A First Traceable Gravimetric Calibration Line in the Swiss Alps

Abstract

In order to determine scale factors or more complex calibration functions of relative gravimeters it is usually necessary to calibrate these instruments regularly on points with known gravity values. Especially well suited are points with absolute measurements and with large gravity differences. This implies that gravimetric calibration lines are usually implemented in north-south direction or on stations with big height differences. The latter has the advantage that traveling time can be kept rather short. In 2013 we established a calibration line in the Swiss Alps between Interlaken (altitude 570 m) and Jungfraujoch (altitude 3,500 m). This line consists of 7 absolute stations and several eccentric points. The total gravity difference is more than 600 mGal. All absolute stations are easily accessible by car or are located in immediate vicinity of a station of the Jungfrau railway. Therefore, it is possible to measure the whole line with relative instruments in a closed loop in 1 day. The absolute stations have been chosen in a way that they are accessible during the whole year and that the gravity difference between two neighboring stations does not exceed 150 mGal. So, it is possible to calibrate as well gravimeters with a limited measuring range or with a non-linear scale factor. The expanded uncertainties (k D 2) of the gravity values at the absolute stations are varying between 5 and 6 Gal. All vertical gravity gradients (VGGs) have been determined by relative measurements on three levels above the marker. The newly established calibration line is free to be used by the whole gravity community and we hope that many institutions will profit. It is to our knowledge worldwide the first traceable gravimetric calibration line satisfying the BIPM Mutual Recognition Arrangement (MRA).The paper describes the details of the establishment of the calibration line, the absolute measurements and a first comparison with relative measurements with three relative gravimeters.

New adjustment of the Croatian first order gravity network

“…For example, the relative gravimeters can be calibrated on foreign calibration lines of sufficient range, e.g. the German or Swiss with gravity ranges of more than 5000 and 6000 µms -2 , respectively (Timmen et al, 2006;Marti et al, 2015). Alternatively, a calibration line of greater range than the present auxiliary line can be established in Croatia.…”

On the effect of the light propagation within the corner-cube reflector of absolute gravimeters

“…Depending on various factors, the vertical gradients can differ by more than 250 E, even for the sites located within the same region [11], so the difference (2) can reach 6 µGal. This value significantly exceeds the precision of modern gravity surveys and could not have been overlooked, if the effect (1) existed.…”