Ground Penetrating Radar (GPR) is shown to be a successful tool in detecting tunnels and voids. Lava tubes are tunnel-like features in volcanic terrains that can be potential safe places for human crews and equipment on the Moon and Mars. We utilize GPR to detect and map lava tubes (Valentines cave, Skull cave and Hercules Leg cave) in Lava Beds National Monument, CA. Our preliminary results show that the ceiling of the lava tubes are readily detectable by GPR. However, due to the strong radar velocity contrast between lava and the air-filled tubes, accurate recovery of the position of the lava tube floor is much more challenging. Careful migration of the GPR data is required to resolve the floor signature and create an image with the tube floor restored to its correct depth. We are developing an optimal workflow for recovering complete lava tube geometries. We can do this because we have collected centimeter-scale LiDAR data from the interior of tubes as well as on the surface along GPR transect lines. Thus we can test the accuracy of GPR migration methods against the LiDAR-measured tube geometry. We are testing conventional 2D migration techniques as well as topographic migration. At selected field sites we have limited 3D ‘grids’ of data. We expect to compare the results of different migration techniques to identify optimal methods for this problem. As a part of this project, we also seek to develop a library of different lava tube geometries and their corresponding GPR image from their migrated sections. The GPR image library will encompass a range lava tube geometries, including tubes of different heights, widths, shapes, and structures (e.g., pillars), plus a variety of floor textures (e.g., smooth, ropey, rubble) and overhead thickness. This library will be an asset for determining the utility of deploying GPR technology in mapping a tube-rich environment.

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.