Seismic measurements - Round 2
LUNA lay silent for almost 4 days – no trucks delivering regolith simulant, no interior works, no rover tests – just some boxes and sensors in a triangular configuration, and lots of cables. What was going on?
This was the second seismic reference measurement, after the initial measurement on the construction site before the hall was built. Comparison between data from the two measurements can show if there are any new sources of noise, if we can still measure the same vibrations within the hall as before, and if we can still resolve the subsurface structure below the hall with the seismic
data. As the seismometers are very sensitive, all other activities in LUNA had to cease during that time window.
First data analyses at MUSC indicate that we still clearly see signals from outside LUNA (cars, airplanes, helicopters…), and even by chance recorded a magnitude 4.2 earthquake near Basel, at about 350 km distance from LUNA. The first seismic waves reached LUNA about 60 s after the quake; maximum ground motion measured in LUNA is around 0.0015 mm. Quakes of this size have also been registered on the Moon with the Apollo instruments of the 1970s. We can also still resolve reverberations in the sediments beneath the hall well, which indicate a thickness of 180-200 m of sand, gravel and clay above the bedrock.
There are specific plans to install seismometers on the Moon again, both with a robotic NASA CLPS mission to Schroedinger Basin on the far side of the Moon in 2025, and with the help of astronauts during the Artemis III landing. The gathered data will for example help to protect astronauts and lunar infrastructure by monitoring meteorite impacts and seismic ground shaking, as well as providing valuable scientific insights into the interior of the Moon. Seismic experiments at a smaller scale can also be used to investigate the shallow subsurface, e.g. in terms of measuring the thickness of the regolith layer, or detecting water ice.
To support instrument testing, LUNA will also host a permanent seismic station, as well as a spatially distributed, engineered fiber-optic cable at the bottom of the hall. Using extremely short laser impulses, this fibers allow to measure deformation due to seismic waves at high spatial resolution. This new technique (Distributed Acoustic Sensing, DAS) is also considered for deployment on the Moon.
