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A system was recently implemented in the Virgo detector to cancel noise in its data produced by seismic waves directly coupling with the suspended test masses through gravitational interaction. The data from seismometers are being filtered to produce a coherent estimate of the associated gravitational noise also known as Newtonian noise. The first implementation of the system uses a time-invariant (static) Wiener filter, which is the optimal filter for Newtonian-noise cancellation assuming that the noise is stationary. However, time variations in the form of transients and slow changes in correlations between sensors are possible and while time-variant filters are expected to cope with these variations better than a static Wiener filter, the question is what the limitations are of time-variant noise cancellation. In this study, we present a framework to study the performance limitations of time-variant noise cancellation filters and carry out a proof-of-concept with adaptive filters on seismic data at the Virgo site. We demonstrate that the adaptive filters, at least those with superior architecture, indeed significantly outperform the static Wiener filter with the residual noise remaining above the statistical error bound.

Terrestrial gravity perturbations caused by seismic fields produce the so-called Newtonian noise in gravitational-wave detectors, which is predicted to limit their sensitivity in the upcoming observing runs. In the past, this noise was seen as an infrastructural limitation, i.e., something that cannot be overcome without major investments to improve a detector's infrastructure. However, it is possible to have at least an indirect estimate of this noise by using the data from a large number of seismometers deployed around a detector's suspended test masses. The noise estimate can be subtracted from the gravitational-wave data; a process called Newtonian-noise cancellation (NNC). In this article, we present the design and implementation of the first NNC system at the Virgo detector as part of its AdV+ upgrade. It uses data from 110 vertical geophones deployed inside the Virgo buildings in optimized array configurations. We use a separate tiltmeter channel to test the pipeline in a proof-of-principle. The system has been running with good performance over months.

Gravitational wave detectors like the Einstein Telescope will be built a few hundred meters under Earth's surface to reduce both direct seismic and Newtonian noise. Underground facilities must be designed to take full advantage of the shielding properties of the rock mass to maximize the detector's performance. A major issue with the Einstein Telescope design are the corner points, where caverns need to be excavated in stable, low permeability rock to host the sensitive measurement infrastructure. This paper proposes a new topology that moves the top stages of the seismic attenuation chains and Michelson beam re-combination in separate excavations far from the beam-line and equipment induced noise while the test mass mirrors remain in the main tunnels. Distributing the seismic attenuation chain components over multiple tunnel levels allows the use of arbitrarily long seismic attenuation chains that relegate the seismic noise at frequencies completely outside the low-frequency noise budget, thus keeping the door open for future Newtonian noise suppression methods. Separating the input-output and recombination optics of different detectors into separate caverns drastically improves the observatory detection efficiency and allows staged commissioning. The proposed scheme eliminates structural and instrumentation crowding while the reduced sizes of excavations require fewer support measures.

The Einstein Telescope (ET) is a proposed next-generation, underground gravitational-wave (GW) detector to be based in Europe. It will provide about an order of magnitude sensitivity increase with respect to currently operating detectors, and furthermore, extend the observation band towards lower frequencies, i.e., down to about 3Hz. One of the first decisions that needs to be made is about the future ET site following an in-depth site characterization. Site evaluation and selection is a complicated process, which takes into account science, financial, political, and socio-economic criteria. In this paper, we provide an overview of the site-selection criteria for ET, provide a formalism to evaluate the direct impact of environmental noise on ET sensitivity, and outline the necessary elements of a site-characterization campaign., Comment: 22 pages, 3 figures

This paper deals with the problem of enforcing modular diagnosability for discrete-event systems that don't satisfy this property by their natural modularity. We introduce an approach to achieve this property combining existing modules into new virtual modules. An underlining mathematical problem is to find a partition of a set, such that the partition satisfies the required property. The time complexity of such problem is very high. To overcome it, the paper introduces a structural analysis of the system's modules. In the analysis we focus on the case when the modules participate in diagnosis with their observations, rather then the case when indistinguishable observations are blocked due to concurrency.