"MOONBIT"—Seismic while Drilling (SWD) Laboratory Testing with Lunar Regolith Simulant

Abstract

The MOONBIT experiment investigated the capability to obtain structural and stratigraphic information of planetary subsoil using Seismic While Drilling (SWD) - a geophysical technique used for Earth subsoil exploration. The basic concept of SWD uses the drill bit as a seismic in-depth source of seismic waves, recorded by geophones as surface receivers gathering digital seismograms at different depths: signal processing provides seismic images known as Reverse Vertical Seismic Profiling (RVSP). MOONBIT's target medium is the regolith layer on the Moon surface, covering basaltic bedrock. As innovatively RVSP SWD method, a passive Downhole Hammering Mechanism (DHM) actuates the drill bit, which becomes the borehole source of seismic waves. Two main tests were conducted in regolith simulant with buried geophones: the first test characterized the propagation velocity of seismic waves; the second test used a 1.5 m drill over a container. Drill control strategies and robustness towards signal transmission problems of future unmanned Moon drills of missions, Earth remote controlled, were also assessed and simulated with an error generator and a remote supervisor prototype. Test results with consistent seismic signals of good quality and resolution and comparative analysis showed that the DHM drill bit is an adequate source of SWD signals for RVSP planetary purposes and may support steering, drilling planning and scientific investigations.