Calibration shifts in a LaCoste-and-Romberg gravimeter: comparison with a Scintrex CG-3M

The reliability and ruggedness of LaCoste-and-Romberg gravity meters has me ant that their use has extended over recent years from gravity mapping to t he investigation of small gravity changes (> 10 mu Gal) with time. When usi ng the instrument at the limit of its capability, limitations become appare nt. Some instruments drift more than others and some are prone to excessive tares. However, with regular servicing these problems are minimized. Here we demonstrate clear evidence for calibration changes of the order of 1000 ppm for LaCoste-and-Romberg meter G513, which, for gravity differences of o ver 50 mGal, represents a highly significant induced error of over 50 mu Ga l. Data collected over 15 years using nearly 4000 mGal of the 7000 mGal ran ge of the instrument illustrates that long periods of stability may be inte rspersed with periods lasting several months while the calibration changes by up to 0.1%. This is comparable with the calibration drift of a new Scint rex CG-3M instrument, although the process causing the change may not be th e same.