Evaluation of a wheel-based seismic acquisition system for a planetary rover

Abstract

Prior to using in-situ planetary resources, efficient mapping of geochemical and physical characteristics of the near surface will be required. As part of an integrated geophysical instrument suite on exploration and prospecting vehicles, we investigated the suitability of seismic piezo-sensors rigidly mounted on the interior of a generic rover wheel. Factors that can compromise proper data acquisition for this system include the natural mechanical resonance of the wheel and wheel-to-ground coupling. We characterized the natural resonance frequency bands of a generic wheel with an electromagnetic shaker. We also collected seismic shot gathers for subsequent seismic surface-wave analysis using a wheel in both a dry, laboratory sand tank (1.8 × 1.8 × 0.6 m) and in frozen loess soils within the United States Army Corps of Engineers Permafrost Tunnel in Alaska. For our wheel, self-weighted coupling to the ground was found to be adequate, although suitable wheelbase dimensions can constrain field acquisition geometries. In unconsolidated sediments, represented by medium sand (0.25–0.5 mm), wheel resonance of 1 kHz does not affect the fundamental mode used in shear-wave-velocity-to-depth inversion. When analyzed, shot-gather data collected from both wheel-mounted sensors and sand-planted sensors, in loose dry sand, effectively captured similar fundamental surface-wave modes. This is evident in frequency-versus-phase velocity dispersion images. Because narrow frequency bands of wheel resonance exhibit a high signal-to-noise ratio, they also readily detect lateral attenuation changes. Thus, wheel resonance can also be used to capture soil attenuation changes, including those produced when pore H2O ice acts to cement the regolith or loose-grained soils.