With the construction of second generation detectors on its way, several R&D, feasibility, and design studies for third generation detectors are focusing on the Einstein Telescope (ET) gravitational observatory project. The ET project is a design study supported by the European Commission under the Framework Programme 7. It concerns the study and the conceptual design for a new research infrastructure that will bring Europe to the forefront of the most promising new development in our quest to understand the history and future of the Universe, the emergence of the field of Gravitational Wave Astronomy.

ET's proposes to improve on the sensitivity of second generations detectors by about a factor 10, across a broader frequency range from 1 Hz to tens of kHz. This places stringent constraints on acceptable noise levels. In particular seismic motion will limit the sensitivity of ET below 10 Hz. As part of the ET design project, Nikhef performed studies to determine seismic noise characteristics at various locations. In the frequency range of interest (above 1 Hz), seismic activity is dominated by atmospheric effects and human activity, such as roads and industrial noise. The seismic wave fields from these sources are excited on, and propagate predominately along the surface, resulting in a reduction in seismic activity with depth. The study includes 15 sites in 9 European countries, USA and Japan, varying in depth from 0 to 1750 m. They also vary in geology from hard rock to salt and clay.

Analytical and computer models have also been developed to better understand the effects of seismic noise on a gravitational wave detector. An important issue for third generation detectors is gravity gradient noise. This involves Newtonian coupling between fluctuating mass densities due to seismic motion in the surrounding soil, and the interferometer test masses. As opposed to normal seismic vibrations, these effects cannot be shielded or otherwise suppressed. Finding a suitably seismically quiet location is therefor the first step in mitigating this noise. The seismic studies show that several orders of magnitude improvement in seismic power can be achieved in comparison to current detector sites.

Another one of Nikhef's main activities within the ET project has been to define the site and infrastructure requirements. The study needs to incorporate the design of such a complex underground facility, featuring multi-kilometer tunnels to host the interferometer arms and cryogenic plants. As such, the design of such a facility requires safety procedures to guarantee a safe working environment for the scientist and engineers.