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The data used in this compilation come from the Canadian Gravity Database (CGDB), which is managed by the Canadian Geodetic Survey (CGS), Surveyor General Branch. CGDB includes more than 755 000 observations, including some 232 000 observations on land. The distribution of the land data represents an average of one gravity point per 40 km². The gravity maps, which are gridded to a 2-km interval with a blanking radius of 20 km, currently include data acquired between 1944 and 2015. CGS and partners continue to supplement the CGDB each year. The surveys are conducted using relative gravimeters that measure the gravity difference between two locations. On the landmass, gravity has been measured primarily using static gravimeters. Although measurements at some offshore locations have been collected using static gravimeters on the ocean floor, most are acquired using dynamic gravimeters aboard moving vessels. The relative nature of the gravimeters require that surveys be tied to base (control) stations with known absolute gravity. The base stations are part of the Canadian Gravity Standardization Network (CGSN), which is tied to the International Gravity Standardization Network 1971 (IGSN71). Today, the traditional base stations are being replaced by new base stations that are measured with an absolute gravimeter having an accuracy of 2µGal (2x10-8 m/s²). All relative gravity measurements are integrated into the IGSN71 datum to create a coherent dataset at the global scale. Normal (theoretical) gravity is calculated using the Geodetic Reference System 1980 (GRS80; https://doi.org/10.1007%2FS001900050278). Bouguer anomalies, which include reductions of the elevation and topographical mass to sea level, are calculated using a vertical gravity gradient of ~0.3086 mGal/m (change slightly with latitude and elevation) and a crustal density of 2670 kg/m³.
The data presented in the radioactivity map of Canada series (Buckle et al., 2014) depict the surface concentrations of three naturally-occurring radioactive elements: potassium (K, %), equivalent uranium (eU, ppm), and equivalent thorium (eTh, ppm); as well as five derived products: natural air absorbed dose rate (NADR, nGy/h) calculated from a linear combination of potassium, equivalent uranium, and equivalent thorium concetrations; the ratios eU/eTh, eU/K, and eTh/K; and the ternary map which uses false colour to illustrate the co-variation of the three measured elements (Broome et al., 1987). This compilation was produced with data from more than 370 airborne gamma-ray surveys flown or supervised by the Geological Survey of Canada between 1969 and 2011. Data was calibrated and acquired in accordance to standards in effect at the time each survey (see Darnley et al., 1975 and IAEA, 1991). Most of the data was acquired using 50 L of Sodium Iodide (NaI) detectors flown at a nominal terrain clearance of 120 m, but line spacings vary from 5000 m to 200 m depending on the specific survey. Potassium is measured directly from the 1460 keV gamma-ray photons emitted by Potassium-40. Uranium and thorium, however, are determined indirectly from gamma-ray photons emitted by daughter products Bismuth-214 (1765 keV) and Thallium-208 (2614 keV) respectively assuming equilibrium between daughter and parent isotopes. For this reason, gamma-ray spectrometric measurements of uranium and thorium are referred to as equivalent uranium (eU) and equivalent thorium (eTh). The measured gamma-rays originate from geological materials in the upper 30 cm of the Earth's surface and their intensity are directly related to the concentrations of K, U and Th in the rocks and minerals present. The geochemical information presented in this compilation is used to support bedrock and surficial geology mapping by outlining lithological variations. It can also indicate mineralization either by association of radio-elements as trace elements with economic minerals or through delineation of their enrichment or depletion due to geochemical alteration resulting from mineralization processes. Overall, this information also contributes to the characterization of the natural radiation environment. Futher information on data acquisition, processing and interpretation and on application can be found in IAEA-TECDOC-1363 (2003), and references therein. These data were also published as Geological Survey of Canada maps, in the Open Files series (7396-7403). References Broome, J., J.M. Carson, J.A. Grant, and K.L. Ford, 1987. A modified ternary radioelement mapping technique and its application to the south coast of Newfoundland, Geological Survey of Canada, Paper 87-14. https://doi.org/10.4095/122382 Buckle, J.L., J.M. Carson, K.L. Ford, R. Fortin and W.F. Miles, 2014, Radioactivity map of Canada, ternary radioelement map, Geological Survey of Canada, Open File 7397. https://doi.org/10.4095/293354 Darnley, A.G., E. M. Cameron and K. A. Richardson, 1975. The Federal-Provincial Uranium Reconnaissance Program, in Geological Survey of Canada, Paper 75-26, p. 49-71. https://doi.org/10.4095/102591 International Atomic Energy Agency, 1991. Airborne Gamma Ray Spectrometer Surveying, International Atomic Energy Agency, Technical Reports Series No. 323. https://www.iaea.org/publications/1427/airborne-gamma-ray-spectrometer-surveying International Atomic Energy Agency, 2003. Guidelines for radioelement mapping using gamma ray spectrometry data; International Atomic Energy Agency, Technical Reports Series No. 1363. https://www.iaea.org/publications/6746/guidelines-for-radioelement-mapping-using-gamma-ray-spectrometry-data
This data set is a compilation of data acquired mostly by airborne surveys in Canada, gridded at 200 m and 1 km resolutions. The Geological Survey of Canada has flown or supervised more than 700 surveys since 1947, generally with a flight-line spacing of 800 m and an altitude of 305 m above the ground, though since 2000 the majority of surveys have been flown with a line spacing of 400 m or less. These aeromagnetic surveys have been leveled to each other to correct for arbitrary datums, slow variations of Earth's magnetic field over time, and differing survey specifications. The magnetic character of a rock depends on its ferromagnetic mineral composition, its concentration and its deformational and metamorphic history. Variations in the magnetic character of Earth's crust cause small magnetic anomalies in the earth's magnetic field. These magnetic anomalies can show geological trends and structural boundaries. The first vertical derivative of magnetic anomalies is calculated from the residual magnetic field and enhances the short wavelength component of the field. It is often used to trace contacts between magnetic domains. These data have also been published as two Geological Survey of Canada Open File maps: Magnetic Anomaly Map, Canada, (Open File 7799) and the First Vertical Derivative of the Magnetic Anomalies Map, Canada, (Open File 7878).