5:15 PM - 6:30 PM
[SGD01-P05] On the sensitivity of superconducting gravimeter CT #036 at Ishigakijima, Japan
Keywords:superconducting gravimeter, Ishigakijima, absolute gravity measurements
The superconducting gravimeter (SG) is a relative instrument in that it solely does not provide information on absolute values of gravity acceleration. Therefore, the user of an SG must calibrate its instrumental sensitivity. The sensitivity of SG CT #036 operated at the VERA Ishigakijima station, Okinawa, Japan, was calibrated by the absolute gravity measurements in January 2015. During the measurements, the gravity control of the SG was switched from RUN to CAL in order to lower the sensitivity. The scale factor before and after this change was estimated to be 57.803 ± 0.110 µGal/V and 1134.41 ± 1.63 µGal/V, respectively.
However, it has been found that tidal analysis of the observed gravity data using the scale factors thus estimated gives significantly different amplitude for tidal signals before and after the sensitivity change. For example, amplitude of the M2 wave is estimated to be 77.631 ± 0.001 µGal for the period 2012–2015 and 77.877 ± 0.001 µGal for the period 2015–2020. The relative difference of these values is about 0.32 %, exceeding the relative estimation errors of the scale factor (0.1–0.2 %).
Because it is unlikely that the amplitude of tidal signals really changed with time, this result implies some kinds of problems in the scale factors estimated with the absolute gravity measurements. We plan to make absolute gravity measurements again at Ishigakijima for more precise determination of the scale factor as well as calibration of the instrumental drift of the SG CT #036.
However, it has been found that tidal analysis of the observed gravity data using the scale factors thus estimated gives significantly different amplitude for tidal signals before and after the sensitivity change. For example, amplitude of the M2 wave is estimated to be 77.631 ± 0.001 µGal for the period 2012–2015 and 77.877 ± 0.001 µGal for the period 2015–2020. The relative difference of these values is about 0.32 %, exceeding the relative estimation errors of the scale factor (0.1–0.2 %).
Because it is unlikely that the amplitude of tidal signals really changed with time, this result implies some kinds of problems in the scale factors estimated with the absolute gravity measurements. We plan to make absolute gravity measurements again at Ishigakijima for more precise determination of the scale factor as well as calibration of the instrumental drift of the SG CT #036.