Exploration of “fairy circles” associated with natural hydrogen seepages with synthetic aperture radar interferometry and backscatter analysis

With the increasing demand for alternative energy sources, natural hydrogen is gaining attention for commercial exploitation. Naturally accumulated hydrogen is only utilized today at the field of Bourakébougou, Mali, highlighting major knowledge gaps in the behaviour of hydrogen systems and in the related exploration-production workflows. Circular depressions called “fairy circles” represent a surface manifestation of hydrogen seeps that commonly occur in continental cratons and are formed relatively quickly (few years). Apart from the topographic imprint of these ~100 m to 2 km diameter depressions, a major signature of the structures is a vegetation anomaly; characterized by a zone of dying vegetation inside the circle, and a ring of healthy, enriched vegetation in their surroundings. Although the connection of surface H₂ seeps to deep-seated H₂ sources has been implied in several case studies, the exact mechanism of fairy circle formation is still largely unknown, together with the underlying generation, migration, and accumulation processes of H₂.

Satellite images are widely used for the mapping of fairy circles, but these observations are mainly restricted to passive satellite sensors without monitoring any temporal changes of the structures. In this study we used Synthetic Aperture Radar (SAR) images acquired by the European Space Agency’s Sentinel-1 satellites to monitor the evolution of fairy circles in terms of morphological and vegetational changes in two demonstration areas: in the Sao Francisco Basin of Brazil, and in the Lublin Basin of SE Poland. In both cases, the duration of the monitoring was ~5 years, with a temporal resolution of ~1 month. We applied the Interferometric Synthetic Aperture Radar (InSAR) method to map ground motions associated with the potentially active surface deformation of fairy circles. We extended the ground motion time series with SAR backscatter analysis to identify changes in the strength of the backscattered signal through time. The aim of the backscatter analysis was to identify any rapid changes associated with the loss/increase of vegetation linked to H₂ degassing. Results show significant ground motion and vegetation anomalies associated with fairy circles in the Sao Francisco Basin (Brazil). Results are not that evident in the Polish area, mostly due to its poorer suitability for InSAR and backscatter analysis (generally lower coherence areas and presence of agricultural and other artificial activities overprinting natural variations). The SAR-based observations were compared with geochemical measurements for monitoring H₂ emissions in the soil in both areas, to better understand the potential link between H₂ degassing and morphological and/or vegetation changes. The detailed understanding of subsurface processes responsible for the detected anomalies and H₂ seeping cannot be inferred, but important constraints on fairy circle formation are achieved. This study demonstrates the applicability and limitations of InSAR and backscatter analysis for the mapping of actively changing fairy circles over two different areas, with important implications of the methodology for further case studies worldwide and constraints on natural hydrogen systems in general.