Rock fall generated seismic signal at the laboratory scale: impact of the nature of the substrate

The seismic signal generated by rockfalls, landslides or avalanches is a unique tool to detect, characterize and monitor gravitational flow activity, with strong implication in terms of natural hazard monitoring. Farin et al. [2015] have shown, theoretically and experimentally, that the properties of an impacting bead (mass and velocity) on smooth surfaces can be recovered from the emitted signal (radiated elastic energy and mean frequency). In this work, we extend this study to rough and erodible surfaces. We dropped steel and glass beads of diameter between 2 and 10 mm, from 10 to 30 cm heights on (i) a smooth PMMA plate, (ii) a rough plate with 3mm-diameter glass beads glued on its surface, and (iii) erodible beds made by adding layers of free beads on the rough plate. The characteristics of the acoustic signal (frequency, energy) generated during the impact is measured thanks to 8 accelerometers (fe = 100kHz) and the bead motion characteristics (kinetic energy lost, bounce angle) is monitored using both these accelerometers and stereoscopy view captured by 2 fast cameras (fe = 500Hz). First, we confirm that the Hertz model well describes the source of generated acoustic waves during bead impacts on smooth surfaces. Then, we establish phenomenologically the link between impacting beads properties and emitted seismic signal for rough and erodible beds. We also quantify the fluctuations of the elastic energy and of the mean frequency in our experiments. Finally, we show that bounce angle actually drives conversion from kinetic energy of the impacting bead into radiated elastic energy.