Precise Observation of the Secular-Gravity Changes by means of the combination of Absolute and Superconducting Gravimeters.

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To obtain so accurate data sets of gravity variations as to be used for the comparison with the satellite gravity data such as GRACE, we studied practical methods to combine superconducting and absolute gravity measurements as well as to improve the accuracies of the individual measurements.For this purpose, we have continued superconducting gravity observations at the observation stations of Kyoto, Esashi, Matsushiro in Japan and Bandung in Indonesia. We also started groundwater measurements to reveal their gravity effects at Kyoto and Bandung stations: The results show a clear correlation between the groundwater variations and the gravity observations, especially at the Bandung station. We confirmed that the accuracy of the gravity measurements are improved by removing the groundwater effects.We carried out several absolute gravity measurements in parallel with the superconducting observations at the Esashi and Matsusiro stations in 2000 and 2001 as well as the Kyoto stations where absolute measurements are periodically conducted. The total observation, time in both stations exceeded more than one month which is long enough for purpose of comparison. Moreover, we developed a new method for the data processing, we finally obtained very precise calibration factors for the superconducting gravimeters. We also determined reference gravity values for future studies of secular gravity changes at the stations.To maintain the accurate absolute gravity measurements, adjustments and validations are very important. We carried out the comparison between the absolute gravity meters in collaboration with Geographical Survey Institute. The comparison also contributed to accumulate the know how of the absolute gravity measurements.In addition to the actual measurements, we also conducted several simulation studies of satellite gravity mission data and ground base measurements, and we established the basic techniques of future utilization of those data sets.