Variace radonu a davkoveho prikonu na tektonickych poruchach v metamorfitech a granitoidech Ceskeho masivu

The radon release from bedrock into dwellings is dependent among other geofactors on the presence of tectonic structures, that, in favourable local conditions, can support the radon migration upwards. The position of selected faults was derived from geological map server of the Czech Geological Survey at 1 : 50 000 scale. Profile measurements of soil gas radon concentrations (222Rn, referred to as Rn) and gamma dose rate (referred to as D´) were performed across detected simple faults (Blanice Furrow area, 19 profiles) and mylonitized faults (Central Bohemian Plutonic Complex, 18 profiles) on a detailed scale - for localization see Fig. 1. The distance of measuring points was 3 m; each profile was measured on 10 points supposing the exact position of fault core at point 5. For Rn measurements the RM2 radonmeter (ionizing chambers principle, 80 cm depth) was used, gamma dose rate measurements were performed using RP2000 radiometer 1 m above soil surface. Mutual variations of Rn and D´ on each profile with respect to Rn and D´ trend (increase or decrease of values) were evaluated in the close vicinity of point 5. The results of trends in the Blanice Furrow area with prevailing lithology of migmatitized paragneisses and two-mica paragneisses are given in Tab. 1. The increase of D´ was observed for 12 of 19 profiles close to fault core and the accordant trend of Rn and D´ was detected for 10 profiles from 19. Similar evaluation was performed on 18 profiles in granitoids of Central Bohemian Plutonic Complex (see Tab. 2), where increase of D´ at the mylonitized faults was observed on 13 profiles and accordant Rn and D´ trend on 7 profiles. The position of maximum values of Rn and D´ was studied for all points in profiles according to rock types. The fault core in migmatitized paragneisses is characterized by occurence of maximum Rn concentrations at point 5 and localization of D´maxima in the distance of 3-6 m apart from the fault core ( see Fig. 2). The position of Rn and D´ maxima in two-mica paragneisses is opposite - D´maxima at the fault core and Rn maxima up to 6 m apart (see Fig. 3). This fact can be explained by the different character of tectonic filling (clayey minerals) in the above mentioned rock types. The variations of Rn and D´ in mylonitized faults in granitoids (see Fig. 4) look similar as in two-mica paragneisses however the mylonitized faults are characterized by lack of maxima both Rn and D´ at the supposed position of fault core. The Rn and D´variations around the fault plane are illustrated in Fig. 5. It can be concluded that behaviour of Rn and D´at the faults is influenced by lithology of rocks, type of structure and level of weathering processes accompanying the fault.

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