A Seismic Sensor Array for Newtonian-Noise Subtraction at a Gravitational-Wave Observatory: Sensor Count Computed from Coherence and Wiener Filtering

Newtonian noise — gravity fluctuations from moving mass in the ground and air — is the one noise source at a gravitational-wave observatory that cannot be shielded, only measured and subtracted. A subtraction array is designed here from coherence arithmetic: 200 seismometers per interferometer vertex, 600 in total, with a stated single-sensor coherence of 0.05 against the test-mass Newtonian noise at 10 Hz, give a Wiener-filter suppression factor of 3.3, reducing 8.0e-25 per root hertz of Newtonian noise to 2.41e-25 per root hertz. The array totals 95 tonnes and 130 kW. All figures are computed from the stated coherence model; the design is simulated, not measured.