First field test of linear gas sensor net for planar detection of CO2 leakages in the unsaturated zone

Gases from subsurface sources can present risks for the biosphere. To extend the lead time in managing such risks, reliable gas detection in the subsurface is required. A monitoring system was developed that is able to: (i) gather information from large areas, (ii) work efficiently within the subsurface; insensitive to changing subsurface environmental conditions while sensitive to changes in target gas concentrations, and (iii) provide a fast response. We report the first field test of linear membrane-based gas sensors: Fourteen 40 m long sensors were installed in two horizontal nets one above the other within a homogenized soil and scanned from a control station. 10 L/min of CO2 was injected into a point-like port 25 cm below the lower sensor net. Two operation modes were successfully tested to detect the CO2 plume: an active mode, which is sensitive to the concentration, and a passive mode, which is sensitive to changes in concentration. By conversion of the monitoring results the minimum contact range of seeping gas and sensor could be determined. Based on this valuable approach, an unexpectedly high level of lateral gas spread within the soil and the formation of a gas pillow below the soil surface could be observed.