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Results of the European Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2007
Abstract and Figures
The second international comparison of absolute gravimeters was held in Walferdange, Grand Duchy of Luxembourg, in November 2007, in which twenty absolute gravimeters took part. A short description of the data processing and adjustments will be presented here and will be followed by the presentation of the results. Two different methods were applied to estimate the relative offsets between the gravimeters.
We show that the results are equivalent as the uncertainties of both adjustments overlap. The absolute gravity meters agree with one another with a standard deviation of 2 μgal (1 gal = 1 cm/s2).
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... It incorporated interferometry, a laser, clock source and an evacuated dropping chamber (Faller, 1967 (Niebauer, Sasagawa, Faller, Hilt, & Klopping, 1995). In 1985 the third generation of the JILA gravimeters, which are now known as the JILAg series, achieved an accuracy of 3-5 µGal, with some of these gravimeters still attending absolute gravimeter comparisons (Francis, O. 2010). The culmination of these advances in technology is the FG5 absolute gravimeter, with measurement accuracy around 10 -9 , which is explained in detail in the next chapter. ...
... During the duration of the comparison, each instrument is swapped around the scheduled piers; depending on the facilities available at a comparison, a determination of rubidium clock frequency, barometer and laser collimation may be available (Schmerge, et al., 2012) (Francis & vanDam, 2010). ...
... A constraint is placed upon the solution, such that the mean of all of the offsets is equal to zero to provide the relative offsets between the AGs; the constraint ensures that the problem is numerically stable, giving limited results rather than the infinite set of solutions that would be generated without the condition. with the condition Where is the gravity value at the site k given by the instrument i, is the adjusted gravity value at the site k, is the offset of the gravimeter i and is the stochastic error (Francis & vanDam, 2010) (Francis, et al., 2014) (Palinkas, et al., 2015). This equation was also used in the NACAG 2010 but a different sign convention is shown in the relevant paper (Schmerge, et al., 2012). ...
Measuring the acceleration due to gravity at the NERC Space Geodesy Facility (SGF), Herstmonceux, provides a complimentary geodetic technique to the long established satellite laser ranging and global navigation satellite system measurements. The gravimetry measurements at the SGF were added to conform to the gravity field objective of the Global Geodetic Observing System and the European Combined Geodetic Network. Since both SLR and GNSS measurements are used in the computation of the International Terrestrial Reference Frame any un-modelled movements in the ground stations are undesirable. Gravity measurements can be used in the identification of such signals. This thesis reports the establishment of the technique at the SGF and contains a description of the installation experiences, a maintenance guide for the instrument and a comprehensive analysis of the gravity measurements over the ten years of operation to date, from 2006- 2016. Environmentally driven influences on the gravity measurements are investigated. The precision of the measurements are shown to be seasonally dependent and of varying magnitude. The hydrological influence on the gravity data from groundwater variation is calculated to be approximately 3.14 µGal, determined from temporal measurement of groundwater depth and an estimation of soil properties. A maximum influence from soil moisture content is estimated resulting in an influence of less than a microgal, which confuses correlation studies between local tide gauge data and an intermittent periodic signal seen in the gravity data. The high frequency data taken at the SGF highlights bias corrections two explainable and one of unknown origin. The bias corrections, of magnitude +1.5, -2 and -7.33 µGal, are shown to be critical to the interpretation of the time series, and, simulated campaign style measurements, using one set of measurements on an annual basis, prove that the data would be easily misinterpreted if the bias offsets found are not applied.
... Germany. The different CRVs solutions are presented by[23] for ECAG2003,[24] for ECAG2007, and by[25] for the others. ...
In 2004, first absolute gravity (AG) measurements were performed on the mountain tops of Mt. Zugspitze (2 sites) and Mt. Wank (1 site), and at the Wank foot (1 site). Wank (summit height 1780 m) and Zugspitze (2960 m) are about 20 km apart from each other and belong geologically to different parts of the Northern Limestone Alps. Bridging a time span of 15 years, the deduced gravity variations for Zugspitze are in the order of 0.30 μm/s² with a standard uncertainty of 0.04 μm/s². The Wank stations (foot and top) show no significant gravity variation. The vertical stability of Wank summit is also confirmed by results of continuous GNSS recordings. Because an Alpine mountain uplift of 1 or 2 mm/yr cannot explain the obtained gravity decline at Zugspitze, the dominating geophysical contributions are assumed to be due to the diminishing glaciers in the vicinity. The modelled gravity trend caused by glacier retreat between epochs 1999 and 2018 amounts to -0.012 μm/s²/yr at both Zugspitze AG sites. This explains more than half of the observed gravity decrease. Long-term variations on inter-annual and climate-relevant decadal scale will be investigated in the future using as a supplement superconducting gravimetry (installed in 2019) and GNSS equipment (since 2018).
Absolute gravity measurements taken on a near-weekly basis at a single location is a rarity. Twelve years of data at the UK’s Space Geodesy Facility (SGF) provides evidence to show that the application of results from international comparisons of absolute gravimeters should be applied to data and are critical to the interpretation of theSGF gravity time series of data from 2007 to 2019. Though residual biases in the data are seen. The SGF time series comprises near weekly data, with exceptions for manufacturer services and participation in international instrument comparisons. Each data set comprises hourly data taken over 1 day, with between 100 and 200 drops per hour. Environmental modelling indicates that the annual groundwater variation at SGFof some 2 m influences the gravity data by 3.1 μGal, based upon some measured and estimated soil parameters. The soil parameters were also used in the calculation of the effect of an additional telescope dome, built above the gravity laboratory, and have been shown to be realistic. Sited in close proximity to the long-established satellite laser ranging (SLR) system and the global navigation satellite systems (GNSS) the absolute gravimetry (AG) measurements provide a complimentary geodetic technique, which is non space-based. The SLR-derived height time series provides an independent measurement of vertical motion at the site which may be used to assess the AG results, which are impacted by ground motion as well as mass changes above and below the instruments.
In 2004, first absolute gravity (AG) measurements were performed on the top of Mt. Zugspitze (2 sites) and at the foot (1 site) and top (1 site) of Mt. Wank. Mt. Wank (summit height 1780 m) and Mt. Zugspitze (2960 m) are about 15 km apart from each other and belong geologically to different parts of the Northern Limestone Alps. Bridging a time span of 15 years, the deduced gravity variations for Zugspitze are in the order of 0.30 μm/s² with a standard uncertainty of 0.04 μm/s². The Wank stations (foot and top) show no significant gravity variation. The vertical stability of Wank summit is also confirmed by results of continuous GNSS recordings. Because an Alpine mountain uplift of 1 or 2 mm/yr cannot explain the obtained gravity decline at Zugspitze, the dominating geophysical contributions are assumed to be due to the diminishing glaciers in the vicinity. The modelled gravity trend caused by glacier retreat between epochs 1999 and 2018 amounts to 0.012 μm/s²/yr at both Zugspitze AG sites. This explains more than half of the observed gravity decrease. Long-term variations on inter-annual and climate-relevant decadal scale will be investigated in the future using as supplement superconducting gravimetry (installed in 2019) and GNSS equipment (since 2018).
Postglacial rebound in Fennoscandia causes striking trends in gravity measurements of the area. We present time series of absolute gravity data collected between 1976 and 2019 on 12 stations in Finland with different types of instruments. First, we determine the trends at each station and analyse the effect of the instrument types. We estimate, for example, an offset of 6.8 μgal for the JILAg-5 instrument with respect to the FG5-type instruments. Applying the offsets in the trend analysis strengthens the trends being in good agreement with the NKG2016LU_gdot model of gravity change. Trends of seven stations were found robust and were used to analyse the stabilization of the trends in time and to determine the relationship between gravity change rates and land uplift rates as measured with global navigation satellite systems (GNSS) as well as from the NKG2016LU_abs land uplift model. Trends calculated from combined and offset-corrected measurements of JILAg-5-and FG5-type instruments stabilized in 15 to 20 years and at some stations even faster. The trends of FG5-type instrument data alone stabilized generally within 10 years. The ratio between gravity change rates and vertical rates from different data sets yields values between − 0.206 ± 0.017 and − 0.227 ± 0.024 µGal/mm and axis intercept values between 0.248 ± 0.089 and 0.335 ± 0.136 µGal/yr. These values are larger than previous estimates for Fennoscandia.
Die Geräteentwicklung im Bereich der Gravimetrie der vergangenen Jahrzehnte zeichnete sich in erster Linie durch eine Digitalisierung der Datenerfassung und -verarbeitung sowie einer Reduktion instrumenteller Fehlerquellen bei einer gleichzeitigen Miniaturisierung der Absolut- und Relativgravimeter aus. Als gänzlich neue Entwicklung des letzten Jahrzehnts, wenn auch schon als technische Möglichkeit in den 1990er Jahren demonstriert, tritt eine steigende Anzahl von Absolutgravimeter (AG) auf Grundlage der Atominterferometrie, also der Interferometrie mit Materiewellen, in Erscheinung. Diese sogenannten Quantengravimeter (QG), vom transportablen Laborinstrument bis hin zum ersten kommerziellen Produkt, stellen den ersten vollständig unabhängigen Ansatz zur Messung von g seit der Ablösung der Pendelapparate durch Feder- und Freifallgravimeter dar.
Die vorliegende Arbeit untersucht zunächst den Stand der Technik der klassischen, im Sinne der Newton’schen Physik, Absolut- und Relativgravimeter des Institut für Erdmessung (IfE). Die auf dem LaCoste & Romberg-Prinzip basierenden Federgravimeter ZLS Burris B-64 und Micro-g LaCoste gPhone-98 werden in dieser Arbeit als Referenz zu einem QG, in der Unterstützung von Absolutmessungen und zur Prüfung von Methoden zur Modellierung des Schwerefeldes eingesetzt. Es wurden instrumentelle Eigenschaften untersucht, z. B. der Kalibrierfaktoren der Gravimeter die über sechs Jahre hinweg mit einer relativen Unsicherheit von 1.3×10e−3/ 2.7×10e−4 für das gPhone-98/B-64 bestimmt werden konnten. Anhand von Gezeitenanalysen wurde ermittelt, dass innerhalb von etwa zehn Wochen die Parameter der amplitudenstärksten Wellengruppen mit einer Güte bestimmt werden können, dass der nicht korrigierte Gezeitenanteil unterhalb von 1 nm/s² liegt. Anhand des im Jahr 2012 aufgerüsteten Micro-g LaCoste FG5X-220 wurde die Rückführung der Messungen auf die SI-Einheiten (Internationales Einheitensystem) demonstriert. Die Untersuchungen haben eine Anfälligkeit des Rubidium Oszillators für Helium aufgedeckt, die zu systematisch verfälschten Messungen führen kann. Durch die Fortsetzung von episodischen Messungen an verschiedenen Stationen konnte das FG5X-220 eine höhere Präzision im Vergleich zu dem FG5-220 demonstrieren. Diese Gravimeter wurden in einem Projekt mit dem Gravimetric Atom Interferometer (GAIN), einem an der Humboldt-Universität zu Berlin entwickelten QG, eingesetzt. Im Verlauf des Projektes konnte GAIN eine hohe Präzision in der Erfassung von Zeitreihen demonstrieren und gleichzeitig systematische Vorteile, z. B. zur Zeit höherer mikroseismischer Aktivität, gegenüber den klassischen Gravimetern aufzeigen. Es wurde deutlich, dass diese neuartigen Quantensensoren höhere Anforderungen an die Reduktionen zeitlicher Schwereänderungen stellen. Am Beispiel der atmosphärischer Massenänderungen wurde eine Reduktion auf Grundlage von 3D-Wettermodellen erstellt und an zwei Stationen getestet. Neben den transportablen QG entstehen vereinzelt stationäre Atominterferometer, wie das Very Large Baseline Atom Interferometer (VLBAI) an der Leibniz Universität Hannover. Für diese 10m Atomfontäne wurden Modellierungen zur Berechnung von g innerhalb des Instrumentes vorgenommen und durch gravimetrische Messungen getestet. Bei einem Test der verwendete Methode zur Modellierung des Schwereeffektes von geometrischen Körpern am Beispiel einer Kraftmaschine der Physikalisch-Technische Bundesanstalt in Braunschweig konnte eine Übereinstimmung zwischen Modell und Messung innerhalb der Messunsicherheit der verwendeten Gravimeter erreicht werden.
The development of gravimetric instruments in the past decades was characterised primarily by the digitalisation of data acquisition and processing as well as a reduction of instrumental error sources and simultaneous miniaturisation of absolute and relative gravimeters. As an entirely new development of the last decade, even if already demonstrated as a technical possibility in the 1990s, an increasing number of absolute gravimeters (AG) based on atomic interferometry, i.e. interferometry with matter waves, have appeared. These so-called quantum gravimeters (QG), from portable laboratory instruments to the first commercial product, represent the first completely independent approach to measuring g since spring and free-fall gravimeters replaced the pendulum apparatus.
This thesis examines the state of the art of classical, in the sense of Newtonian physics, absolute and relative gravimeters of the Institut für Erdmessung (IfE). The spring gravimeters ZLS Burris B-64 and Micro-g LaCoste gPhone-98, which are based on the LaCoste & Romberg principle for spring gravimeters, are used as a reference for QG, in support of absolute gravity measurements, and for testing the methods of modelling the gravitational field. Instrumental properties are investigated, e.g. the calibration factors of the gravimeters which were determined over six years with a relative uncertainty of 1.3×10e−3 /2.7×10e−4 for the gPhone-98/B-64. Based on tidal analyses, it was determined that, within about ten weeks, the parameters of the largest wave groups could be identified with a quality that the uncorrected tidal effect is below 1nm/s². Using the Micro-g LaCoste FG5X-220, which was upgraded in 2012 to the latest generation of AG, the traceability of the measurements to the SI units (International System of Units) is demonstrated. The investigations revealed a susceptibility of the rubidium oscillator to helium, which can lead to a systematic error in the measurements. By continuing the FG5-220s time series of episodic measurements at several stations, the FG5X-220 could demonstrate a higher precision than the former. The forementioned gravimeters were used in a project with the Gravimetric Atom Interferometer (GAIN), a QG developed at Humboldt-Universität zu Berlin. In the course of the project, GAIN was able to demonstrate its high precision in the acquisition of gravity time series and, at the same time, to show systematic advantages compared to classical gravimeters, e.g., at times of high microseismic activity. It became obvious that these novel quantum sensors place higher demands on the reductions of temporal gravity changes. As an example, the gravity effect of atmospheric mass changes based on 3D weather models was created and tested at two stations. In addition to the transportable QGs, a small number of stationary atomic interferometers such as the Very Large Baseline Atom Interferometer (VLBAI) at Leibniz University Hannover are developed. For this 10m atomic fountain, a model was implemented to calculate g within the instrument and tested by gravimetric measurements. The method to model the gravity effect of arbitrary geometric shapes was applied to a force standard machine at Physikalisch-Technische Bundesanstalt, Braunschweig, as a proof of concept. The agreement between the model and the measurement is within the measurement uncertainty of the gravimeters used.
For the first time, we present a complete, processed compilation of all repeated absolute gravity (AG) observations in the Fennoscandian postglacial land uplift area and assess their ability to accurately describe the secular gravity change, induced by glacial isostatic adjustment (GIA). The data set spans over more than three decades and consists of 688 separate observations at 59 stations. Ten different organizations have contributed with measurements using 14 different instruments. The work was coordinated by the Nordic Geodetic Commission (NKG). Representatives from each country collected and processed data from their country, respectively, and all data were then merged to one data set. Instrumental biases are considered and presented in terms of results from international comparisons of absolute gravimeters. From this data set, gravity rates of change (ġ) are estimated for all stations with more than two observations and a timespan larger than 2 yr. The observed rates are compared to predicted rates from a global GIA model as well as the state of the art semi-empirical land uplift model for Fennoscandia, NKG2016LU. Linear relations between observed ġ and the land uplift, ḣ (NKG2016LU) are estimated from the AG observations by means of weighted least squares adjustment as well as weighted orthogonal distance regression. The empirical relations are not significantly different from the modelled, geophysical relation ġ=0.03-0.163(±0.016) ḣ. We also present a ġ-model for the whole Fennoscandian land uplift region. At many stations, the observational estimates of ġ still suffer from few observations and/or unmodelled environmental effects (e.g. local hydrology). We therefore argue that, at present, the best predictions of GIA-induced gravity rate of change in Fennoscandia are achieved by means of the NKG2016LU land uplift model, together with the geophysical relation between ġ and ḣ. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.
In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field.Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial-body and Earth attractions, to the mass in the direct vicinity of the instruments, and vertical displacement of the instrument itself on the ground.
In this paper, we review the geophysical questions that can be addressed byground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources and good practices. We also discuss the next challenges to be met by ground gravimetry,the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.
We have compiled and analyzed FG5 absolute gravity observations between 1993 and 2014 at 21 gravity sites in Norway, and explore to what extent these observations are applicable for glacial isostatic adjustment (GIA) studies. Where available, raw gravity observations are consistently reprocessed. Furthermore, refined gravitational corrections due to ocean tide loading and non-tidal ocean loading, as well as atmospheric and global hydrological mass variations are computed. Secular gravity trends are computed using both standard and refined corrections and subsequently compared with modeled gravity rates based on a GIA model. We find that the refined gravitational corrections mainly improve rates where GIA, according to model results, is not the dominating signal. Consequently, these rates may still be considered unreliable for constraining GIA models, which we trace to continued lack of a correction for the effect of local hydrology, shortcomings in our refined modeling of gravitational effects, and scarcity of observations. Finally, a subset of standard and refined gravity rates mainly reflecting GIA is used to estimate ratios between gravity and height rates of change by ordinary and weighted linear regression. Relations based on both standard and refined gravity rates are within the uncertainty of a recent modeled result.
Since 2006 the gravity change at 17 points in Sweden has been observed with Lantmäteriet’s absolute gravimeter FG5-233. The main purpose of the observations is to study the postglacial rebound in Fennoscandia. In 2010, a suspected jump of a few μGal can be seen in the gravity time series that significantly affects the estimated gravity rate of changes. It is shown that if the jump is not considered, then the absolute value of the gravity rate of change is systematically underestimated compared to the land uplift model NKG2014LU_test. In this paper two different ways to estimate and apply corrections of the jump are demonstrated. The first is to estimate the jump from the observations themselves within a least squares adjustment, while the second is to assume the instrument bias obtained in international comparisons of absolute gravimeters. The best agreement between land uplift model and estimated rates of change of gravity is achieved by correcting the data with the official biases reported from the international comparisons.
The results of an international comparison of absolute gravimeters held in Walferdange Luxembourg in November 2003 are presented.
The absolute meters agreed with one another with a standard deviation of less than 2 µGal (1 Gal = 1 cm/s2) (if we exclude one prototype instrument from the analysis). For the first time, the ability of the operators was put to
the test. The comparison indicates that the errors due to the operator are less than 1 µGal, i.e. within the observational
errors.