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  • 1
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    Noise Levels of Superconducting Gravimeters: Updated Comparison and Time Stability (2011)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2011-06-01
    Description: Since the beginning of the Global Geodynamics Project (GGP) in 1997, the number of superconducting gravimeters (SGs) has increased to reach 30 operating sites today. Data from this network allow a comparison of the noise levels of the different contributing stations. The knowledge of the noise levels at each site is important in any combination of data to determine global Earth parameters, for example, the stacking of the data in the search for elusive signals, like the gravity variations associated with the translational mode of the inner core. We use a standardized procedure based on the computation of the residual power spectral densities (PSDs) over a quiet time period in order to evaluate the combined instrument plus site noise in the long-period seismic band (0.3 mHz-1 mHz). The experience at Strasbourg (France) has shown some improvements from the TT70-T005 full-size instrument to the C026 compact model in terms of noise reduction, while the most recent Observatory SG types, OSG044 at Bad Homburg (Germany) and OSG052 at Sutherland (South Africa), do not show any further improvement with respect to the compact models, respectively CD30 and CD037, operating at the same stations. At Black Forest Observatory (BFO) in Germany, the experience of the dual-sphere OSG with a lower sphere heavier than usual has shown that the instrumental and site conditions make this station the least noisy one at frequencies larger than 0.1 mHz. The noise analysis using the longest time-series available has shown that the noise level at these sites is mostly stable (within 1{sigma}) over the years. The comparison with some seismological noise models shows that the best SG sites are less noisy than long-period seismometers below 1 mHz. However, the noise level of the best SGs is still at the limit of detection of the subseismic translational mode of the inner core.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 2
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    Estimation of the gravimetric pole tide by stacking long time-series of GGP superconducting gravimeters (2016)
    Oxford University Press
    Details
    Publication Date: 2016-02-20
    Description: We compute the gravimetric factor at the Chandler wobble (CW) frequency using time-series from superconducting gravimeters (SG) longer than a decade. We first individually process the polar motion and data at each individual gravity station to estimate the gravimetric factor amplitude and phase, then we make a global analysis by applying a stacking method to different subsets of up to seven SG stations. The stacking is an efficient way of getting rid of local effects and improving the signal-to-noise ratio of the combined data sets. Using the stacking method, we find a gravimetric factor amplitude and phase of 1.118 ± 0.016 and –0.45 ± 0.66 deg, respectively, which is smaller in amplitude than expected. The sources of error are then carefully considered. For both local and global analyses, the uncertainties on our results are reliably constrained by computing the standard deviation of the estimates of the gravimetric factor amplitude and phase for increasing length of the time-series. Constraints on the CW anelastic dissipation can be set since any departure of the gravimetric factor from its elastic value may provide some insights into the dissipative processes that occur at the CW period. In particular, assuming given rheological models for the Earth's mantle enables us to make the link between the gravimetric factor phase and the CW quality factor.
    Keywords: Gravity, Geodesy and Tides
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Constraints provided by ground gravity observations on geocentre motions (2016)
    Oxford University Press
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    Publication Date: 2016-07-08
    Description: The geocentre motion is the motion of the centre of mass of the entire Earth, considered an isolated system, in a terrestrial system of reference. We first derive a formula relating the harmonic degree-1 Lagrangian variation of the gravity at a station to both the harmonic degree-1 vertical displacement of the station and the displacement of the whole Earth's centre of mass. The relationship is independent of the nature of the Earth deformation and is valid for any source of deformation. We impose no constraint on the system of reference, except that its origin must initially coincide with the centre of mass of the spherically symmetric Earth model. Next, we consider the geocentre motion caused by surface loading. In a system of reference whose origin is the centre of mass of the solid Earth, we obtain a specific relationship between the gravity variation at the surface, the geocentre displacement and the load Love number $h^{\prime }_1$ , which demands the Earth's structure and rheological behaviour be known. For various networks of real or fictitious stations, we invert synthetic signals of surface gravity variations caused by atmospheric loading to retrieve the degree-1 variation of gravity. We then select six well-distributed stations of the Global Geodynamics Project, which is a world network of superconducting gravimeters, to invert actual gravity data for the degree-1 variations and determine the geocentre displacement between the end of 2004 and the beginning of 2012, assuming it to be due to surface loading. We find annual and semi-annual displacements with amplitude 0.5–2.3 mm.
    Keywords: Gravity, Geodesy and Tides
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    Earth's core and inner-core resonances from analysis of VLBI nutation and superconducting gravimeter data (2016)
    Oxford University Press
    Details
    Publication Date: 2016-11-14
    Description: Geophysical parameters of the deep Earth's interior can be evaluated through the resonance effects associated with the core and inner-core wobbles on the forced nutations of the Earth's figure axis, as observed by very long baseline interferometry (VLBI), or on the diurnal tidal waves, retrieved from the time-varying surface gravity recorded by superconducting gravimeters (SGs). In this paper, we inverse for the rotational mode parameters from both techniques to retrieve geophysical parameters of the deep Earth. We analyse surface gravity data from 15 SG stations and VLBI delays accumulated over the last 35 yr. We show existing correlations between several basic Earth parameters and then decide to inverse for the rotational modes parameters. We employ a Bayesian inversion based on the Metropolis–Hastings algorithm with a Markov-chain Monte Carlo method. We obtain estimates of the free core nutation resonant period and quality factor that are consistent for both techniques. We also attempt an inversion for the free inner-core nutation (FICN) resonant period from gravity data. The most probable solution gives a period close to the annual prograde term (or S 1 tide). However the 95 per cent confidence interval extends the possible values between roughly 28 and 725 d for gravity, and from 362 to 414 d from nutation data, depending on the prior bounds. The precisions of the estimated long-period nutation and respective small diurnal tidal constituents are hence not accurate enough for a correct determination of the FICN complex frequency.
    Keywords: Gravity, Geodesy and Tides
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Continuous high resolution gravity measurements at a geothermal field in Northern Iceland (2018)
    In:  Abstracts
    Details
    Publication Date: 2020-02-12
    Type: info:eu-repo/semantics/conferenceObject
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  • 6
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    Continuous high resolution gravity measurements at a geothermal field in Northern Iceland (2018)
    In:  Geophysical Research Abstracts
    Details
    Publication Date: 2020-02-12
    Type: info:eu-repo/semantics/conferenceObject
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  • 7
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    Continuous high resolution gravity measurements at a geothermal field in Northern Iceland (2019)
    In:  Geophysical Research Abstracts, Vol. 21, EGU2019-16472
    Details
    Publication Date: 2020-10-13
    Description: For a better understanding of the sustainability of geothermal resources, we want to quantify subsurface masschanges caused by production and injection of fluids at the Theistareykir geothermal field in Northeast Iceland.For this purpose, we installed three superconducting gravity meters (iGrav006, iGrav015 and iGrav032) and twospring gravity meters (gPhone061 and gPhone128) in vicinity to the new geothermal power plant that started op-eration in October 2017.Prior to the Iceland installation, all gravity meters were setup at the gravimetric observatory J9 in Strasbourg forsimultaneous side-by-side measurements. The obtained data were used for instrumental calibration, comparisonof noise levels and tidal analysis. In Theistareykir, three of our measuring sites are set up close to the geother-mal production and injection wells. The fourth site is located outside the geothermal field, to provide referencemeasurements that are unaffected by the activities of the power plant. At each site additional physical parameters,which influence the local gravity signal, are measured. This includes the continuous monitoring of GPS-positions,rainfall, soil moisture and snow thickness. Moreover, snow weight and snow water equivalent are measured at thesite close to the production wells.Here, we present the results of the unique intercomparison of three superconducting gravity meters and twogPhones at Strasbourg and the initial time series obtained at the geothermal site in Iceland. A preliminary in-terpretation of the gravity variations with regard to the geothermal activities and the hydro-metrological dynamicsis given.
    Language: English, Old (ca. 450-1100)
    Type: info:eu-repo/semantics/conferenceObject
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  • 8
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    Superconducting Gravimeter Data from Rustrel (LSBB) - Level 1 (2020)
    GFZ Data Services
    Details
    Publication Date: 2020-10-13
    Description: The International Geodynamics and Earth Tide Service (IGETS) was established in 2015 by the International Association of Geodesy. IGETS continues the activities of the Global Geodynamics Project (GGP) between 1997 and 2015 to provide support to geodetic and geophysical research activities using superconducting gravimeter (SG) data within the context of an international network. As part of this network, the Rustrel station (code RU) was established in 2015 thanks to the financial support of the EQUIPEX MIGA (Matter wave-laser based Interferometer Gravitation Antenna) ANR-11-EQPX-0028 and by the European FEDER 2006-2013 “PFM LSBB - Développement des qualités environnementales du LSBB”. Continuous time-varying gravity and atmospheric pressure data from RU are integrated in the IGETS data base hosted by ISDC (Information System and Data Centre) at GFZ. The operation and maintenance of the RU instrumentation is done by staff at EOST/IPG Strasbourg. The RU station is located about 500 m beneath the surface in the underground research laboratory of the LSBB (« Laboratoire Souterrain à Bas Bruit ») built in 1997. The LSBB is located within the Regional Natural Park of Luberon (longitude: 5.4837 E, latitude: 43.9411 N, height above MSL: 499.3 m). The LSBB gallery is located in the unsaturated zone of the Fontaine de Vaucluse karst hydrosystem. There is no industry around the site so the anthropogenic noise is very low. The time series of gravity and barometric pressure from iOSG-24 starts in October 2015 and is going on. The time sampling of the raw gravity and barometric pressure data of IGETS Level 1 is 1 minute. For a detailed description of the IGETS data base and the provided files see Voigt et al. (2016, https://doi.org/10.2312/GFZ.b103-16087).
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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  • 9
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    Continuous high resolution gravity measurements at a geothermal field in Northern Iceland (2019)
    In:  Abstracts
    Details
    Publication Date: 2021-01-05
    Type: info:eu-repo/semantics/conferenceObject
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  • 10
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    Intercomparing Superconducting Gravimeter Records in a Dense Meter-Scale Network at the J9 Gravimetric Observatory of Strasbourg, France (2022)
    In:  Pure and Applied Geophysics
    Details
    Publication Date: 2022-07-18
    Description: 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.
    Type: info:eu-repo/semantics/article
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