Optimisation des méthodes à induction électromagnétique pour l'ingénierie des sols

Abstract : This study has been conducted in the framework of the CIFRE doctoral contract, in collaboration with the Ginger CEBTP Geophysics pole and the “Laboratoire Magmas et Volcans”. The main objective of this work is to improve the performance of the sub-surface electromagnetic induction method.The principle is to retrieve more information from electromagnetic data keeping a prospecting method close to current field methods. Electromagnetic (EM) methods are based upon magnetic fields variations in order to to measure electrical characteristics of soils. EM devices are inductive and so don’t need contact with the ground. Thus they can be implemented faster than most of others geophysical methods (seismic, electrical profiles, gravimetry). In this framework, several developments have been performed to respond to the improvement of EM prospecting methods. The field survey was improved by the development of an automated acquisition system including a conductivimeter (EM-31) mounted on a fibreglass cart with a GPS receiver and a computer running a special Python program which ensure continuous data recording and geo-reference. In this thesis we present a correction procedure for the EM-31 conductivimeter values, in particular the device height above the ground. A Matlab program was also specifically conceived for automated EM data processing. It combines basic data processing and visualization tools. Two case studies was conducted during this doctoral work. The first one is about a hundred kilometers of linear prospecting along the Loire protection dykes (France). This study highlights the difficulties of large scale geophysical prospecting and allows to identify specific issues such as management of large data number. This influence the prospecting methodology and allows implementation of adapted automatized data processing. The second case study is about the implementation of EM devices on volcanic fields. Several archaeological sites has been mapped using EM methods. However in volcanic regions, soils and rocks are known to have strong magnetic effects. In this specific case of an archaeological study the purpose is to determine precisely the magnetic effects of soils on EM measures.
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Mathias Pareilh-Peyrou. Optimisation des méthodes à induction électromagnétique pour l'ingénierie des sols. Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2016. Français. ⟨NNT : 2016CLF22781⟩. ⟨tel-02129301⟩

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