Magnetic, electromagnetic, gravity and seismic techniques measure physical properties of the earth. Variations in magnetic character, conductivity or density tell us the depth, position and shape of rocks or mineral deposits, based on interpretive models. Depths to sources may be considerable - EM to hundreds of metres, mag to tens of kilometres, gravity and seismic to hundreds of kilometres. However, relating the responses obtained to our understanding of the surface (or near-surface) geology can be difficult, especially where anomalies relate to buried sources, not exposed on the surface. We require additional geoscientific information, such as local rock properties, to constrain models used for interpretation - without those constraints, an unambiguous analysis may not be possible.

Gamma-Ray Spectrometry (GRS) provides a direct measurement of the surface of the earth, with no significant depth of penetration. This at-surface characteristic allows us to reliably relate the measured radioelement contrasts to mapped bedrock and surficial geology, and alteration associated with mineral deposits. All rocks, and 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.

Why do we need to know about K, U, Th?

Potassium (K), uranium (U) and thorium (Th) are the three most abundant, naturally occurring radioactive elements. K is a major constituent of most rocks and is the predominant alteration element in most mineral deposits. Uranium and thorium are present in trace amounts, as mobile and immobile elements, respectively. As the concentration of these different radioelements varies between different rock types, we can use the information provided by a gamma-ray spectrometer to map the rocks. Where the 'normal' radioelement signature of the rocks is disrupted by a mineralizing system, corresponding radioelement anomalies provide direct exploration guidance.

Airborne methods provide valuable, systematic coverage of large areas. Ground spectrometry greatly improves the resolution of individual radioelement sources. By relating radioelement variations measured by a properly calibrated ground spectrometer to relevant lithogeochemical variations, based on a control group of samples, analytical costs can be substantially reduced.

Ground surveys do not require a corresponding airborne survey. They are easily conducted by one person as a reconnaissance survey or more formally using a series of grid lines. The resulting geochemical information provides an important additional layer of information significantly improving bedrock and surficial mapping and ore vectoring.

The primary acquisition data set is a multichannel gamma-ray energy spectrum. This diagram shows a typical natural radiation energy spectrum, depicting the relative count rates at each energy level, from 0 to 3 MeV. The area from 0 to 0.4 MeV is not used and consists of counts created by Compton scattering. For geological mapping, the K⁴⁰ (potassium), Bi²¹⁴ (uranium) & Tl²⁰⁸ (thorium) peaks are of interest. During the aerial survey, the full spectrum of counts is recorded once per second, using a 256-channel histogram. During post-flight data processing, the counts for the radioelements of interest (K⁴⁰, Bi²¹⁴, Tl²⁰⁸) are accumulated. The summation includes the counts for a range of energies (a 'window' or 'region of interest') centred on each peak:

The accumulated count rates are then converted to equivalent ground concentrations of potassium, uranium & thorium using a set of calibration constants that are a characteristic of each spectrometer system.

Note the use of the term 'equivalent' for uranium & thorium concentrations. These concentrations (by weight) are determined indirectly from their daughter products (Bi²¹⁴ & Tl²⁰⁸ respectively) that are assumed to be in equilibrium with their parent isotope. Potassium concentration is determined directly from K⁴⁰.

The recorded counts are subject to a certain amount of Compton scattering that results in extra counts being recorded in each of the 3 regions (peaks) of interest. The effects of this scattering can be removed if the spectrometer has been properly calibrated to determine a set of 6 constants called 'stripping ratios'. This diagram illustrates the relationship between the stripping ratios and the measured count rates. Ground and aircraft gamma-ray spectrometer systems can be calibrated by recording a series of measurements of count rates on concrete calibration pads containing known, but small concentrations of radioactive elements (Grasty et al, 1991).

Both airborne and ground instruments are calibrated using international standards developed by the GSC, to ensure consistent, accurate estimates of K, eU and eTh. Four calibration pads are required for proper spectrometer calibration, including three containing known concentrations of potassium (K⁴⁰), uranium (Bi²¹⁴) and thorium (Tl²⁰⁸). A fourth pad, made from the same concrete used for the others, but with no additional material, acts as a blank or 'control'.

The same pads are used for calibrating both handheld portable spectrometers and larger systems installed in fixed-wing aircraft or helicopters.

This calibration procedure yields:

• the 6 stripping ratios for the spectrometer
• the sensitivity of the spectrometer (count rate per unit of concentration of potassium, uranium & thorium) measured in cps/ppm or cps/%.

These values are only applicable for a small source at ground level and can be used to convert counts measured at ground level to equivalent ground concentrations of potassium, uranium & thorium.

Airborne gamma-ray spectrometers

Additional calibration is needed for airborne spectrometers. It is necessary to know the variation of the calibration constants at different terrain clearance (height above ground, not elevation above sea level):

• the variation of the stripping ratios with height (the stripping ratios determined from the calibration pad are only applicable at ground level)
• the variation of the count rate with height (attenuation factors)
• the sensitivity of the spectrometer (for an infinite source) at the planned flying height of the survey (the sensitivities determined from the calibration pad are only applicable to a small source at ground level)

These calibration factors are determined by relating the count rates measured in a series of flights (at different heights) over a test strip to the equivalent ground concentrations of potassium, uranium & thorium, that are determined from measurements taken at ground level with a properly calibrated hand-held spectrometer. The computed sensitivities are only applicable at a specific 'planned' flying height for the survey, typically 120 m. (Charbonneau & Darnley, 1970).

	A typical day of calibration at the airport, using the Section's portable calibration pads. The Section's Skyvan aircraft (retired 1996) is in the background. A scintillometer and Exploranium GR-256 spectrometer are currently recording data on the potassium calibration pad (red paint). Another spectrometer is also recording on the 'blank' pad (white paint) to the right. The pads are separated by 5 metres to eliminate counts from adjacent pads. Yes, in case you're wondering, the calibration process is much faster if several people watch the instrument during the counting process ;)
	Calibration of portable spectrometer on the potassium calibration pad (red paint) inside an aircraft hangar. The uranium (blue paint) and blank (white paint) pads are on the left.
	Calibration of an airborne spectrometer system (mounted inside the helicopter). The uranium pad (blue paint) has been placed directly below the detectors on the helicopter (grey box with red straps). The rack-mounted computer system is behind the box of detectors.
	Compare the compact helicopter system (above) with the GSC's original Nova-based original airborne gamma-ray spectrometer used in the Skyvan (left). The helicopter system uses 8 NaI crystals (32 L), compared with the Skyvan's 12+2 crystals (50 L) on the right. Therefore, the helicopter system has less sensitivity than the Skyvan system, but can partially compensate for it by flying at lower terrain clearances than the Skyvan.

The following discussion is a simplified outline of the processing that is applied to raw gamma-ray spectrometry measurements to obtain equivalent ground concentrations of potassium, uranium & thorium. For further details, refer to International Atomic Energy Agency (IAEA) Technical Report 323 (Grasty et al, 1991)

You can also try out the Gamma Calculator (requires lightweight JavaScript, but not Java), which allows you to experiment with some sample data: change calibration constants and/or data values to see the effect on calculated equivalent ground concentrations.

After the accumulated counts for the 3 regions of interest (K⁴⁰, Bi²¹⁴ & Tl²⁰⁸) have been accumulated from the spectrum, they are processed as follows:

• subtract background counts
  The background counts can be determined using upward-looking detectors and/or by flying over bodies of water. These background counts are then subtracted from the accumulated counts.

• compute stripping ratios at flying height
  The stripping ratios computed by the pad calibration procedure are applicable only at ground level: the equivalent stripping ratios at the observed terrain clearance are derived from the ground-level stripping ratios, using the factors derived from the test strip.

• apply stripping ratio corrections
  The effects of Compton scattering are removed by applying the stripping ratio corrections computed at the observed height.

• adjust count rate for attenuation with height
  Using the count rate attenuation factors (determined from the test strip), the count rate at the observed terrain clearance is converted to an equivalent count rate at the planned flying height.

• compute equivalent ground concentration
  The final background-corrected, stripped, height-adjusted count rates are converted to equivalent ground concentration of potassium (K, %), uranium (eU, ppm) & thorium (eTh, ppm) by dividing by the sensitivities (measured in cps/ppm or cps/%).

• compute ratios and exposure
  The diagnostic ratios (eU/eTh, eU/K & eTh/K) and Exposure rate are computed from the K, eU & eTh concentrations.

The complete display of all of these data requires a minimum of 7 contour maps or images (one for each measured or derived variable). Several techniques are used for correlating these variables:

	the Ternary Radioelement Map (K, eU, eTh) is a special product that provides a unique view of the composite data set
	a multivariable stacked profile display of the variables using an application like SurView is an essential tool for the analysis of anomalies

The gamma-ray spectrometry data is rounded out by also recording total field magnetometer and VLF total field and quadrature (where possible).

• Surficial materials
  • bedrock
  • surficial

• Mineral exploration
  • uranium
  • granophile elements (Sn, W, etc.)
  • rare elements (Be, Zr, Y, etc.)
  • carbonatites (rare earth elements, P, Nb, etc.)
  • precious metals (Au, Ag, etc.)
  • base metals (Cu, Zn, etc.)

• Environmental radiation monitoring
  • radon risk potential
  • nuclear accidents
  • nuclear waste disposal
  • snow cover, soil moisture

Myth 1:

AGRS is a uranium-only tool: "I'm not looking for uranium, so why do I need a radioactivity survey?"

Fact: In Canada and worldwide, airborne and ground GRS techniques have been successfully applied to: exploration for base, precious and strategic metals, industrial minerals bedrock and surficial geological mapping environmental problems (snow-water equivalence, radon, nuclear reactors/accidents)

AGRS has a continuous 'depth of penetration', the response decreasing with increasing depth (similar to EM, magnetic, gravity, seismic, etc)

Fact: AGRS measures only the top 30 cm (or less) of the earth's surface. There is essentially, no 'penetration'.

Overburden is evil: "Our project area has variable, often thick, overburden covering the bedrock, so gamma-ray surveys will not be useful"

Fact: This is true only where the radioelement composition of the overburden bears no relationship to the underlying bedrock. Our experience is that this is rare in Canada, even in heavily glaciated areas. In the vast majority of cases, the overburden is locally derived, thus reflecting the underlying bedrock geochemistry, including the three radioelements K, eU and eTh. In areas of the world where tropical weathering modifies the chemical composition of the original bedrock sources, interpretation is more complex, requiring an understanding of how the radioelements have been affected. Even in these more severe cases, however, the gamma-ray data provide important information on geomorphic processes and soil/regolith properties, that help refine geochemical models used in mineral exploration.

• AGRS is a remote sensing, geophysical technique that provides 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 '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.

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

• A single AGRS measurement provides an average surface concentration for an area of several thousand square metres, composed of variable proportions of bedrock, overburden, soil moisture, water and vegetation.

• Typically, bedrock concentrations are higher than AGRS values - concentrations in glacial drift are only slightly higher than AGRS values. AGRS ratio values (eU/eTh, eU/K and eTh/K) tend to closely approximate ratio values at ground level. Ratio patterns can enhance subtle variations in elemental concentrations due to lithological changes or 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 to geological mapping and mineral exploration depends on two factors:
  • the extent to which radioelement distribution relates to bedrock differences and the extent to which these are recognizably modified by mineralizing processes

  • the extent to which bedrock radioelement content is reflected in surficial materials that can be spatially related to bedrock sources

• Finally, it is important to emphasize that properly calibrated airborne and ground gamma-ray spectrometry surveys produce quantitative geochemical data. The combination of gamma-ray spectrometry with magnetic and electromagnetic sensors, in a modern digital system, yields powerful mapping and exploration tools for all end-users, from individual prospectors using analogue output (maps), to companies and regional mappers with GIS capabilities.