Underground radon gas concentrations related to earth tides

Over several years, radon concentrations have been recorded in a former gypsum mine near Walferdange (Luxembourg). Because of the exceptional quality of the site (thermal stability better than 0.01 7C/year, no running water, easy access, far enough from the oceans), today the mine hosts an underground laboratory for geodynamics and seismology with more than 25 permanent instruments that continuously record earth tides, earth quakes and meteorological parameters. One of the main interests in monitoring radon concentrations in this mine was to check the ability of earth tide effects on radon concentrations in the mine atmosphere. First results show that besides outside temperature and atmospheric air pressure, radon concentrations seem to be influenced by earth tides. Power spectra, calculated for different time series of radon concentrations, show the presence of both O1 (lunar declination) and M2 (principal lunar) tides. The increase in vertical extension and the decrease in gravity induced by earth tides in the bulk of the rocks may open supplementary pathways for radon and thus induce an increase of the radon transport through the rocks.