Subsurface Cavities Characterization from X-Band VHR Spaceborne SAR Images

Cave systems are one of the last frontiers of human exploration on both Earth and other celestial bodies. In this context, lava tubes are natural subsurface tunnels, which are not visible from the surface, that are ubiquitous on Earth as well as on the Moon and Mars. Skylights are one of the surface evidences of the presence of such conduits in the form of overhanging collapses of the cave ceiling, making these cavities partially observable and potentially accessible. In this paper, we propose a method for imaging and characterizing subsurface structures by spaceborne VHR SAR imaging. We performed several acquisitions over different Earth locations by exploiting Capella's X-band VHR SAR imaging radars in spotlight mode. The obtained results show that the proposed methodology is able to characterize the main geometric parameters of the first section of a lava tube (e.g. width, height) in the surroundings of a skylight and it provides an indication of the actual subsurface accessibility. The proposed methodology has several important implications for exploration and could be applied to different type of cavities other than lava tubes.