Elastic waves generated by impact and vibration in confined granular media

TL;DR

This study investigates how seismic waves propagate in confined granular media under different pressures, using experiments and simulations to understand asteroid-like materials' mechanical properties relevant for space exploration and impact risk assessment.

Contribution

The paper introduces a novel experimental setup and numerical model to analyze impact and vibration wave propagation in granular media under confining pressure, revealing pressure-dependent wave speed and attenuation behaviors.

Findings

Seismic wave speed increases with the square root of confining pressure.

Granular media behaves like a solid with a constant elastic modulus during compression.

Wave attenuation is approximately 3.4 Np/m.

Abstract

Observational data of asteroids can be explained by considering them as an agglomerate of granular material. Understanding the mechanical properties of these objects is relevant for many scientific reasons: space missions design, evaluation of impact threats to our planet, and understanding the nature of asteroids and their implication in the origin of the solar system. In-situ measurements of mechanical properties require complex and costly space missions. Here a laboratory-scale characterization of wave propagation in granular media is presented using a novel experimental setup as well as numerical simulations. The pressure inside an asteroid is still a matter of debate, but it definitely presents a pressure gradient towards the interior. This is why impact characterization needs to be performed as a function of the confining pressure. Our experimental setup allows for the simultaneous measurement of the external confining pressure, internal pressure, total strain, and acceleration in a 50 cm side squared box filled up with a billion grains. We study the propagation of impact-generated and shaker-born seismic body waves in the 500 Hz range. Through subsequent compression-relaxation cycles, it was observed that the granular media behaves on average like a solid with a constant elastic modulus during each compression. Effective medium theory (EMT) for granular media explains the data at low pressure. After each compression-relaxation cycle, the elastic modulus increases, and a high hysteresis is observed: relaxation shows a more complex behavior than compression. We show that seismic waves generated by both impact and vibration travels at the pressure wave speed. Thanks to a numerical model, we measure a strong wave attenuation $\alpha\sim3.4$ Np/m. We found that the wave speed increases with the confining pressure with a $p^{1/2}$.