The analysis of temporal and spatial variations in the flux of soil gases across the soil–air interface is a useful tool to study geo-dynamical processes associated with volcanic and/or seismic activity. However, many of these variations are induced by external variables, such as temperature, barometric pressure, rainfall and other meteorological variables. In an attempt to filter out non-endogenous variations in the emissions of gases, the optimal choice of the monitoring sites with numerical filtering techniques based on multi-variate and frequency domain analysis of the time series for gaseous emissions were combined, in the case of radon (Rn-222). Monitoring sites are located in underground galleries in the volcanic island of Tenerife, Canary Islands, Spain. Since the effect of wind, rainfall and temperature variations are very small inside galleries, a first natural filtering process of external parameters in the emissions of gases was achieved. This new approach has been successfully tested and as a result, the background level for radon emissions at various locations has been defined, by which correlations between gaseous emissions and the volcanic and/or seismic activity could be carried out.

This work has been funded under Spanish MCyT projects PB98-0643 and PGC2000-2177-CE, the European Union Project EVR1-CT-1999-40010) and project VULMAC-1 of the European Programme INTERREG-IIIB. AED thanks the Spanish MEC for the contract under the programme ‘Ramón y Cajal’.

15 pages, 12 figures, 3 tables.-- Available online Oct 12, 2006.