GOCE OBSERVATIONS IN EXPLORATION GEOPHYSICS Carla Braitenberg, Patrizia Mariani, Tommaso Pivetta Department of Geosciences, University of Trieste, via Weiss 1, 34100, Trieste, Italy, email: berg@units.it Proccedings of the 4th International GOCE User Workshop, Technische Universität München (TUM), Munich, Germany 31 March - 1 April 2011 (Trieste, 29 March 2011) ABSTRACT The satellite GOCE has produced an extraordinary global gravity field with a spatial resolution of 80 km at a precision of 1-2 mGal. When considering geologic structures, the wavelengths of interest for exploration purposes are smaller. We show that the GOCE data produce a new quality assessment tool for fields of higher resolution, which all necessarily rely on terrestrial data. The space-borne control of terrestrial data is necessary in order to obtain 100% reliability of existing data or to assess the quality of newly acquired data. We propose a scheme for controlling and testing the quality of higher resolution data, for example airborne campaigns or the EGM08 global gravity field. EGM08 has the higher resolution of 10 km, but with varying quality, depending on the terrestrial data availability. We show how the quality assessment can be made using the GOCE data, giving confidence in the successive modelling results. Specifically we consider the African continent. Africa is an example where terrestrial data area scarce and where the reliability of EGM08 is very variable, jeopardizing the usefulness of the field if the quality assessment with the GOCE data is not fulfilled. 1. INTRODUCTION At time of writing the second edition of the GOCE derived Earth Gravity Models have been published ([1],[2],[3]), with maximum degree of the spherical harmonic expansion of N = 250, 240 and 210, respectively. All three fields are independent of terrestrial data, as they have been developed incorporating the satellite missions GRACE and GOCE. The GOCE observations are global, except for a circular area above the two poles, and thus independent of the particular conditions of a geographical area of interest. The maximum degree of N = 250 limits the spatial resolution to about 80 km (20000 km/250). Compared to existing global gravity fields as EGM08 [4] and EIGEN05C [5], with maximum degree N=2159 and 360, respectively, this may seem of no advantage. This argument does not consider the fact that the higher resolution is nominal, tied to the degree of the spherical expansion and does not express the varying availability of the terrestrial data. The realistic local resolution of these latter fields above the level (20000/120 km) depends entirely on the database of terrestrial data that entered the calculations. The models are given as gridded data at a certain height level or in terms of Stokes coefficients, with which the gravity values can be globally calculated. The models are accompanied with the error values of the Stokes coefficients, which give an average estimate of the error levels for each degree, but which are difficult to translate into a local quality assessment of the fields. The lack of knowledge of the local error hampers the use of these data in geophysical exploration, where reliability of the data is an important issue. We show that the GOCE data are adequate to assess the quality of terrestrial observations, also if the spatial resolution of the terrestrial data is higher than that of GOCE. We show with the example of Central North Africa, that the study of geological structures requires a field with higher resolution than that of GOCE, as is the EGM08 field, but that not all areas are reliable and that GOCE is necessary to assess in which areas the EGM08 field can be used to study the structures. 2. SIZE OF GEOLOGICAL STRUCTURES IN AFRICA Africa is a continent where geophysical data are insufficiently known over large areas. The social and economic growth of the country relies on a better knowledge of the crustal and lithospheric structure, which is essential in the exploration of geo-resources and in the risk assessment due to seismic and volcanic hazard. The geological units are the essential starting point for further studies, and can be seen in the geological map [6]. The full extent of the geological units cannot be evaluated when these are concealed below a superficial cover, because the geological mapping relies on identifying surface rocks. The gravity field is one means to delineate the full extent of the structures as long as they bear a density change with respect to the neighbouring rocks. In order to distinguish the units the gravity field must have a resolution that is smaller than the units we are interested to map. The main geological structures that are presumably accompanied by a density variation are rifts and their associated basins, sedimentary basins in general, magmatic intrusions and deposits, mobile belts and fold belts, orogenic ranges, faults. In Fig. 1 we show a schematic map of the geology in North-Central