Elastic Properties of Lunar Regolith from Vertical Seismic Profiling

To reveal shallow subsurface structures as well as velocity distribution of the lunar regolith layer, we propose a survey of Reverse Vertical Seismic Profiling (RVSP) under a program of SELENE-2. In the RVSP experiment, seismic sources generated within the borehole are recorded by receivers deployed on the Moon surface. In Lunar Soil Mechanics (LSM) Investigation System, drilling operations and shear strength measurements (1 m in total depth) are planned for the regolith layer. We attach a vibrator at the shear experiment devices and generate an active source signal within the borehole. MEMS sensors are further attached at legs of the lander and record the wavefields derived from the vibrator within the borehole. Here we show the effectiveness of RVSP survey using the proposed simple (small-size) acquisition system from simulation study and laboratory experiments. First, we conducted simulation study for the designed survey geometry in order to determine the survey parameters (e.g., source signal). This simulation study demonstrates that the source signal should contain high-frequency components in order to resolve shallow velocity structure from short offset data (less than 3m). Since the high-frequency signal is much attenuated, we used a sweep waveform for the source signal. To examine the ability of the sweep waveform for the high-attenuation regolith layer, we conducted small-scale RVSP experiments in laboratory using lunar soil simulant. The laboratory experiments demonstrated that the seismic signal can be retrieved by cross-correlation between source (sweep) waveform and recorded waveform. From the direct P-waves and reflected waveforms observed on the processed gathers, P-wave velocity of the regolith was estimated as ~85m/s. This study shows that the proposed simple RVSP survey could reveal the shallow subsurface structures as well as velocity within the regolith layer.