  Structured soils with regular meshes of metric size holes implemented in
first ten meters of the ground have been theoretically and experimentally
tested under seismic disturbance this last decade. Structured soils with rigid
inclusions embedded in a substratum have been also recently developed. The
influence of these inclusions in the ground can be characterized in different
ways: redistribution of energy within the network with focusing effects for
seismic metamaterials, wave reflection, frequency filtering, reduction of the
amplitude of seismic signal energy, etc. Here we first provide some time-domain
analysis of the flat lens effect in conjunction with some form of external
cloaking of Rayleigh waves and then we experimentally show the effect of a
finite mesh of cylindrical holes on the ellipticity of the surface Rayleigh
waves at the level of the Earth's surface. Orbital diagrams in time domain are
drawn for the surface particle's velocity in vertical (x,z) and horizontal
(x,y) planes. These results enable us to observe that the mesh of holes locally
creates a tilt of the axes of the ellipse and changes the direction of particle
movement. Interestingly, changes of Rayleigh waves ellipticity can be
interpreted as changes of an effective Poisson ratio. However, the proximity of
the source is also important to explain the shape of the ellipses. We analyze
these observations in terms of wave mode conversions inside the mesh and we
propose to broaden the discussion on the complexity of seismic wave phenomena
in structured soils such as soils foundations and on the coupling effects
specific to the soil-structure interaction.
