Mapeamento de uma interface separando dois meios homogêneos através da reconstituição gravimétrica compacta: aplicação a dados do LEPLAC-IV

Abstract

Mapping an arbitrary interface separating two homogeneous media, traditionally uses the downward continuation of the observed gravity anomaly. This procedure requires either a low-pass filtering or the introduction of a damping parameter. In both cases there is a loss in the resolution of the estimated interface relief. Moreover, the use of the downward continuation operator implies the unnecessary assumption that the interface is a harmonic surface. As a result, the use of the downward continuation operator constrains the relief mapping to interfaces which are smooth relative to its average depth. This difficulty is reduced in the inversion procedure presented in this thesis, whose aim is to map the crust-mantle interface using the minimum moment of inertia method by assuming a single horizontal axis of mass concentration, coinciding with the earth's surface. Horizontal and vertical density variations may be taken into account so that the method may be extended to complex geological settings involving the interpretation of adjacent bodies having different origins such as those present in crustal transitional zones. The perfomance of the inversion method was evaluated using sinthetic data and the results confirmed its efficiency in recovering the shape of horizontally elongated sources whose top or bottom may be represented by an arbitrary surface. The results also confirmed the method's efficiency in mapping the shape of bodies presenting lateral density variation, reflecting, therefore, a geological situation where the nature of the basement of the crustal transitional zone is being investigated. Based on these tests, the method was applied to the gravity inversion of five profiles from LEPLAC-IV, offshore the southeastern brazilian continental margin. The results permitted to infer the type of bodies making up the structural framework of the region comprising the São Paulo Ridge and the Porto Alegre Fracture Zone; in addition, it was possible to map the Moho in this region.