Resolution of Lava Tubes with Ground Penetrating Radar: The TubeX project

Remote sensing surveys of the Moon and Mars show evidence of lava tubes, which are potential safe havens for human crews and their equipment. Ground penetrating radar (GPR) can be used to map tubes because the void/rock interface at tube ceilings and floors strongly reflects radar pulses. We have tested the capacity of GPR to sense lava tube geometry at Lava Beds National Monument (LBNM) in California, USA. GPR and detailed LiDAR data are presented for two tubes: Skull Cave, with a few meters of overburden, diameter ~10-20 m, and a rubbly floor; and Valentine Cave, with similarly thin overburden, diameter ~1-3 meters, and a flatter smoother floor. On both caves GPR clearly resolves the ceiling and permits good estimates of the cave width as validated with LiDAR data. Where GPR fails, the primary cause is inferred to be strong out-of-plane effects due to complex 3D geometries. Recovery of the floor position requires migrating the GPR data with a 2D velocity model, as signal velocity is faster in void space. We find that floor position is recoverable in caves whose voids are taller than the radar wavelength (~3 m in this study). Forward modeling assuming planetary parameters suggests GPR should be similarly successful on the Moon or Mars.