Ground penetrating radar for detecting subsurface features of active gas vents — mofettes in Slovenia

Mofettes are natural gas vents where vast amounts of CO
₂
migrate from the subsurface and discharge into the atmosphere. Some mofettes can easily be identified by bubbles forming in the water collected in gas vents or by reduced vegetation in the surrounding area, however these indicators are not always present in the field. Geophysical investigations of the shallow subsurface can help detect possible gas vents also in dry areas without visible changes to the vegetation. The objective of this study was to test the suitability of the GPR method for researching subsurface features of two mofette sites in NE Slovenia by using three different antenna frequencies. The best ratio between resolution and depth penetration was acquired with the 500 MHz antenna. The results show that high CO
₂
concentrations coincide well with areas of high signal attenuation seen in GPR profiles. Where mofettes were visible at the surface and the vegetation was reduced, concave reflectors were identified underneath the high attenuation horizon. Below these concave reflectors, narrow vertical zones without reflections were also observed. These could represent fractures serving as pathways for seeping gas. On both sites, 3D models also provided information about the spatial extend of the mofettes.