A group from the Institut Fresnel in Marseille and the ground improvement specialist company, Menard, both in France, say they’ve built and tested a seismic invisibility cloak in an alluvial basin in southern France. That’s the first time such a device has been constructed.
Future versions of this system could be used to protect hospital, nuclear power plants and other key facilities.
The secret of invisibility cloaks lies in engineering a material on a scale smaller than the wavelength of the waves it needs to manipulate. The appropriate sub-wavelength structures can then be arranged in a way that steers waves.
The French team created its so-called metamaterial by drilling three lines of empty boreholes 5 metres deep in a basin of silted clay up to 200 metres deep. They then monitored the area with acoustic sensors.
The experiment consisted of creating waves with a frequency of 50 Hertz and a horizontal displacement of 14 mm from a source on one side of the array. They then measured the way the waves propagated across it.
The French team say its metamaterial strongly reflected the seismic waves, which barely penetrated beyond the second line of boreholes.
One problem with this kind of array is that the reflected waves could end up doing more damage to buildings nearby. That’s why some groups are looking at metamaterials that absorb energy rather than steer or reflect it.