Second Projet de Renforcement Institutionnel du Secteur Minier de la Republique Islamique de Mauritanie (prism-ii)
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Figure 11. Color-shaded relief image of gridded depth estimates using the Extended Euler method (Phillips 2002). A structural index of 1 and a window size of 7 (1225 m) were used.
3 Radiometric DataWhile many naturally occurring elements contain radioactive isotopes, only potassium and the uranium and thorium decay series, possess radioisotopes that produce gamma rays of sufficient energy and intensity to be measured by gamma ray spectrometry. This is because they are relatively abundant in the natural environment. The actual elements measured and their units are: K: potassium, i.e. K40 (units: %), eU: equivalent uranium concentration by weight (the spectrometer directly measures Bi214, that is an indirect measure of uranium) (units: ppm) and eTh: equivalent thorium concentration by weight (the spectrometer directly measures Tl208, that is an indirect measure of thorium) (units: ppm). The depth of penetration of the radioelement data is about 30 cm, so it is most valuable as a surficial geologic mapping tool. Average crustal abundances of these elements quoted in the literature are in the range 2-2.5% K, 2-3 ppm U and 8-12 ppm Th (Nicolet and Erdi-Krausz 2003). The radiometric data were collected along with the aeromagnetic data and reduced by the contractors (FUGRO and Sanders). The gridded K, Th, and U data from the 1:200,000 scale sheets were merged together (Figs. 12-14) and added together to produce a ternary radioelement map (Fig. 15, Plate 3). A ternary radioelement map is a color composite image generated by modulating the red, green and blue phosphors of the display device or yellow, magenta and cyan dyes of a printer in proportion to the radioelement concentration values of the K, Th, and U grids. The use of red, green and blue for K, Th and U, respectively, is standard for displaying gamma ray spectrometric data. Blue is used to display the U channel, since this is the noisiest channel and the human eye is least sensitive to variations in blue intensity. White regions contain high concentrations of all 3 elements, whereas blue and other dark colors indicate low concentrations of all 3 elements. 
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