Terrestrial Gravity Fluctuations
Abstract
Different forms of fluctuations of the terrestrial gravity field are observed by gravity
experiments. For example, atmospheric pressure fluctuations generate a gravity-noise foreground
in measurements with super-conducting gravimeters. Gravity changes caused by high-magnitude
earthquakes have been detected with the satellite gravity experiment GRACE, and we expect
high-frequency terrestrial gravity fluctuations produced by ambient seismic fields to limit the
sensitivity of ground-based gravitational-wave (GW) detectors. Accordingly, terrestrial gravity
fluctuations are considered noise and signal depending on the experiment. Here, we will focus
on ground-based gravimetry. This field is rapidly progressing through the development of GW
detectors. The technology is pushed to its current limits in the advanced generation of the LIGO
and Virgo detectors, targeting gravity strain sensitivities better than 10–23 Hz–1/2 above a few
tens of a Hz. Alternative designs for GW detectors evolving from traditional gravity gradiometers
such as torsion bars, atom interferometers, and superconducting gradiometers are currently
being developed to extend the detection band to frequencies below 1 Hz. The goal of this article
is to provide the analytical framework to describe terrestrial gravity perturbations in these
experiments. Models of terrestrial gravity perturbations related to seismic fields, atmospheric
disturbances, and vibrating, rotating or moving objects, are derived and analyzed. The models
are then used to evaluate passive and active gravity noise mitigation strategies in GW detectors,
or alternatively, to describe their potential use in geophysics. The article reviews the current
state of the field, and also presents new analyses especially with respect to the impact of seismic
scattering on gravity perturbations, active gravity noise cancellation, and time-domain models
of gravity perturbations from atmospheric and seismic point sources. Our understanding of
terrestrial gravity fluctuations will have great impact on the future development of GW detectors
and high-precision gravimetry in general, and many open questions need to be answered still as
emphasized in this article.
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