Fig 5 - uploaded by Alessandro Germak
Content may be subject to copyright.
1 Picture taken during the comparison of absolute gravimeters in the Underground Laboratory for Geodynamics in Walferdange
Source publication
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 applie...
Similar publications
We present the results of the third European Comparison of Absolute Gravimeters held in Walferdange, Grand Duchy of Luxembourg, in November 2011. Twenty-two gravimeters from both metrological and non-metrological institutes are compared. For the first time, corrections for the laser beam diffraction and the self-attraction of the gravimeters are im...
Citations
... 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).
... 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.
... Intercomparison campaigns [e.g. Francis et al., 2005;2010;2013;2015;Jiang et al., 2012;Schmerge et al., 2012;Vitushkin et al., 2002] showed that differences between FG5 and JILAg gravimeters are commonly of the order of 100-150 nm/s² . A difference as large as 461 nm/s² was reported for one of the A10 instruments that participated in the ICAG-2001 intercomparison [Vitushkin et al., 2002]. ...
We estimate the signature of the climate-induced mass transfers in repeated absolute gravity measurements based on satellite gravimetric measurements from the GRACE mission. We show results at the globe scale, and compare them with repeated absolute gravity (AG) time behavior in three zones where AG surveys have been published: Northwestern Europe, Canada and Tibet. For 10 yearly campaigns, the uncertainties affecting the determination of a linear gravity rate of change range 3-4 nm/s²/a in most cases, in absence of instrumental artefacts. The results are consistent with what is observed for long term repeated campaigns. We also discuss the possible artefact that can results from using short AG survey to determine the tectonic effects in a zone of high hydrological variability. We call into question the tectonic interpretation of several gravity changes reported from stations in Tibet, in particular the variation observed prior to the 2015 Gorkha earthquake.
... 6 http://kcdb.bipm.org. Figure 4 shows the results of all comparisons of absolute gravimeters the FG5(X)-220 participated in (Francis et al. 2005(Francis et al. , 2010Jiang et al. 2012;Francis et al. 2013Francis et al. , 2015. The listed results of RICAG are from comparisons carried out at the Geodetic Observatory Wettzell by the Federal Agency for Cartography and Geodesy (BKG, Germany). ...
The absolute measurement of g is currently realized through the laser interferometric measurement of a free falling retro-reflector. The Micro-g LaCoste FG5X is a free-fall gravimeter with a laser interferometer in Mach-Zehnder configuration which uses simultaneous time and distance measurements to calculate the absolute value of g. Because the instrument itself contains the necessary working standards for precise time and length measurements, it is considered independent of external references. The timing is kept with a 10 MHz rubidium oscillator with a stability of . The length unit is realized by the laser interferometer. The frequency calibrated and iodine stabilized helium-neon laser has a wavelength of 633 nm and an accuracy of .
In 2012 the FG5-220 of the Institut für Erdmessung (IfE) was upgraded to the FG5X-220. The upgrade included a new dropping chamber with a longer free fall and new electronics including a new rubidium oscillator. The metrological traceability to measurement units of the Système International d’unités (SI unit) is ensured by two complementary and successive approaches: the comparison of frequencies with standards of higher order and the comparison of the measured g to a reference measured by absolute gravimeters defined as primary standards within the SI. A number of experiments to test the rubidium oscillator were performed. The oscillator showed a linear drift of per month (= per month) in the first 18 months of use. A jump in the frequency of 0.01 Hz (= ) was revealed recently and the drift rate changed to /month.
Since the upgrade of the absolute gravimeter the instrument participated in several international comparisons, which showed no significant measuring offset between the instrument prior and after the upgrade.
... Hence, the pillar may cause some vibration during the FG5 measurements. When we excluded the FG5 outside stations, we obtained the differences at the inside stations to be between −3.9 and 5.5 μGal which are similar to the results reported by the other researchers (Vitushkin et al. 2002;Timmen 2010;Jiang et al. 2011Jiang et al. , 2012aFrancis et al. 2010Francis et al. , 2013Schmerge et al. 2012). ...
In order to define gravity datum and gravity scale in the Kingdom of Saudi Arabia (KSA), an absolute gravity network, called KSA Absolute Gravity Network (KSA-AGN), comprising of 25 sites distributed countrywide was observed from January 2013 to February 2013. Two stations were installed at each network site: one inside the building and one outside. Micro-g A10 (#029) portable absolute gravimeter was used for data acquisition of two setups of ten sets each at both inside and outside stations. Set scatters for A10 setups are usually less than ±3 μGal, and the differences between two setups vary in the range of −8 to 5 μGal. The weighted mean of the two setups were calculated as unique absolute gravity value and its uncertainty at the stations. Seven of the stations (five inside and two outside) were collocated by Micro-g FG5 (#111) absolute gravimeter having 24 sets for each setup. Set scatters for FG5 setups are less than ±4 μGal almost like A10 setups. However, we obtained the total uncertainty of FG5 and A10 measurement about ±2 and ±6 μGal, respectively. Furthermore, to reduce measured absolute gravity from the reference height to any height, gravity gradients over both inside and outside stations were measured by using two Scintrex CG5 (#922 and #924) relative gravimeters. Average CG5 gradient at the outside stations is about 3.1 μGal/cm, satisfying the free air gradient in the country. Differences between A10 and FG5 absolute gravities at 72 cm vary between −3.8 and 9.5 μGal at seven stations. Excluding the outside stations, we obtained the differences from −3.8 to 5.5 μGal at inside stations.
... Absolute gravimeter comparisons are expected to be one of the primary functions of TMGO. In the last decade, formalized comparisons of absolute gravimeters have been conducted only in Europe at the Bureau International des Poids et Mesures (BIPM) and the Underground Laboratory for Geodynamics in Walferdange (WULG) (Vitushkin et al. 2002;Francis and van Dam 2006;Francis et al. 2010;Jiang et al. 2011); these comparisons were well organized with more than 10 gravimeters participating. Smaller comparisons of two absolute gravimeters have often occurred in the last decade, and they have often been performed in other smaller observatories; these smaller comparisons also were primarily performed in Europe, but the Canadian Absolute Gravity Site (CAGS) in Canada was also utilized (Liard et al. 2003;Van Camp et al. 2003;Schmerge and Francis 2006;Simon et al. 2006;Wilmes and Falk 2006). ...
... According to the rules of key comparisons in metrology established by BIPM, one of the objectives of comparisons is to test not only the performance of the meters, but also the capability of the operators to provide a value with the associated uncertainty consistent with the other operators. Having the operators each process their own data instead of having one scientist process all the data was first implemented in the protocol for the comparison held in Luxembourg in 2007 (Francis et al. 2010). In advance of the comparison, it was agreed upon by the participants that this rule would be part of the protocol of NACAG-2010. ...
... As each gravimeter measured at only three of the nine sites, not all g-values can be directly compared. For NACAG-2010, the procedure used in the 2007 comparison in Walferdange was adopted (Francis et al. 2010). A global weighted leastsquare adjustment was performed using as input the g-values given by the operators and the associated variances. ...
The first North American Comparison of absolute gravimeters (NACAG-2010) was hosted by the National Oceanic and Atmospheric Administration at its newly renovated Table Mountain Geophysical Observatory (TMGO) north of Boulder, Colorado, in October 2010. NACAG-2010 and the renovation of TMGO are part of NGS’s GRAV-D project (Gravity for the Redefinition of the American Vertical Datum). Nine absolute gravimeters from three countries participated in the comparison. Before the comparison, the gravimeter operators agreed to a protocol describing the strategy to measure, calculate, and present the results. Nine sites were used to measure the free-fall acceleration of g. Each gravimeter measured the value of g at a subset of three of the sites, for a total set of 27 g-values for the comparison. The absolute gravimeters agree with one another with a standard deviation of 1.6 μGal (1 Gal ≡ 1 cm s
−2). The minimum and maximum offsets are −2.8 and 2.7 μGal. This is an excellent agreement and can be attributed to multiple factors, including gravimeters that were in good working order, good operators, a quiet observatory, and a short duration time for the experiment. These results can be used to standardize gravity surveys internationally.
... [6] Intercomparison campaigns have shown systematic errors (offsets) between the different absolute gravimeters that are larger than the declared uncertainties [Vitushkin et al., 2002[Vitushkin et al., , 2010Francis et al., 2005Francis et al., , 2010. Although the offsets can be determined by comparing the instruments, this is always within uncertainties and not always logistically feasible. ...
... Other malfunctioning components may also bias the AG [Wziontek et al., 2008]; this is why, before and after each campaign, the instrument measured at the Membach reference station, where it was compared with the continuously measuring superconducting gravimeter [Van Camp et al., 2005;Van Camp and Francis, 2006]. Finally, the FG5#202 was regularly compared to other AG meters [Robertsson et al., 2001;Vitushkin et al., 2002Vitushkin et al., , 2010Van Camp et al., 2003;Francis et al., 2005Francis et al., , 2010Baumann et al., 2010]. The FG5#202 also benefited from maintenances by the manufacturer in 1998, 2000, 2003, 2005, 2007 and 2010, where it was also compared to other FG5s. ...
In continental plate interiors, tectonic deformations are small and the associated ground surface movements remain close to or below the accuracy of current geodetic techniques, and at the limit of the noise level. An absolute gravimeter is an appropriate tool to quantify slow vertical movements, as this instrument, based on length and time standards, is drift free and does not depend on any terrestrial reference frame. Repeated absolute gravity (AG) measurements have been performed in Oostende (Belgian coastline) and at 8 stations along a southwest-northeast profile across the Belgian Ardennes and the Roer Valley Graben (Germany), in order to estimate the tectonic deformations in the area. After 7-13 years (depending on the station), we find evidence that the movements are no larger than a few millimeter per year and result from a combination of anthropogenic, climatic, tectonic, and Glacial Isostatic Adjustment (GIA) effects. This demonstrates the importance of precisely modeling the GIA effects in order to investigate intraplate tectonic deformations at the sub-millimeter level. This study also shows that AG measurements, repeated once or twice a year, can resolve vertical velocities at the 1.0 mm/yr level after 10 years, even in difficult conditions, provided that the gravimeter is carefully maintained.
... Comparing the measurement results of absolute gravimeters of the highest metrological quality in the ICAGs at the BIPM as well as in the Regional Comparisons of Absolute Gravimeters (RCAG) (see, for example, Vitushkin et al., 2002;Boulanger et al., 1981;Francis et al., 2007) is currently the only way to test the uncertainty in absolute g-measurements and to determine the offsets of individual gravimeters with respect to Comparison Reference Values (CRV) (Vitushkin, 2008;). The CRVs in the ICAGs and RCAGs are the g-values obtained at one or more gravity stations at BIPM or at the sites of RCAGs. ...
The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sèvres, France in September 2005. The organization of ICAG-2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented.
The g-values for the sites A and B of the BIPM gravity network at a height of 0.90 m are (980,925,702.2 ± 0.7) μGal and (980,928,018.5 ± 0.7) μGal, respectively. This result is in a good agreement with that obtained in ICAG-2001.
KeywordsGravimetry-Absolute gravimetry-International comparison
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).
