Radioactivity data - Airborne gamma-ray spectrometry

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 types of radiation (alpha, beta, gamma) at specific energy levels. Only gamma-ray radiation has sufficient energy to be useful for geological mapping or exploration. However, gamma-ray spectrometry provides a method of measuring concentrations of individual radioactive elements (in particular, K, U, Th) as the basis for mapping rocks and soils by virtue of their characteristic radioactivity signatures.

Gamma rays are released through the spontaneous decay of radioactive elements. The 3 most common, naturally occurring radioactive elements are potassium, uranium and thorium, which are found at some concentration in most rock-forming minerals. Potassium, for example, occurs mainly in the mineral feldspar, which is an abundant and widespread mineral in the earth's crust, and a prominent component of granitic rock. Uranium and thorium are generally present in low concentrations (measured in parts per million (ppm)) in a wide range of minerals. High concentrations of uranium may represent a target of economic interest to mining companies. Indirectly, the three radioactive elements can provide an indication of economic concentrations of many other metals. The concentrations of potassium, uranium and thorium have been mapped over about 40% of Canada, using airborne detectors to measure the distinctive gamma ray spectra from the three radioactive elements. These radioactivity maps provide information about the 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) 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 radioactive element contrasts to mapped bedrock and surficial geology, 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.

Airborne methods provide valuable, systematic coverage of large areas. Ground spectrometry greatly improves the resolution of individual radioactive element sources. By relating the radioactive element variations, measured with 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 an orientation or reconnaissance survey. This may be followed by a more formal survey using a series of grid lines if warranted. The resulting geochemical information provides an important additional layer of information significantly improving bedrock and surficial mapping and ore vectoring

Airborne surveys are typically flown at a planned terrain clearance of 120 m, with flight line spacing ranging between 200 m and 5,000 m, depending on the purpose of the survey and available funds. Typically most reconnaissance scale surveys were flown at a line spacing of 5,000 m, while regional scale surveys were flown with a line spacing of 1,000 m. For detailed mapping or deposit scale applications, surveys are flown with a line spacing of 200 to 500 m.

Stripping ratios, attenuation coefficients and sensitivities must be carefully determined for each acquisition system. Proper application of these constants during processing results in seamless data sets across individual survey boundaries, regardless of the acquisition system used. For further information, see Practical theory at Radiation Geophysics.

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

Often, depending on the complexity of the geology, subtle variations in K, U and/or Th may not be readily apparent. For these reasons, the proper interpretation of gamma-ray spectrometry data requires the examination of all of the measured variables and associated derived products. Ratio maps can enhance or reinforce subtle variations in the measured variables. This can be particularly important, especially when dealing with varying intensities of alteration associated with a mineralizing process. For example, potassium alteration (enrichment) associated with certain types of mineral deposits, may occur in host rocks with normally low to moderate levels of K resulting in a high K signature. This would be easily recognized if this alteration occurred in isolation. However there may be normal, high K rock types in close proximity to the altered rocks. In this situation, the K associated with the alteration may not be distinguishable from other high K rock types. The ratio maps, in particular the eTh/K ratio, can be a sensitive indicator of K alteration associated with mineralization and can be used as a direct indicator of mineralization. Examples of this would include AGRS data over porphyry deposits such as Casino, YT. On a regional scale, subtle variations in the measured variables that are clearly reinforced on the ratio maps are shown by the eU, eTh and eU/eTh maps for southern Nova Scotia. Within the peraluminous granitic rocks of southern Nova Scotia, uranium concentrations generally increase and thorium concentrations decrease with increasing magmatic differentiation, resulting in abnormally high eU/eTh ratios associated with the most evolved parts of these granitic intrusions.

The ternary (three-component) radioactive element map is an effective method of displaying variations in total radioactivity and in the relative abundances of the three radioactive elements. Areas of the image with the same colour will have similar ratios of K, eU, eTh, and the intensity of that colour is a measure of the total radioactivity. This allows the map to represent the radioactive element distribution better than any of the other single variable maps. The ternary map is often easier to work with to get an overview of the distribution of radioactivity, however, it does not replace the more detailed, quantitative information available on the other 7 maps (total radioactivity, K, eU, eTh, eU/eTh, eU/K and eTh/K). For more information, see:

Broome, J., Carson, J.M., Grant, J.A. and Ford, K.L., 1987
A Modified Ternary Radioelement Mapping Technique and its Application to the South Coast of Newfoundland; Geological Survey of Canada, Paper 87-14.

Most of the airborne data for Canada was collected through Federal/Provincial co-operative programs to support ongoing and future geological mapping activities and mineral resource exploration. For these reasons survey areas were initially chosen because of their higher mineral potential, for example, the Canadian Shield. Most of Canada's major population centers occur within 200 kilometers of the border with the United States in areas of low mineral potential. Therefore there is little coverage around major population centers in Canada.

The airborne gamma-ray spectrometry (AGRS) data used to create these images were acquired by a series of several hundred airborne surveys over 30 years, from about 1970 to 1999, using a variety of aircraft and instrument packages. The aircraft flew along a pattern of parallel flight lines typically at 120 m terrain clearance (height above the ground). The flight line spacing includes 200-500 m, 1000 m, 5000 m and some at 25000 m. Typically aircraft fly at a speed of ~120 knots (190 km/h). Most data were acquired by sampling (counting) at 1 second intervals (some of the oldest data were sampled every 2.5 seconds), which is equivalent to about 60 m on the ground.

These 250 m grids were prepared using survey data from the NATGAM database. More detailed survey data supersede the regional survey data in these grids.

For further details of data processing and practical theory related to gamma-ray spectrometry, visit the Radiation Geophysics Web site.