Surface Clues for Subsurface Hydrogen: Tracking the Deformation of Sub Circular Depressions with InSAR

Abstract

Natural hydrogen (H2) is an emerging energy resource with significant exploration potential, and sub-circular depressions (SCDs) have been identified as a possible surface proxy for subsurface hydrogen systems. This study applies multi-temporal Interferometric Synthetic Aperture Radar (InSAR) to characterize surface deformation at the Gassola SCD, located near the Bourakébougou natural hydrogen field in Mali, using Small Baseline Subset (SBAS) and Persistent Scatterer (PS) processing of a 64-scene Sentinel-1 ascending orbit time series spanning 2019 through 2023. The SBAS mean LOS velocity field reveals a spatially heterogeneous deformation signal concentrated within and around the depression, with velocities ranging from -32 to +30 mm/year after quality control masking. A transect across the depression resolves central uplift of approximately +15 mm/year flanked by subsidence of -20 to -30 mm/year, with a topographic diameter-to-depth ratio of ~1% consistent with published SCD morphology. Displacement time series show alternating subsidence and uplift phases on timescales of less than a year, with a broadly anti-phase relationship between depression center and rim. Annualized epoch-to-epoch displacement maps reveal episodic deformation activity, with periods of spatially diffuse regional displacement giving way to progressively localized, depression-centered deformation that contracts before decaying to inactivity. This recurring pattern is interpreted as the surface expression of pulsed gas migration converging on the SCD through preferential subsurface pathways. SBAS and PS solutions agree on deformation spatial patterns during most active intervals but disagree in magnitude by a factor of approximately five, attributed to fundamental differences in scatterer sampling between the two methods. No consistent correspondence between rainfall and displacement is identified, supporting a gas-dynamic rather than hydrological interpretation. The results demonstrate the potential of InSAR as a first-order screening tool for natural hydrogen exploration but integration with geochemical and subsurface datasets remains necessary for definitive interpretation.