The two end-member concept of mantle plume-driven versus far field stress-driven continental rifting anticipates high volumes of magma emplaced close to the rift-initiating plume, whereas relatively low magmatic volumes are predicted at large distances from the plume where the rifting is thought to be driven by far field stresses. We test this concept at the Guinea Plateau, which represents the last area of separation between Africa and South America, by investigating for rift-related volumes of magmatism using borehole, 3D seismic, and gravity data to run structural 3D inversions in two different data areas. Despite our interpretation of igneous rocks spanning large areas of continental shelf covered by the available seismic surveys, the calculated volumes in the Guinea Plateau barely match the magmatic volumes of other magma-poor margins and thus endorse the aforementioned concept. While the volcanic units on the shelf seem to be characterized more dominantly by horizontally deposited extrusive volcanic flows distributed over larger areas, numerous paleo-seamounts pierce complexly deformed pre and syn-rift sedimentary units on the slope. As non-uniqueness is an omnipresent issue when using potential field data to model geologic features, our method faced some challenges in the areas exhibiting complicated geology. In this situation less rigid constraints were applied in the modeling process. The misfit issues were successfully addressed by filtering the frequency content of the gravity data according to the depth of the investigated geology. In this work, we classify and compare our volume estimates for rift-related magmatism between the Guinea Fracture Zone (FZ) and the Saint Paul's FZ while presenting the refinements applied to our modeling technique.

Made available in DSpace on 2015-03-13T17:08:33Z (GMT). No. of bitstreams: 1 MarinaBG_DISSERT.pdf: 4802721 bytes, checksum: 33f005bb55a08c628d93bc270cde7787 (MD5) Previous issue date: 2012-06-20 / The gravity inversion method is a mathematic process that can be used to estimate the basement relief of a sedimentary basin. However, the inverse problem in potential-field methods has neither a unique nor a stable solution, so additional information (other than gravity measurements) must be supplied by the interpreter to transform this problem into a well-posed one. This dissertation presents the application of a gravity inversion method to estimate the basement relief of the onshore Potiguar Basin. The density contrast between sediments and basament is assumed to be known and constant. The proposed methodology consists of discretizing the sedimentary layer into a grid of rectangular juxtaposed prisms whose thicknesses correspond to the depth to basement which is the parameter to be estimated. To stabilize the inversion I introduce constraints in accordance with the known geologic information. The method minimizes an objective function of the model that requires not only the model to be smooth and close to the seismic-derived model, which is used as a reference model, but also to honor well-log constraints. The latter are introduced through the use of logarithmic barrier terms in the objective function. The inversion process was applied in order to simulate different phases during the exploration development of a basin. The methodology consisted in applying the gravity inversion in distinct scenarios: the first one used only gravity data and a plain reference model; the second scenario was divided in two cases, we incorporated either borehole logs information or seismic model into the process. Finally I incorporated the basement depth generated by seismic interpretation into the inversion as a reference model and imposed depth constraint from boreholes using the primal logarithmic barrier method. As a result, the estimation of the basement relief in every scenario has satisfactorily reproduced the basin framework, and the incorporation of the constraints led to improve depth basement definition. The joint use of surface gravity data, seismic imaging and borehole logging information makes the process more robust and allows an improvement in the estimate, providing a result closer to the actual basement relief. In addition, I would like to remark that the result obtained in the first scenario already has provided a very coherent basement relief when compared to the known basin framework. This is significant information, when comparing the differences in the costs and environment impact related to gravimetric and seismic surveys and also the well drillings / Esta disserta??o apresenta resultados de invers?es gravim?tricas realizadas com dados p?blicos na Bacia Potiguar Emersa. A invers?o gravim?trica ? uma processo matem?tico que pode ser usado para estimar o relevo do embasamento de uma bacia sedimentar. Embora este seja um problema matematicamente mal-posto, devido ? n?o unicidade e/ou instabilidade das solu??es, ele pode ser estabilizado atrav?s do uso de funcional a ser minimizado de modo que, al?m do ajuste das observa??es geof?sicas, alguns v?nculos geologicamente significativos sejam incorporados. O processo de invers?o gravim?trica aqui utilizado admite que o contraste de densidade entre o embasamento e as rochas sedimentares ? constante e conhecido. A metodologia pressup?e ainda a discretizac?o do pacote sedimentar num conjunto de prismas (3D), cujas espessuras correspondem ?s profundidades do embasamento e s?o os par?metros a serem estimados. A estabiliza??o do problema ? feita com os v?nculos de (1) suavidade na varia??o espacial da profundidade, (2) de proximidade a um modelo de refer?ncia, bem como (3) de honrar desigualdades conhecidas sobre a profundidade em locais espec?ficos, a exemplo de po?os. Esses v?nculos de desigualdade foram incorporados na forma de barreiras logar?tmicas, as quais permitem agregar informa??es n?o s? de po?os que atingiram o embasamento como tamb?m de po?os conclu?dos na se??o sedimentar. Os cen?rios de aplica??o da metodologia de invers?o foram escolhidos de modo a simular diferentes fases ou etapas de explora??o de uma bacia ao longo do tempo. A invers?o no primeiro cen?rio foi realizada utilizando somente dados gravim?tricos e um plano como modelo de refer?ncia. A invers?o no segundo cen?rio foi subdividida em dois casos: agregando aos dados gravim?tricos os v?nculos oriundos de po?os ou de um modelo s?smico de refer?ncia. Por fim, na invers?o no ?ltimo cen?rio foram agregados dados de po?os e um modelo de refer?ncia s?smico. Para todos os cen?rios, as estimativas obtidas do relevo do embasamento revelaram nitidamente a forma interna do Rifte Potiguar, evidenciando a presen?a de fei??es estruturais alinhadas com dire??o principal NE-SW e segmentadas por componentes de dire??o NW em alguns locais. Em rela??o aos valores absolutos de profundidade, a inclus?o dos v?nculos dos po?os aprimorou os resultados. Por sua vez, a introdu??o do modelo s?smico de refer?ncia aumentou a resolu??o global dos resultados. A flexibilidade de incluir na invers?o uma variedade de informa??es de fontes distintas gerou resultados mais robustos e que atendem simultaneamente aos dados gravim?tricos, s?smicos e geol?gicos (po?os). A consist?ncia e confiabilidade dos resultados comprovaram a potencialidade da invers?o gravim?trica como ferramenta de delineamento da geometria do embasamento. ? relevante destacar que a invers?o realizada apenas com dados gravim?tricos, e numa malha com espa?amento regional, Cen?rio 1, j? forneceu uma estimativa de relevo do embasamento bastante coerente com o arcabou?o conhecido da bacia. Essa ? uma importante informa??o, se levarmos em conta diferen?as de custo financeiro e de impacto ambiental dos levantamentos gravim?tricos em rela??o ao m?todo s?smico, bem como ? perfura??o de po?os