Scientific challenges

The META-FORET project aims to experimentally test two geophysical configurations from which metamaterial physics - inducing seismic cloaking and/or seismic protection - can be conclusively demonstrated. An essential route from physics to geophysics, will be to demonstrate through numerical simulations and laboratory scale experiments that such up-scaling is meaningful, that is, we will precisely show that the metamaterial designs applied using small objects at the laboratory scale actually lead to large-scale demonstrations at the geophysics scale.
Manipulating, at the geophysics scale, the use of a collection of long vertical tree-like resonators outside the ground or concrete-like inclusions inside the ground, with both systems behaving as a seismic metamaterial, is the main goal of the META-FORET project and completely fundamental for future understanding of wave cancellation or wave bending for seismic surface waves.

The META-FORET project is prospective and exploratory, for the following three reasons:

The challenging translation from the laboratory scale to the geophysics scale

The translation of the required physics from the laboratory scale to the geophysics scale is highly challenging. In the plate, only one plate mode was excited in the kHz range (the A0 flexural mode). The A0 mode is vertically polarized, and as no leakage was possible, the elastic wave was forced to interact with the vertical rods. At the geophysics scale, the medium is no longer 2D as in the plate, but is fully 3D, with the possibility of leakage of surface waves deep into the bedrock. (…)

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Vertical vs. elliptical component

On the laboratory scale, the A0 mode has a dominant vertical component that perfectly matches the vertical displacement associated with the compressional resonance in the rods. The bandgaps were spectacular because of this strong coupling. In geophysics, Rayleigh waves are elliptically polarized, with both radial and vertical components. This means that the excitation of the longitudinal resonances within the resonators (buried or not) is far from obvious, and the flexural resonance of the (…)

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Trees as simple vertical rods might be an overly simplistic approach

Finally, considering the trees as simple vertical rods might be an overly simplistic approach, even though the seismic wavelength for Rayleigh waves (between 10 and 20 m) will probably not be sensitive to (i) the decomposed subsurface soil (the width of organic deposits and humus should be <0.3 m) or the roots system architecture in the case of maritime pine tree (Fig. 1a), or (ii) the effects of lateral branches on the compressional/ flexural resonance in the trees (Fig. 1b). As a matter (…)

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