Interpreting Synthetic Aperture Radar Backscatter During Effusive Eruptions: Insights From Rift and Summit Activity at Kīlauea, Island of Hawai'i

Synthetic aperture radar (SAR) backscatter has particular advantages for monitoring the progression of effusive eruptions. It does not rely on cloud‐free or daytime satellite acquisitions and is particularly sensitive to surface roughness and local slope changes, providing complementary information to other sensors. However, it is currently less commonly used for volcano monitoring than differential phase, optical or infrared sensors due largely to ambiguities in interpreting radar backscatter signals: the physical properties of volcanic deposits are not simply relatable to magnitude or sign of backscatter change. Nevertheless, SAR backscatter has value for mapping and monitoring flow progression. Here, we carry out a retrospective SAR backscatter analysis of Kīlauea's 2021 summit lava lake and Pu‘u‘ō‘ō crater, including its lava lake and three eruptive episodes from 2011 to 2013, to test the potential of backscatter for mapping lava flow emplacement and tracking subsequent cooling, alteration, and lava lake changes. We examine the inflation and collapse events at Pu‘u‘ō‘ō crater during the same period to assess the geometrical limitations of extracting height measurements using radar shadows. We investigate corrections needed for backscatter analysis, including principal component analysis to reduce instrument‐dependent noise. Our analysis shows that SAR backscatter can provide temporally dense timeseries of crater depth estimates, estimate lava lake area, track lava flow emplacement, and potentially provide information about post‐emplacement changes. This makes backscatter a valuable data set, especially where ground‐based measurements are restricted and optical observations are limited due to cloud coverage, and is complementary to radar phase, coherence, and other SAR sensor data.