GPR imaging of lava tubes with the TubeX project

Lava tubes can offer protection for human crews and their equipment on other solar system bodies, in particular from radiation threats and extreme surface temperatures. Developing strategies to survey regions of other terrestrial bodies (such as the Moon or Mars) for tubes suitable for potential habitation will likely become an important part in planning future space exploration projects. A variety of surface geophysical techniques, such as ground penetrating radar (GPR) have the potential to help recognize and map tubes. GPR shows promise for providing high resolution information on tube geometries. To investigate GPR's capacity and limitations, we use GPR, as well as comparative methods of seismic and magnetic surveys, in conjunction with LiDAR mapping of tube interiors at the Lava Beds National Monument (LBNM) in California, USA. LBNM offers a wide variety of tube geometries and textures.

We have collected 2D GPR profiles and small 3D GPR grids (of parallel 2D lines) with antenna frequencies of 100 and 200 MHz on four lava tubes with different geometries, textures and at different depths. Challenges in recovering tube geometries include wave scattering in fractured rock covering tubes, irregular and "drippy" ceilings and walls, and blocky floors. Our primary results show that the top of the LBNM tubes can generally be resolved in the GPR data, while resolving the bottom is more challenging. The utility of various GPR processing techniques can be directly assessed by comparing resolved GPR images against the LiDAR-measured tube geometries.