Radiometric Characterization of Soils in North-Central Region of Goias And Its Implications for Estimates of Crustal Radiogenic Heat

In this work we present progress obtained in analysis airborne geophysical survey data for the north-central region of the state of Goiás. It included both aeromagnetic and gamma spectrometric surveys. Analysis of aeromagnetic data reveals the presence of a large number of magnetic lineaments, indicative of distinct structural features in subsoil layers. The analytic signal values point to the existence of substantial small scale variations related to lithologic changes. Analysis of radiometric data allowed determination of relative abundances of natural radioactive elements (Uranium, Thorium and Potassium) of the main soil types and subsoil layers The results obtained indicate that most of the common soil types are characterized by Uranium contents of greater than 1ppm. Only argi soil and lato soil are found to have Uranium contents lower than 1ppm. Thorium abundances also have trends similar to those of Uranium. The distribution of Potassium does not follow a similar pattern, but the K/U ratios fall in the expected range of values. On the other hand, the Th/U ratios are higher than normal. This has been considered as indicative of underestimation of Uranium in soil layers of the study area. Systematic differences in sampling depths of gamma rays and non-equilibrium conditions of the radioactive series in soft sedimentary layers are considered to be possible mechanisms. The results have also allowed determination of radiogenic heat of the main soil types and subsoil layers. Its value varies significantly with the soil types. Thus cambi soil in this region is found to have a mean heat production of 3.32 ± 5.9 μWm-3 while that of argi soil is only 0.36 ± 0.3 μWm-3. The mean heat production of soil layers at the surface is 0.68 ± 0.4 μWm-3. Heat production values of basement rocks are estimated to be more than 1.3 μWm-3, after corrections for density changes and nonequilibrium conditions of Uranium series.