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sur le développement des zones arides et semi-arides

Titre : Entwicklung eines Verfahrens zur Abbildung krustaler Leitfähigkeitsstrukturen anhand von Magnetotellurikdaten aus Namibia

Auteur : Weckmann, Ute

Université de soutenance : Freie Universität Berlin

Grade : Doktorgrades im Fachbereich Geowissenschaften 2002

Sommaire partiel
This PhD thesis deals with different imaging methods for interpretation of magnetotelluric data. Iterative inversion schemes are commonly used for this purpose. Nowadays there are several algorithms available, which assume a one- or two-dimensional subsurface. The results of the inversion strongly depend on site spacing, the applied modelling grid and the regularization parameters. Usually a unique model cannot be expected as the data can be explained by a variety of models. In case of a data set, which indicates a 3D subsurface, a 2D model can only be an oversimplifying approximation. Generally, the usage of a 3D forward modelling algorithm is possible, however, due to the complexity and the huge amount of modelling parameters only simplified modelling studies are feasible. As the modelling grid and its corresponding conductivities have a strong impact on the convergency of the model, complicated anomalies have to be simplified or enlarged by keeping the product of conductivity and thickness of the anomaly. In applying suitable imaging methods, however, we have the possibility to convert MT data directly in a physical property and to show them as co-ordinate system invariants. The derived imaging methods are applied on MT data from Namibia, which were recorded at 60 sites with GPS based instruments (S.P.A.M. MK III) in 1999. The main purpose of these measurements was a detailed study of the Waterberg Fault / Omaruru Lineament (WF/OL), a major tectono-stratigraphic zone boundary in the Damara Belt. The results show strong 3D effects : Above and north of the fault we observe phases over 90o and a strong correlation of parallel electric and magnetic field components resulting in a poor determination of one off-diagonal impedance component. These effects are certainly not caused by instrument failure or strong cultural noise. They provide indications of a complex conductivity distribution in the subsurface, which results in current channelling and deflection. The described 3D effects are not only observable at one single site but at all sites north of the WF/OL. Decomposition methods indicate that the implied distortion model is not applicable to this data

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