Spatial distribution characteristics of soil radon in the southern Tangyin graben

This paper discussed the spatial distribution characteristics of radon in the soil gas and their relationship with faults, geological structures, lithology, and sediment thickness based on the radon concentrations obtained by the field mobile measurement at the gridding layout observation points in the southern Tangyin graben. The measurements showed that the soil radon concentrations in the Tangyin graben varied from 3.09 to 78.54 kBq/m³ with a mean value of 27.22 kBq/m³, and the anomalous threshold was 48.40 kBq/m³. Spatially, the studied area was divided into two parts based on the contour of Quaternary system （50 m thickness）, the distribution characteristics of soil gas presented that radon background concentrations were higher in western region than that in eastern evidently because of the difference of lithology units made up the local strata and the influence of human mining activity. Accordingly, the radon concentration anomalies of soil gas in western region were patchily scattered on the periphery of Tangxi fault belt besides of distributed along the fault belt itself. Nevertheless, in eastern region, the most of radon concentration anomalies mainly presented along Tangzhong and Tangdong fault belts. Similarly, the contours map of radon concentrations also indicated the azimuth of concentration anomalous belts were consistent with the strike of Tangzhong and Tangdong faults in east region, which implied the emanation of deep-seated source gas was controlled by fault structures. In addition, the radon concentrations contours map also suggested there was a radon anomalous band of NE strike that was almost parallel to contours of local Quaternary system thickness, by which we speculated there was a buried fault. Furthermore, in this studied area, though the release intensity of soil radon in the western part was significantly higher than that in the middle-eastern part, the radon concentrations anomalous extent showed a trend of increasing from west to east, which revealed that the Tangdong fault was more active than others. The comprehensive analysis indicated that the spatial distribution of soil radon concentrations was mainly controlled by fault structures, lithology formation, thickness of sedimentary layer, and human mining activities, and variations of radon concentrations were mainly dominated by the background tectonic activity of southern Tangyin graben. Our results imply that soil radon is an effective indicator for tectonic activity observation of Tangdong fault. While it is appiled to Tangxi fault which is located in the transition region between the uplift and the subsidence the influence of environmental background needs to be fully considered, because of the impact of bedrock cropping out partially and human mining activities.