International audienceNatural gas seeps provide a unique opportunity for testing techniques and strategies of environmental baseline assessment (EBA) for exploration or exploitation activities of conventional and unconventional methane. We report field acquisitions performed at a gas seep in the French Alps where thermogenic, CH4-dominated (up to 85% vol.) and CO 2-rich (up to 11.5 % vol.) gas emanates over a localized 8x4 m emission area with complex geometry (Gal et al., 2016) that characterizes many naturally occurring gas vents (e.g. Etiope et al., 2010). We tested a portable system of in-situ soil gas flux measurements, combining an accumulation chamber and a monitoring system, recording CH4 and CO 2 enrichments as well as O 2 depletion over 3 minutes, the typical duration of soil flux measurements. Methane fluxes have been quantified on 70 monitoring points emplaced along a 2x1 m mesh grid. The potential time evolution of the CH4 and CO 2 emission has been assessed through complementary soil gas concentration measurements at 1 m depth. Only minor changes of the location of the emitting points with respect to previous measurements have been observed. Correlations of CO 2 , O 2 and CH4 indicate distinct processes of CH4-oxidation in the area surrounding the main CH4-vent and in the vent itself. The most active part of the gas vent only exhibits high CH4 fluxes whereas adjacent areas show moderate to high CH4 and CO 2 fluxes and more distal areas are only characterized by a CO 2 flux. Multi-gas flux measurements in the vicinity of a seep or supposed leak is an efficient and straightforward onsite method to be integrated in EBA, but spatial extension and resolution of survey grids will depend much on the expected scale of impact of industrial point sources (exploration/exploitation wells). Natural fluxes can be addressed both for methane and CO 2 and reduction-oxydation processes assessed through correlation with O 2. Other vents in similar geological positions in the French subalpine chains on a vast transect from the Swiss border to the Provence were identified, based on historical records, and some of them rediscovered and measured
