Radiometric Characterization of soils in North-Central Region of Goias and its implications for estimates of crustal radiogenic heat
Suze Nei P. Guimarães and Valiya M. Hamza, Observatório Nacional – ON/MCT, Rio de Janeiro.
Copyright 2011, SBGf - Sociedade Brasileira de Geofísica
This paper was prepared for presentation during the 12th International Congress of the Brazilian Geophysical Society held in Rio de Janeiro, Brazil, August 15-18, 2011.
Contents of this paper were reviewed by the Technical Committee of the 12th International Congress of the Brazilian Geophysical Society and do not necessarily represent any position of the SBGf, its officers or members. Electronic reproduction or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibited.
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 non-equilibrium conditions of Uranium series.
The intensities of nuclear radiations in any particular terrestrial environment are in general related to the abundances of natural radioactive elements in rocks and soils of that locality (Adams, 1961; Wilford et al., 1997). Gamma ray spectrometric methods have been widely used in measurements of radioactive minerals in soils and basement rocks (Ex: Adams and Gasparini, 1970). It has also been used for mapping area extent of geologic rock formations associated with such radioactive elements. Examples of such studies carried out in Brazil include the works of Blum et al (2003), Sapucaia (2005).
Measurements of Gamma radiations at ground levels provide information on the abundances of radioactive elements in the top soil layer of <40cm. Nevertheless results of airborne measurements are considered representative of radiometric characteristics of basement rocks beneath the soil cover. The reason is that weathering and surface processes promote large scale interactions between soil cover and basement rocks. Hence results of gamma spectrometric methods have been widely used as supplementary tools in geologic mapping. Examples of such studies carried out in Brazil include Vasconcelos et al (1994), and Maas et al (2003). With the advent of computer processing facilities it has been possible to analyse the results of large scale air-borne radiometric data. In the present work we report progress obtained in analysis of airborne gamma spectrometric data carried in the state of Goiás. The purpose is to map the radiometric characteristics of the soil and estimate radiogenic heat production of subsoil layers on regional scale.