Sensitivity Calibration of Superconducting Gravimeters CT #036 and iGrav #028 with Absolute Gravity Measurements at Matsushiro

Abstract

　In June 2023, we carried out absolute gravity measurements at the Matsushiro station, Japan, with an FG5 absolute gravimeter. Thanks to the good weather conditions, we succeeded in obtaining very high-quality data over a total of four days. The drop-to-drop scatter for the whole experiment was 4.25 μGal, and the gravity acceleration at 130 cm above the floor was estimated to be 979,772,780.451±0.027 μGal. Using this data, we made calibration of the sensitivity for two superconducting gravimeters （CT #036 and iGrav #028） currently operating simultaneously at Matsushiro. Special care was taken in the treatment of the system delay of the superconducting gravimeters as well as the timing of absolute gravity measurements. Detailed analysis of the data from both types of gravimeters revealed that the absolute gravimeter recorded, though imperfectly, 20-s-period surface waves after a teleseismic earthquake. The final estimates of the scale factors（expressed as positive values） were 1134.65±0.69 μGal/V for CT #036 and 2043.05±1.23 μGal/V for iGrav #028, respectively, achieving a relative accuracy of approximately 0.06% for both gravimeters. These results were further verified by comparing amplitude of tidal signals derived from tidal analysis of the data from the two superconducting gravimeters.