Sensitivity analysis of the gravity geoid estimation: A case study on the Azores plateau

A numerical investigation into the effects of high-resolution heterogeneous data on the estimation of the geoid is presented for the area of the Azores plateau. Gravity data were complemented with high-resolution digital terrain models, mass density models from geophysical information, and the most recent geopotential model from the CHAMP mission (GGM02) to derive a high-precision gravity model. Gravity data were reduced from the residual terrain model effects and the GGM02C model, and a grid with 1.5′ spatial resolution was generated by least squares collocation. The gravimetric geoid model was computed by the direct evaluation of Stokes’ integral with a spherical cap radius of 0.8°, with gravity data reduced by Helmert's second condensation method. The geoid model was evaluated over sea by comparison with 6 years of Topex/Poseidon altimeter data and an absolute agreement of 14 cm RMS was achieved after removing the bias. Over land, the geoid was compared with a total of 223 GPS/levelling points corresponding to five different vertical datums. Locally adjusted solutions were determined for each island with a four-parameter model, and a maximum RMS of 12 cm (on Pico) and a minimum of 2.1 cm (on Faial) were determined. As a result of persistent volcanic and tectonic activity on most of the Azores islands, this geoid residual standard deviation must not be viewed as purely geoid error but containing, in addition, levelling and GPS position errors and vertical displacement. The effect on geoid accuracy of the digital terrain model resolution, of the spatial density of gravity data, and of the mass density was evaluated and two important conclusions were derived. First, the digital terrain model resolution has a clear effect on the geoid accuracy, with a maximum RMS of 3.4 cm on S. Miguel. Second, the mass density model does not affect the accuracy of the geoid. This second result indicates that the effect of variable density models on geoid determination may apply only to extensive areas of high terrain and not to smaller topographic units such as single volcanoes. The high-precision gravity model and gravimetric geoid for Azores (AZGEOD07) generated in this study will allow the conversion of old orthometric heights to ellipsoidal heights at precisions useful for the monitoring of vertical deformation. The geoid will also have application to investigate ocean circulation modulation and dynamic processes of Earth's mantle.