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Arrays of surface resonators forming so-called metabarriers are capable of converting seismic Rayleigh waves into less dangerous shear waves traveling in the bulk, attenuating the ground motion at the surface. However, this phenomenon pertains to resonant materials inserted in homogeneous media which feature properties rather different to those of natural soils. In this work, we experimentally investigate the ability of a resonant metabarrier embedded below the surface of an inhomogenous medium, i.e. a granular unconsolidated material, to attenuate the incoming surface waves. Although unable to diverge their propagation into the bulk, we find that surface resonators in granular media achieve attenuation of Rayleigh waves by up to 50 % around their resonant frequency. The findings can be used as a starting point for future real-scale implementations for seismic protection systems in stratified soils presenting an inhomogeneous stiffness profile at the seismic Rayleigh waves scale.

Precision engineering tasks require active isolation systems that are efficient especially at low frequencies. The limitations of such control systems include the resolution of the sensor used and the magnetic coupling between the sensor and the actuator. In order to bypass these limitations, inertial sensors using Michelson interferometer are being developed. A first prototype has been built and tested. It has been shown that it has a sub-nanometer resolution over a large frequency range, extending from 0.1 Hz to 100 Hz. To further improve the resolution, a new optical design will be presented in this paper. The elements of the setup are chosen to lower the noise of the whole system. Actually, two main sources of noise can be reduced. The first one is due to the optical components, inducing a phase shift which is converted into a displacement error. The second is a consequence of the pendulum movement of the piece/spring holding the moving mass. It couples the vertical translation and the rotation. By choosing correctly the optical components, the first source can be diminished. The resolution reached is compared with that predicted by the optical model implemented on MATLAB. With the best optical resolution of the setup achieved, the interferometer had been integrated into a STS1 seismometer to reduce the pendulum movement. The optical sensor replaces the conventional capacitive sensor of the device without disturbing the mechanical parts. The resulting modified STS1 has a spectral resolution below 10^(-13) m/rtHz, while at the same time is insensitive to magnetic field. The final objective of this research consists in introducing the inertial sensor into a single-axis isolation system equipped with a voice coil actuator. In feedback configuration, the setup will allow to reach an unprecedented high level of isolation, opening a new window in gravimetry and gravitational wave detection.

The Laser Interferometer Gravitational wave Observatories (LIGO) host kilometers long vacuum equipment. At each site, five large chambers and support structures are used to store the detector’s core instrumentation. This article discusses the vibrations of these large vacuum chambers and equipment. It presents and summarizes transmissibility measurements performed at the LIGO test facilities and observatories. In addition, it presents detailed models and compares them with experiments and discusses vibration mitigation techniques and control options. It concludes with recommendations for future LIGO observatories.