Laboratory calibration of Lacoste-Romberg type gravimeters by using a heavy cylindrical ring

Abstract

SUMMARY A calibration device developed for Lacoste-Romberg (LCR) gravimeters in the Geodynamical observatory of the Eotvos Lorand Geophysical Institute of Hungary is described. The equipment generates gravity variations of about 112 μGal with the use of a suspended cylindrical ring of mass 3200 kg. A ring with an inner diameter slightly bigger than the width of the instrument to be calibrated is raised and lowered over the gravimeter installed on a column. The advantages of this solution with respect to others are: the gravimeter remains stationary during the calibration process, the gravity variations generated by the ring have smooth extrema, they are larger than variations caused by bodies with other geometry, the experiment is symmetrical with respect to the gravimeter and no ground loading by the ring occurs. The device allows a calibration accuracy for LCR instruments of the order of 0.1 per cent to be attained. This is a considerable improvement for absolute calibrations in a very narrow range, especially in the context of tidal measurements. To reach such an error level the instrumental drift, tidal variation, the microseisms and air pressure must be removed most accurately from the measured data. The position of the ring in space and time is measured with an accuracy of 0.1 mm and ***1 s. The tilt of the frame and of the instruments must be recorded and excluded from the experimental results. Calibration results obtained with the device are compared to the calibration values obtained at broad and medium ranges along two calibration lines with different ranges.