Airborne Geophysical Surveys in the North-Central Region of Goias (Brazil): Implications for Radiometric Characterization of Subtropical Soils

In this work we present progress obtained in analysis airborne geophysical survey data for the north-central region of the state of Goias (Brazil). The results obtained indicate that most of the subtropical soil types are characterized by Uranium contents of greater than one parts per million (ppm). Only ultisol and oxisol soils are found to have Uranium contents lower than one ppm. Thorium and Potassium abundances also display trends similar to those of Uranium. The K/U ratios fall in the expected range of values for common soils while the Th/U ratios are higher than normal. This latter observation may indicate a characteristic feature of subtropical soils. Alternatively it may be considered as indicative of disequilibrium conditions in radioactive series and consequent underestimation of Uranium in soil layers of the study area. In this context we point out the possibility of using results of radiometric surveys as a convenient complementary tool in identifying geochemical zoning of soils in subtropical environments. Analysis of aeromagnetic data reveals the presence of a large number of magnetic lineaments. The analytic signal values point to the existence of substantial small scale variations related to lithologic changes. There are indications that deposition of soil types are controlled to a large extent by the system of NE-SW trending faults and fracture systems. According to results of radiogenic heat production calculations the cambisol soil in this region are found to have a mean heat production of 3.32 \pm 5.9 \mu Wm-3 while that of ultisol soil is only 0.36 \pm 0.3 \mu Wm-3. The mean heat production of soil layers at the surface is 0.68 \pm 0.4 \mu Wm-3. Heat production values of basement rocks are estimated to be more than 1.3 \muWm-3, after corrections for density changes and non-equilibrium conditions of Uranium series.