The Jurassic Quiet Zone (JQZ) is a region of low amplitude, difficult-to-correlate magnetic anomalies located over Jurassic oceanic crust. We collected 1200 km of new deep-tow magnetic anomaly profiles over the Pacific JQZ that complement 2 deep-tow profiles reported in Sager et al. (1998). Our primary goals were to extend the correlation of deep-tow magnetic anomalies farther back in time, to evaluate the correlatability and repeatability of anomalies, and to refine the Jurassic geomagnetic polarity reversal time scale (GPTS). Correlations of anomalies were excellent over M34 and over supposedly older seafloor to the south of ODP Site 801. In contrast, the correlation in the region between M34 and Site 801 was difficult. Using anomaly correlation models, we made magnetic polarity block models to establish a revised Jurassic GPTS extending until 169.4 Ma. Age calibration was accomplished with radiometric dates from two ODP holes. Systematic changes in anomaly amplitudes occur along the survey lines with the amplitudes decreasing backward in time and then increasing again in the oldest part of survey area. The zone of the most difficult to correlate anomalies corresponds to a period of ~4 m.y. that appears to have an abrupt end. This low amplitude zone suggests unusual magnetic behavior during the Jurassic. It has been said that many of the larger anomalies are likely caused by changes in polarity, whereas smaller anomalies may be intensity fluctuations. Although it is impossible to identify which anomalies are caused by reversals and which are not, magnetization structures observed in ODP Hole 801C suggest that many of the smallest anomalies, particularly around Hole 801C indicate polarity reversals. We concluded that (1) the new data demonstrates repeatability and correlatability of the JQZ magnetic anomalies implying that they are seafloor spreading lineations and (2) good correlations made new GPTS models extending back to 169.4 Ma; and (3) the origin of the JQZ may be a combination of rapid polarity reversals in the Jurassic low magnetic dipole field and closely spaced, tilted magnetization structure in the oceanic crust.

The Jurassic Quiet Zone (JQZ) is a region of low amplitude, difficult-to-correlate magnetic anomalies located over Jurassic oceanic crust. We collected 1200 km of new deep-tow magnetic anomaly profiles over the Pacific JQZ that complement 2 deep-tow profiles reported in Sager et al. (1998). Our primary goals were to extend the correlation of deep-tow magnetic anomalies farther back in time, to evaluate the correlatability and repeatability of anomalies, and to refine the Jurassic geomagnetic polarity reversal time scale (GPTS). Correlations of anomalies were excellent over M34 and over supposedly older seafloor to the south of ODP Site 801. In contrast, the correlation in the region between M34 and Site 801 was difficult. Using anomaly correlation models, we made magnetic polarity block models to establish a revised Jurassic GPTS extending until 169.4 Ma. Age calibration was accomplished with radiometric dates from two ODP holes. Systematic changes in anomaly amplitudes occur along the survey lines with the amplitudes decreasing backward in time and then increasing again in the oldest part of survey area. The zone of the most difficult to correlate anomalies corresponds to a period of ~4 m.y. that appears to have an abrupt end. This low amplitude zone suggests unusual magnetic behavior during the Jurassic. It has been said that many of the larger anomalies are likely caused by changes in polarity, whereas smaller anomalies may be intensity fluctuations. Although it is impossible to identify which anomalies are caused by reversals and which are not, magnetization structures observed in ODP Hole 801C suggest that many of the smallest anomalies, particularly around Hole 801C indicate polarity reversals. We concluded that (1) the new data demonstrates repeatability and correlatability of the JQZ magnetic anomalies implying that they are seafloor spreading lineations and (2) good correlations made new GPTS models extending back to 169.4 Ma; and (3) the origin of the JQZ may be a combination of rapid polarity reversals in the Jurassic low magnetic dipole field and closely spaced, tilted magnetization structure in the oceanic crust.

La thèse porte sur la génération de fronts d’ondes de test en milieu réverbérant pour la mesure d’antennes Ultra Large Bande. L’objectif est de générer des fronts d’ondes déterministes convergents, respectant les standards de mesure d'antennes, dans un milieu diffusif caractérisé par une décohérence spatiale, temporelle et fréquentielle. Les performances intéressantes, visées pour l’application, sont la génération temps réel, l’utilisation d’un nombre limité de sources et l’obtention d’un rendement de conversion d’énergie élevé. Tout d’abord, nous présentons des concepts de mesure d’antennes et montrons, les limites des méthodes actuelles pour la réalisation de caractérisations rapides permettant un niveau de rapport signal à bruit satisfaisant et n'utilisant pas de sources multiples. Puis, nous introduisons notre moyen d’essai, le système Time reversal Electromagnetic Chamber (TREC), constitué notamment d’une chambre réverbérante, dans laquelle un principe non standard de retournement temporel est utilisé pour générer des fronts d’ondes cohérents et convergents d’espace libre. Nous cherchons alors à étendre les capacités de la TREC à la génération des fronts d’ondes de test convergents et localement plans. Dans une phase d'étude préalable, nous développons deux approches pour synthétiser la propagation en espace libre des fronts d’ondes de mesure. Les techniques utilisées reposent sur l’utilisation de l’opérateur Slepian, permettant la résolution des contraintes imposées par les propriétés spécifiques des distributions de champ. La solution optimale de caractérisation a permis de générer des fronts d'ondes comportant une résolution angulaire. La synthèse des fronts d'ondes de test est utilisée dans des simulations électromagnétiques de TREC 2D et 3D. Les résultats ont validé la faisabilité de la génération de fronts d’ondes convergents, localement plans et résolus angulairement. / This contribution corresponds to the generation of test wavefronts in reverberation chamber for antenna Ultra Wide Band characterization. We were interested in generating deterministic convergent wavefronts, fulfilling the antenna measurement standard, in a diffusive medium characterized by a spatial, temporal and frequency decoherence. The interesting performances referred to the application, are the real-time synthesis, the use of a limited number of sources and the high energy conversion efficiency. First we present the concepts of antenna measurement and the limitations of current methods for achieving quick characterization with a sufficient signal to noise ratio without multiple sources. Then, we introduce our system, the Time Reversal Electromagnetic Chamber (TREC), which is mainly constituted by a reverberation chamber, in which a non-standard time reversal principle is used for coherent convergent free space wavefronts generation. Then we try to extend the TREC capacities to the generation of free space convergent and locally plan wavefronts. In a preliminary study, we develop two approaches to synthesize the free space propagation of the wavefronts. The used techniques are based on the utilization of the operator Slepian, allowing the resolution of the constraints determined by the specific properties of the field distribution. The optimal solution allowed generating wavefronts characterized by angular resolution. Test wavefronts synthesis is used in 2D and 3D TREC electromagnetic simulations. Results validated the feasibility of the generation of locally plan convergent wavefronts with angular resolution.