MODELLING OF SECULAR VARIATIONS IN GRAVITY AND IN GEOIDAL UNDULATIONS

The known time variation of the absolute gravity and that of the height above sea level enables us to compute the vertical shift of the equipotential surfaces i.e. that of the mean sea level and by this way to determine the true vertical displacement of the earth's surface (i.e the true recent crustal movement). By a further development of Barta's ideas and numeric results, a model has been computed for the time variation of gravity which served to compute the ver-tical shift of the geoid. Model computations with different density of known gravity variations (i.e. absolute gravity stations) have shown that a world network of 36 stations seems to be sufficient to represent the characteristic features of the global distribution of the variation of reoidal undulations, but determining the magnitude of the variations needs 62 stations at least (30 0 X 30 0 density) in the case of the model used by the authors. Also the numeric value of the gatio c = og/oN between gravity variation and change in the geoidal undulation has been in-vestigated and a rather varied global distribution between- 00 and + 00 has been stated. All these experiences are valid even if the numeric values (i.e. the magnitudes) of the variations computed by the model were overestimated. 1