Abstract
Change of the liquid helium level affects the position change of the proof mass of superconducting gravimeter (SG). We estimated its effect on three different types of SGs (TT-70#016, CT#043, and OSG#058) by approximating the spatial relationship between the SG sensor unit and liquid helium reservoir using a double-layered cylinder model where the inner cylinder and outer one are assumed SG sensor unit and liquid helium reservoir tank, respectively.
From these calculations, it was predicted for SG TT-70#016 that a gravity change of 0.88 µGal (1 µGal=10-8m•s-2) results when the liquid helium level in the reservoir is increased from 0% to 100%. Similar gravity changes of 0.25 µGal and 0.35 µGal were also predicted for SGs CT#043 and OSG#058, respectively. In addition, artificially-induced, rapid gravity changes of approximately 0.9 µGal over the course of two hours likely occur during the filling of SGs with liquid helium from 20% to 100% for TT-70#016. Actual gravity changes correlated with the rapid liquid helium level changes were also found in the observed gravity record of SG CT#043 in 2003, although the actual gravity changes were greater than the predicted ones.
Gravity changes of approximately 0.1 µGal due to fluctuations of 5% to 14% in the liquid helium level change are also possible during the normal continuous observation with SGs. It is therefore preferable to monitor the liquid helium level routinely when measuring small gravity signals in the order of sub-micro Gal to correct for these effects.
