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Short-term temporal variations of soil gas radon concentration have been studied using different measuring techniques--instantaneous methods (grab sampling) using Lucas cells, continuous monitors, and integral nuclear track-etch detectors. A relatively low variability appeared during a 72-h follow-up. Different temporal changes were observed by using different methods. A substantial part of these changes was probably caused by fluctuations and errors connected with measuring methods themselves and did not reflect real variations of the measured parameter.

S U M M A R YResearch of geochemical and petrophysical changes in rocks over theŽdánice hydrocarbon field, SE Moravia, the Czech Republic, with hydrocarbon accumulation at the depth of 900 m, was performed by surface geophysical methods along two regional profiles A and B in the period of 2003-2005. The methods of ground investigation were gamma-ray spectrometry, the measurement of radon and thoron in soil air, thermometry and measurement of soil gas permeability of rocks. The measurement was conducted at 174 stations along profile A, at the length of 6880 m, and at 214 stations along profile B, at the length of 8335 metres. Geophysical data vary significantly due to alternating sandstones and claystones in sedimentary cover and statistical methods were applied to distinguish the signatures between rocks of oil-productive zone (OPZ) and its surroundings. The OPZ exhibits decreases of K, U and Th, which can be ascribed to enhanced leaching of natural radioelements caused by hydrocarbon-generated groundwater acids. The decrease of Th does not support the hypothesis of Th geochemical stability. Increases in radon and thoron over an OPZ may be caused by enhanced emanation power of unbound U-and Th-containing minerals due to acids. Low-magnitude radiometric signatures, reflecting petrophysical changes, can contribute to estimates of hydrocarbon prospects in studied areas.

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