A geophysical investigation of the fairy circles in Nebraska for geologic hydrogen exploration

Fairy circles are subcircular surface depression features that are widely distributed all over the world and they are commonly considered as a potential indicator to the presence of subsurface geologic hydrogen. Understanding the relationship between fairy circles and geological structures could assist in understanding potential hydrogen migration pathways and accumulations. We have carried out an integrated geophysical investigation over a group of such circles in the central U.S. We integrate digital elevation model (DEM), airborne gamma-ray spectrometry (AGRS) data, and ground electrical resistivity data to understand the connection between the surficial expressions and the underlying geology of the fairy circles. The study has shown that correlation between the locations of the fairy circles with either radiometric data or subsurface mafic or ultramafic rocks cannot be established. Meanwhile, the imaged geo-electrical structure within 500 m depth from the surface shows that the rim of the studied circle is directly above a ring of more resistive gap in a conductive layer. The low water saturation or lower salinity imaged by the high resistivity in this ring would allow the subsurface H2 to seep through more easily and, therefore, can explain the elevated H₂ concentration from soil gas sampling.