Physically Constrained Dual-Polarimetric SAR Decomposition for Volcanic Surface Characterisation: Analogue Studies for the Venus EnVision mission

Synthetic Aperture Radar (SAR) observations provide valuable information for volcanic surface characterisation due to their sensitivity to surface roughness and dielectric properties. Accurate characterisation of volcanic terrains is important for understanding SAR scattering behaviour and surface physical properties, including roughness and dielectric variability, in both terrestrial and planetary environments. Radar remote sensing is particularly valuable for volcanic investigations in regions where optical observations are limited by cloud cover, ash, or dense atmospheres, such as on Venus. Conventional model-based dual-polarimetric SAR decomposition methods, including the framework proposed by Lorenzo Mascolo et al. [1], often overestimate volume scattering in volcanic terrains because depolarised returns arising from rough-surface interactions and multiple scattering are incorrectly interpreted as random volume scattering. Since volcanic surfaces are predominantly non-vegetated and characterised by blocky lava flows, rough interfaces, and coherent depolarisation mechanisms, a significant portion of the surface scattering contribution is absorbed into the estimated volume scattering component. This leads to systematic underestimation of surface scattering power and reduced physical interpretability of decomposition results. To address these limitations, this study proposes a physically constrained model-based dual-polarimetric SAR decomposition framework for volcanic surface characterisation using ALOS PALSAR-2 datasets acquired over terrestrial volcanic analogue sites in Iceland. The proposed approach constrains the unpolarised scattering component using polarisation-state information derived from Stokes parameters. A hybrid formulation combining the degree of polarisation and a phase-sensitive randomness index is introduced to distinguish stochastic volume scattering from coherent rough-surface depolarisation. In addition, an intensity-based constraint is incorporated to suppress residual overestimation in high-backscatter regions while preserving total power conservation. The methodology is particularly relevant for forthcoming dual-polarimetric SAR observations from the ESA EnVision mission, which will enable investigation of volcanic surface scattering behaviour and dielectric variability on planetary surfaces. Experimental results demonstrate improved discrimination of volcanic surface units and more physically consistent decomposition compared with conventional dual-polarimetric approaches.

Keywords: Volcanos;  Synthetic Aperture Radar; Dual-polarimetric decomposition; ALOS-PALSAR-2; EnVision

References

[1] L. Mascolo, S. R. Cloude, and J. M. Lopez-Sanchez, “Model-based decomposition of dual-pol SAR data: Application to Sentinel-1,” IEEE Transactions on Geoscience and Remote Sensing, vol. 60, pp. 1–19, 2021.

Acknowledgement: The authors would like to acknowledge the Japan Aerospace Exploration Agency (JAXA) for providing the ALOS PALSAR-2 datasets to carry out this research work.