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The sustainable exploitation of a geothermal reservoir is usually assessed through continuous field monitoring and structural exploration of the hydrothermal reservoir. However, accurate subsurface mass and fluid displacement as well as energy transfer model of the hydrothermal reservoir is most of the time not resolved enough both in space and time. Since 2017, both at Krafla and Theistareykir powerplants (northern Iceland), we use several multi-parameter stations each equipped with a gravity meter (superconducting or spring relative meter), a broad band seismometer, a GNSS receiver and other meteorological and hydrological sensors. With this set-up, we aim to model mass and stress transfer through the combination of absolute and micro gravity measurements, continuous signals measured at the multi-parameter stations and seismic measurements.We present results from the 2022 micro gravity and absolute gravity campaigns conducted at Theistareykir geothermal field. Through inversion and interpretation of such results, as well as the analysis of the continuous measurements, and injection and production data, we aim to assess the anthropogenic contribution in the mass and energy transfer models of the investigated area. Furthermore, we show the first continuous measurements and accurate Earth tide model for the Krafla area. The final goal of this study is to verify the conditions of sustainable exploitation of the reservoirs, and establish reservoir parameters, such as permeability, that regulate the response of the geothermal system to changes in production and injection rates.

We report the results of about 20 years of relative gravity measurements acquired on Mt. Somma-Vesuvius INGV monitoring network, together with about 9 months of continuous gravimetric recordings collected with the new generation relative gravimeter gPhoneX#116, specifically designed for continuous gravity recording. We also present the outcomes of an intercomparison experiment of the gPhone#116 conducted at the J9 gravity observatory in Strasbourg (France). In this intercomparison, we were able to check the meter scale factor with a high degree of precision by comparing them with 2 superconducting gravimeters and a FG5-type absolute ballistic gravimeter. It was also possible to carry out a detailed study of instrumental drift, a crucial topic for reliable monitoring of the long-term gravity variations in active volcanic areas. In fact, a challenge in time lapse gravimetry is the proper separation of the instrumental variations from real gravity changes eventually attributable to recharge or drainage processes of magma or fluids in the feeding systems of active volcanoes.Since 1980s the relative gravity network of Mt- Somma-Vesuvius has evolved over time becoming progressively larger and denser. We discuss the results of the time-lapse monitoring since 2003, when the INGV network reached an almost stable configuration. The retrieved field of time gravity change shows a pattern essentially related to the ground deformation detected by the permanent GNSS network. Vesuvius is currently experiencing subsidence at a variable rate. A clear topographic effect emerges with a strong correlation with altitude, whereby higher stations subside at a greater rate, up to 7 mm/year, than those at lower altitudes. Most of the observed gravity changes can be explained by this dynamics; only a residual positive gravity is detected in the western sector of the volcano, which could be likely due to hydrological effects. A reliable tidal gravity model was derived from the analysis of the gravity records. We believe that this result should help improve the accuracy of the volcano monitoring as it will be useful for the correct reduction of tidal effects for all relative and absolute gravity measurements acquired in the area.

Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach that provides highly precise continuous measurement of gravity acceleration. However, large-scale effects (e.g. tides, polar motion, atmospheric loading) limit the fine time-scale interpretation of this data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters located 512 m apart on the same vertical. The difference calculation makes it possible to remove shared large-scale effects. Daily variation of this gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p-value = 4.10-10). However, this approach is effective only during rain-free periods. In the future, comparison with direct ET measurements (e.g. eddy-covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing will allow to extend this approach to other experimental sites equipped with a single superconducting gravimeter.

The excitation of long-period signals has been reported for large eruptions such as El Chichon (1982) and Mount Pinatubo (1991).We report on series of pressure and gravity signals associated with the series of violent explosions at the Hunga-Tunga eruption, on 14-15th January 2022 at different locations and set-up configurations.We present records from various stations all over the world. Those stations have different set up and local configurations.At most stations, gravity signals records reveal clearly surface waves associated to the main eruption at ~4:30, 15th January and pressure wave up to Iceland.We analyze similarity and difference in the signals in association with the location and the station configurations

The Theistareykir geothermal field is located in North Iceland on the Mid-Atlantic ridge. A power plant produces 90 MWe using two 45 MWe turbines in operation since autumn 2017 and spring 2018, respectively. We performed hybrid microgravity measurements from 2017 to 2019 to monitor the short-term mass redistribution induced by geothermal production. Time-lapse microgravity surveys conducted each summer with a Scintrex CG5 gravimeter reveal the spatial gravity variations with respect to a reference, where the temporal gravity changes are monitored by absolute gravity measurements done with FG5#206 from Micro-g Solutions. In parallel, continuous gravity changes are recorded by a network of several GWR Instruments iGrav superconducting gravimeters and spring gravimeter, located in the injection and production areas. A height correction is applied to the gravity data using InSAR and GNSS measurements. We notice a regular residual gravity decrease in the production area versus a stable behaviour in the injection area. Time-lapse gravity measurements reveal a minimum residual decrease of − 38 ± 10 µGal (1 µGal = 10–8 m s−2) in 2019 with respect to 2017. Simplistic forward modelling of the produced geothermal fluid using a multiple Mogi sphere model can partly explain the residual gravity decrease. This suggest that a significant part of the injected geothermal fluid flows away, maybe drained by the Tjarnarás fault to the South where an increase of the water table level is observed. However, further modelling work is needed to confirm this.

Superconducting gravimetry data is accessible using the following links: surface SG iGrav#031 (http://igets.u-strasbg.fr/SG/ru031/Level1/) and deep SG iOSG#024 (http://igets.u-strasbg.fr/SG/ru024/Level1/). Superconducting gravimetry data is accessible using the following links: surface SG iGrav#031 (http://igets.u-strasbg.fr/SG/ru031/Level1/) and deep SG iOSG#024 (http://igets.u-strasbg.fr/SG/ru024/Level1/).; International audience; Key Points:•For the first time, two vertically spaced gravimeters allow to interpret small gravity hydrologically induced signal (

International audience; The Earth's mass distribution is continuously changing due to physical processes taking place either beneath the subsurface or on the surface. Some of the primary sources for these mass variations are tides in the ocean and solid Earth, atmospheric disturbances and seasonal climate changes. The development of Superconducting Gravimeters (SGs) has made it possible to characterize and monitor such mass variations at micro scales. Our study focuses on the LSBB karst catchment's hydrodynamics using a unique configuration of two SGs located 520 m depth apart. The installation of a SG (iGrav-31) at the surface of the LSBB several years after the installation of the first (iOSG-24) inside the tunnel has provided several new insights into the understanding of hydrological processes occurring in the LSBB. In this work, we compare differential and residual gravity time-series together with the ERA5 global hydrological loading model. In the subsequent section, we implement a rectangular prism method to compute forward gravity responses using input from the hydrological model. We also numerically evaluate and validate the already published hypotheses (Mouyen et al., 2019) about uncertainties related to groundwater storage's location in this catchment. Based on our observations, we find that most water-storage changes occur in the unsaturated karst zone between both SGs. The misfit between the residual gravity time-series and the local hydrogravity effect computed from ERA5 model shows large lateral fluxes and rapid runoff occurring in the LSBB. Finally, we conclude this work by examining the radial and depth sensitivity of water masses' effect near the SGs, and enlisting some recommendations for further studies.

SUMMARY Assessing the spatial and temporal heterogeneity in subsurface water storage has strong societal and environmental implications, as it is key to assess the water availability for the ecosystem and society. This challenge is especially significant in mountainous areas, where the local population totally depends on springwater as a freshwater resource, while water storage dynamics is complex to evaluate because it exhibits spatiotemporal heterogeneities on all scales as a result of the topography. In this study, we compare the water balance of a headwater granitic catchment (CWB) with water storage changes assessed from in situ continuous gravity monitoring using an iGrav superconducting gravimeter (SGWSC) located at the summit of the catchment. We show that SGWSC and CWB exhibit a similar annual cycle, although they deviate in the months following winter peak flow events. We investigate the reasons for these discrepancies using a tank model adjusted to the SG signal. This shows that during these events, the effective discharge in the SG footprint area is much lower than the catchment streamflow. We attribute this difference in the drainage term to a lower contribution of the upper part of the catchment to the generation of peak flow, compared to the lower part.

Continuous high-resolution gravimetry is increasingly used to monitor mass distribution changes in volcanic, hydrothermal or other complex geosystems. To quantify the often small target signals, gravity contributions from, e.g. atmospheric mass changes, global and local hydrology should be accounted for. We set up three iGrav superconducting gravity meters for continuous monitoring of the Þeistareykir geothermal field in North Island. Additionally, we installed a set of hydrometeorological sensors at each station for continuous observation of local pressure changes, soil moisture, snow and vertical surface displacement. We show that the contribution of these environmental parameters to the gravity signal does not exceed 10 µGal (1 µGal = 10–8 m s−2), mainly resulting from vertical displacement and snow accumulation. The seasonal gravity contributions (global atmosphere, local and global hydrology) are in the order of ± 2 µGal at each station. Using the environmental observations together with standard gravity corrections for instrumental drift and tidal effects, we comprehensively reduced the iGrav time-series. The gravity residuals were compared to groundwater level changes and geothermal mass flow rates (extraction and injection) of the Þeistareykir power plant. The direct response of the groundwater levels and a time-delayed response of the gravity signal to changes in extraction and injection suggest that the geothermal system is subject to a partially confined aquifer. Our observations indicate that a sustainable “equilibrium” state of the reservoir is reached at extraction flow rates below 240 kg s−1 and injection flow rates below 160 kg s−1. For a first-order approximation of the gravity contributions from extracted and injected masses, we applied a simplified forward gravity model. Comparison to the observed gravity signals suggest that most of the reinjected fluid is drained off through the nearby fracture system.

Full author list: B Canuel, S Abend, P Amaro-Seoane, F Badaracco, Q Beaufils, A Bertoldi, K Bongs, P Bouyer, C Braxmaier, W Chaibi, N Christensen, F Fitzek, G Flouris, N Gaaloul, S Gaffet, C L Garrido Alzar, R Geiger, S Guellati-Khelifa, K Hammerer, J Harms, J Hinderer, M Holynski, J Junca, S Katsanevas, C Klempt, C Kozanitis, M Krutzik, A Landragin, I Làzaro Roche, B Leykauf, Y-H Lien, S Loriani, S Merlet, M Merzougui, M Nofrarias, P Papadakos, F Pereira dos Santos, A Peters, D Plexousakis, M Prevedelli, E M Rasel, Y Rogister, S Rosat, A Roura, D O Sabulsky, V Schkolnik, D Schlippert, C Schubert, L Sidorenkov, J-N Siemß, C F Sopuerta, F Sorrentino, C Struckmann, G M Tino, G Tsagkatakis, A Viceré, W von Klitzing, L Woerner and X Zou, Gravitational waves (GWs) were observed for the first time in 2015, one century after Einstein predicted their existence. There is now growing interest to extend the detection bandwidth to low frequency. The scientific potential of multi-frequency GW astronomy is enormous as it would enable to obtain a more complete picture of cosmic events and mechanisms. This is a unique and entirely new opportunity for the future of astronomy, the success of which depends upon the decisions being made on existing and new infrastructures. The prospect of combining observations from the future space-based instrument LISA together with third generation ground based detectors will open the way toward multi-band GW astronomy, but will leave the infrasound (0.1-10 Hz) band uncovered. GW detectors based on matter wave interferometry promise to fill such a sensitivity gap. We propose the European Laboratory for Gravitation and Atom-interferometric Research (ELGAR), an underground infrastructure based on the latest progress in atomic physics, to study space-time and gravitation with the primary goal of detecting GWs in the infrasound band. ELGAR will directly inherit from large research facilities now being built in Europe for the study of large scale atom interferometry and will drive new pan-European synergies from top research centers developing quantum sensors. ELGAR will measure GW radiation in the infrasound band with a peak strain sensitivity of 3.3 × 10-22/√Hz at 1.7 Hz. The antenna will have an impact on diverse fundamental and applied research fields beyond GW astronomy, including gravitation, general relativity, and geology., This work was realized with the financial support of the French State through the ‘Agence Nationale de la Recherche’ (ANR) in the frame of the ‘MRSEI’ program (Pre-ELGAR ANR-17-MRS5-0004-01) and the ‘Investissement d’Avenir’ program (Equipex MIGA: ANR11-EQPX-0028, IdEx Bordeaux—LAPHIA: ANR-10-IDEX-03-02). AB acknowledges support from the ANR (project EOSBECMR), IdEx Bordeaux—LAPHIA (project OE-TWR), the QuantERA ERA-NET (project TAIOL) and the Aquitaine Region (projets IASIG3D and USOFF). The work was also supported by the German Space Agency (DLR) with funds provided by the Federal Ministry for Economic Affairs and Energy (BMWi) due to an enactment of the German Bundestag under Grant Nos. 50WM1556, 50WM1956 and 50WP1706 as well as through the DLR Institutes DLR-SI and DLR-QT. XZ thanks the China Scholarships Council (No. 201806010364) program for financial support. JJ thanks ‘Association Nationale de la Recherche et de la Technologie’ for financial support (No. 2018/1565). PA-S, MN, and CFS acknowledge support from contracts ESP2015-67234-P and ESP2017-90084-P from the Ministry of Economy and Business of Spain (MINECO), and from contract 2017- SGR-1469 from AGAUR (Catalan government). LAS thanks Sorbonne Universit´es (Emergence project LORINVACC) and Conseil Scientifique de l’Observatoire de Paris for funding. RG acknowledges Ville de Paris (Emergence programme HSENS-MWGRAV), ANR (project PIMAI) and the Fundamental Physics and Gravitational Waves (PhyFOG) programme of Observatoire de Paris for support. We also acknowledge networking support by the COST actions GWverse CA16104 and AtomQT CA16221 (Horizon 2020 Framework Programme of the European Union). DS gratefully acknowledges funding by the Federal Ministry of Education and Research (BMBF) through the funding program Photonics Research Germany under contract number 13N14875. SvAb, NG, SL, EMR, DS, and CS gratefully acknowledge support by ‘Nieders¨achsisches Vorab’ through the ‘Quantum- and Nano-Metrology (QUANOMET)’ initiative within the project QT3, and through ‘Förderung von Wissenschaft und Technik in Forschung und Lehre’ for the initial funding of research in the new DLRSI Institute, the CRC 1227 DQ-mat within the projects A05, B07 and B09, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—EXC-2123 QuantumFrontiers—390837967 (B2), and the German Space Agency (DLR) with funds provided by the Federal Ministry for Economic Affairs and Energy (BMWi) due to an enactment of the German Bundestag under Grants No. DLR 50WM1641 (PRIMUSIII), 50WM1952 (QUANTUS-V-Fallturm), and 50WP1700 (BECCAL), 50WM1861 (CAL), 50WM2060 (CARIOQA) as well as 50RK1957 (QGYRO). FS, GMT and AV gratefully acknowledge support by the Italian ‘Ministero dell’Istruzione, Universit`a e Ricerca’ through the funding program PRIN, under contract number 2015L33WAK_003. BL, VS, MK, and AP gratefully acknowledge support by the Berlin School of Optical Sciences and Quantum Technology (BOS.QT) and by the German Space Agency (DLR) with funds provided by the Federal Ministry for Economic Affairs and Energy (BMWi) due to an enactment of the German Bundestag under Grants No. 50WP1432 (QUANTUS-IV-MAIUS), 50WP1953 (QUANTUS-V-Fallturm), and 50WP1702 (BECCAL).

Lake Chad, the largest freshwater lake of north-central Africa and one of the largest lakes of Africa, is the relict of a giant Quaternary lake (i.e., Megalake Chad) that developed during the early- to mid-Holocene African Humid Period. Over the drylands of the Sahara Desert and the semi-arid Sahel region, remote sensing (optical satellite imagery and digital elevation models) proved a successful approach to identify the paleo-shorelines of this giant paleo-lake. Here we present the first attempt to estimate the isostatic response of the lithosphere due to Megalake Chad and its impact on the elevation of these paleo-shorelines. For this purpose, we use the open source TABOO software (University of Urbino, Italy) and test four different Earth models, considering different parameters for the lithosphere and the upper mantle, and the spatial distribution of the water mass. We make the simplification of an instantaneous drying-up of Megalake Chad, and compute the readjustment related to this instant unload. Results (i.e., duration, amplitude, and location of the deformation) are then discussed in the light of four key areas of the basin displaying prominent paleo-shoreline morpho-sedimentary features. Whatever the Earth model and simplification involved in the simulations, this work provides a strong first-order evaluation of the impact on hydro-isostasy of Megalake Chad. It demonstrates that a water body similar to this megalake would induce a significant deformation of the lithosphere in the form of a vertical differential uplift at basin-scale reaching up to 16 m in the deepest part of the paleo-lake, and its shorelines would then be deflected from 2 m (southern shorelines) to 12 m (northern shorelines), with a maximum rate of more than 1 cm y&minus, 1. As such, any future study related to the paleo-shorelines of Megalake Chad, should integrate such temporal and spatial variation of their elevations.

International audience

In mountainous area, spring water constitutes the only drinking water resource and local economy is highly dependent on forest health and productivity. However, climate change is expected to make extreme water shortage episodes more and more frequent. Forest is therefore more and more exposed to water stress. It appears necessary to quantify the drought induced by water deficit to evaluate forest vulnerability and to plan the future of forest management. In this study we quantified the 2018 water deficit experienced by the forest in the Strengbach catchment, located in the French Vosges mountains. Three methods for estimating catchment water storage changes (WSC) have been compared. The first relies on superconducting gravimeter monitoring while the second relies on catchment water balance. The third one relies on global hydrological model MERRA2. We show that WSC estimated from measured gravity changes correlate well with WSC estimated from catchment water balance while WSC inferred from MERRA2 significantly differs. The Strengbach catchment water cycle is mostly annual but exhibits significant interannual variability associated with the 2018 drought episode: August 2018 has a water deficit of 37 mm (as inferred from catchment water balance) or 76 mm (as seen with superconducting gravimetry) compared to August 2017. We illustrate here the use of superconducting gravimeter monitoring as an independent proxy for WSC in a mountainous catchment while most of hydro-gravimetric studies have been conducted on relatively flat areas. We therefore contribute to expand the area of use of high precision gravity monitoring for the hydrological characterization of the critical zone in mountainous context. This innovative method may help to assess forest vulnerability to drought in the context of climate change.

New land gravity data results acquired in northern Alsace were presented. Compared to the available old Bouguer anomaly, we recovered an accurate Bouguer anomaly field showing data uncertainties [Formula: see text]. A qualitative data analysis using pseudotomographies reveals several negative anomalies suggesting a decrease of the bulk density at the depth of geothermal interest. We have performed a quantitative study on the basis of the existing 3D geologic model derived from a reinterpretation of the vintage seismics. The theoretical gravity response indicates a great mismatch with the observed Bouguer anomaly. The stripping approach was applied, and the stripped Bouguer anomaly indicates that the density values of the Jurassic, but especially for the Triassic, the Buntsandstein, and the upper part of the basement, were overestimated even using the density values measured in the deep geothermal borehole. This suggests that the borehole density values do not reflect the density variations occurring at larger scale. To reduce the Bouguer anomaly during stripping, a negative density contrast should be affected to the Buntsandstein layer overlaying the basement, suggesting that the part located between the Buntsandstein and the upper part of the basement presents a low-density value compared to the reference density, which is not necessarily expected and is not observed in the densities measured in the borehole. Interestingly, a correlation is found between the gravity analyses and the thermal gradient boreholes in the northern part of the study area. For two boreholes, the gravity interpretation suggests a huge density decrease in the Buntsandstein, which may arise from a combination of high-density fracturing and the important quantity of geothermal fluid significantly affecting the bulk density. Analysis of the thermal borehole data suggests that these two boreholes indicate higher geothermal potential compared with the other boreholes.

International audience; We investigate a nearly 8-year record (2010-2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). We first perform a tidal analysis with ET34-ANA v7.1 software that leads to the gravimetric amplitude and phase factors for all separable waves according to the available time duration. We test nine different ocean tide models for the main eleven tidal constituents (Ssa, Mm, Mf, Q1, O1, P1, K1, N2, M2, S2, K2). After correction for ocean tidal loading we obtain the real and imaginary parts of the residual vector. We also investigate atmospheric loading which is dominated in this equatorial location by the thermal waves S1 and S2 that are modulated in amplitude by annual and semi-annual components. After correction for ocean loading, we test different air pressure corrections on the tidal gravimetric factors for the waves Sa, Ssa, S1 and S2. We show the rather large discrepancy that exists between the classical single admittance pressure reduction and a hybrid model using global atmospheric models everywhere except in the local zone where the model pressure is replaced by the observed pressure.

SUMMARY We analyse a nearly 8-yr record (2010–2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). After tidal analysis removing all solid Earth and ocean loading tidal contributions and correcting for the long-term instrumental drift and atmospheric loading, we obtain a gravity residual signal which is essentially a hydrological signal due to the monsoon. This signal is first compared to several global hydrology models (ERA, GLDAS and MERRA). Our superconducting gravimeter residual signal is also superimposed onto episodic absolute gravity measurements and to space gravimetry GRACE data. A further comparison is done using local hydrological data like soil moisture in the very superficial layer (0–1.2 m), water table depth and rainfall. The temporal evolution of the correlation coefficient between the gravity observation and both the soil moisture and the water table is well explained by the direct infiltration process of rain water together with the lateral transfer discharging the water table. Finally, we compute the water storage changes (WSC) using a simulation based on the physically based Parflow-CLM numerical model of the catchment, which solves the water and energy budget from the impermeable bedrock to the top of the canopy layer using the 3-D Richards equation for the water transfers in the ground, the kinematic wave equation for the surface runoff and a land surface model (CLM) for the energy budget and evapotranspiration calculation. This model forced by rain is in agreement with evapotranspiration and stream flow data and leads to simulated water storage changes that nicely fit to the observed gravity signal. This study points out the important role played by surface gravity changes in terms of a reliable proxy for water storage changes occurring in small catchments.

The abstract is available here: https://uscholar.univie.ac.at/o:1054129

We revisit a number of details that arise when doing joint AG–SG (absolute gravimeter–superconducting gravimeter) calibrations, focusing on the scale factor determination and the AG mean value that derives from the offset. When fitting SG data to AG data, the choice of which time span to use for the SG data can make a difference, as well as the inclusion of a trend that might be present in the fitting. The SG time delay has only a small effect. We review a number of options discussed recently in the literature on whether drops or sets provide the most accurate scale factor, and how to reject drops and sets to get the most consistent result. Two effects are clearly indicated by our tests, one being to smooth the raw SG 1 s (or similar sampling interval) data for times that coincide with AG drops, the other being a second pass in processing to reject residual outliers after the initial fit. Although drops can usefully provide smaller SG calibration errors compared to using set data, set values are more robust to data problems but one has to use the standard error to avoid large uncertainties. When combining scale factor determinations for the same SG at the same station, the expected gradual reduction of the error with each new experiment is consistent with the method of conflation. This is valid even when the SG data acquisition system is changed, or different AG’s are used. We also find a relationship between the AG mean values obtained from SG to AG fits with the traditional short-term AG (‘site’) measurements usually done with shorter datasets. This involves different zero levels and corrections in the AG versus SG processing. Without using the Micro-g FG5 software it is possible to use the SG-derived corrections for tides, barometric pressure, and polar motion to convert an AG–SG calibration experiment into a site measurement (and vice versa). Finally, we provide a simple method for AG users who do not have the FG5- software to find an internal FG5 parameter that allows us to convert AG values between different transfer heights when there is a change in gradient.

In spite of the large stability of the fused quartz elastic system, which is utilized in modern relative gravimeters such as Scintrex CG-5, precise gravity measurements need regular calibration mostly because of weak changes in time of the elastic properties of the spring. Calibration of the relative gravimeters is important to avoid the systematic error of scale in relevant observations. In this study, due to the establishment of a large amplitude (of about 1200 mGal) calibration line in Iran, the stability of three CG-3 M and three CG-5 gravimeters has been continuously investigated based on the results of a 12 year (2005–2017) observations. The absolute gravity values at calibration stations were measured during the period 2005–2007 and more recently in 2017–2018 (for most of the stations) with absolute FG5 gravimeters. The results show that the Scintrex gravimeters exhibit different behaviors on the calibration line. The accuracy of determining the calibration coefficient of the gravimeters was better than 40 ppm. According to our results there is no effect of the gravity amplitude itself on the calibration factors. CG-5 #83 and CG-5 #87 have the largest changes in calibration factor (more than 1000 ppm) over the 12 year observation period while CG-3 M #20 and CG-3 M #60 have the smallest range (less than 200 ppm). The misclosure of relative gravity measurements in the first-order gravity network of Iran has been calculated before and after calibration corrections and it is shown that applying the scale factor correction reduced significantly the misclosures on the gravity network.

International audience; Geodetic tools monitor the earth's deformation and gravity field. They are presently sensitive enough to record subtle changes triggered by hydrological processes, thus providing complementary data to standard hydrological measurements. Among these tools, superconducting gravimeter (SG) have proven useful to unravel groundwater redistribution, which significantly alter the gravity field. In the frame of the EquipEx MIGA (Matter wave-laser based Interferometer Gravitation Antenna) project, one SG (iOSG-24) was set up in July 2015 in the Low-noise Underground Laboratory (LSBB) at Rustrel, France, in a gallery located 500 m beneath the surface. In this work, we analyse the underground iOSG-24 gravity time series together with hydro-meteorological data and basic gravity modelling. We find that the gravimeter recorded the redistribution of water in the ground and that most of this redistribution occurs in the unsaturated zone located above the gravimeter. Nevertheless, residuals between our model and the gravity data suggest the occurrence of large lateral fluxes and rapid runoff not considered in our model. We discuss how the setting of a second SG, planned in July 2018, at the surface of the LSBB could help unravelling such hydrological processes.

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.

International audience; We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates.

International audience; Since July 2015 a Superconducting Gravimeter (SG) of the latest generation, the iOSG-24, has been recording continuously the time-varying gravity field at the low noise underground laboratory (LSBB URL) of Rustrel, France. This instrument designed for observatory purpose has a levitated Niobium sphere weighting 17.7 g instead of 4.3 g. The advantage of increasing the mass of the sphere is to reduce the thermal noise due to Brownian motion inside the sensor. A comparison of the noise levels shows that the combination of this iOSG-24 with the environmental condition at the LSBB makes this site one of the quietest worldwide SG sites. Parallel measurements with an absolute FG5 gravimeter give a calibration factor of −451 ± 3 nm/s2/V and a negligible linear drift is observed 3 months after the installation. Tidal analyses are consistent with theoretical predictions and have shown a strong S1 thermal wave. Influence of the hydrological water content, mostly related to the Fontaine de Vaucluse catchment, is clearly visible on the SG residuals after tides removal. However the hydrological influence modelled using the MERRA2 products does not fully explain the observed noise level at periods between 1 and 10 days. Longer gravimetric time-series are necessary to further study the seasonal hydrological effects.

The volume collects papers submitted to Pure and Applied Geophysics following a call on the topic “Geodynamics and Earth Tides”. Partly, the authors had participated in the 18th Geodynamics and Earth Tides Symposium held inTrieste, Italy, in June 2016. TheEarth tides constitute the leading thread through the book, since instrumentation sensitive enough to observe them, also records a broad spectrum of signals generated by Earth dynamic processes.

International audience; The resonance associated with the Free Core Nutation (FCN) has been widely studied in Very Long Base Interferometry (VLBI) network measurements and in superconducting gravity records, but few experiments have been done with tiltmeters. In this study we use records collected with a pair of about 100 m long hydrostatic silica tiltmeters, orthogonally installed in an abandoned silver mine at Sainte Croix aux Mines (Alsace, in North-Eastern France). Main difficulties in retrieving FCN parameters from tidal analysis arise from the weak amplitude of PSI1 tidal wave (the closest in frequency to the FCN), as well as from the inaccuracy of the available ocean loading correction. Moreover because of the closeness in frequency of the single constituents of the diurnal tidal band, long (>1 year) records are needed for resolving K1, PSI1 and PHI1 waves. Hence we analyze a 10-year dataset of tilt records, hich has preliminarily required a critical review and a relevant editing for making records suitable for tidal analysis and subsequent inversion of the tidal parameters. A Bayesian inversion is used for a preliminary retrieval of the FCN parameters.

International audience; Continuous gravity observations of ocean and solid tides are usually done with land-based gravimeters. In this study we analyze a 426-day record of time-varying gravity acquired by an ocean-bottom Scintrex spring gravimeter between August 2005 and November 2006 at the Troll A site located in the North Sea at a depth of 303 m. Sea-bottom pressure changes were also recorded in parallel with a Paroscientific quartz pressure sensor. From these data, we show a comparison of the noise level of the sea-floor gravimeter with respect to two standard land-based relative gravimeters: a Scintrex CG5 and a GWR Superconducting Gravimeter that were recording at the J9 gravimetric observatory of Strasbourg (France). We also compare the analyzed gravity records with the predicted solid and oceanic tides. The oceanic tides recorded by the seafloor barometer are also analyzed and compared to the predicted ones using FES2014b ocean model. Observed diurnal and semi-diurnal components are in good agreement with FES2014b predictions. Smallest constituents reflect some differences that may be attributed to non-linearity occurring at the Troll A site. Using the barotropic TUGO-m dynamic model of sea level response to ECMWF atmospheric pressure and winds forcing, we show a good agreement with the detided ocean-bottom pressure residuals. About 4-hPa of standard deviation of remaining sea-bottom pressure are however not explained by the TUGO-m dynamic model.

This paper is devoted to the problem of the ground-satellite comparison of time-variable gravity. We first review the different methods used to validate satellite gravity observations (ground truth experiment) and point out possible issues. We first show results obtained in Europe where a moderate amplitude hydrological signal exists using both satellite (GRACE) and surface gravity measurements from the GGP (Global Geodynamics Project) network of superconducting gravimeters. We show also the nice agreement between ground and GRACE observations in Djougou (West Africa) where hydrological changes due to monsoon are important. We finally present on-going projects for the so-called ‘null test’ which is related to observations in a region with no (or very small) hydrological contribution to gravity. First results for the Sahara (Algeria) are reported and future missions in Egypt and Saudi Arabia briefly introduced.

International audience; The paper in question by Van Camp and co-authors [MVC] challenges previous work showing that ground gravity data arising from hydrology can provide a consistent signal for the comparison with satellite gravity data. The data sets used are similar to those used previously, that is, the gravity field as measured by the GRACE satellites versus ground-based data from superconducting gravimeters (SGs) over the same continental area, in this case Central Europe. One of the main impediments in this paper is the presentation that is frequently confusing and misleading as to what the data analysis really shows, for example, the irregular treatment of annual components that are first subtracted then reappear in the analysis. More importantly, we disagree on specific points. Two calculations are included in our comment to illustrate where we believe that the processing in [MVC] paper is deficient. The first deals with their erroneous treatment of the global hydrology using a truncated spherical harmonic approach which explains almost a factor 2 error in their computation of the loading. The second shows the effect of making the wrong assumption in the GRACE/hydrology/surface gravity comparison by inverting the whole of the hydrology loading for underground stations. We also challenge their claims that empirical orthogonal function techniques cannot be done in the presence of periodic components, and that SG data cannot be corrected for comparisons with GRACE data. The main conclusion of their paper, that there is little coherence between ground gravity stations and this invalidates GRACE comparisons, is therefore questionable. There is nothing in [MVC] that contradicts any of the previous papers that have shown clearly a strong relation between seasonal signals obtained from both ground gravity and GRACE satellite data.

International audience; Long gravity records are of great interest when performing tidal analyses. Indeed, long series enable to separate contributions of near-frequency waves and also to detect low frequency signals (e.g. long period tides and polar motion). In addition to the length of the series, the quality of the data and the temporal stability of the noise are also very important. We study in detail some of the longest gravity records available in Europe: 3 data sets recorded with spring gravimeters in Black Forest Observatory (Germany, 1980-2012), Walferdange (Luxemburg, 1980-1995) and Potsdam (Germany, 1974-1998) and several superconducting gravimeters (SGs) data sets, with at least 9 years of continuous records, at different European GGP (Global Geodynamics Project) sites (Bad Homburg, Brussels, Medicina, Membach, Moxa, Vienna, Wettzell and Strasbourg). The stability of each instrument is investigated using the temporal variations of tidal parameters (amplitude factor and phase difference) for the main tidal waves (O1, K1, M2 and S2) as well as the M2/O1 factor ratio, the later being insensitive to the instrumental calibration. The long term stability of the tidal observations is also dependent on the stability of the scale factor of the relative gravimeters. Therefore we also check the time stability of the scale factor for the superconducting gravimeter C026 installed at the J9 Gravimetric Observatory of Strasbourg (France), using numerous calibration experiments carried out by co-located absolute gravimeter (AG) measurements during the last 15 years. The reproducibility of the scale factor and the achievable precision are investigated by comparing the results of different calibration campaigns. Finally we present a spectrum of the 25 years of SG records at J9 Observatory, with special attention to small amplitude tides in the semi-diurnal and diurnal bands, as well as to the low frequency part.

[1] The acquisition of reliable data sets representative of hydrological regimes and their variations is a critical concern for water resource assessment. For the subsurface, traditional approaches based on probe measurements, core analysis, and well data can be laborious, expensive, and highly intrusive, while only yielding sparse data sets. For this study, an innovative field survey, merging relative microgravimetry, magnetic resonance soundings, and hydrological measurements, was conducted to evaluate both surface and subsurface water storage variations in a semiarid Sahelian area. The instrumental setup was implemented in the lower part of a typical hillslope feeding to a temporary pond. Weekly measurements were carried out using relative spring gravimeters during 3 months of the rainy season in 2009 over a 350 × 500 m2 network of 12 microgravity stations. Gravity variations of small to medium amplitude (≤220 nm s−2) were measured with accuracies better than 50 nm s−2, revealing significant variations of the water storage at small time (from 1 week up to 3 months) and space (from a couple of meters up to a few hundred meters) scales. Consistent spatial organization of the water storage variations were detected, suggesting high infiltration at the outlet of a small gully. The comparison with hydrological measurements and magnetic resonance soundings involved that most of the microgravity variations came from the heterogeneity in the vadose zone. The results highlight the potential of time lapse microgravity surveys for detecting intraseasonal water storage variations and providing rich space-time data sets for process investigation or hydrological model calibration/evaluation.

International audience; In the last few years, the performance of the cryogenic gravity instruments has been further improved by the development of a new generation of superconducting gravimeter (SG): the so-called iOSG which is a superconducting gravimeter designed for observatory purpose with a heavier sphere than previous SGs. The first iOSG (iOSG-024) has been installed in July 2015 at the LSSB low background noise underground research laboratory in Rustrel (France), funded by the EQUIPEX MIGA (Matter wave-laser based Interferometer Gravitation Antenna) project and by the European FEDER 2006-2013 " PFM LSBB – Développement des qualités environnementales du LSBB ". This instrument is operational since September 2015. We present the first tidal analyses of the 7-month time-varying gravity records of this newly installed instrument as well as the calibration results performed by parallel FG5 absolute gravity measurements. We also show the performances of iOSG-024 in terms of noise levels in the seismic (in the millihertz frequency range) band using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The obtained noise levels are compared with other SG sites and with seismological reference noise models. The combination of the instrumental performance of the iOSG with the LSBB site properties makes this gravimetric station one of the quietest in the world, comparable to the lower sensor of the OSG-56 at BFO, at seismic frequencies.

International audience; The increasing number of hydro-gravimetry studies proves the rising interest of the hydrology community towards this monitoring method. The accuracy of Superconducting Gravimeters (SG) potentially allows the retrieval of small Water Storage Changes (WSC) down to a few millimeters of equivalent water thickness However, the importance of these corrections applied to SG data to achieve such a precision in gravity residuals should be recalled. The Djougou permanent gravity station presented in this paper and located in northern Benin, West-Africa, provides a good opportunity to review these considerations. This station is equipped since July 2010 with the superconducting gravimeter SG-060 aimed at deriving WSC at different time-scales, daily to inter-annual. In this area, WSC are 1) part of the control system for evapotranspiration (ET) process, a key variable of the West-African monsoon cycle and 2) the state variable for resource management, a critical issue in storage-poor hard rock basement contexts such as in northern Benin. The potential for deriving WSC from time-lapse gravity data partly depends on environmental features such as topography and the instrument shelter. Therefore, this issue is addressed first, with the background idea that such sensitivity analysis should be undertaken before setting up any new instrument. In Djougou, local topography is quite flat leading to a theoretical straightforward relationship between gravity changes and WSC, close to the standard Bouguer value. However, the shelter plays a significant masking role, which is the principal limitation to the retrieval of fast hydrological processes such as ET following a rain event. Several issues concerning classical gravity corrections are also addressed in the paper. These include gap-filling procedures during rain-events and drift estimates for short time series. Special attention is provided to atmospheric corrections, and different approaches are tested: a simple scalar admittance, a filtered scalar admittance, a frequency-dependent admittance and direct atmospheric loading calculations. It is shown that only the physically-based approach of direct loading calculations performs better in both residual minimization and ET retrieval. Moreover, non-local hydrological effects are investigated and account for about 20% of the gravity residuals. Finally, gravity residuals are briefly analyzed at two distinct time scales: rapid (up to a few days) and seasonal. At the rapid time-scale, it is shown that ET retrieval is hardly achievable given shelter size and state-of-the-art atmospheric corrections. Still, mean values retrieved from this study are in accordance with known values of potential ET and constant lateral flow. Direct comparison of gravity changes with hydrological data (neutron probe monitoring and water table levels) show some discrepancies, particularly for the hydrological year of 2011, for which all hydrological data show a deficit, but SG and FG5 data do not. This preliminary analysis both provides a basis and call for further hydro-gravity modeling, to comprehensively investigate the water-cycle at the Djougou station.

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 Earthmodel. 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 ' 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.

pyGrav is a Python-based open-source software dedicated to the complete processing of relative-gravity data. It is particularly suited for time-lapse gravity surveys where high precision is sought. Its purpose is to bind together single-task processing codes in a user-friendly interface for handy and fast treatment of raw gravity data from many stations of a network. The intuitive object-based implementation allows to easily integrate additional functions (reading/writing routines, processing schemes, data plots) related to the appropriate object (a station, a loop, or a survey). This makes pyGrav an evolving tool. Raw data can be corrected for tides and air pressure effects. The data selection step features a double table-plot graphical window with either manual or automatic selection according to specific thresholds on data channels (tilts, gravity values, gravity standard deviation, duration of measurements, etc.). Instrumental drifts and gravity residuals are obtained by least square analysis of the dataset. This first step leads to the gravity simple differences between a reference point and any point of the network. When different repetitions of the network are done, the software computes then the gravity double differences and associated errors. The program has been tested on two specific case studies: a large dataset acquired for the study of water storage changes on a small catchment in West Africa, and a dataset operated and processed by several different users for geothermal studies in northern Alsace, France. In both cases, pyGrav proved to be an efficient and easy-to-use solution for the effective processing of relative-gravity data. Display Omitted Aggregation of multiple processing steps in an open-source, user-friendly program.Object-oriented programming to handle and process structured relative-gravity data.Simple code structure which allows fast and easy implementation of new functions.Simple data-selection step through object-oriented GUI.

International audience; We compute the gravimetric factor at the Chandler wobble (CW) frequency using time-seriesfrom superconducting gravimeters (SG) longer than a decade. We first individually processthe polar motion and data at each individual gravity station to estimate the gravimetric factoramplitude and phase, then we make a global analysis by applying a stacking method todifferent subsets of up to seven SG stations. The stacking is an efficient way of getting ridof local effects and improving the signal-to-noise ratio of the combined data sets. Using thestacking 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 oferror are then carefully considered. For both local and global analyses, the uncertainties onour results are reliably constrained by computing the standard deviation of the estimates of thegravimetric factor amplitude and phase for increasing length of the time-series. Constraintson the CW anelastic dissipation can be set since any departure of the gravimetric factor fromits elastic value may provide some insights into the dissipative processes that occur at the CWperiod. In particular, assuming given rheological models for the Earth’s mantle enables us tomake the link between the gravimetric factor phase and the CW quality factor.

International audience; This paper is devoted to an overview of the use of hybrid gravimetry in Earth and Environmental Sciences. We first recall the concept of hybrid gravimetry which relies on the simultaneous use of different types of gravimeters either superconducting, absolute or relative spring gravimeters. This combination of instruments provides a complete tool for time-lapse gravimetry: while superconducting gravimeters and/or absolute gravimeters are used to obtain temporal gravity changes at one or several base stations, relative gravity surveys provide spatial differences with respect to these base stations, and allow to cover a much wider area than base stations only. Hybrid gravimetry therefore provides time-lapse gravity changes at a survey scale. We present here an overview of different published applications in hydrology, glaciology, volcanology and geothermics in order to point out that hybrid gravimetry is a powerful tool to monitor spatially and temporarily surface and underground mass changes.

We present a comparison in the various tidal bands, between two different spectral analyses of long gravimetric time series. The first one is performed using a long gravity series recorded by superconducting gravimeters at J9 Observatory (Strasbourg) and the second one uses a theoretical series of the same length, almost 28 years, computed for the same location according to the Hartmann and Wenzel tidal potential development.

We analyse data from seven superconducting gravimeter (SG) stations in Europe from 2002 to 2007 from the Global Geodynamics Project (GGP) and compare seasonal variations with data from GRACE and several global hydrological models—GLDAS, WGHM and ERA-Interim. Our technique is empirical orthogonal function (EOF) decomposition of the fields that allows for the inherent incompatibility of length scales between ground and satellite observations. GGP stations below the ground surface pose a problem because part of the attraction from soil moisture comes from above the gravimeter, and this gives rise to a complex (mixed) gravity response. The first principle component (PC) of the EOF decomposition is the main indicator for comparing the fields, although for some of the series it accounts for only about 50per cent of the variance reduction. PCs for GRACE solutions RL04 from CSR and GFZ are filtered with a cosine taper (degrees 20–40) and a Gaussian window (350km). Significant differences are evident between GRACE solutions from different groups and filters, though they all agree reasonably well with the global hydrological models for the predominantly seasonal signal. We estimate the first PC at 10-d sampling to be accurate to 1 μGal for GGP data, 1.5 μGal for GRACE data and 1 μGal between the three global hydrological models. Within these limits the CNES/GRGS solution and ground GGP data agree at the 79per cent level, and better when the GGP solution is restricted to the three above-ground stations. The major limitation on the GGP side comes from the water mass distribution surrounding the underground instruments that leads to a complex gravity effect. To solve this we propose a method for correcting the SG residual gravity series for the effects of soil moisture above the station.

The Bureau International des Poids et Mesures (BIPM), Sèvres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005. ICAG 2005 was prepared and performed as a metrological pilot study, which aimed: (1) To determine the gravity comparison reference values; (2) To determine the offsets of the absolute gravimeters; and (3) As a pilot study to accumulate experience for the CIPM Key Comparisons. This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG–RGC comparison was held at the BIPM in 1980–1981 and since then meetings have been organized every 4 years. In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports. Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.

International audience; 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 1σ) over the years. The comparison with some seismological noise models shows that the best SG sites are less noisy than longperiod 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.

The Svalbard archipelago, Norway, is affected by both the present-day ice melting (PDIM) and Glacial Isostatic Adjustment (GIA) subsequent to the Last Pleistocene deglaciation. The induced deformation of the Earth is observed by using different techniques. At the Geodetic Observatory in Ny-Alesund, precise positioning measurements have been collected since 1991, a superconducting gravimeter (SG) has been installed in 1999, and six campaigns of absolute gravity (AG) measurements were performed between 1998 and 2007. Moreover, the Gravity Recovery and Climate Experiment (GRACE) satellite mission provides the time variation of the Earth gravity field since 2002. The goal of this paper is to estimate the present rate of ice melting by combining geodetic observations of the gravity variation and uplift rate with geophysical modelling of both the GIA and Earth's response to the PDIM. We estimate the secular gravity variation by superimposing the SG series with the six AG measurements. We collect published estimates of the vertical velocity based on GPS and VLBI data. We analyse the GRACE solutions provided by three groups (CSR, GFZ, GRGS). The crux of the problem lies in the separation of the contributions from the GIA and PDIM to the Earth's deformation. To account for the GIA, we compute the response of viscoelastic Earth models having different radial structures of mantle viscosity to the deglaciation histories included in the models ICE-3G or ICE-5G. To account for the effect of PDIM, we compute the deformation of an elastic Earth model for six models of ice-melting extension and rates. Errors in the gravity variation and vertical velocity are estimated by taking into account the measurement uncertainties and the variability of the GRACE solutions and GIA and PDIM models. The ground observations agree with models that involve a current ice loss of 25 km3 water equivalent yr−1 over Svalbard, whereas the space observations give a value in the interval [5, 18] km3 water equivalent yr−1. A better modelling of the PDIM, which would include the precise topography of the glaciers and altitude-dependency of ice melting, is necessary to decrease the discrepancy between the two estimates.