. Only gamma-ray radiation has sufficient range to be useful for airborne geophysical surveying, providing a method of measuring concentrations of individual radioactive elements (in particular potassium, uranium, and thorium) in support of bedrock and surficial geological mapping, exploration for a variety of mineral commodities, and for environmental radiation monitoring. Airborne gamma-ray spectrometric (AGRS) surveys provide valuable, systematic coverage of large areas, whereas ground gamma-ray spectrometry (GGRS) greatly improves the resolution of individual radioactive element sources.

Gamma rays are released through the spontaneous decay of radioactive elements. The three most common, naturally occurring radioactive elements are potassium, uranium, and thorium, which are found in varying concentrations in most rock-forming minerals. Potassium occurs mainly in the mineral feldspar, which is an abundant and widespread mineral in the Earth's crust. It also occurs in other common rock-forming minerals, such as biotite and muscovite. Uranium and thorium are generally present in low concentrations (measured in parts per million, ppm) in a wide range of minerals. The range of radioactive element concentrations in most common rock types is shown in Table 1 (Killeen 1979).

Directly or indirectly, these three radioactive elements can provide an indication of economic concentrations of many other metals. For example, high concentrations of uranium may directly represent a target of economic interest, but may also be used as indirect indicators of other types of mineralization if they occur as trace elements associated with that mineralization, or if they are enriched or depleted due to alteration. For example, gamma-ray surveys are effective in mapping potassic alteration associated with certain mineral deposit types, including porphyry Cu (–Au–Mo), volcanogenic massive sulphide and iron-oxide–copper–gold (IOCG; Shives et al. 1997). Maps that show radioactive element distribution provide information about fundamental mineralogical and geochemical properties of bedrock and surficial deposits, and have proven highly useful for geological mapping, mineral exploration, and environmental studies, often indicating geological features not seen by other techniques.

Gamma-ray spectrometry (GRS) is a surface technique only, providing a direct measurement of radioactive element concentrations at the surface of the Earth. There is no significant depth of penetration because the source of most gamma radiation is limited to the top 0.30 m of the Earth's surface. This surface-only characteristic allows geologists to reliably relate the measured radioactive element contrasts to the subjacent bedrock and surficial materials, and alteration associated with mineral deposits. All rocks, and the materials derived from them, are radioactive, containing detectable amounts of a variety of radioactive elements. A gamma-ray spectrometer is designed to detect the gamma rays associated with these radioactive elements and to accurately sort the detected gamma rays by their respective energies. It is this sorting ability that distinguishes the spectrometer from instruments that measure only total radioactivity, e.g. scintillometers.

Data acquired by gamma-ray spectrometric methods are normally portrayed as a multi-channel gamma-ray energy spectrum. A typical natural radiation energy spectrum (Fig. 1) depicts the relative count rates at each energy level from 0 to 3 mega-electron volts (MeV). The interval from 0 to 0.4 MeV is not used because it is dominated by Compton scattering of higher energy radiation (Grasty 1979). For geological mapping, the ⁴⁰K (potassium), bismuth-214 (²¹⁴Bi, a decay product of uranium), and thallium-208 (²⁰⁸Tl, a decay product of thorium) peaks are of interest. Concentrations of uranium and thorium are determined indirectly from their daughter products (²¹⁴Bi and ²⁰⁸Tl, respectively), which are assumed to be in equilibrium with their parent isotope. Potassium concentration is determined directly from ⁴⁰K. The term 'equivalent' or the abbreviation 'e' is used for reporting concentrations of uranium and thorium determined by gamma-ray spectrometry. During an airborne survey, the full spectrum is recorded once per second, using a 256-channel histogram. During post-flight data processing, the counts for the radioelements of interest (i.e. ⁴⁰K, ²¹⁴Bi, ²⁰⁸Tl) are accumulated; this summation includes the counts for a range of energies (a 'window' or 'region of interest') centred on each peak. The standard energy windows are shown in Table 2. For presentation purposes, the accumulated count rates are converted to equivalent ground concentrations of potassium, uranium, and thorium using a set of calibration constants unique to the spectrometer system being used.

Airborne geophysical surveys are normally flown in a direction perpendicular to the main geological strike of the survey area. For AGRS surveying, the line spacing depends on the objectives of the survey: in reconnaissance surveys it may vary between 2000 and 5000 m; in regional surveys it may vary between 500 and 2000 m; and in detailed surveys flown for mineral exploration purposes, flight line spacing can be as little as 100 or 200 m. The above-ground flight elevation (terrain clearance) in fixed-wing spectro-metric surveys is normally 120 m, although it may vary between 100 and 150 m. For helicopter-borne surveys, the detectors are mounted in the helicopter, and the nominal terrain clearance varies between 60 and 120 m, depending on local terrain conditions and the configuration of other survey equipment. Typically, aircraft fly at a speed of approximately 120 knots (190 km/h).

A typical gamma-ray system includes a 256-channel spectrometer that records data at 1-second intervals, which is equivalent to about 60 m on the ground. Surveys flown with a fixed-wing aircraft usually employ sensors consisting of 50 l of thallium-doped sodium iodide NaI(Tl) crystals in the main detector array, and 8 to 12 l of NaI(Tl) crystals in the upward-looking detector array. When using a helicopter platform, the main detector array will usually consist of 33 l of NaI(Tl) crystals, and the upward-looking array from 4 to 8 l. After energy calibration of the spectra, counts from the main detector are recorded in the five windows listed in Table 2. Radiation in the upward-looking detector is recorded in a radon window (1660– 1860 kilo-electron volts (keV)). Comprehensive descriptions of airborne and ground gamma-ray spectrometric surveys, including fundamentals, instrumentation, calibration, data processing, and interpretation, are presented by Grasty (1979), Grasty et al. (1991), Grasty and Minty (1995), Shives et al. (1995), Dickson and Scott (1997), Horsfall (1997), Minty (1997), Minty et al. (1997), Wilford et al. (1997), and the International Atomic Energy Agency (2003), and references therein.

Spectral enhancement techniques such as Noise Adjusted Singular Value Decomposition (NASVD) or Minimum Noise Fraction (MNF) can be used in GRS to improve the signal-to-noise ratio compared to recorded raw spectra (International Atomic Energy Agency 2003). These spectral-enhancement techniques impose rigorous constraints on the means of measuring raw spectra (see Grasty and Minty 1995). The use of these techniques will not change the average response for an area but will reduce the noise in individual readings. It has been estimated (Hovgaard and Grasty 1997) that NASVD yields an improvement similar to that obtained by increasing the detector volume by a factor of between three and four.

Constants such as stripping ratios, attenuation coefficients, and sensitivities must be carefully determined for each acquisition system. Proper application of these constants during processing usually results in seamless datasets across individual survey boundaries, regardless of the acquisition system used. Over-water values should not be deleted from a dataset or set to zero, because such values contain useful data-quality information. The statistical nature of the GRS technique means that 'negative' concentrations are acceptable in low average count-rate areas. If background corrections are properly determined, and correctly applied, then radioactive element concentrations should yield a mean concentration of zero over large bodies of water.

Radioelement ratios can offer a useful tool for lithological mapping. However, due to statistical uncertainties in the individual radioelement measurements, some care should be taken in the calculation of these radioelement ratios. Ratios should always be determined by using radioactive element concentrations because concentrations are computed using instrument-specific calibration constants, ensuring that results from other (calibrated) spectrometer systems will be consistent. An acceptable method of determining ratios neglects any data points for which the potassium concentration is less than a predetermined threshold, usually 0.25%, as these measurements are likely to have been collected over water. Next, the element concentrations of pairs of adjacent points on either side of the data point are progressively summed until the total accumulated concentration in both the numerator and denominator exceed values equivalent to at least 100 counts prior to conversion to concentrations. The ratios are then calculated using the accumulated sums of concentration values. With this method, the errors associated with the calculated ratios will be similar for all data points.

Most of the airborne GRS data for Canada were collected through a variety of federal and federal/provincial cooperative programs to support ongoing and future geological mapping and mineral resource exploration activities. For these reasons, survey areas were typically chosen because of their high mineral potential. Systematic AGRS data have been acquired from about 1970 to the present, using a variety of aircraft and instrument packages. Figure 2 shows an index map of AGRS coverage for Canada.

Gamma ray spectrometric data for Canada are easily accessible through the internet using the Geo-science Data Repository (GDR) at [http://gdr.nrcan.gc.ca]. These data are derived from the digital archives of the Radiation Geophysics Section. Locations of the original flight lines and gridded data for over 300 surveys are currently available from the GDR web-site. Grids of Canada at 250 m cell size, using all digitally archived survey data, are also available from the GDR website. More information regarding the collection, distribution, and applications of GRS data can be found by visiting the Radiation Geophysics web-site at [http://gsc.nrcan.gc.ca/gamma].

Unlike conventional magnetic, electromagnetic, gravity, and seismic techniques that measure physical rock properties (often in response to induced effects), and that may detect variations from source depths of tens to thousands of metres, AGRS and GGRS measure variations in radioactivity emanating from the top 0.30 m of the Earth's surface. The three most abundant natural radioactive elements, potassium, uranium and thorium, occur as major, mobile and immobile elements, respectively, in varying proportions, in all rocks and soils.

Because the concentration of each radioactive element varies between different rock types, a gamma-ray spectrometer can be used to measure these variations and effectively map the different rock types. Where the normal radioactive signature is altered by a mineralizing process, the resulting alteration-related signature provides exploration guidance. Interpretation becomes complicated when these bedrock-related signatures become obscured by overlying transported surficial material or by extreme weathering effects. In Canada, extensive glaciation has mechanically reworked and transported bedrock-derived material. However, the resulting overburden usually reflects local bedrock chemistry. Only when the surficial material is composed predominantly of reworked material, such as glaciofluvial or glaciolacustrine deposits, does the resulting radioactivity signature not reflect the underlying bedrock. Effective interpretation, therefore, requires some knowledge not only of the radioactive element contents of rock types expected to be found in the survey area, but also knowledge of the glacial history of that area.

Interpretation of AGRS data requires understanding of some basic concepts of gamma-ray spectrometry and the importance of proper controls on system calibration, data acquisition, data processing, and presentation methods. Some basic concepts are listed here:

• AGRS is a remotely sensed geophysical technique providing information about the distribution of K, U, and Th that is directly interpretable in terms of surface geology.

• AGRS is a surface technique only; interpretation requires an understanding of the nature of the surficial materials and their relationship to bedrock geology.

• Although AGRS measures a physical phenomenon, it is, for geological and exploration purposes, best considered in geochemical terms, i.e. as 'Airborne Geochemistry'.

• Unlike a camera lens, a gamma-ray spectrometer does not have a 'fixed field of view' – a highly radioactive point source may be detected even when it lies outside some nominal field of view.

• The gamma-ray flux decreases exponentially with distance from the source.

• A single AGRS measurement provides an estimate of the average surface concentration for an area of several thousand square metres; this area is commonly underlain by variable proportions of bedrock, overburden, soil moisture, open water, and vegetation.

• Typically, bedrock concentrations are higher than AGRS values, whereas concentrations in glacial drift are only slightly higher than AGRS values. However, AGRS ratios (eU/eTh, eU/K, and eTh/K) are normally a close approximation of those ratios at ground level.

• Ratio patterns can enhance subtle variations in elemental concentrations related to lithological changes or to alteration processes associated with mineralization.

• Not all ratio anomalies are created equal; caution should be used when interpreting ratio patterns, as questionable high-ratio anomalies can be generated by low count rates in the denominator.

The usefulness of AGRS in geological mapping and mineral exploration depends on two factors: 1) the extent to which radioelement distribution relates to bedrock differences and how this distribution may be modified by mineralizing processes; and 2) the extent to which bedrock radioelement content is reflected in surficial materials that can be spatially related to bedrock sources. Therefore, effective application of AGRS to geological mapping and mineral exploration must rely on knowledge of: i) how the acquired radioactive element distributions correlate with normal lithologic signatures; ii) how these normal litho-logic signatures are recognizably modified by mineralizing processes (alteration signatures); iii) how these litho-logical and alteration signatures are incorporated into surficial materials; and iv) how these surficial materials can be spatially related to bedrock sources. Remote predictive mapping (RPM) applications and methods should be directed toward improving our ability to address these four basic problems of effective AGRS interpretation.

The ability of gamma-ray spectrometry to map the distribution of potassium, uranium, and thorium at the surface of the Earth provides powerful assistance for regional and local bedrock and surficial geological mapping. Important direct and indirect exploration guidance in a wide variety of geological settings is also provided, as is important information for environmental radiation monitoring and land-use planning.

Data enhancement and presentation methods can provide valuable interpretation assistance for gamma-ray spectrometric data. Ratio maps, including ternary K-U-Th colour presentations, are useful but, in the absence of corresponding radioelement concentration maps, are ambiguous. Flight line data, presented as stacked profiles, provide more detail than the inherently smoothed contour maps.

Proper interpretation of gamma-ray spectrometric data requires a clear understanding of petrology, surficial and bedrock geology, and geo-chemistry, as well as the gamma-ray spectrometric method itself. Despite the availability of large quantities of airborne gamma-ray spectrometric data collected since the 1970s and the development and publication of numerous case histories that clearly demonstrate the application to mapping and mineral exploration, the technique remains under-utilized and poorly understood by many potential users. The challenge remains to further develop techniques and case histories that continue to illustrate the broad range of applications, and to effectively disseminate this knowledge.