Satellite InSAR Data for Monitoring Ground Displacement in Salt Cavern Underground Gas Storage Sites

Natural gas remains a critical transitional energy source in the shift towards renewable systems, addressing seasonal demand fluctuations and supporting energy security. Underground Gas Storage (UGS) facilities, particularly salt caverns, play a key role in this framework by enabling rapid injection and withdrawal cycles. However, UGS activities can induce ground deformation, including subsidence and seasonal displacements. This raises questions about geomechanical stability, long-term sustainability, and environmental impacts. In this context, Interferometric Synthetic Aperture Radar (InSAR) represents a promising technology for large-scale, cost-effective, and high-precision monitoring of surface displacements associated with UGS operations. This study introduced a novel methodological framework for UGS monitoring by the use of Sentinel-1 SAR images and the advanced SqueeSAR algorithm. Two case studies from Lower Saxony (Germany), Jemgum and Nüttermoor, were selected as representative salt cavern UGS sites characterised by high injection/withdrawal rates and long operational histories. Multi-temporal InSAR analyses revealed subsidence velocities of up to 28 mm/year, resulting in distinct cone-shaped deformation that encompasses both UGS facilities. Decomposing the ascending and descending datasets allowed the vertical and east-west horizontal components of displacement to be quantified. The results confirm that salt cavern convergence induces a long-term subsidence trend, while operational cycles generate seasonal displacement patterns correlated to injection and withdrawal phases. To standardise the detection of UGS-affected areas, a semi-automatic thresholding procedure was implemented within a GIS environment, combining displacement velocity, cumulative deformation, and seasonal correlation criteria. This approach allowed the systematic identification of areas affected by UGS operations, including subsidence zones close to storage wells and seasonal deformation fields further away. Building on this, the interpretation of displacement time series in relation to UGS curves of gas in storage was refined using a cross-correlation technique. The RTK parameters, correlation (R), time delay (T) and proportionality (K), allowed the isolation of displacement signals directly attributable to UGS operations, filtering out unrelated processes. High R values (>0.8) and positive K indices close to the centres of the caverns highlighted the strong correlation between the volumes of gas injected/withdrawn and measured surface displacements. T values quantified the temporal lag in the surface response. The integrated methodology demonstrates the operational value of InSAR for UGS monitoring, offering insights into both the static subsidence regime and the dynamic seasonal behaviour of salt caverns. These results provide operators with a robust basis for optimising injection and withdrawal strategies, mitigating geomechanical risks, and extending the operational lifetime of storage assets. From a regulatory perspective, the proposed framework supports the adoption of standardised monitoring best practices, enabling proactive risk management and guaranteeing adherence to environmental safety standards. In addition, the proposed approach can be adapted to other geological contexts, including depleted reservoirs, aquifers, and emerging applications such as Carbon Capture and Storage (CCS) and Underground Hydrogen Storage (UHS). In conclusion, this research demonstrates the potential of InSAR as a primary monitoring tool for UGS activities. The study establishes a reproducible, scalable and cost-effective monitoring framework that integrates multi-temporal satellite data, automated threshold-based mapping and cross-correlation analyses.