Human gravity-gradient noise in interferometric gravitational-wave detectors

摘要: When ambient seismic waves pass near and under an interferometric gravitational-wave detector, they induce density perturbations in the Earth, which turn produce fluctuating gravitational forces on interferometer’s test masses. These mimic a stochastic background of thus constitute noise source. This gravity-gradient has been estimated discussed previously by Saulson using simple model Earth’s motions. In this paper, we develop more sophisticated these motions, based theory multimode Rayleigh Love propagating multilayer medium that approximates geological strata at LIGO sites, use to reexamine gravity gradients. We characterize transfer function, T(f )≡x(f )/W(f ), from spectrum rms displacements averaged over vertical horizontal directions, W(f test-mass x(f )≡Lh(f ); here L is interferometer arm length, h(f ) spectrum, f frequency. Our predicts function with essentially same functional form as derived Saulson, T≃4πGρ(2πf )-2β(f where ρ Earth masses, G Newton’s constant, β(f )≡γ(f )Γ(f )β′(f dimensionless reduced whose components γ≃1 Γ≃1 account for weak correlation between two corner masses slight reduction due height above surface. paper’s primary foci are (i) study how β′(f )≃β(f depends various modes present (ii) attempt estimate actually sites quiet times noisy times, (iii) corresponding magnitude times. conclude β′≃0.35–0.6 β′≃0.15–1.4. (For comparison, Saulson’s gave β=β′=1/sqrt[3]=0.58.) By folding our resulting into “standard spectrum,” typical obtain spectra. At below benchmark level “advanced interferometers” all frequencies (though not much ∼10 Hz); it may significantly exceed advanced 10 Hz. The lower edge quiet-time constitutes limit, beyond there would be little gain further improvements vibration isolation thermal noise, unless one can also reduce gradient noise. Two methods such briefly discussed: monitoring each mass, computing produce, correcting data those forces; constructing narrow moats around interferometers’ end stations shield out fundamental-mode waves, suspect dominate