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Radiation Geophysics
Practical theory

A perspective on Gamma-Ray Spectrometry

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 gamma-ray energy spectrum

spectrum 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:

Typical gamma-ray spectrometry energy windows
Name Element Peak
(keV)
Energy Range
(keV)
K K40 1460 1360 - 1560
U Bi214 1760 1660 - 1860
Th Tl208 2615 2410 - 2810
Total Count     410 - 2810

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.

Calibration of gamma-ray spectrometers

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 stripping ratios 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:

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):

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).

Calibration pads

calibration pads 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 ;)


gr256 calibration

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.

helicopter calibration

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.

NOVA in Skyvan

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.


Detector boxes in Skyvan

Survey data corrections

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:


Variables

Primary measured & derived variables
for AGRS surveys
Name Symbol Units
Potassium K %
equivalent Uranium eU ppm
equivalent Thorium eTh ppm
Total count   Ur
Exposure   µR/h
Natural air absorbed dose rate   nGy/h
equivalent Uranium/equivalent Thorium eU/eTh  
equivalent Uranium/Potassium eU/K ppm/%
equivalent Thorium/Potassium eTh/K ppm/%

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:

Ternary Map the RGS logo Ternary Radioelement Map (K, eU, eTh) is a special product that provides a unique view of the composite data set

stacked profile sections+geology

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).

Secondary variables
for AGRS surveys
Name Units
Magnetic total field nT
Magnetic vertical gradient nT/m
VLF total field %
VLF quadrature %

Applications of airborne gamma-ray spectrometry


Myths about airborne gamma-ray spectrometry (AGRS)

Myth 1:

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

RGS logoFact:

In Canada and worldwide, airborne and ground GRS techniques have been successfully applied to:


Myth 2:

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

RGS logoFact:

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


Myth 3:

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

RGS logoFact:

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.


Some basic concepts...


2006-08-03
http://www.gsc.nrcan.gc.ca/gamma/theory_e.php