Terrestrial gravity fluctuations

Abstract

Terrestrial gravity fluctuations are a target of scientific studies in a variety of fields within geophysics and fundamental-physics experiments involving gravity such as the observation of gravitational waves. In geophysics, these fluctuations are typically considered as signal that carries information about processes such as fault ruptures and atmospheric density perturbations. In fundamental-physics experiments, it appears as environmental noise, which needs to be avoided or mitigated. This article reviews the current state-of-the-art of modeling high-frequency terrestrial gravity fluctuations and of gravity-noise mitigation strategies. It hereby focuses on frequencies above about 50 mHz, which allows us to simplify models of atmospheric gravity perturbations (beyond Brunt-Väisälä regime) and it guarantees as well that gravitational forces on elastic media can be treated as perturbation. Extensive studies have been carried out over the past two decades to model contributions from seismic and atmospheric fields especially by the gravitational-wave community. While terrestrial gravity fluctuations above 50 mHz have not been observed conclusively yet, sensitivity of instruments for geophysical observations and of gravitational-wave detectors is improving, and we can expect first observations in the coming years. The next challenges include the design of gravity-noise mitigation systems to be implemented in current gravitational-wave detectors, and further improvement of models for future gravitational-wave detectors where terrestrial gravity noise will play a more important role. Also, many aspects of the recent proposition to use a new generation of gravity sensors to improve real-time earthquake early-warning systems still require detailed analyses.

Newtonian-noise reassessment for the Virgo gravitational-wave observatory including local recess structures

“…Love waves are surface shear waves guided in near surface layered media. Love waves are not of concern here since they do not produce NN [8]. The body * a.singha@maastrichtuniversity.nl † stefan.hild@maastrichtuniversity.nl ‡ jan.harms@gssi.it waves, comprised of compressional and shear waves, can propagate through media in all directions.…”

Earthquake early warning using future generation gravity strainmeters

“…The response of different types of gravity strainmeters to gravity-gradient fluctuations is not identical (Harms, 2015). The consequence is that instrumental noise spectra differ qualitatively between detector types.…”

“…But a gravity noise foreground is common to all the detectors: the local gravity noise (LGN). The LGN has several contributions: seismic LGN produced by density changes in the ground due to seismic waves; atmospheric LGN generated by density fluctuations in the atmosphere due to, for instance, infrasounds, temperature changes, and turbulences; and LGN associated with human activity (Harms, 2015). This noise couples with the detector in a way completely equivalent to the earthquake signal: it is then impossible to shield the detector from it.…”

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

“…Generally, the hope is that gravitational coupling makes unique predictions about the Wiener filters. This is certainly true for isotropic fields where, at LHO, the Wiener filter of sensors within about 3 m to the test mass must have small amplitudes, increasing with distance up to some point (depending on seismometer SNR and number of seismometers) and then decreasing again toward greater distances [8]. A consequence is that optimal placement of seismometers in isotropic fields for NN cancellation does not include any sensors close to the test mass.…”

“…Environmental noise couplings involve seismic [1,2], acoustic [3,4], and electromagnetic fields [5,6]. When ambient fields, and also moving and vibrating objects, produce mass-density fluctuations, then direct gravitational coupling with the test masses gives rise to gravity noise in the detector, also known as Newtonian noise (NN) [7,8]. Newtonian noise of seismic and acoustic origin is predicted to limit sensitivity of LIGO and Virgo detectors in the frequency range 10-20 Hz once the sensitivities have progressed toward their design targets [8][9][10].…”