著者

若林環 風間卓仁

出版者

北海道大学大学院理学研究院

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.84, pp.11-20, 2021-03-26

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Campaign relative gravity measurement is one of the most powerful methods to monitor spatiotemporal mass variations associated with volcanic activities. In order to quantify volcanic gravity variations accurately, the systematic gravity error due to scale factor should be corrected from original gravity data. We therefore determined relative scale factors of portable relative gravimeters, using relative gravity values measured at six gravity points between Kyoto University and Mt. Hiei in 2020 frequently. We here calculated the relative scale factor of each gravimeter by dividing the gravity value of the LC-G534 gravimeter by that of the corresponding gravimeter. The scale factor of the LC-G680 gravimeter relative to the LC-G534 gravimeter was obtained to be 0.999900 ± 0.000067; although the gravity difference between Kyoto University and Mt. Hiei (about 167 mGal) was not so large for the scale factor calibration, we succeeded in determining the G680’s relative scale factor with small standard deviation because the measurement error of the gravity data became smaller thanks to the frequent gravity measurements. However, the relative scale factors for LC-G680 and D-58 as of 2020 were found to be different from those in 2018 by more than 0.0002. In addition, the standard deviations of the relative scale factors for LC-D58 and CG-5 were obtained to be greater than 0.0002. These results may be related to significant temporal variations in reading values, originating from the overhaul in 2019 (LC-G680), the manual adjustment of the reading range (LC-D58) and the large instrumental drift (CG-5). Our results also imply that scale factor values for LaCoste-type gravimeters can depend on their reading values, as mentioned in a previous study for Scintrex-type gravimeters.