Vliv horninovych kontaktu na objemovou aktivitu radonu a davkovy prikon gama zareni hornin

The profile measurements of soil gas radon (²²²Rn) concentration and gamma dose rate (D´) were carried out along the contact of rocks characterized by contrasting radon index (Neznal et al. 2004). A total of 45 localities investigated were situated in four areas: Klatovy, Kytlice, Veselí nad Lužnicí and Chrudim showing a variable geology. Each profile comprised 3 measuring sites (25–50 m apart) with radon measurements done at a depth of 30, 40, 50, 80 cm, and gamma dose rate measurement 1 m above the surface. The local trend in profiles was compared with regional data from radon database of the Czech Geological Survey and with a radiometric map (Manová – Matolín 1998). This comparison resulted in mean 79 % accordance of both local and regional trends (range 67–100 %) in 4 areas with different geological environment. The direct evidence of rock contact at a scale of particular profiles was observed on 68 % of the measured localities. The lowest accordant radon concentration and gamma dose rate increase or decrease (55 %) on the immediate rock contact was observed in the Klatovy area, which can be explained by the presence of a wide belt of hornfelses situated in the contact zone of Variscan granitoids and Neoproterozoic metasediments. The highest accordance (80 %) was found in the area of Kytlice (a contact of trachytes with Cretaceous sediments), which is illustrated in Fig. 1 (joined profiles of 2 localities). The increase in soil gas radon concentration with depth was observed in the majority of measured profiles. However, in some cases, especially in the Veselí nad Lužnicí area (a contact of weathered Variscan granitoids or Moldanubian migmatites with Cretaceous sandstones and claystones), the upward release of radon was influenced by impermeable barriers. The mean radon concentration at depths of 50 and 80 cm was compared separately in 4 areas (Fig. 2). The significant difference in depth trend can be observed in the area of Klatovy (possible influence of contact metamorphism). The results of the project confirm the role of radiometric parameters as a useful tool for detection of rock contacts at a scale of tens of meters.

References

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IAEA-TECDOC-1363 (2003): Guidelines for radioelement mapping using gamma ray spectrometry data. – IAEA Vienna.

Manová, M. – Matolín, M. (1998): Radiometrická mapa České republiky 1 : 500 000. – GEOČR 500, CD-ROM. – Čes. geol. služba. Praha.

Neznal, Matěj – Neznal, Martin – Matolín, M. – Barnet, I. – Mikšová, J. (2004): The new method for assessing the radon risk of building sites. – Spec. Pap. Czech Geol. Survey 16, 48 str.

sine (2013): Doporučení SÚJB: Stanovení radonového indexu pozemku přímým měřením. http://www.sujb.cz/fileadmin/sujb/docs/radiacni-ochrana/121031_Doporuceni_RIP.pdf.