First seismic in-situ characterization of regolith simulants in LUNA

DLR and ESA are jointly operating the Moon analogue facility LUNA in Cologne, Germany, to provide a venue for end-to-end testing of instruments, experiments, procedures and operations in a controlled, standardized environment. The facility consists of a large-scale testbed filled with mare regolith simulant EAC-1A, nominally to a depth of 60 cm, but extending to 3 m in the so-called deep-floor area (DFA), as well as a smaller dust lab filled to about 60 cm depth with the Lumina250 highland simulant. Both simulants have been characterized with a focus on mineralogical and geological properties, but for EAC-1A, lab data on shear-wave velocities as well as electric properties are also available. For both of these properties, compaction, which is in-situ unknown, plays an important role.

Here, we report on the first attempts of in-situ characterization of the elastic properties of EAC-1A in LUNA by 12 single-station ambient vibration measurements that were analysed in terms of the H/V spectral ratios. In addition to a peak at 0.76 Hz consistently observed at all locations that is related to local geology (sediment-bedrock interface at about 150 m depth), measurements in areas covered by the regolith simulants show additional high-frequency peaks between 12 and 55 Hz, dependent on regolith thickness. As the regolith thickness at each measurement location is known, the common trade-off between layer velocity and thickness in the inversion of the H/V peak frequency is resolved and measurements at different regolith thicknesses can be used to constrain the vertical velocity profile of EAC-1A. However, the task is complicated by strong surface topography as well as the structure of the DFA and buried exploration targets within, which could potentially result in 2D and 3D site effects for some measurement locations. Hence, careful data selection based on the directivity of the observed H/V peaks is performed. First results indicate very similar velocities for both mare and highland simulants, pointing to the dominant effect of granular texture as compared to chemical composition.

We compare fits to the data for different types of velocity laws and also discuss our results in light of the laboratory measurements as well as in comparison to in-situ data from the Moon.