Gravity field implied density modeling of topography, for precise determination of the geoid


1 Associate Professor, Faculty of Geodesy and Geomatics Engineering, K.N.Toosi University of Technology, Tehran, Iran

2 Senior technical staff, National Cartographic Center (NCC), Tehran, Iran

3 Land seismic manager, National Iran Oil Company (NIOC), Exploration management Dept., Tehran, Iran


Precise determination of the geoid using the Stokes-Helmert approach requires a density distribution model within the topography. The model is used for precise evaluation of topographical indirect effects on gravity and potential applied in transforming between the real and the Helmert spaces. The range of mass density variation within the topography is between 1000 and 3100 kg.m-3. Assigning the global average value of  kg.m-3 at a point, instead of its real point value, may cause errors of decimeter magnitude in the geoid determination. A regional-local gravity anomaly separation technique using Bouguer gravity anomaly (BA) along with the Free air gravity Anomaly (FA) in the region of Iran are used to estimate the local topographical effect on gravity from the observed anomalies after eliminating the non-density origin long wavelength including isostatic features into the observed BA. A Global Geopotential Model (GGM) is also used to eliminate the deep sited density-origin long wavelength features from the observed anomalies as well. Then, the power spectral analysis, apparent density mapping, and forward modeling techniques are used to convert the local topographical effect on gravity into the corresponding 3-D density model (GRADEN) model in the region. The model showed thorough correlation with the superficial geological density (GEODEN) model at the surface level, provided that the reliable digitization of the model is in order. The GRADEN model minus the constant density demonstrates contributions up to a meter in mountainous areas and 7cm in the RMS scale to the geoid in the region.