Optimization of seismometer arrays for the cancellation of Newtonian noise from seismic body waves

Abstract

Newtonian noise from seismic fields is predicted to become a sensitivity limiting noise contribution of the gravitational-wave detectors Advanced LIGO and Virgo in the next few years. It also plays a major role in the planning of next-generation detectors, which might be constructed underground, as planned for the Einstein Telescope, mostly to suppress Newtonian noise. Coherent noise cancellation using Wiener filters provides a way to mitigate Newtonian noise. So far, only the cancellation of Newtonian noise produced by seismic surface waves has been studied in detail due to its relevance for Advanced LIGO and Virgo. However, seismic body waves can still contribute significantly to Newtonian noise in surface detectors, and they might be the dominant source of gravity fluctuations in underground detectors. In this paper, we present the first detailed analysis of coherent cancellation of Newtonian noise from body waves. While the required number of seismometers to achieve a certain level of noise suppression is higher than for seismic surface waves, we show that optimal seismometer arrays can greatly reduce body-wave Newtonian noise. The optimal array configurations and achieved residuals depend strongly on the composition of the seismic field in terms of average compressional-wave and shear-wave content. We propose Newtonian-noise cancellation to achieve the ambitious low-frequency target of the Einstein Telescope.

Simulations of Gravitoelastic Correlations for the Sardinian Candidate Site of the Einstein Telescope

“…Rayleigh waves are predicted to give the dominant contribution to NN in surface detectors (Coughlin et al, 2016;Harms et al, 2020), and even underground detectors can still be limited by gravitational noise from Rayleigh waves depending on the detector depth (Badaracco & Harms, 2019). The Rayleigh field produces surface displacement and density perturbations beneath the surface at the same time (Beccaria 10.1029/2020JB020401 et al, 1998Hughes & Thorne, 1998), which leads to gravity perturbations.…”

“…Another mechanism is by gravitational coupling giving rise to NN and cannot be shielded in any way (Beker et al, 2011(Beker et al, , 2015. A well-explored cancelation scheme is based on Wiener filters (Badaracco & Harms, 2019;Cella, 2000; One of the most important things in NN cancelation is the homogeneity of the seismic field. Scattering of seismic fields from an irregular surface topography can cause heterogeneity of the seismic field.…”

“…Scattering of seismic fields from an irregular surface topography can cause heterogeneity of the seismic field. It can lead to a more complex field structure that is not completely characterized by surface displacement and will likely pose a great challenge even to 3‐D seismic surveys with boreholes where effective placement of seismometers needs to be achieved (Badaracco & Harms, 2019). The scattering will especially be the problem if it is strong enough to alter seismic waveforms significantly over very short propagation distances (Driggers et al, 2012).…”

Observation of a potential future sensitivity limitation from ground motion at LIGO Hanford

“…Only under near ideal conditions, the required information can be obtained from a single point; see, for example, [22]. Generally, one should expect that many sensors are required [23,24]. Current plans for the Virgo NNcancellation system foresee the deployment of 30 seismometers around each test mass.…”

“…The great unsolved problem is how to optimally place sensors to make sure that the information required for NN cancellation is extracted from the field. First optimization results were obtained for simplified models of seismic fields [23,24], but a rigorous optimization based on seismic observations does not exist yet. The scale of this effort is immense, but the sources can be easily monitored as long as they are known, and a major advantage is that the coupling to the detector data is linear, which greatly facilitates noise cancellation.…”