Measuring topographic change after volcanic eruptions using multistatic SAR satellites: Simulations in preparation for ESA’s Harmony mission

Volcanoes are dynamic systems whose surfaces constantly evolve. During volcanic eruptions, which can pose great threat to local communities, significant changes to the local topography occur as edifices build up and/or collapse and lava, tephra and other eruptive products are deposited. Monitoring such changes in topography is crucial to risk assessment and the prediction of further eruptive behaviour. Multistatic Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing modality particularly suited to this task as it allows for the creation of digital elevation models (DEMs) that can accurately map out three-dimensional changes in the topography, regardless of weather conditions and temporal decorrelation caused by volcanic activity. Few such missions are however currently operational. Harmony is an upcoming ESA mission that will be operating alongside Sentinel-1 and will provide multistatic InSAR capabilities for the measurement of stress and deformation across the cryosphere, the oceans and the solid earth, with the monitoring of topographic change due to volcanic eruptions being one of the specific areas of focus for the mission. In this work we demonstrate the use of high resolution bistatic interferometric data from TanDEM-X for the measurement of topographic change after recent eruptions in El Reventador, Ecuador and La Soufrière, St. Vincent and the Grenadines. Additionally, we simulate data at the lower, 20 m resolution of Harmony so as to gain insights into its capability in quantifying topographic change. Our results demonstrate that Harmony’s resolution can be sufficient to resolve and measure accurately topographic change such as the emplacement of lava flows, but may be challenged in areas of steep topography where unwrapping errors can occur. The experimental results highlight the effect of acquisition pass direction with respect to local topography, the challenges arising in areas of steep topography and the importance of masking results based on estimates of precision and resolution. Finally we discuss some of the challenges, as well as implications of the Harmony mission for the future of volcano monitoring.