Surface identification of undermined spaces with the use of soil radon emanometry
Several surface geophysical methods were used to study the displays of undermined spaces in their physical fields and to try to distinguish undermined from non-undermined parts of underground mining area. The studied area of the Čáry lignite mine in the Western Slovakia represents an actively subsided place with high risk to the population. Despite very low radioactivity and relatively high gas permeability of building geological formations, the results of soil radon emanometry show the possibility of radon gas accumulation inside the undermined spaces, but their permanent subsidence causes loosing of overlying material and escape of radon gas. The boundaries (edges) of undermined and sunken areas were identified as the only places with increased values of 222Rn activity, probably due to the presence of vertical supporting mine walls allowing radon gas accumulation and upward movement. Thus, the soil radon emanometry clearly indicates the borders between undermined or sunken and non-undermined parts.