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Magnetic susceptibility of 72 cambisol profiles from the vicinity of the Vír dam, NW Moravia was measured. The enhanced susceptibility of topsoil, particularly of the horizon O, was assessed from the aspect of vegetation setting and magnetic mineralogy. Magnetic susceptibility variations with field and temperature as well as frequency dependent susceptibility were applied to indicate magnetic carriers. It was found that the enhanced magnetic susceptibility is caused very likely by the presence of maghemite and magnetite of various grain sizes. Magnetic minerals are pedogenic in origin in all three horizons, while in the O and A horizons they are mostly anthropogenic in origin. Magnetic susceptibility was correlated with contents of trace elements Mo, Cu, Pb, Zn, Ag, As, Cd, Sb, Bi, Hg, Se, furthermore with TOT/C, TOT/S, and 137 C. The close association of susceptibility with Pb, Sb and Hg, or also with Mo, As, Se and their similar depth variations suggest a slight anthropogenic input in soils from a long-distant source. The same is valid for 137 C, its strong correlation with magnetic susceptibility has been found.

Oil at the Zdanice field in the SE Czech Republic is reservoired at a depth of 900 m in weathered Precambrian granitoids and Lower Miocene sandstones and conglomerates. Tertiary claystones, siltstones and sandstones form the top seal. Surface gamma‐ray spectrometry carried out along two profiles across the field (6,880 m and 8,335 m in length, respectively) found perceptible decreases of K, U, and Th concentrations relative to background values. This can be explained in terms of enhanced leaching of these radionuclides by groundwater acids derived from hydrocarbons. Observed increases in radon and thoron in soil gas above the oil‐producing zone at Zdanice may be caused by enhanced gas emanation from U‐ and Th‐bearing minerals decomposed by groundwater acids. Laboratory analyses of 444 samples collected from 18 shallow boreholes (20 m deep) pointed to a trend of decreasing K, U and Th concentrations above the oil‐producing zone at Zdanice. Decreases in rock porosity and organic carbon content were also noted together with an increase in mineralogical density, magnetic susceptibility and sulphur content. Petrophysical analyses of 330 samples from deep boreholes showed lower values of K and U in samples from oil‐producing wells. These observations of minor petrophysical alterations in rocks overlying the Zdanice oilfield may be characteristic and may be applicable to hydrocarbon exploration operations elsewhere.