Analysis and modeling of the effect of tides on the hydrostatic leveling system at CERN

International audienceTo meet alignment tolerances that are becoming tighter and tighter (¡10 mm for the Compact Linear Collider (CLIC) project), the surveyors in the Survey Section at European Organization for Nuclear Research (CERN) must master the tilt effects exerted on their hydrostatic levelling system (HLS) networks. These effects are many and have varied consequences, although the majority of them tilt the ground and also the water surface present inside HLS (HLS sensors in a homogeneous way. In order to model all inclinations together as a block, we have adjusted, at each time t, the line through the seven sensors in the Transfer Tunnel 1 (TT1) experiment. After removal of this signal, the residual amplitudes of the readings are less than the HLS alignment tolerances of the proposed CLIC main accelerator CLIC. In addition, the residual signals have lost their semidiurnal and diurnal periodic components, proving that any local effects in the TT1 facility cannot be detected with the accuracy of our systems. Further progress has to be made however, to master the effect of temperature on the HLS. The periods remaining in the residual HLS signal proves the presence of uncorrected thermal effects

On the calibration of a superconducting gravimeter using absolute gravity measurements

International audienceA 24 hr continuous parallel registration between an absolute free-fall gravimeter and a relative cryogenic gravimeter is analysed. Different adjustment procedures (L,, L2 norms) are applied to the sets of absolute and relative readings in order to estimate the value of the calibration factor of the superconducting meter, as well as its uncertainty. In addition, a sensitivity test is performed to investigate the influence of some parameters (like the laser frequency and its short-term drift) upon this factor. The precision in the calibration factor is found to be better than 1 per cent, but systematic effects related to the short time interval may add another one and half per cent uncertainty. From preliminary results, it appears that this calibration experiment leads to a close agreement between the values of the gravimetric factor for the reference tidal wave O1 observed with the superconducting meter and the theoretical value (Dehant-Wahr body tide + ocean loading)

A simple method to retrieve the complex eigenfrequency of the Earth's nearly diurnal-free wobble; application to the Strasbourg superconducting gravimeter data

International audienceWe have analysed more than four years of data from the Strasbourg superconducting gravimeter to retrieve the period and damping of the nearly diurnal-free wobble (NDFW). The removal of noise spikes is found to be crucial for an accurate determination of tidal-wave amplitudes and phases. A new simple algorithm is derived which allows an analytical solution for the NDFW pertod and damping using the complex gravimetric factors of three resonant diurnal waves. The results show a huge reduction of the confidence intervals when compared with a previous investigation from a Lacoste Romberg spring meter operated at the same station. Our results are in close agreement with values obtained from two other European superconducting gravimeters. The results are also compared with respect to values inferred from very long baseline interferometry (VLBI) measurements

Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben

The study is devoted to the monitoring of a geothermal reservoir by hybrid gravimetry combining different types of instruments (permanent superconducting gravimeter, absolute ballistic gravimeter, and micro-gravimeters) and different techniques of measurements (both time-discrete and recording data collection). Using a micro-gravimetric repetition network around a reference station, which is regularly measured, leads to the knowledge of the time and space changes in surface gravity. Such changes can be linked to the natural or anthropic activities of the reservoir. A feasibility study using this methodology is applied to two geothermal sites in the Alsace region (France) of the Rhine graben. We show the results in terms of gravity double differences from weekly repetitions of a network of 11 stations around the geothermal reservoir of Soultz-sous-Forêts, separated into 5 loops during July–August 2013 and 2014 as well as preliminary results from 2 stations near Rittershoffen (ECOGI). We point out the importance of a precise leveling of the gravity points for the control of the vertical deformation. A first modeling of surface gravity changes induced by realistic geothermal density perturbations (Newtonian attraction) is computed in the frame of the existing geological model and leads to gravity changes below the μGal level being hence undetectable. However, and for the same case, borehole gravity modeling showed a significant anomaly with depth that can be used as a complementary monitoring method. We show that in the limit of our uncertainties (SD ~ 5 μGal), we do not detect any significant gravity change on the geothermal site of Soultz in agreement with the fact that there was indeed no geothermal activity during our analysis period. On the contrary, the measurements near Rittershoffen show a signal above the noise level which correlates in time with a production test but cannot be explained in terms of Newtonian attraction effects according to our basic numerical simulation

Strategies in the processing and analysis of continuous gravity record in active volcanic areas: the case of Mt. Vesuvius

This research is intended to describe new strategies in the processing and analysis of continuous gravity records collected in active volcanic areas and to assess how permanent gravity stations can improve the geophysical monitoring of a volcano. The experience of 15 years in continuous gravity monitoring on Mt. Vesuvius is discussed. Several geodynamic phenomena can produce temporal gravity changes. An eruption, for instance, is associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes The amplitude of such gravity variations is often quite small, in the order of 10-102 nms-2, so their detection requires high quality data and a rigorous procedure to isolate from the records those weak gravity signals coming from different sources. Ideally we need gravity signals free of all effects which are not of volcanic origin. Therefore solid Earth tide, ocean and atmospheric loading, instrumental drift or any kind of disturbances other than due to the volcano dynamics have to be removed. The state of the art on the modelling of the solid Earth tide is reviewed. The atmospheric dynamics is one of the main sources precluding the detection of small gravity signals. The most advanced methods to reduce the atmospheric effects on gravity are presented. As the variations of the calibration factors can prevent the repeatability of high-precision measurements, new approaches to model the instrumental response of mechanical gravimeters are proposed too. Moreover, a strategy for an accurate modelling of the instrumental drift and to distinguish it from longterm gravity changes is suggested

Local and global hydrological contributions to gravity variations observed in Strasbourg

International audienceWe investigate the contribution of local and global hydrology to the superconducting gravimeter (SG) installed in the Strasbourg observatory.Adeterministic approach is presented to account for the contribution of water storage variations in the soils in the vicinity of the gravimeter: both amount and distribution of water masses are determined before calculating Newtonian attraction. No adjustment is performed on gravity time series. Two multi-depth Frequency Domain Reflectometer (FDR) probes have been installed to monitor the amount of water stored in the soil layer above the gravimeter. Since August 2005, they have been monitoring the variation of the water content of the entire soil thickness. Several investigations have been undertaken in order to estimate the distribution of water masses: a precise local DEM (Digital Elevation Model) has been determined using differential GPS. The geometry and heterogeneity of the soil layer have been evaluated thanks to geophysical and geomechanical prospections. The comparison between observed and modelled gravity variations shows that daily up to seasonal variations are in good agreement. For long-term variations, deep water storage and other processes have to be modelled to explain recorded gravity variations

A Geophysical Interpretation of the Secular Displacement and Gravity Rates Observed at Ny-Ålesund, Svalbard in the Arctic— Effects of Post-Glacial Rebound and Present-Day Ice Melting

We have analysed the Ny-Ålesund very long baseline interferometry (VLBI) data over the period 1994 August to 2004 May, and we obtain secular displacement rates relative to a NNR-NUVEL-1A reference frame of and for the north, east and vertical directions, respectively. The corresponding global positioning system (GPS) station displacement rates relative to the same reference frame for the north, east, and vertical directions are at NYA1 and at NALL, where these GPS rates were derived from the ITRF2000 velocity solution of Heflin. From the comparison at 25 globally distributed collocated sites, we found that the difference in uplift rate between VLBI and GPS at Ny-Ålesund is mainly due to a GPS reference frame scale rate error corresponding to 1.6 mm yr−1 in the GPS vertical rates. The uplift rate was estimated to be 5.2 ± 0.3 mm yr−1 from the analysis of the tide gauge data at Ny-Ålesund. Hence the uplift rates obtained from three different kinds of data are very consistent each other. The absolute gravity (AG) measurements at Ny-Ålesund, which were carried out four times (period: 1998–2002) by three different FG5 absolute gravimeters, lead to a decreasing secular rate of . In this analysis, the actual data obtained from a superconducting gravimeter at Ny-Ålesund were used in the corrections for the gravity tide (including the ocean tide effect) and for the air pressure effect. We have estimated three geophysical contributions to examine the observed rates: (1) the effect of the sea-level (SL) change on a timescale of a few decades, (2) the effect of the present-day ice melting (PDIM) in Svalbard and (3) the sensitivity of the computed post-glacial rebound (PGR) effects to different choices of the models of past ice history and Earth\u27s viscosity parameters. Our analysis indicates that the effect of SL change can be neglected as the main source of the discrepancy. On the other hand, the effect of PDIM cannot be ignored in explaining the mutual relation between the observed horizontal and vertical rates and the predicted ones. A large melting rate of the order of −75 cm yr11 (i.e. roughly 1.6 times larger than the mean rate derived from glaciology over Svalbard) would explain the observed uplift but only half of the gravity changes. Our comparison results clearly point out the importance of both the estimation accuracy of the elastic deformations and better observation accuracy to constrain the size of PGR effects in the northwestern Svalbard more tightly

Monitoring of groundwater redistribution in a karst aquifer using a superconducting gravimeter

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