Polarimetric Characterisation of Volcanic Surfaces Using Dual and Full Polarimetric Spaceborne SAR Datasets: Analogue Studies for the Venus’s EnVision Mission

Radar remote sensing is essential for investigating Venus’ surface due to its dense CO₂-rich atmosphere and permanent cloud cover. The forthcoming ESA EnVision mission, equipped with the S-band VenSAR instrument operating in dual polarimetric HH and HV modes, will provide high-resolution observations to characterise surface scattering mechanisms, surface roughness, and dielectric properties. These observations are expected to enable the identification of signatures associated with active volcanic processes, including recent lava flow emplacement and surface alteration driven by thermal and chemical weathering. However, interpretation of polarimetric SAR observations over volcanic terrains remains challenging due to strong surface roughness, structural anisotropy, and orientation angle-induced depolarisation effects. Terrestrial volcanic analogues therefore provide a suitable framework for the development and validation of physically consistent polarimetric models prior to the availability of VenSAR data. This study presents a technical analysis of polarimetric scattering mechanisms at the Sundhnúksgígar and Holuhraun volcanic sites in Iceland using dual and full polarimetric ALOS-PALSAR-2 L-band SAR datasets. Fully polarimetric observations are used to quantify dominant scattering contributions and to evaluate the performance of conventional model-based decomposition approaches, including Freeman-Durden and Yamaguchi decomposition, over rough and structurally complex lava surfaces. To address the systematic overestimation of volume scattering, which can cause rough aa lava flows to be misclassified as vegetation, a modified model-based decomposition technique is introduced. By redistributing cross-polarised backscatter as a function of surface roughness, the proposed approach improves the separation of scattering mechanisms and enables more accurate discrimination of lava flow units and volcanic surface textures across both study areas. In addition, a dual polarimetric analogue of the proposed model-based decomposition technique is developed to enable volcanic surface characterisation using reduced polarimetric configurations consistent with the EnVision VenSAR acquisition mode. Multi-temporal ALOS PALSAR 2 dual polarimetric acquisitions are analysed to investigate surface evolution and volcanic dynamics associated with lava emplacement, flow cooling, and post-eruptive surface modification at the Sundhnúksgígar volcano site. The dual polarimetric formulation demonstrates strong correspondence with full polarimetric results in terms of dominant scattering behaviour and spatial variability, supporting the applicability of the proposed framework for future Venus observations. These results provide a validated polarimetric approach for characterising volcanic surfaces and contribute directly to the scientific preparation and exploitation of EnVision VenSAR data.

Keywords: Volcanos; SAR Polarimetry; Polarimetric SAR Decomposition; EnVision; ALOS-PALSAR-2; VenSAR