Potential of Synthetic Aperture Radar backscatter for monitoring volcanic eruptions

Radar backscatter has been shown to be useful for observing volcanic eruptions, especially for remote or dangerous eruptions, as it is not limited by access to the volcano or cloud-coverage. Currently it is still being less widely used for volcano monitoring than radar phase measurements. This is in part because of the ambiguity in the interpretation of backscatter signals: there is not always a simple link between the magnitude or signal of the backscatter and the physical properties of the fresh volcanic deposits. In this thesis, I present three case studies (1) the 2018 explosive eruption of Volcán de Fuego, Guatemala, (2) the 2011-2013 effusive eruptions from Puʻu ʻŌʻō Crater, Kīlauea, Hawai`i, and (3) the 2021 dome growth at La Soufrière, St. Vincent and using a range of SAR sensors (COSMO-SkyMed, TerraSAR-X, Sentinel-1, and ALOS) demonstrate how radar backscatter can be used to monitor volcanic eruptions and quantify the changes to the ground surface. Radar backscatter is dependent on the scattering properties of the ground surface (i.e., surface roughness, local incidence angle, and dielectric properties). All of which can be altered during a volcanic eruption and provide information about specific deposits and processes. The pyroclastic density currents and lahars during the 2018 eruption of Volcán de Fuego and the emplacement and development of lava flows in 2010-2013 in Hawai`i were dominated by changes in the surface roughness. I identify deposits and variations within these based on their different morphologies, calculating the lengths of flows and areas affected by the eruptions. Where a deposit is emplaced over a period of multiple SAR acquisitions, I can map the progression and development of the deposit through time. While the backscatter signals associated with the eruptions in Hawai’i and Volcán de Fuego were dominated by changes to the surface roughness, backscatter changes during dome growth at St. Vincent were dominated by changes in the local incidence angle. The analysis at La Soufrière is therefore driven by this gradient-dominated signal, which provided the opportunity to extract topographic profiles from the SAR backscatter. I test different SAR backscatter method to increase the signal-to-noise ratio and improve the identification of SAR backscatter change related to volcanic deposits. I found that using a combination of (1) spatial filters, (2) extended timeseries, (3), radiometric terrain corrections, and (4) understanding the pre-eruption land cover work to improve the signal, but depends on the type of deposit and volcanic setting. Further, the addition of supplementary datasets (e.g., high-resolution DEM, rainfall data, pre-eruption land cover) are important when interpreting backscatter changes. Through the three case studies, I demonstrate the ways backscatter can be used to understand and monitor a range of volcanic eruption styles. I highlight a number of quantitative volcanic outcomes (e.g., flow lengths, deposit thicknesses, areas and volumes), a variety of SAR methods (e.g., change difference, extended timeseries, flow mapping, pixel offset tracking) and corrections (e.g., radiometric terrain correction, satellite dependency).