Nondestructive imaging of hypervelocity impact‐induced damage zones beneath laboratory‐created craters by means of ultrasound travel‐time tomography

Abstract

Since the 1960s, hypervelocity impact experiments have been conducted to study the complex deformation mechanisms which occur in the subsurface of meteorite craters. Here, we present ultrasound tomography measurements of the damage zone underneath seven experimentally produced impact craters in sandstone cubes. Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network and the NEOShield Project, decimeter‐sized sandstone targets were impacted by aluminum and steel projectiles with radii of 2.5, 4, and 5 mm at velocities between ~3.0 and ~7.4 km s−1. The 2‐D ultrasound tomography clearly shows a correlation between impact energy and the damaged volume within the target blocks. When increasing impact energies from 805 to 2402 J, a corresponding increase in the damage radius from ~13.1 cm to ~17.6 cm was calculated. p‐Wave velocity reductions up to 18.3% (for the highest impact energy) were observed in the vicinity of the craters. The reduction in seismic velocity decreased uniformly and linearly with increasing distance from the impact point. The damage intensities correspond to peak damage parameters of 0.4–0.51 compared to undamaged target blocks. In addition to the damage zone below the crater, we could identify weakened zones at the sandstone walls which represent precursors of spalling. The volume of the damaged subsurface beneath experimentally produced craters determined through ultrasound tomography is larger than that obtained from previously reported p‐wave velocity reductions or to microscopic and microcomputed tomography observations of crack densities in experimentally produced craters.

Subsurface deformation of experimental hypervelocity impacts in quartzite and marble targets

“…For example, Moser et al. () found for sandstone targets that a reduction of ultrasound velocities correlated with subsurface damaging at much deeper depths within the target than observed by microanalysis (see also Raith et al, ).…”

“…Hence, further microscopic studies, as well as ultrasound measurements, could hint at the reliability of the model data in predicting deep-lying deformation features. For example, Moser et al (2013) found for sandstone targets that a reduction of ultrasound velocities correlated with subsurface damaging at much deeper depths within the target than observed by microanalysis (see also Raith et al, 2018).…”

Experimental impact cratering: A summary of the major results of the MEMIN research unit

“…Micro‐computed tomography is capable of tracing large spall cracks and concentric fractures below the crater surface of the experimentally impacted sandstone blocks. Ultrasound tomography revealed a hemispherical damage zone of reduced p‐wave velocities beneath the crater (Raith et al. Forthcoming).…”

Borehole Seismic Observations From the Chicxulub Impact Drilling: Implications for Seismic Reflectivity and Impact Damage

“…D P and D S have been particularly useful metrics to track changes in the degree of disruption extending radially from impacts into large target blocks. This is accomplished by measuring V P directly on samples cut from the shocked blocks (Ahrens & Rubin, 1993; Ai & Ahrens, 2007) or indirectly via wave speed tomography (Moser et al., 2013; Raith et al., 2018; Xia & Ahrens, 2001). D P has been particularly useful in connecting damage to attenuation and induced micro‐crack densities (Liu & Ahrens, 1997) in materials from these experiments, although these the magnitudes of these damage measures were not linked more directly to levels of strain experienced by the sample.…”