Intercomparing Superconducting Gravimeter Records in a Dense Meter-Scale Network at the J9 Gravimetric Observatory of Strasbourg, France.

This study is a metrological investigation of eight superconducting gravimeters that have operated in the Strasbourg gravimetric Observatory. These superconducting gravimeters include an older compact C026 model, a new observatory type iOSG23 and six iGravs (6, 15, 29, 30, 31, 32). We first compare the amplitude calibration of the meters using measurements from FG5 #206 absolute gravimeter (AG). In a next step we compute the amplitude calibration of all the meters by time regression with respect to iOSG23 itself carefully calibrated by numerous AG experiments. The relative calibration values are much more precise than absolute calibration for each instrument and strongly reduce any tidal residual signal. We also compare the time lags of the various instruments with respect to iOSG23, either by time cross-correlation or tidal analysis for the longest records (about 1 year). The instrumental drift behavior of the iGravs and iOSG23 is then investigated and we examine the relationships observed between gravity and body temperature measurements. Finally, we compare the noise levels of all the instruments. A three-channel correlation analysis is used to separate the incoherent (instrumental) noise from the coherent (ambient) noise. The self-noise is then compared to a model of thermal noise (Brownian motion) using the known instrumental parameters of the damped harmonic oscillator. The self-noise of iGrav instruments is well-explained by the thermal noise model at seismic frequencies (between 10-3 and 10-2 Hz). As expected, the self-noise of iOSG23 with a heavier sphere is also lower than that of iGravs at such frequencies.

Limits of Detection of Gravimetric Signals on Earth.

Gravimetry is a well-established tool to probe the deep Earth's processes. Geophysical signals coming from the deep Earth, like the inner core free oscillations, have however never been detected. Challenging quests raise the question of the limits of detection of elusive signals at the Earth's surface. Knowledge of the instrumental limits and of the environmental noise level at a site is fundamental to judge the true sensitivity of an instrument. We perform a noise level comparison of various gravimeters and a long-period seismometer at the J9 gravimetric observatory of Strasbourg (France) to provide a reference of instrumental performances. We then apply a three-channel correlation analysis of time-varying surface gravity from superconducting gravimeter records to isolate the instrumental self-noise from the environmental noise. The self-noise coherence analysis shows that the instrumental noise level remains flat towards lower frequencies till 10-4 Hz. At seismic frequencies, the self-noise is well explained by a Brownian thermal noise model. At daily and sub-daily time-scales, self-noise is increasing with the period but to a much lesser extent than observed noise level. Observed Earth's ambient noise level at sub-seismic frequencies is hence mostly due to unmodeled geophysical processes. At hourly time-scales, our ability to detect elusive signals coming from the deep Earth's interior is not limited by the instrument capability but is mostly due to the environmental effects.

Exploring gravity with the MIGA large scale atom interferometer.

We present the MIGA experiment, an underground long baseline atom interferometer to study gravity at large scale. The hybrid atom-laser antenna will use several atom interferometers simultaneously interrogated by the resonant mode of an optical cavity. The instrument will be a demonstrator for gravitational wave detection in a frequency band (100 mHz-1 Hz) not explored by classical ground and space-based observatories, and interesting for potential astrophysical sources. In the initial instrument configuration, standard atom interferometry techniques will be adopted, which will bring to a peak strain sensitivity of [Formula: see text] at 2 Hz. This demonstrator will enable to study the techniques to push further the sensitivity for the future development of gravitational wave detectors based on large scale atom interferometers. The experiment will be realized at the underground facility of the Laboratoire Souterrain à Bas Bruit (LSBB) in Rustrel-France, an exceptional site located away from major anthropogenic disturbances and showing very low background noise. In the following, we present the measurement principle of an in-cavity atom interferometer, derive the method for Gravitational Wave signal extraction from the antenna and determine the expected strain sensitivity. We then detail the functioning of the different systems of the antenna and describe the properties of the installation site.