FROM CHANDLER-WOBBLE TO FREE OSCILLATIONS - COMPARISON OF CRYOGENIC GRAVIMETERS AND OTHER INSTRUMENTS IN A WIDE PERIOD RANGE

The cryogenic gravimeter was developed to overcome the mechanical defi ciencies of spring gravimeters. The stability of the instrument permit s the study of long-term gravity variations for the first time. Short- term, especially periodical gravity variations have been well observed with spring gravimeters. This investigation tries to determine up to what level and frequency the various types of instrument have equivale nt quality. Side-by-side runs of two cryogenic instruments, and compar isons with relative spring gravimeters and long-period seismometers, s hould help to answer this question. From the intercomparison of two cr yogenic gravimeters in Bad Homburg, Germany (TT40 and TT60) and in Mia mi, USA (TT40 and UCSD SG B) one can see that, in general, the gravity signals coincide within several nanometres per second squared, but al so intervals are noted where the differences are 10-30 nm s(-2) over a timespan of 1-3 months. Usually, known instrumental problems are the cause for these discrepancies. Data analysis as well as comparisons wi th absolute gravity measurements in parallel demonstrate that the drif t of the meters is monotonic and in general has exponential characteri stics. This allows investigations in the long-period tide band as well as in the range of long-term gravity variations, e.g. the gravitation al effects of polar motion, water table and air pressure variations. A t the other end of the spectrum-in the period range from 15 min to 1 d ay-a side-by-side run of the cryogenic gravimeter SG-102 and the sprin g gravimeter LCR-ET-19 in the Black Forest Observatory in Schiltach, G ermany, shows that the noise spectrum of the spring gravimeter is slig htly lower than that of the cryogenic one. Analysis of the free oscill ations of the Earth from several large earthquakes shows that this spr ing gravimeter and modern seismometers are at least of equal quality. Data sets recorded with cryogenic gravimeters open the possibility of investigating gravity variations in a wide frequency range. However, e specially for investigations in the high-frequency band (1 c day(-1) a nd above), one should also make use of well-maintained conventional sp ring gravimeters and possibly modern force-balance seismometers, which may have at least the same or better quality.