Séminaire de Phlippe Roux: Evidence of metamaterial physics at the laboratory and the geophysics scale: the METAFORET project

Evidence of metamaterial physics at the laboratory and the geophysics scale: the METAFORET project

Philippe Roux, ISTerre, Grenoble

The METAFORET project was designed to demonstrate that complex wave physics phenomena classically observed at the meso- and micro-scales in acoustics and in optics also apply at the geophysics scale. In particular, the experiment performed in Oct. 2016  shows that a dense forest of trees can behave as a locally resonant metamaterial for seismic surface waves. The dense arrangement of trees anchored into the ground creates anomalous dispersion curves for surface waves, which highlight a large frequency band-gap around one resonant frequency of the trees, at ~45 Hz. This demonstration is carried out through the deployment of a dense seismic array of ~1000 autonomous geophones providing seismic recordings under vibrating source excitation at the transition between an open field and a forest. Insights and interpretations on the observed seismic wavefield are validated with 2D numerical simulations of trees over a layered halfspace. At the laboratory scale, we also describe the manifestation of localized states through coherent and incoherent analyses of a diffuse elastic wave field inside a two-dimensional metamaterial made of a collection of vertical long beams glued to a thin plate. We demonstrate that localized states arise due to multi-wave interactions at the beamplate attachment when the beams acts as coupled resonators for both compressional and flexural resonances on the metasurface. Due to the low-quality factor compressional resonance of the beams, inside the main bandgap the modal density of the system drops to near a high-quality factor flexural resonance of the beams, and blocks the diffusion process of the wavefield intensity. This lab scale experiment physically highlights the tight-binding-like coupling in the localized regime for this two-dimensional metamaterial.