FRACTURE MAPPING LAVA TUBES: COMPARING VISUAL VERSUS INFRARED DATA SETS

Lava tubes on the moon may provide readymade shelters for future explorers. To better understand and characterize lava tubes, the GEODES project seeks to apply geological and geophysical measurements toward characterizing subsurface void spaces. Of importance is understanding the long term stability of lava tubes. The GEODES team recently completed field data collection in Lava Beds National Monument, CA, with characterizing lava tubes from multiple sources one of the major goals. We report here on visual versus infrared imagery within two lava tubes (Valentine and Hercules Leg caves). We seek to determine if (1) fractures on cave walls can be observed through either visual or infrared (thermal) imagery, (2) if the thermal camera was uniquely suited to highlight fractures that were permeable (wet), and (3) whether fractures can be mapped from the imagery to a level needed to perform structural stability analyses.

Comparisons with in situ fracture mapping within the lava tube are ongoing. However, several observations were apparent between the visual and thermal cameras. Visual imagery is useful where there has been prior mineralization events, as the vein minerals are typically a different color than the surrounding basaltic host rock. Similarly, wet or large aperture fractures are often apparent in visual imagery. However, not all small or hairline fractures were easily identified. Also, visual imagery required a significant amount of light aimed at the cave wall to capture data. Infrared camera data shows that fractures can be seen in some of the thermal imagery. In particular, wet or large aperture fractures are often warmer than the surrounding rock. This effect was most notable close to the cave entrance, with the effect diminishing with depth into the lava tube. This is likely an effect of temperature homogenization within the cave, creating similar temperatures for the walls, air and water. Additionally, the resolution of the thermal imagery is coarser than the visual. However, the thermal data provided valuable information regarding which fractures were conducive and currently permeable by highlighting temperature differences of fractures with air or water flowing through them. This work was funded by the NASA GEODES project (N. Schmerr [UMD], PI).