THE GRAVITY-ANOMALY FIELD IN THE GULF-OF-BOTHNIA SPATIALLY CHARACTERIZED FROM SATELLITE ALTIMETRY AND IN-SITU MEASUREMENTS

The gravity anomaly field in the Gulf of Bothnia has been investigated using (1) in situ high-precision measurements conducted on the sea ic e during cold winters, and (2) gravity anomaly profiles computed from collinear satellite radar altimeter data from the Geosat ERM and the T opex/Poseidon missions. The in situ measurements were obtained from a collaboration between the Finnish Geodetic Institute, the Geological S urvey of Sweden (SGU) and the National Survey of Sweden (LMV), and wer e processed with the geostatistical method called kriging. These data were used to calibrate the altimetric gravity. Altimetry generally res olves features of 20 km wavelength or longer, and in some cases detect s shorter features when a sampling interval of 10 Hz is used. The prec ision of the along-track one-dimensional altimetric profiles correspon ds to a gravity uncertainty of 2-3 mGal, and comparison with in situ m easured gravity show 4 mGal discrepancy. The precision of the in situ measurements is better. However, depending on the sampling distance, t he estimation uncertainty interior the in situ data areas may be up to 5 mGal between neighbouring data points. In regions with in situ data gaps, the estimation uncertainty of the in situ gravity measurements is rapidly increasing to a maximum of 9 mGal. An improved estimation u ncertainty of 4-9 mGal was obtained in the same data gap regions with the support of satellite altimetry. Altimetric gravity is therefore us ed to estimate the gravity field in such regions, and to spatially cha racterize the gravity field in the Gulf of Bothnia.