Transformation seismology: composite soil lenses for steering surface elastic Rayleigh waves

Abstract

Metamaterials are artificially structured media that can focus (lensing) or reroute (cloaking) waves, and typically this is developed for electromagnetic waves at millimetric down to nanometric scales or for acoustics or thin elastic plates at centimeter scales. Extending the concepts of Kadic et al. 2013 we show that the underlying ideas are generic across wave systems and scales by generalizing these concepts to seismic waves at frequencies, and lengthscales of the order of hundreds of meters, relevant to civil engineering. By applying ideas from transformation optics we can manipulate and steer Rayleigh surface wave solutions of the vector Navier equations of elastodynamics; this is unexpected as this vector system is, unlike Maxwell's electromagnetic equations, not form invariant under transformations. As a paradigm of the conformal geophysics that we are creating, we design a square arrangement of Luneburg lenses to reroute and then refocus Rayleigh waves around a building with the dual aim of protection and minimizing the effect on the wavefront (cloaking) after exiting the lenses. To show that this is practically realisable we deliberately choose to use material parameters readily available and this metalens consists of a composite soil structured with buried pillars made of softer material. The regular lattice of inclusions is homogenized to give an effective material with a radially varying velocity profile that can be directly interpreted as a lens refractive index. We develop the theory and then use full 3D time domain numerical simulations to conclusively demonstrate the validity of the transformation seismology ideas: we demonstrate, at frequencies of seismological relevance 3-10 Hz, and for low speed sedimentary soil ($v_s$: 300-500 m/s), that the vibration of a structure is reduced by up to 6 dB at its resonance frequency.