Propagation and attenuation of pulses driven by low velocity normal impacts in granular media

We carry out experiments of low velocity normal impacts into granular materials that fill an approximately cylindrical 42 liter tub. Motions in the granular medium are tracked with an array of 7 embedded accelerom-eters. Longitudinal pulses excited by the impact attenuate and their shapes broaden and become smoother as a function of travel distance from the site of impact. Pulse propagation is not spherically symmetric about the site of impact. Peak amplitudes are about twice as large for the pulse propagating downward than at 45 degrees from vertical. An advection-diffusion model is used to estimate the dependence of pulse properties as a function of travel distance from the site of impact. The power law forms for pulse peak pressure, velocity and seismic energy depend on distance from impact to a power of-2.5 and this rapid decay is approximately consistent with our experimental measurements. Our experiments support a seismic jolt model, giving rapid attenuation of impact generated seismic energy into rubble asteroids, rather than a reverberation model, where seismic energy slowly decays. We apply our diffusive model to estimate physical properties of the seismic pulse that will be excited by the forthcoming DART mission impact onto the secondary, Dimorphos, of the asteroid binary (65803) Didymos system. We estimate that the pulse peak acceleration will exceed the surface gravity as it travels through the asteroid.

Automated summary

In this study, we conducted low velocity normal impacts on granular materials and found that the longitudinal pulses excited by impact attenuate and spread as they travel away from the impact site. Using a model, we estimated the relationship between pulse properties and travel distance from the impact site, and observed a decay in peak pressure, velocity, and seismic energy as distance increases. Our experimental results support a seismic jolt model and we applied our model to estimate the physical properties of the seismic pulse generated by the upcoming mission impact on the asteroid Dimorphos.