NEW DEVELOPMENTS TOWARDS SEISMIC METAMATERIALS

Why should complex wave physics be limited to experiments at very small scales? As waves obey similar propagation equations whatever their origin, why are there so few observations of the effects of advanced wave physics at the geophysics scale, for example? Of course, the answers to these deliberately provocative questions are multiple. What can be achieved in terms of wave manipulation and control at the laboratory scale turns out to be very tedious, if not impossible, at larger scales, where the deployment of numerous autonomous sensors is often another practical limitation. In recent years, however, wide areas of the sciences involved with large-scale data have been through a technological revolution. Indeed, Earth sciences, and especially geophysics, can now benefit from continuous data acquisition on very dense arrays of seismometers, with sometimes more than 10,000 sensors. These sensor deployments were nearly unconceivable only 10 years ago, and up to very recently, they were limited to vastly expensive geophysics experiments that could only be funded by the oil & gas industries. However, the technology has now progressed and academic institutions can now obtain experimental data using thousands of seismic sensors at an affordable cost. Our current research aims to fill this gap in terms of large-scale wave manipulation with a multidisciplinary approach devised by a team composed of physicists, geophysicists and engineers who share a common interest in wave propagation in complex media. In practice, we aim to experimentally test two geophysics configurations from which metamaterial physics - inducing seismic cloaking and/or seismic protection - can be demonstrated. For example, the first configuration deals with the interaction between a surface wave and a natural forest. Each tree within the forest can react as a resonator that traps a small part of the seismic waves propagating at the Earth surface. The collective behavior of the trees when arranged in a dense forest is then analogous to the physics observed at a very small scale in optical metamaterials. In the second experimental configuration, we aim to show that a particular spatial distribution (deduced from a geometric transform) of long and thin vertical inclusions in the ground (concrete columns) surrounding a structure can create a special seismic lens that diverts surface waves away from the inner region leaving the protected object almost untouched. We believe that this work could lead to important geophysical and civil engineering applications. For example, forbidden frequency bands could be exploited for the cancellation of ambient seismic noise at locations where ground vibrations are an issue for highly sensitive scientific measurements (e.g., local vibrations of large astronomic lenses). Similarly, bandgaps might be used to protect sensitive structures, like power plants, from potentially destructive surface waves caused by earthquakes. The design of a forest that can ‘hide’ any kind of manmade structure from surface seismic wave through a natural ‘cloak of invisibility’ would be a societal revolution in the field of seismic hazards protection.