Field study shows possibility of deflecting seismic waves around desired geologic surface areas

Apr 09, 2014 by Bob Yirka weblog
(a) Scheme of the testing device for seismic metamaterials. An oscillating probe generates acoustic waves at 50 Hz in front of a mesh of cylindrical boreholes. An array of sensors monitors the intensity of the waves at various positions. (b) Experimental results. The map (black rectangles: sensors; white circles: boreholes; red cross: source) plots the difference of energy after and before drilling the boreholes. The dark blue region behind the holes indicates that the presence of the borehole mesh results in a decrease of elastic energy transmitted to that area. The red region indicates the area close to the source in which the wave intensity increases because of reflection. Credit: S. Brûlé et al., Phys. Rev. Lett. (2014)

(Phys.org) —A team of researchers in France has successfully demonstrated small-scale reflection of seismic waves around a desired geological area. In their paper published in the journal Physical Review Letters, the researchers describe an experiment they conducted at an outdoor site to test whether lessons learned in creating cloaking devices using metamaterials (and other techniques) might be applied to seismic waves generated by earthquakes.

The construction of tiny has made headlines around the world in recent years, though they are still very much in their infancy. In this new field study, the team in France sought to learn whether a similar cloaking device could be built on a much larger scale. More specifically, they wanted to know if it might be possible to cloak parts of the surface of the Earth, from destructive seismic waves, i.e. earthquakes.

To find out, they noted the frequencies at which seismic waves tend to move through the ground and then devised a grid system that in theory could cause the waves to change course due to diffraction among the elements that made up the grid. To test their theory they dug an array of boreholes five meters deep (.32 meters diameter) in a section of ground to serve as the cloaking device. They then buried several sensors around and near the grid to test how well the cloak worked. Finally, a crane was used to lower a vibrating pole into a hole a meter and a half away from the grid to cause the creation of seismic waves.

In analyzing data from the sensors, the researchers found that the bore-hole grid did indeed reflect a significant portion of the seismic waves, leaving the ground around them nearly unshaken. They also noted that areas next to the bore-hole grid had nearly double the amount of seismic energy—they were exposed to the initial waves from the source and also from the waves that were reflected off the grid.

The experiment opens the door to the possibility of protecting populated areas from the devastation of earthquakes. Many questions must still be addressed however. First, if a massive grid were constructed to protect a city, where would the waves be sent instead. Also, there is the question of how well the idea would scale—much more work needs to be done to find the answers to such questions, though it will be a near certainty that the results by this team will cause others to join in as well.

Explore further: Detection of supershear rupture in 2013 Craig, Alaska, earthquake

More information: Experiments on Seismic Metamaterials: Molding Surface Waves, Phys. Rev. Lett. 112, 133901 – Published 31 March 2014, dx.doi.org/10.1103/PhysRevLett.112.133901

ABSTRACT
Materials engineered at the micro- and nanometer scales have had a tremendous and lasting impact in photonics and phononics. At much larger scales, natural soils civil engineered at decimeter to meter scales may interact with seismic waves when the global properties of the medium are modified, or alternatively thanks to a seismic metamaterial constituted of a mesh of vertical empty inclusions bored in the initial soil. Here, we show the experimental results of a seismic test carried out using seismic waves generated by a monochromatic vibrocompaction probe. Measurements of the particles' velocities show a modification of the seismic energy distribution in the presence of the metamaterial in agreement with numerical simulations using an approximate plate model. For complex natural materials such as soils, this large-scale experiment was needed to show the practical feasibility of seismic metamaterials and to stress their importance for applications in civil engineering. We anticipate this experiment to be a starting point for smart devices for anthropic and natural vibrations.

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antialias_physorg
not rated yet Apr 09, 2014
Does this work equally well for P and S waves? The probe they use would, if I am not mistaken, only simulate P waves.
Doug_Huffman
not rated yet Apr 09, 2014
Does this work equally well for P and S waves? The probe they use would, if I am not mistaken, only simulate P waves.
See MODE CONVERSION that inter-converts P/S at interface obliquities.
Pawe_Guzik
not rated yet Apr 09, 2014
"First, if a massive grid were constructed to protect a city, where would the waves be sent instead."

Why should they need to be sent? Try to convert them into electricity.
russell_russell
not rated yet Apr 09, 2014
And who cloaked us? (The earth) And with what? And why?
Attempted humor.
russell_russell
not rated yet Apr 09, 2014
Hint: Gravitational lensing using the meta material BHs.
More attempted humor.
Nik_2213
not rated yet Apr 09, 2014
Be careful what you try to patent, guys; I remember this trick from a science-fiction tale set on a truly weird planet in a multi-star system where they finally prevented 'heavy birds' undermining everything by making a barrier of air-bricks, ie with air-gaps...

IIRC, there was also a ghastly pun on 'I Quit / Ike Witt'
antialias_physorg
5 / 5 (2) Apr 10, 2014
Why should they need to be sent? Try to convert them into electricity

It's a bit like the problem with harnessing lightning strikes for energy. Earthquakes have tremendous energy - but they are so infrequent that the average power you could deliver from such an "earthquake energy generator station" would be incredibly low.

It's also rather tricky, as if you build such a 'protective skirt' then you're effectively putting large areas outside the city off limits for further development of any kind - as they'd get twice the earthquake damage (with many cities being surrounded by suburbs this means that's a no-go in any case).
And once set the geometry of the pillars can't be altered easily to accomodate a city's growth.

It's a neat idea and useful for isolated, high value structures...but I think for most cities it wouldn't be sensible (maybe in extreme risk areas like Japan)