ANALYSIS OF LAVA TUBE FRACTURES IN VALENTINE AND HERCULES LEG CAVES, LAVA BEDS NATIONAL MONUMENT

Lava tubes on the Moon may provide readymade shelters for future explorers. To better understand and characterize lava tubes, the Geophysical Exploration of the Dynamics and Evolution of the Solar System (GEODES) team recently completed field data collection in Lava Beds National Monument, CA. We report here on fracture mapping within two lava tubes, Valentine and Hercules Leg caves. We seek to determine if fractures can be mapped from remote imagery (visible and/or thermal) to the level needed to perform structural stability analyses.

Fractures were mapped, measured, and photographed at multiple locations within each cave. First sites were just inside the cave, with subsequent measurement sites 30 m along the wall until the back of the cave was reached. At each measurement site, multiple 3 m fracture profiles were acquired by laying a measuring tape along the wall and noting the distance to each subsequent fracture. Fracture profiles were taken ~1 m apart. Visual and thermal images were acquired of each measurement site.

We found that visual imagery was most useful where there had been prior mineralization events, or where the fracture aperture was wet and/or large. However, not all small or hairline fractures were easily identified. The visual imagery also required a significant amount of light aimed at the cave wall to capture data.

The infrared camera data showed that some fractures can be seen in thermal imagery. Although the resolution of the thermal imagery is coarser than the visual, 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. In particular, wet or large aperture fractures were often colder than the surrounding rock.

In Valentine cave, 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. In Hercules Leg cave, the temperature differences between cave wall and cracks remained observable throughout. This is possibly because the entire length of the Hercules Leg lava tube is close to the surface, with multiple roof collapses, so that the temperature never homogenized.