DATA available for Academic Research

Each data set proposed below may be quite large. Be aware that the download could last a certain amount of time. The data come in Matlab format with one or a few codes to read / plot the data. There is also a Readme file and some pictures that explains very shortly the acquisition set-up and, sometimes, one scientific paper that was published from this data set.

The idea is to promote collaborative research. Of course, you can download the data and process them the way you want. Whatever the data utilization, for research or for teaching purposes, the only requirement is an acknowledgement to:

Philippe ROUX, ISTerre, Université Grenoble Alpes, CNRS UMR 5275, Grenoble (France):
Of course, I would be more than happy to describe the data in more details and, why not, participate one way or another to your research investigation. So feel free to interact with me as much as possible….

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METAFORET data

The first METAFORET experiment consists in the deployment of a very dense array of three-component geophones at the interface between an open field and a forest. To find out the optimal forest configuration, a collaboration was started with R. de Lary, Head of the “Centre National de la Propriété Forestière d’Aquitaine” (CNPF) which manages the 1 million hectare of pine tree forests in the Landes department. Different sites were investigated and the best configuration appeared to be a maritime pine tree forest with tree height of 15 m with a density of 800 trees per hectare (unit mass of 0. 3 t per tree and one tree every 12.5 m2). The CNPF contributed to the METAFORET experiment with a technician whose tasks will be to (1) manage the administrative authorizations for the seismic array deployment on the selected sites and (2) clean the ground from possible brambles or blackberry bushes in order to facilitate the seismic sensors installation.

Picture of the maritime pine forest where seismic data were collected in Oct. 2016

The seismic array was made up of 31 × 31 = 961 sensors in a regular grid with 4-m inter-element spacing, thus covering an area of 120 m × 120 m (Figure below). The spacing has been chosen to match as closely as possible the half-wavelength spatial sampling requirement of the seismic field, both in the forest and in the open field. With the expectation of surface-wave velocities between 150 m/s and 300 m/s (depending on the ground composition: sand or clay) in the frequency range [5 Hz to 50 Hz], the choice of 4-m as the inter-element spacing appears to be the optimal compromise.

This spatial density of geophones is mandatory to accurately measure the dispersion curves outside and inside the forest, and it also needs to be at the transition between an open field and the forest. We believe that comparisons between the dispersion curves in these different areas will confirm the metamaterial behavior of the forest, and will demonstrate in particular the hybridization effect that is associated with the resonances in the trees. Note that whether periodic or random, the spatial arrangement of the trees is of little importance, as metamaterial physics is not based on the Bragg scattering effect that is classically observed in periodic crystals, but on the collective behavior of resonators that are arranged at a sub-wavelength scale.

Top: Configuration of the META-FORET experiment. The goal is to install 961 three-component geophones on a 120 m × 120 m grid with 4-m inter-element spacing. The seismic array (red) is placed across the interface of an open field and a dense pine-tree forest (80 trees per 400 m2). Bottom left: Continuous ambient noise recording will be carried out over 12 days with wireless seismic sensors from Geokinetics. Bottom right: As well as this ambient noise, active source signals will be recorded using a vibrometer source positioned at different locations (top: blue elipses) in the open field (inside and outside the array) and inside the forest.

Ambient noise recording. In the first step, long intervals of continuous ambient noise was recorded by the seismic array. As ambient seismic noise is dominated by surface waves, this ambient noise might be a good candidate for working with the waves that are trapped close to the subsurface. In the case of omnidirectional noise sources, the dispersion curve can be obtained from a kx-ky frequency–wavenumber transformation along the two directions of the seismic array. However, this isotropic noise distribution is in practice difficult to obtain at frequencies above 5 Hz, because of the predominance of human-generated seismic noise sources (e.g., cars, industry). This means that long-term recordings might be necessary to reach this condition, and especially day and night recordings. For the extraction of dispersion curves from ambient noise data, the global seismic array should be divided into two sub-arrays, as the open-field array and the forest array. We expect to observe seismic bandgaps inside the forest, using the dispersion curve in the open field as a reference. As well as the bandgaps, the dispersion curves should reveal any typical abnormal behavior due to hybridization close to the resonant frequencies of the trees. If the aims are achieved, the dispersion curves inside the forest should reveal the metamaterial nature of the forest.

Finally, note that the wind might be another source of excitation for the trees, with potential flexural excitation transmitted to the ground. Wave excitation from the ground to the trees or from the trees to the ground might lead to different results, as different types of resonance can be induced in these two cases: compressional resonance, if vertical displacement associated with Rayleigh waves excites the trees; flexural resonance, which can lead to horizontal ground motion if the wind is the dominant excitation source.

Active shots. In a second step, a vibrometer source (property of ISTerre) was used as an active source to complement the ambient-noise recordings. The vibrometer is fully programmable and works efficiently between 10 Hz and 500 Hz. With the restriction to frequencies between 10 Hz and 100 Hz, we generated 1-min-long frequency-modulated sweeps, to obtain a sufficient signal-to-noise ratio for all of the seismic sensors. These active shots will be synchronized with GPS and will be stacked over a large number of repetitive acquisitions. The use of active shots represent a spatio-temporal wavefield as it penetrates into the forest.

Snapshots similar to those obtained at the laboratory scale (see below) were then reproduced at the geophysics scale. We observe strong damping of the incident field when it is filtered in the frequency bandgaps revealed by the dispersion curves. At the same time, we should also obtain sub-wavelength and supra-wavelength modes as in locally resonant metamaterials. The use of an active source also provides measurements of the dispersion curves in the frequency band of the excitation, which can be compared to those obtained from the ambient-noise measurements.

When compared to the laboratory experiment performed with rods attached to a plate (see below), the great advantage of the forest configuration is the possibility to move the source inside the array, and especially inside the forest. The excitation of the metamaterial from inside is of foremost importance, to understand the local coupling with the resonators and the emergence of high-frequency wavenumbers within the medium. To date, the theoretical interpretation of such a locally resonant metamaterial was performed through Fano interference between the propagating and scattered waves; i.e., an approach that neglects near-field effects. The possibility to excite the forest and to record the near-field contribution close to the resonators will shed new light on the interpretation of metamaterial physics.

– To access the METAFORET data set, you should connect to ftp://metaforet ist-ftp.ujf-grenoble.fr/

Note that you will need a password to reach the data. This password will be send to you upon request at the following mail address:
Philippe.roux univ-grenoble-alpes.fr

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Laboratory data

The purpose of this web page is to make acoustic or seismo-acoustic data recorded at the laboratory scale available to the public. The following data set is unique in the sense that they would be very difficult to acquire with numerical modeling. This is the main purpose of analog experiments at the lab scale: it takes a few minutes to be generated at the lab scale and it would take forever, if possible, to create the equivalent data set with numerical simulations.

Experimental configuration at the laboratory scale

We demonstrate the experimental realization of a multi-resonant metamaterial for Lamb waves, i.e. elastic waves propagating in plates (see figure above). The metamaterial effect comes from the resonances of long aluminum rods that are attached to an aluminum plate. Using time-dependent measurements, we experimentally prove that such a medium exhibits wide band gaps as well as sub- and supra-wavelength modes for both a periodic and a random arrangement of the resonators. The extraction of the metamaterial dispersion relation allows us to predict this physics through hybridizations between flexural and compressional resonances in the rods and slow and fast Lamb modes in the plate. We finally underline how the various degrees of freedom of such system paves the way to the design of metamaterials for the control of Lamb waves in unprecedented ways.

– DATA available at: ftp://ist-ftp.ujf-grenoble.fr/users/rouxphi/mesoscopic_metamaterial/

Updated on 16 September 2022