Investigation of Topographic Reductions for Marine Geoid Determination in the Presence of an Ultra-High Resolution Reference Geopotential Model

Abstract

During the last decade, the realization of the satellite gravity missions of CHAMP and GRACE, the acquisition of new gravity data and the development of novel processing methodologies has led to the determination of more accurate and higher in resolution global geopotential models. The spatial scale of ~110 km that EGM96 could represent has improved today with EGM2008 to the level of ~16 km (full wavelength). This advance in the representation of higher frequencies by the geopotential models may signal the need to reassess the methodologies and techniques traditionally used for local and regional geoid determination. The traditional procedure followed is that of the remove-compute-restore method. The input functionals related to the Earth's gravity field are first reduced to a reference geopotential model, then the topographic effects are taken into account through one of the available reduction methods, computations follow using the reduced observations, and finally the contribution of the global geopotential model and the topographic indirect effects are added back to the computed reduced geoid values. One crucial point to this operation is that the attraction of the masses considered with a topographic reduction scheme is supposed to represent the medium and high frequencies in the gravity field, which still remain in the data, in principle even after they have been reduced to a geopotential model. Given that the best available digital depth models have a resolution of 30 arcsec, which translates to roughly 1 km spatial wavelength, it becomes apparent that the contribution of such a model to the reduction of gravity and geoid data, when a high resolution geopotential model is used as reference, is questionable or should be at least investigated. This final point is the main goal of this paper, i.e., to investigate the contribution of the available digital depth models to the reduction of gravity anomalies and geoid heights when a geopotential model with the resolution of EGM2008 is used. To this extent, marine gravity anomalies and satellite altimetry sea surface heights are used off-shore the Atlantic coast of Argentina. EGM2008 is used as a reference surface to reduce the available gravimetric and altimetric observations, and the latest bathymetry model from the Scripps Institute of Oceanography group (SIOv11.1) is employed in order to compute topographic reductions based on the Residual Terrain Model (RTM) scheme. The results acquired are validated in terms of the reduction they provide to the available input data, both the mean and the standard deviation of the residuals, as well as in terms of the spectral content of the residual signal spectrum. Conclusions and recommendations on the use of topographic reductions and the treatment of topographic effects for geoid modelling in the presence of a high-resolution geopotential model are also drawn so as to ensure the consistency between data used and results acquired.