Seasonal gravity variation at Ny-Alesund, Svalbard observed from a superconducting gravimeter

As the most north observation station of the GGP network (Global Geodynamics Project, Crossley et al., 1999), the observation at Ny-Alesund with a superconducting gravimeter was started in September 1999. We have analyzed the short- and long-period tides using the 3-year data obtained from the beginning of the observa-tion. The residual gravity signals after subtracting the short- and long- period gravity tides including the effects of annual and semiannual ocean tides and of the polar motion show a clear seasonal variation. Behavior of the observed seasonal variation is regular in time, i.e. it starts on at around May-June in the sense of decreasing in gravity and the sense changes to increasing at around July--August. At a frequency of once a year, the SG was calibrated three times by comparing with the absolute gravimeter (FG5 absolute gravimeters). The calibration results suggest that the scale factor of the SG (i.e. micro Gal/Volt) is changing at a rate of about -0.2%/yr, even though the SG is protected against to the tilting motion of the observation pier by using an automatic leveling system called thermal leveler. This change is also confirmed by the temporal variation in the O1 amplitude obtained from the analysis using the original data of Volt unit, i.e. decreasing in the amplitude at a rate of about -0.3%/yr. The change in the scale factor is far large in magnitude compared with a usual value obtained from other SGs, for example, 0.01%/yr of the SG at Esashi, Japan. On the other hand, the tilt measurement was conducted at the same pier for one year from 2000 to 2001 (Kuempel and Fabian, 2002), and its data indicates a similar seasonal variation in its time behavior. It has not been analyzed enough the exact relation between the sensitivity change of the SG and the tilt of the observation pier, but both observation results may indicate seasonal instability of permafrost in Ny-Alesund as pointed out by Kuempel and Fabian ( 2002).