Results of the European Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2007

Abstract

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/s 2 ). In 1999, a laboratory ( Fig. 5.1) dedicated to the comparison of absolute gravimeters was built within the WULG. The laboratory lies 100 m below the surface at a distance of 300 m from the entrance of the mine. The WULG is environmentally stable (i.e., constant temperature and humidity within the lab), and is extremely well isolated from anthropogenic noise. It has the power and space requirements to be able to accommodate up 16 instruments operating simultaneously. IntroductionMultiple absolute gravimeter comparisons are regularly carried out. Being absolute instruments, these gravimeters cannot really be calibrated. Only some of their components (such as the atomic clock and the laser) can be calibrated by comparison with known standards. The only way one currently has to verify their good working order is via a simultaneous comparison with other absolute gravimeters of the same and/or if possible even of a different model, to detect possible systematic errors.During a comparison, we cannot estimate how accurate the meters are: in fact, as we have no way to know the true value of g, we can only investigate the relative offsets between instruments. This means that all instruments can suffer from the same unknown and undetectable systematic error. However, differences larger than the uncertainty of the measurements, is usually indicative of a possible systematic error.For the second comparison in Walferdange, a few new procedures have been introduced. First, some of the participants accepted to take part in a

Results of the first North American comparison of absolute gravimeters, NACAG-2010

“…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.…”

“…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).…”

Results of the Seventh International Comparison of Absolute Gravimeters ICAG-2005 at the Bureau International des Poids et Mesures, Sèvres

“…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;Vitushkin et al 2007). 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.…”

Repeated absolute gravity measurements for monitoring slow intraplate vertical deformation in western Europe

“…[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.…”