The upgrade of the Virgo gravitational wave detector, which will increase its sensitivity by an order of magnitude, has recently commenced. With its improved capability, Advanced Virgo is expected to herald the era of gravitational wave astronomy.

The Earth's surface randomly moves in all directions with amplitudes often exceeding the micrometer level with a broadband spectrum dominated, around 0.3 Hz, by a microseismic peak caused by swell waves, that moves the ocean's sea floor and coasts. At 10Hz the observed seismic displacement noise level is up to 10^10 times larger than the allowed displacement (10^-18 m/√Hz) of the main mirrors of detectors like Advanced Virgo.

Detailed noise studies have shown that, in order to achieve the enhanced sensitivity, in Advanced Virgo also the optical benches, hosting ancillary optics and optical sensors for the alignment of the main interferometer, need to be isolated from seismic noise.

New bench support structures, capable of attenuating seismic motion with 60 dB (> 10 Hz) in all six degrees of freedom have been designed and constructed at Nikhef. Their design makes use of technology developed at LIGO and AEI. Other seismic isolation structures,  having a different layout for better performance (90 dB above 10 Hz), have also been designed. These systems, which are to be operated in ultra high vacuum, will be constructed in the next 3 years and installed in Advanced Virgo.

The seismic attenuation is obtained passively using the properties of mechanical oscillators which attenuate, as second-order low pass filters, above their natural frequency. Negative stiffness technology is employed to lower the system eigenfrequencies below 0.5 Hz: for horizontal attenuation inverted pendulums are used, vertical isolation is provided by geometric anti-springs (see figures inserted to the right).

A photo of the system is shown below. Three inverted pendulums support a box that contains three geometric anti-spring filters. Together they attenuate the motion of the bench top by 60 dB in all 6 degrees of freedom.

The system attenuates motion above the eigenfrequencies of the filters, but can still move at these frequencies and those corresponding to internal modes of the system.
Motion at the eigenfrequencies of the filters is sensed with velocity and displacement sensors (geophones and LVDTs)  and are actively damped with a feedback system.

Internal modes of the system have been identified by comparing extensive measurements to FEM models. The horizontal modes are actively damped. The vertical modes are damped with dedicated eddy current dampers like the one shown in the figure to the right.