High precision gravity measurements with the dual sphere superconducting gravimeter in Sutherland (South Africa)

Abstract

Since March 2000, the reconstructed Dual Sphere Superconducting Gravimeter (SG) with two gravity sensors is continuously recording gravity data at the newly constructed South African Geodynamic Observatory Sutherland of GFZ (SAGOS). Additionally, meteorological sensors and a ground water table sensor were installed for estimation of gravity changes induced by the atmosphere and hydrosphere. The observatory is designed for high precision geodynamic observations of the Earth with space techniques and ground instruments. The Noise Magnitude characterising the quality of the site Sutherland is close and even smaller than the “New Low Noise Model” values at frequencies below 1 mHz. This result characterises the excellent quality of the site for the observation of weak gravity signals. For calibration of the SG gravity sensors, absolute gravity measurements have been carried out at SAGOS. The calibration factor could be fixed with an accuracy of 0.2%. Tidal amplitudes, amplitude factors and phase leads have been determined for Sutherland with high resolution and accuracy for the first time. After applying ocean loading correction for the main diurnal and semidiurnal waves, there are still discrepancies between the measured and model tidal parameters. These discrepancies have to be minimised by improving the models and data correction for non-tidal induced gravity effects. The maximal vertical surface shift caused by the earth tides and mass redistribution in the atmosphere during the time from March 27th 2000 to August 1st 2001 has been determined to 41.9 cm by using the Love numbers h2 and k2 derived from the tidal and deformation potential. The data of the earthquake near the coast of Peru on June 23rd 2001 have been analyzed for detection of the free oscillation modes of the Earth. Especially the long periodic modes 0S3, 2S1 and 0S2 are very well marked after this earthquake and less disturbed because of the low noise site. They confirm the model frequencies for these modes.