High precision levelling supporting the International Comparison of Absolute Gravimeters

Abstract

High precision levelling is an indispensable method used to monitor benchmark and terrain stability at the Bureau International des Poids et Mesures (BIPM). Associated with the International Comparison of Absolute Gravimeters (ICAG), levelling measurements were carried out repeatedly over the past decades. A local gravity field strongly depends on vertical terrain deformation. 1 cm displacement implies about 2 µGal change in the vertical gravity acceleration. The precision of absolute and relative gravimetry nowadays is about 1 µGal.At the beginning of the century, a strategy was outlined that the ICAG should be upgraded to a metrological Key Comparison of the CIPM MRA (Mutual Recognition Arrangement) recognized officially by the designated governmental organizations. As a result of this decision, BIPM site B was constructed and completed in Spring 2001. The site B pillar is 4 m × 6 m × 1.5 m in dimension and more than 80 ton in weight. Such a large, newly built concrete body produces local deformation due to its sinking or tilting. This in turn influences the local gravity field. Rigorous levelling measurements have been performed by the Bureau de Recherches Géologiques et Minières (BRGM), France, since 2001 and repeated together with the 4-year ICAGs of 2001, 2005 and 2009. ICAG-2009 was characterized by becoming the first Comité International des Poids et Mesures (CIPM) Key Comparison which supported meanwhile the BIPM watt balance (WB) project. The WB pillar was built in Spring 2009. The Institut Géographique National (IGN), France, has hence been invited to participate in the levelling. The latter also measured the link between the BIPM local network and external stations of the French national height reference system IGN69.In this paper, we report the final results of the levelling programmes of 2001, 2005 and 2009 and compare the results. We investigate stability of the BIPM gravity-levelling stations. We conclude that the existing and the newly built stations can be considered as stable for the purposes of the ICAGs and the WB, although further repeat measurement may be required for the WB site.For the first time, detailed and complete levelling data and results are publishedOnly the final result of the 2005 levelling mission was published in [4] where, instead of NGF, the height reference system used was IGN69. This is incorrect and is corrected in this paper.. After 30 years of organizing and holding eight ICAGs, the BIPM will hand over the KC ICAG-2013 to another CIPM MRA designated institute. This paper serves as a technical and historical report of precision levelling, a sub-task of the ICAGs.