Experimental Rock Physics Characterization of the Geophysical Properties of Lunar Analog Material at Skull Cave in Lava Beds National Monument, California

The lunar subsurface is a primary science target for future missions to the Moon and serves as a potential host location for void spaces for astronaut shelter, resources such as water ice, and important ore-bodies for in-situ manufacturing and building materials. Geophysical methods allow us to access the near subsurface with a remote sensing approach and requires an understanding of physical variations in the structural properties of target resources. Here we investigate how the seismic signature of the lunar environment is affected by changes in material properties using analog samples of basalts collected from Skull Cave at Lava Beds National Monument in California to measure hand sample geophysical properties and ultimately relate these back to local seismic surveys of the sample locale. This will be key for the subsurface exploration in future lunar surface missions.

Many void spaces in the lunar subsurface are likely formed from lava flows that cool and harden near the top while molten lava continues to drain below, forming a hollow structure known as a lava tube. Studying terrestrial lava tubes can provide valuable information on their structure, dimensions, and volcanic history that can aid in the investigation of lunar lava tubes. Skull Cave was selected as a field site in order to conduct co-located geophysical surveys which consist of seismic transects, ground-penetrating radar surveys, LiDAR topography and sampling above and inside the cave. This research focuses on characterizing the samples collected where these geophysical measurements took place.

Experiments are conducted using the Autolab 1500 deformation apparatus and two piezoelectric velocity transducers in order to capture ultrasonic velocities under hydrostatic conditions. Samples are cored into cylinders with a height of 38.1 mm and a diameter of 18.4 mm. During the experiments confining pressure is increased from 0 MPa to 60 MPa to 0 MPa and seismic velocities of the analog samples are collected at 2 MPa intervals. Pore volume is calculated from Computed Tomography scans and porosity is used to interpret the seismic data. The laboratory characterization of the Skull Cave samples will aid in interpretation of larger scale geophysical features and can provide realistic physical properties of lava tubes to be used in future modeling studies.