If a rock is to retain a geologically meaningful magnetic record of its history, it is essential that it contains magnetic minerals which are capable of carrying stable magnetic remanence. Of the natural occurring magnetic minerals, magnetite is the most important because of its abundance and strong magnetic signature. The stability, i.e., the resistance to demagnetisation or reorientation, of magnetic remanence is related to grain size; in smaller grains the magnetic moments align to have single domain (SD) structures, in larger grains complex magnetic patterns are formed (multidomain (MD)). “Classical” domain theory predicts that SD remanence is stable, whilst MD remanence is not. However experimental evidence has shown that both SD and MD grains can have stable remanences. In this thesis the origin of stable MD remanence is examined. There are two opposing theories; one suggests that the stability is due to independent SD-like structures, the other postulates that the stability is due to metastable MD structure. A series of experiments were designed to examine the stability using a selection of characterised synthetic and natural samples. Low-stress hydrothermal recrystallised samples where grown for this study. For the first time, the stability of thermoremanence induced in hydrothermal crystals to cooling was examined. The results agree with previous observations for crushed and natural magnetites, and support kinematic models. The behaviour of SIRM and thermoremanences in MD magnetite to low-temperature cooling to below the crystallographic Verwey transition at 120-124 K (T_v) and the cubic magnetocrystalline anisotropy isotropic point (T_k) at 130 K was investigated. On cooling through T_v, SIRM was observed to decrease and demagnetise, however thermoremanence was found to display a large increase in the magnetisation at T_v, which was partially re- versible on warming. The size of the anomaly is shown to be dependent on the temperature at which the thermoremanence is acquired, internal stress and grain size. The anomaly is attributed to the large increase in the magnetocrystalline anisotropy which occurs on cooling through T_v . It is postulated that low-temperature cycling demagnetisation is due to kinematic processes which occur on cooling between room temperature and T_k. Characterisation of low-temperature treated remanence and partially alternating field demagnetised remanence, suggest that the stable remanence is multidomain. Low-temperature cooling of remanence in single sub-micron crystals was simulated using micromagnetic models. The models predict the observed anomaly for thermoremanence on cooling through T_v, and also the relative behaviour of SIRM and thermoremanence. The single domain threshold was calculated for the low-temperature phase of magnetite, and was found to be 0.14 microns, compared to 0.07 microns at room temperature.

La ré-identification de véhicules permet d’estimer deux paramètres clés en gestion dynamique de trafic : les temps de parcours et les matrices origine-destination. Dans cette thèse, nous avons choisi d'effectuer cette ré-identification par analyse des signatures magnétiques mesurées par des capteurs tri-axiaux placés sur la chaussée. La signature magnétique est générée par l'aimantation du véhicule : elle est alors susceptible de varier en fonction de l'orientation du véhicule par rapport au champ magnétique terrestre (à cause de l'aimantation induite dans le plan horizontal), et en fonction de sa position latérale relative par rapport aux capteurs. Les expérimentations que nous avons menées nous ont permis d'obtenir une base de données de signatures magnétiques, et ainsi d'évaluer les performances des deux méthodes de ré-identification que nous avons élaborées.La première méthode consiste à comparer directement des paires de signatures magnétiques mesurées par les capteurs. Les calculs de distances entre les paires sont effectués avec des algorithmes classiques comme la distance euclidienne. Les résultats obtenus sont très bons, et baissent peu lorsque le véhicule change d'orientation. Toutefois, ils sont très sensibles à la déformation des signaux due au décalage latéral du véhicule, et nécessitent donc de positionner un capteur tous les 0.20m sur toute la largeur de la voie.Dans un second temps, nous proposons une méthode de ré-identification qui compare des paires de modèles magnétiques de véhicules. Ces modèles sont composés de plusieurs dipôles, et sont calculés à partir des signatures mesurées. La modélisation a pour but de s’affranchir du décalage latéral du véhicule, en remontant à la position relative du véhicule par rapport aux capteurs. Avec deux fois moins de capteurs que la méthode précédente, les résultats obtenus sur signaux réels sont également très bons, même s'ils sont un peu plus sensibles au changement d'orientation du véhicule. De plus, une simulation nous permet d'extrapoler qu'il est effectivement possible de s'affranchir du décalage latéral avec cette méthode. / Vehicle re-identification gives access to two essential data for dynamic traffic management: travel times and origin-destination matrices. In this thesis, we chose to re-identify vehicles by analysing their magnetic signatures measured with several 3-axis magnetic sensors located on the road. A magnetic signature is created by the vehicle magnetization. Therefore, the vehicle orientation to the Earth’s magnetic field (which determines the induced magnetization) and the variation of the lateral position of the vehicle relative to the sensors’ one might both have an impact on the magnetic signature. We gathered our experiments’ results into a database of magnetic signatures that we used to evaluate the performances of the two vehicle re-identification methods we developed.The first method is a direct comparison of pairs of magnetic signatures measured by the sensors. Distances between pairs of signatures are computed using classic algorithms such as the Euclidean distance. This method’s results are very positive and the vehicle change of orientation has only a slight impact on them. However, the distortion of signals due to a lateral offset in the vehicle position has a strong impact on the results. As a consequence, sensors have to be placed every 0.20m over the road’s entire width.The second proposed method compares pairs of vehicles’ magnetic models. Those models are composed of several magnetic dipoles and are determined from the measured signatures. Magnetic modelling aims to suppress the influence of the vehicle lateral position on the results by assessing the relative position of the vehicle above the sensors. Although the vehicle orientation has slightly more impact on the performances than with the first method, the overall results are more promising. This method also allows us to divide by two the number of sensors used.

La ré-identification de véhicules permet d’estimer deux paramètres clés en gestion dynamique de trafic : les temps de parcours et les matrices origine-destination. Dans cette thèse, nous avons choisi d'effectuer cette ré-identification par analyse des signatures magnétiques mesurées par des capteurs tri-axiaux placés sur la chaussée. La signature magnétique est générée par l'aimantation du véhicule : elle est alors susceptible de varier en fonction de l'orientation du véhicule par rapport au champ magnétique terrestre (à cause de l'aimantation induite dans le plan horizontal), et en fonction de sa position latérale relative par rapport aux capteurs. Les expérimentations que nous avons menées nous ont permis d'obtenir une base de données de signatures magnétiques, et ainsi d'évaluer les performances des deux méthodes de ré-identification que nous avons élaborées.La première méthode consiste à comparer directement des paires de signatures magnétiques mesurées par les capteurs. Les calculs de distances entre les paires sont effectués avec des algorithmes classiques comme la distance euclidienne. Les résultats obtenus sont très bons, et baissent peu lorsque le véhicule change d'orientation. Toutefois, ils sont très sensibles à la déformation des signaux due au décalage latéral du véhicule, et nécessitent donc de positionner un capteur tous les 0.20m sur toute la largeur de la voie.Dans un second temps, nous proposons une méthode de ré-identification qui compare des paires de modèles magnétiques de véhicules. Ces modèles sont composés de plusieurs dipôles, et sont calculés à partir des signatures mesurées. La modélisation a pour but de s’affranchir du décalage latéral du véhicule, en remontant à la position relative du véhicule par rapport aux capteurs. Avec deux fois moins de capteurs que la méthode précédente, les résultats obtenus sur signaux réels sont également très bons, même s'ils sont un peu plus sensibles au changement d'orientation du véhicule. De plus, une simulation nous permet d'extrapoler qu'il est effectivement possible de s'affranchir du décalage latéral avec cette méthode. / Vehicle re-identification gives access to two essential data for dynamic traffic management: travel times and origin-destination matrices. In this thesis, we chose to re-identify vehicles by analysing their magnetic signatures measured with several 3-axis magnetic sensors located on the road. A magnetic signature is created by the vehicle magnetization. Therefore, the vehicle orientation to the Earth’s magnetic field (which determines the induced magnetization) and the variation of the lateral position of the vehicle relative to the sensors’ one might both have an impact on the magnetic signature. We gathered our experiments’ results into a database of magnetic signatures that we used to evaluate the performances of the two vehicle re-identification methods we developed.The first method is a direct comparison of pairs of magnetic signatures measured by the sensors. Distances between pairs of signatures are computed using classic algorithms such as the Euclidean distance. This method’s results are very positive and the vehicle change of orientation has only a slight impact on them. However, the distortion of signals due to a lateral offset in the vehicle position has a strong impact on the results. As a consequence, sensors have to be placed every 0.20m over the road’s entire width.The second proposed method compares pairs of vehicles’ magnetic models. Those models are composed of several magnetic dipoles and are determined from the measured signatures. Magnetic modelling aims to suppress the influence of the vehicle lateral position on the results by assessing the relative position of the vehicle above the sensors. Although the vehicle orientation has slightly more impact on the performances than with the first method, the overall results are more promising. This method also allows us to divide by two the number of sensors used.

The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the Ceará Plateau, as the largest one. This is a very complex and poorly known area, with very few available research data. The geophysical approach was used to achieve a better understanding, and to guide further surveys. Gravity and magnetic data from the Equant I Project, seismic published lines and previous studies were applied to make several models and analyses. The gravity and the magnetic anomaly sources seemed to be the same, related to a denser and magnetized basement. It is estimated the basement surface between 800 and 6000 m and the Mohorovičić discontinuity, which is about 22-23 km below the Ceará Plateau. It is also presented and discussed the position of the continent-ocean boundary, recognized at approximately 40 km from the continental shelf, that locate the plateau at the oceanic crust. The area of the transitional crust, with an extension of 40-50 km, represents the altered continental crust, formed during the rift phase, previous to the Atlantic Ocean. The lack of data gives some limitation to the analysis, and add some uncertainties in the results, which are discussed along this thesis. / A Margem Equatorial Brasileira é uma margem passiva transformante com longas zonas de fratura e diversos montes submarinos, incluindo o Platô do Ceará, o maior deles. Essa é uma área muito complexa e pouco conhecida, com poucos dados disponíveis. A abordagem geofísica foi utilizada para alcançar uma melhor compreensão da área e para guiar próximas pesquisas. Dados gravimétricos e magnéticos do Projeto Equant I, linhas sísmicas publicadas e estudos prévios foram aplicados para desenvolver os modelos e as análises. A fonte das anomalias gravimétricas e magnéticas pareceram ser as mesmas, relacionadas a um embasamento mais denso e magnetizado. É estimada a superfície do embasamento entre 800 e 6000 m e a descontinuidade de Mohorovičić, aproximadamente a 22-23 km abaixo do Platô do Ceará. Foi também apresentada e discutida a posição do limite crosta continental-oceânica, aproximadamente a 40 km da plataforma continental, colocando o platô na crosta oceânica. A área da crosta transicional, com uma extensão de 40-50 km, demarca uma crosta continental mais alterada, formada durante a fase rifte, anterior à abertura do Oceano Atlântico. A falta de dados limitou as análises, e certamente implicou em diversas incertezas nos resultados, que são discutidas durante esta dissertação.

The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the Ceará Plateau, as the largest one. This is a very complex and poorly known area, with very few available research data. The geophysical approach was used to achieve a better understanding, and to guide further surveys. Gravity and magnetic data from the Equant I Project, seismic published lines and previous studies were applied to make several models and analyses. The gravity and the magnetic anomaly sources seemed to be the same, related to a denser and magnetized basement. It is estimated the basement surface between 800 and 6000 m and the Mohorovičić discontinuity, which is about 22-23 km below the Ceará Plateau. It is also presented and discussed the position of the continent-ocean boundary, recognized at approximately 40 km from the continental shelf, that locate the plateau at the oceanic crust. The area of the transitional crust, with an extension of 40-50 km, represents the altered continental crust, formed during the rift phase, previous to the Atlantic Ocean. The lack of data gives some limitation to the analysis, and add some uncertainties in the results, which are discussed along this thesis. / A Margem Equatorial Brasileira é uma margem passiva transformante com longas zonas de fratura e diversos montes submarinos, incluindo o Platô do Ceará, o maior deles. Essa é uma área muito complexa e pouco conhecida, com poucos dados disponíveis. A abordagem geofísica foi utilizada para alcançar uma melhor compreensão da área e para guiar próximas pesquisas. Dados gravimétricos e magnéticos do Projeto Equant I, linhas sísmicas publicadas e estudos prévios foram aplicados para desenvolver os modelos e as análises. A fonte das anomalias gravimétricas e magnéticas pareceram ser as mesmas, relacionadas a um embasamento mais denso e magnetizado. É estimada a superfície do embasamento entre 800 e 6000 m e a descontinuidade de Mohorovičić, aproximadamente a 22-23 km abaixo do Platô do Ceará. Foi também apresentada e discutida a posição do limite crosta continental-oceânica, aproximadamente a 40 km da plataforma continental, colocando o platô na crosta oceânica. A área da crosta transicional, com uma extensão de 40-50 km, demarca uma crosta continental mais alterada, formada durante a fase rifte, anterior à abertura do Oceano Atlântico. A falta de dados limitou as análises, e certamente implicou em diversas incertezas nos resultados, que são discutidas durante esta dissertação.