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Network of Superconducting Gravimeters Benefits a Number of Disciplines
Abstract
A global network of superconducting gravimeters (SGs) is compiling significant data for a range of important studies spanning a number of disciplines concerned with the Earth's gravity, tides, environment, and geodetics. Among phenomena being looked at are seismic normal modes, the Slichter triplet, tidal gravity, ocean tidal loading, core nutations, and core modes. Hydrologists and volcanologists also may benefit from SG data.
The network was set up by the Global Geodynamics Project (GGP),an international program of observations of temporal variations in the Earth's gravity field. Observations began in 1997 and will continue until 2003. Eighteen SGs currently are in operation in the network (see Figures 1 and 2).
... An estimate of the combined instrument + site noise in the short-period (seismic) band has become a standard tool for SG station data analysis, introduced by Banka and Crossley (1999) and elaborated in Rosat and Hinderer (2011) and Valco and Palinkas (2015). Rosat and Hinderer quoted a value of 1.17 for the SNM at AP which placed it sixth best out of seven OSG instruments, but based only on 59 d of data compared to more than 200 d for the longest OSG (at that time) which was at Black Forest Observatory in Germany, one of the world's quietest seismic sites with an SNM of 0.9. ...
... ICET (International Center for Earth Tides) provided a long-running service for the gravity community by publishing regular reports on SG data from about 1975 to 2016, after which it was incorporated into the IGETS service of IAG (Voigt et al. 2016(Voigt et al. , 2018Boy et al. 2020). These reports were provided for all GGP stations (Crossley et al. 1999), and gave an assessment of the residual noise in the tidal band for each station and sensor. ICET also performed corrections of the 1 min data (code 22) that were fairly severe in terms of eliminating offsets and disturbances to provide a cleaned series suitable for tidal analysis (e.g. ...
... Banka, D.& Crossley, D., 1999. Noise levels of superconducting gravimeters at seismic frequencies, Geophys. ...
The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) project began in 2006 using a 3.5 m telescope on a high peak in the Sacramento Mountains of New Mexico. It has been one of the best performing (in terms of returned photons per viewing period and range uncertainty) of a handful of similar sites worldwide. The purpose of installing a superconducting gravimeter (SG) in 2009 was to improve the determination of local deformation, directly affecting the telescope motions, as part of an effort to reduce the error in lunar distance to the mm level. We have now accumulated 10 years of gravity measurements, together with data from a nearby permanent GPS Plate Boundary Observatory (P027) site. This paper describes the traditional analysis of SG data to produce a local tidal model and to determine a number of necessary dynamic corrections that are specific to the site, of which local atmospheric and hydrology attraction and loading are the most important. Loading corrections are available through the Ecole et Observatoire des Sciences de la Terre (EOST) website as part of the International Earth Rotation Service, and we consider 12 different EOST loading series for global atmosphere and hydrology loading and attraction. Nonetheless, local hydrology can only be addressed at the required level of detail using local models determined directly from the gravity data. We devote a major portion of the paper to the development of several water storage and conceptual tank models that succeed in reducing the large seasonal variance of the SG residuals by more than 90%. Yet there remain episodes in the corrected residuals that depart by up to 4 μGal from the hydrological models, and the mean error of our fit remains between 1.5-2.0 μGal for our two data sets, 2009-2012 and 2013-2018 respectively. Unfortunately the vertical GPS data relies on the UNAVCO processing, and has typically poor vertical accuracy. We find the correlation between the GPS and gravity residuals to be correspondingly weak, and then discuss briefly the incorporation of the displacement and gravity data into the Planetary Ephemeris Program for solving for the lunar distance.
... The stability of SGs has been very well-validated for studying geophysical phenomena over a very wide/broad period range, from a few seconds to a few years. Its high-accuracy data covers the Earth's tides, normal seismic modes, outer and inner core oscillations, hydrological effects, volcanic eruptions, reservoir monitoring, subsidence episodes, and the polar motion signatures etc (Richter et al., 1995a;Crossley et al., 1999;Van Camp et al., 2017). The SG records continuous temporal variation of gravity to sub-microGal level and performs as a long and very long period seismometer. ...
... Several studies focus on the comparison of global ocean tide models on the basis of terrestrial gravimetry (stationary sensors) with analysed OTL signals. Boy et al. (2003) as well as Baker & Bos (2003) use data from worldwide distributed SGs from the Global Geodynamics Project (GGP; Crossley et al. 1999) which has been transferred into the IAG Service International Geodynamics and Earth Tide Service (IGETS; Boy et al. 2020). Boy et al. (2003) find differences of a few nm s −2 between OTL amplitudes from various global ocean tide models and software packages as well as from SG time-series which are significantly larger than the uncertainty estimates from the SG time-series of a few 0.01 nm s −2 . ...
The superconducting gravimeter GWR iGrav 047 has been installed on the small offshore island of Heligoland in the North Sea approximately at sea level with the overall aim of high-accuracy determination of regional tidal and non-tidal ocean loading signals. For validation, a second gravimeter (gPhoneX 152) has been setup within a gravity gradiometer approach to observe temporal gravity variations in parallel on the upper land of Heligoland. This study covers the determination of regional ocean tide loading (OTL) parameters based on the two continuous gravimetric time-series after elimination of the height-dependent gravity component by empirical transfer functions between the local sea level from a nearby tide gauge and local attraction effects. After reduction of all gravity recordings to sea level, both gravimeters provide very similar height-independent OTL parameters for the eight major diurnal and semidiurnal waves with estimated amplitudes between 0.3 nm s−2 (Q1) and 11 nm s−2 (M2) and RMSE of 0.1–0.2 nm s−2 for 2 yr of iGrav 047 observations and a factor of 2 worse for 1.5 yr of gPhoneX 152 observations. The mean absolute OTL amplitude differences are 0.3 nm s−2 between iGrav 047 and gPhoneX 152, 0.4 nm s−2 between iGrav 047 and the ocean tide model FES2014b and 0.7 nm s−2 between gPhoneX 152 and FES2014b which is in good agreement with the uncertainty estimations. As by-product of this study, OTL vertical displacements are estimated from the height-independent OTL gravity results from iGrav 047 applying proportionality factors ${\rm d}h/{\rm d}g$ for the eight major waves. These height-to-gravity ratios and the corresponding phase shifts are derived from FES2014b. The OTL vertical displacements from iGrav 047 are estimated with amplitudes between 0.4 mm (Q1) and 5.1 mm (M2) and RMSE of 0.1–0.7 mm. These OTL amplitudes agree with FES2014b within 0.0 (M2) and 0.8 mm (K1) with a mean difference of 0.3 mm only. The OTL amplitudes from almost 5 yr of GNSS observations show deviations of up to 6 mm (M2) compared to vertical displacements from both iGrav 047 and FES2014b, which suggests systematic effects included in the estimation of OTL vertical displacements from GNSS. With the demonstrated accuracy, height-independent sensitivity in terms of gravity and vertical displacements along with the high temporal resolution and the even better performance with length of time-series, iGrav 047 delivers the best observational signal for OTL which is representative for a large part of the North Sea.
Gravimetry is a versatile metrological approach in geophysics to accurately map subterranean mass and density anomalies. There is a broad diversification regarding the working principle of gravimeters, wherein atomic gravimeters are one of the most technologically progressive class of gravimeters which can monitor gravity at an absolute scale with a high-repetition without exhibiting drift. Despite the apparent utility for geophysical surveys, atomic gravimeters are (currently) laboratory-bound devices due to the vexatious task of transportation. Here, we demonstrated the utility of an atomic gravimeter on-site during a gravity survey, where the issue of immobility was circumvented with a relative spring gravimeter. The atomic gravimeter served as a means to map the relative data from the spring gravimeter to an absolute measurement with an effective precision of 7.7μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\upmu }$$\end{document}Gal. Absolute measurements provide a robust and feasible method to define and control gravity data taken at different sites, or a later date, which is critical to analyze underground geological units, in particular when it is combined with other geophysical approaches.
We present a mathematically rigorous procedure for obtaining spectrograms of tectonic plate eigenmodes at extremely low frequencies, corresponding to oscillation periods of 15 minutes to 8 hours. The data is sourced from superconducting gravimeters of International Geodynamics and Earth Tide Service (IGETS) network. The motivation for this research stems from the proposal by Pavlov et al. to use this spectral data in monitoring of the state of active zones located along plate boundaries.
In this paper, we present observations of free oscillations of the Earth after major earthquakes in Chile (February 27, 2010, MW8.8) and Japan (April 11, 2011, MW9.1) using data from the dual-sphere superconducting gravimeter (SG - 055), installed at Badargadh (23°0.47 N, 70°0.62 E), Kutch, Gujarat, India in March 2009. To see the noise characteristics, we calculated the power spectral density of the gravity time series of 5 quiet days in the frequency band 0.05–20 mHz using the new low noise model (NLNM) as a reference. We compared the noise level of the Badargadh site to other SG sites around the world. This shows that the Badargadh SG is in a low noise state. We find that the noise increases at frequencies below 1 mHz. Such a characteristic is also observed in Djougou (Afrique, Benin) and Strasbourg (France). Using theoretical tides for Gujarat, we estimated a scale factor of about −814 nm/s²/V for Grav1 (lower-sphere) and about −775 nm/s²/V for Grav2 (upper-sphere). We corrected the influence of atmospheric pressure from the one-second gravity data before switching to the frequency domain. We extracted a total of 53 Earth's Free Oscillations (EFO) modes during the earthquake in Japan and about 47 EFO modes during the earthquake in Chile. We are able to extract the lowest 0S2 spheroidal mode (0.30945 mHz or 54 min) and 0S0 radial mode (0.81439 mHz or 20 min). The longer time series shows individual 0S2 singlets and 0S3 (0.46855 mHz) singlets due to the Coriolis splitting effect. We cross-referenced the frequencies of these modes using the PREM model and previous global observations. The correlation coefficient between the observed and the PREM model for these two events are 0.999 for Japan earthquake and 0.993 for Chile earthquake. This validates the quality of the data useful for low-frequency studies in seismology. We also calculated the relative deviations of our observed fundamental modes with previously determined observed and theoretical values. We found that the relative deviations of our observed free oscillations do not exceed 0.5%, indicating good correlations.
The processes occurring on the Earth are controlled by several gradients. The surface of the Planet is featured by complex geological patterns produced by both endogenous and exogenous phenomena. The lack of direct investigations still makes Earth interior poorly understood and prevents complete clarification of the mechanisms ruling geodynamics and tectonics. Nowadays, slab-pull is considered the force with the greatest impact on plate motions, but also ridge-push, trench suction and physico-chemical heterogeneities are thought to play an important role. However, several counterarguments suggest that these mechanisms are insufficient to explain plate tectonics. While large part of the scientific community agreed that either bottom-up or top-down driven mantle convection is the cause of lithospheric displacements, geodetic observations and geodynamic models also support an astronomical contribution to plate motions. Moreover, several evidences indicate that tectonic plates follow a mainstream and how the lithosphere has a roughly westerly drift with respect to the asthenospheric mantle. An even more wide-open debate rises for the occurrence of earthquakes, which should be framed within the different tectonic setting, which affects the spatial and temporal properties of seismicity. In extensional regions, the dominant source of energy is given by gravitational potential, whereas in strike-slip faults and thrusts, earthquakes mainly dissipate elastic potential energy indeed. In the present article, a review is given of the most significant results of the last years in the field of geodynamics and earthquake geology following the common thread of gradients, which ultimately shape our planet.
A continuous Galerkin method based approach is presented to compute the seismic normal modes of rotating planets. Special care is taken to separate out the essential spectrum in the presence of a fluid outer core using a polynomial filtering eigensolver. The relevant elastic-gravitational system of equations, including the Coriolis force, is subjected to a mixed finite-element method, while self-gravitation is accounted for with the fast multipole method. Our discretization utilizes fully unstructured tetrahedral meshes for both solid and fluid regions. The relevant eigenvalue problem is solved by a combination of several highly parallel and computationally efficient methods. We validate our three-dimensional results in the non-rotating case using analytical results for constant elastic balls, as well as numerical results for an isotropic Earth model from standard “radial” algorithms. We also validate the computations in the rotating case, but only in the slowly-rotating regime where perturbation theory applies, because no other independent algorithms are available in the general case. The algorithm and code are used to compute the point spectra of eigenfrequencies in several Earth and Mars models studying the effects of heterogeneity on a large range of scales.
This paper presents a novel Nb superconducting joint with an ultralow resistance of 7.9 × 10-16 Ω, fabricated using the electron beam welding (EBW) method. After the EBW process, the two Nb filaments formed a single joint with a much larger grain size and smaller grain misorientation. More importantly, the resistance of the EBW Nb joint was nearly one magnitude lower than that of most conventional pressing joint. The ultralow resistance is essential for superconducting gravimeters, which require an extremely low drift rate. The EBW Nb joint allowed the superconducting gravimeter to have a much better performance when applied in the field of structural geology, geodesy, microgravity, and metrology. We believe that the EBW method could be one of the most promising joint fabrication methods for achieving maximum stability (less than 1 μgal/yr).
Precise temperature control is crucial for the development of sensitive instruments, as temperature fluctuations will introduce many negative effects. In temperature control systems,
$1/{f}$
noise is dominant in the low-frequency range, which limits the temperature control precision. To suppress the
$1/{f}$
noise, AC bridges based on the modulation-demodulation method are commonly used. While the
$1/{f}$
noise is suppressed, other types of noise rise to domination, further limiting the precision of the temperature control system. The response of the AC-bridge readout circuit to temperature fluctuation is the most serious. This paper proposes a method for suppressing the response of the AC-bridge readout circuit to temperature fluctuations. Using this method, the temperature coefficient of the readout circuit is reduced by matching the temperature coefficients of the used electro-components according to their weights of impact. This method has been verified in the test of a precise temperature control system designed for a superconducting gravimeter. The temperature coefficient of the AC-bridge circuit was reduced by approximately 57.4 times compared with the unmatched AC-bridge circuit, and the temperature fluctuation in the vacuum chamber of the superconducting gravimeter was controlled within
$\pm 5~\mu \text{K}$
.
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