To convert ground concentrations of K, U and Th to “natural exposure” (given by Grasty, 1984), one should use

NE (uR/h) = 1.5 * K (%) + 0.625 eU (ppm) + 0.31 eTh (ppm);


Or, one could use the following formulae to calculate the “natural air absorbed dose rate”:

ADR (nGy/h) = 13*K (%) + 5.4 * eU(ppm) + 2.7 * eTh (ppm)


See for definitions for instance http://www.empr.gov.bc.ca/Mining/Geoscience/MapPlace/thematicmaps/pages/Clisbako.aspx