Exploration and monitoring of "fairy circles" associated with natural hydrogen seepages with synthetic aperture radar backscatter analysis and interferometry in the São Francisco basin (Brazil)

Circular depressions with distinctive vegetation pattern called “fairy circles” can represent a typical type of hydrogen seeps. Vegetation/soil moisture changes and ground motions are mapped using Synthetic Aperture Radar (SAR) backscatter analysis and interferometry in the São Francisco basin (Brazil) exhibiting numerous fairy circles, including the H 2 -emitting Campinas and Baru structures. Results show that some of the fairy circles are active features in terms of vegetation/soil moisture variations and ground motion on relatively short timescales (<5 years). These changing factors, in general, do not seem to be directly linked to each other, and the underlying processes remain unexplored. While large-scale ground motion anomalies are also detected, the Campinas structure represents the most pronounced subsidence signal with up to 1 cm/year, implying its active formation potentially connected to an underlying H 2 system. SAR-based mapping proved to be successful in monitoring the dynamics of fairy circles, providing constraints towards the understanding of potential H 2 seeps. • SAR-based remote sensing is applicable to monitor (H 2 -emitting) “fairy circles”. • Vegetation/soil moisture change does not correlate with active H 2 seeping. • The H 2 emitting Campinas structure is actively subsiding. • Surface changes imply underlying processes potentially connected to subsurface H 2 .