INTERPRETAÇÃO MAGNETOTELÚRICA TRIDIMENSIONAL DA PORÇÃO ONSHORE DA BACIA DO ESPÍRITO SANTO E DE SUA ZONA CRISTALINA ADJACENTE (ORÓGENO ARAÇUAÍ) ASSOCIADA A DADOS GRAVIMÉTRICOS E MAGNETOMÉTRICOS

Abstract

The Espírito Santo Basin, one of the Brazilian sedimentary basins with passive margin, is inserted in the context of the Araçuaí Orogen - that composes its Precambrian basement. Several geophysical and geological studies have been carried out in the region for various purposes. Among them, it is worth mentioning the study by COSTA (2005) that used magnetotelluric and transient electromagnetic data, acquired in 2001 and 2002 by teams from the National Observatory, along an approximate W-E profile in the onshore portion of the Espírito Santo Basin, to generate geoelectric models using a two-dimensional magnetotelluric inversion methodology. The broadband magnetotelluric data acquired along this profile are the same as those used in this work. The transfer functions used by the magnetotelluric method were obtained using the robust technique developed by EGBERT e BOOKER (1986) through the routines developed by Marcelo Banik (INPE). There were problems in the TS1 band of some stations, to solve this the processing of the TS1 band of these stations were performed by the WinGLink program. Subsequently, dimensionality analysis of the data was performed, using WAL invariants and the Phase Tensor. These techniques indicated that the data are essentially three-dimensional in nature, revealing the need to apply a three-dimensional inversion approach for interpretation. The 3D inversion was performed with the parallelized version of the ModEM code, which performs objective function minimization via the nonlinear conjugate gradient. Several inversions were performed to better define the geophysical resistivity model. For the selected result, an initial resistivity mesh was set to a value of 100 ohm m. As the data are located near the coast, to consider a more realistic model, the bathymetry was incorporated into the inversion as a priori information and a resistivity of 0.3 ohm m was set for the ocean. Inversions were performed using data from the Impedance Tensor, the Magnetic Transfer Function - Tipper, and jointly between the Tipper and the Impedance Tensor. The selected result for interpretation was obtained through the data inverted by the impedance tensor. Convergence of the model was obtained after 69 iterations and showed a satisfactory overall fit (nRMS = 1.68). The presence of several conductive and resistive anomalies in the subsurface was verified in the selected model, and a consistency could be identified between what would be expected for a more resistive Precambrian zone and a more conductive sedimentary zone. These anomalies were interpreted in association with geological information, potential data (gravity and magnetic) and well data. They were correlated with local structures and formations. It was possible to delimit the basin basement from the generated model, in coherence with data from wells in the study region. Furthermore, it was also possible to map three strong crustal conductors in this Precambrian terrain, which are probably related to recent circulations of saline fluids, correlated to events that originated the Abrolhos Bank, the Vitória-Trindade Chain, and onshore magmatic provinces. The potential data from the region were qualitatively correlated with the geoelectric model and the geological structures observed in the region. It was possible to make an association between fault systems and lithologies, from the previous geological knowledge, with the potential anomalies and the resistivity model. Finally, it was verified that the selected three-dimensional model allowed to expand the understanding about the complex regional structure of the area at crustal scale. Still, a coherence is noted with the model obtained with the two-dimensional inversion presented in COSTA (2005)’s research. It can be concluded that the interpretation of the resistivity model shows that the three-dimensional magnetotelluric inversion is a suitable tool for crustal imaging, since the model showed good consistency with the geology of the region and with the results obtained from other methodologies.