A magic cloak of earthquake protection

 

The editor thought Harry Potter was so last decade, so this Cosmos article got majorly changed.  Thought the original wasn’t bad though…

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Harry Potter’ invisibility cloak might just have been trumped by a French team, who aim to make entire buildings invisible. Scientists from the Institut Fresnel in Marseilles are teaming up with geo-engineering company Ménard for an experiment in the shape of a circle, two hundred metres across – enough to protect the whole of Hogwarts School (or more realistically a sensitive site such as a nuclear plant).

The team are not striving for invisibility to human eyes: instead of light waves they will divert the waves that travel through the surface of the earth during earthquakes and other seismic disturbances. As they reported in last month’s Physics Review Letters they have already succeeded with an area the size of Harry Potter’s Gryffindor common room.

“We managed to stop the propagation of the waves,” says the leader of the team, physicist Sebastien Guenneau. “This is the first proof of concept of a seismic metamaterial, a structure which can scatter and deflect wave trajectories. You can build on this knowledge to create an invisibility cloak which will actually protect a specific site from seismic waves.”

Metamaterials were first developed in the optics domain. They are substances comprised of an array of small elements, whose regular pattern leads to unusual behaviour on a large scale. Guenneau studied at Imperial College London with the pioneer of metamaterials, Sir John Pendry; Pendry shook the optics world in the early 2000s when he proposed an invisibility cloak based on metamaterials, and then, in collaboration with David Smith from Duke University, built such a device that operated at microwave wavelengths.

The strange effects that metamaterials have on waves rely on geometric structures that are smaller than the waves they are influencing. For Pendry’s centimeter-scale microwaves the patterns were millimetres in size,  but for Guenneau’s team, dealing with earthquake wavelengths of around a metre and a half, the structures were 30cm wide boreholes, spaced roughly a metre apart.

However, unlike metamaterials based on pure, man-made substances, the earth is much less homogeneous.

“Soil is a different story,” says Guenneau. “Its properties are difficult to characterise, and depend on different things, such as the weather! It makes the mathematical models much more difficult.”

Overcoming more than just mathematical hurdles – other scientists initially ridiculed the theory – Guenneau fortuitously met geo-engineer at Ménard, Stephane Brûlé, who was open minded and influential enough to persuade his company to collaborate on the idea – albeit during the summer holiday period.

So it was that the team of twenty people studied the weather forecasts carefully and chose three sunny days in August 2012, to take the measurements at a site near Grenoble.

Using a seismic source vibrating the ground at 50 times a second they first measured the propagation of the waves in the undisturbed soil. Then after carefully drilling three rows of five metre deep holes, they repeated the experiment. Sure enough, as the model predicted, most of the energy was reflected by the hole pattern; behind the array the detectors only measured one fifth of the energy that had reached the detector before the holes were drilled.

“It’s interesting because these are the first experimental results on this topic,” says University of Sydney physicist, Professor Boris Kuhlmey, who studies electromagnetic metamaterials that function at the nanometer wavelengths of light. “It’s the very beginning of the field: the modelling is quite extensive, but the experiment is quite limited in scope. The structure they have explored will only work over a narrow band of frequencies, but if your aim is to stop an earthquake you don’t get to choose the frequency.”

However Kuhlmey says Guenneau’s mathematical models offer the possibility of a phenomenon known as a zero stop band, which can cut out a wide range of earthquake waves. “These exist in electromagnetic metamaterials – the paper suggests that for seismic waves they are possible too. That would be really key to get it to work well. Maybe it’s possible, on the scale of a city, to diminish the impact of an earthquake considerably.”

Guenneau’s next experiment will certainly push back the boundaries. The team will inflict earthquakes measuring six on the Richter scale, with frequencies of between 2 and 12 vibrations per second on their test site, protected by a ring of boreholes 200 metres in diameter.

“It would be a dream for me to see this done for real one day, not just tests,” muses Guenneau. But he is not precious about his idea. “I am sure that the civil engineers will come up with better ways to make it work, I don’t have the expertise,” he says.

In the meantime he is already turning his considerable skills to other problems, such as tsunami control. “Imagine some columns of wood, 200 m from the sea shore, arranged in a similar fashion to the bore holes in the seismic experiments. The effect will be that you deflect or guide the tsunami to a nonsensitive coastal area.”

“Also, I’d like to do some work in biology…” he throws in.


Originally published in Cosmos Magazine 28/4/14