Academic literature on the topic 'Polarisation horizontale'

En terrain volcanique, les eaux de pluie s'infiltrent jusqu'à leur rencontre avec un niveau imperméable qui correspond le plus souvent au socle cristallin. Ce sont les talwegs et les lignes de crête des paléo- reliefs de ce socle dont la profondeur peut dépasser la centaine de mètres qu'il convient de détecter, parfois avec une précision décamétrique. La méthode géophysique la plus utilisée en hydrogéologie des terrains volcaniques est la prospection électrique qui fournit des coupes verticales des résistivités électriques. La morphologie du substratum imperméable ou saturé peut aussi être obtenue en mesurant en surface les potentiels électriques de polarisation spontanée (en abrégé PS) qui se forment par la percolation de l'eau infiltrée dans le terrain poreux. La base de la zone non saturée, appelée surface SPS, est calculée par une relation faisant intervenir les données PS, les altitudes et deux coefficients définis à partir des données géologiques. Cette surface indique directement les axes de circulation et les lignes de partage des eaux. Deux exemples pris sur des sites bien documentés montrent la validité de la méthode pour localiser les axes de circulation de l'eau souterraine et les limites entre bassins versants. Un troisième exemple montre les résultats PS comparés à ceux des méthodes électromagnétiques VLF et AMT. La méthode PS est légère et offre une bonne précision horizontale, mais elle demande au moins un forage d'étalonnage pour préciser la profondeur des interfaces.

RÉSUMÉ Les plateaux palsiques étudiés sont localisés dans une vallée près de Kangiqsualujjuaq, le long de l'estuaire du George, à quelque 15 km du littoral de la baie d'Ungava. Le tond de la vallée a émergé des eaux de la mer D'Iberville et a été occupé par un lac avant 5400 BP. Une tourbière a commencé à se développer et à progresser au détriment du lac il y a environ 4500-4300 BP. L'aggradation du pergélisol près du lac actuel, au centre de la vallée, a eu lieu vers 1800-1600 BP; le soulèvement gélival a été plus tardif en bordure du plateau palsique, soit vers 800 BP. Selon les méthodes géophysiques employées en surface et dans les trous de forage (résistivité électrique, polarisation provoquée, polarisation spontanée et électromagnétisme), l'épaisseur du mollisol varie de 30 à 80 cm et celle du pergéiisol, de 3,5 m à plus de 15 m. On note également la présence de taliks sous les dépressions topographiques qui percent le plateau. La méthode de calométrie in situ et les mesures de la teneur en eau avec une sonde à neutron ont permis de déterminer la composition du mollisol et du pergéiisol. L'utilisation de toutes ces méthodes sur le même site donne une connaissance précise des propriétés physiques du milieu et une bonne perception de l'extension verticale et horizontale du pergéiisol. Le concept de morcellement des plateaux palsiques en palses isolées le long de « lignes de fusion » dans le pergéiisol est appuyé par les résultats des méthodes d'investigation électriques.

Abstract. Microwave limb sounding in the presence of ice clouds was studied by detailed simulations, where clouds and other atmospheric variables varied in three dimensions and the full polarisation state was considered. Scattering particles were assumed to be horizontally aligned oblate spheroids with a size distribution parameterized in terms of temperature and ice water content. A general finding was that particle absorption is significant for limb sounding, which is in contrast to the down-looking case, where it is usually insignificant. Another general finding was that single scattering can be assumed for cloud optical paths below about 0.1, which is thus an important threshold with respect to the complexity and accuracy of retrieval algorithms. The representation of particle sizes during the retrieval is also discussed. Concerning polarisation, specific findings were as follows: Firstly, no significant degree of circular polarisation was found for the considered particle type. Secondly, for the ±45° polarisation components, differences of up to 4 K in brightness temperature were found, but differences were much smaller when single scattering conditions applied. Thirdly, the vertically polarised component has the smallest cloud extinction. An important goal of the study was to derive recommendations for future limb sounding instruments, particularly concerning their polarisation setup. If ice water content is among the retrieval targets (and not just trace gas mixing ratios), then the simulations show that it should be best to observe any of the ±45° and circularly polarised components. These pairs of orthogonal components also make it easier to combine information measured from different positions and with different polarisations.

Abstract. We have developed a new, high time-resolution, microwave heterodyne spectrometer for observations of water vapour in the middle atmosphere. It measures the rotational transition of water vapour at 22.235 GHz in the vertical and horizontal polarisation. The two polarisations are averaged in order to optimise the signal-to-noise ratio. The different polarisations have separate, but identical, signal chains consisting of a 22 GHz cooled HEMT amplifier, a second, warm, 22 GHz HEMT booster amplifier, an IF stage and a Chirp Transform Spectrometer (CTS) backend. Continuous calibration with two internal loads kept at temperatures close to the observed atmosphere, a wobbling optical table to reduce standing waves in the optical path and the low receiver temperature ensures a time resolution of an order of magnitude better than what has been achieved by earlier instruments. The error sources in the retrieved spectrum are discussed and the data is compared and validated against EOS-MLS on the NASA Aura satellite. The profiles are found to be in good agreement with each other.

Abstract. Detailed simulations of microwave limb sounding in the presence of ice clouds have been performed. It is clarified that, while particle absorption normally gives no significant change of the measured radiance for down-looking measurements, this is not the case for limb sounding. The particles were treated as horizontally aligned oblate spheroids and for this assumption on particle shape, and comparable situations, no significant degree of circular polarisation is generated. Differences between the brightness temperature of the ±45° polarisation components up to 4 K were found, but this difference appears to be small as long as single scattering conditions apply. The cloud extinction is the smallest for the vertically polarised component, but it should be more beneficial to observe any of the ±45° and circularly polarised components if ice water content is also a target of the retrievals. These latter pairs of orthogonal components also make it easier to combine information measured from different positions and with different polarisations. The results indicate that single scattering can be assumed for cloud optical thicknesses below about 0.1, which is thus an important threshold with respect to the complexity and accuracy of retrievals. The representation of particle sizes during the retrieval is discussed.

Abstract. Sea ice melt pond fraction (fp), linked with lower sea ice surface albedo and increased light transmittance to the ocean, is inadequately parameterised in sea ice models due to a lack of observations. In this paper, results from a multi-scale remote-sensing program dedicated to the retrieval of level first-year sea ice (FYI) fp from dual co- and cross-polarisation C-band synthetic aperture radar (SAR) backscatter are detailed. Models which utilise the dominant effect of free-water melt ponds on the VV / HH (vertical transmit and vertical receive / horizontal transmit and horizontal receive) polarisation ratio at high incidence angles are tested for their ability to provide estimates of the subscale fp. Retrieved fp from noise-corrected Radarsat-2 quad-polarisation scenes are in good agreement with observations from coincident aerial survey data, with root mean square errors (RMSEs) of 0.05–0.07 obtained during intermediate and late stages of ponding. Weak model performance is attributed to the presence of wet snow and slush during initial ponding, and a synoptically driven freezing event causing ice lids to form on ponds. The HV / HH (horizontal transmit and vertical receive / horizontal transmit and horizontal receive) ratio explains a greater portion of variability in fp, compared to VV / HH, when ice lids are present. Generally low HV channel intensity suggests limited applications using dual cross-polarisation data, except with systems that have exceptionally low noise floors. Results demonstrate the overall potential of dual-polarisation SAR for standalone or complementary observations of fp for process-scale studies and improvements to model parameterisations.

Abstract. Understanding the evolution of melt ponds on Arctic sea ice is important for climate model parameterisations, weather forecast models and process studies involving mass, energy and biogeochemical exchanges across the ocean–sea ice–atmosphere interface. A field campaign was conducted in a region of level first-year sea ice (FYI) in the central Canadian Arctic Archipelago (CAA), during the summer of 2012, to examine the potential for estimating melt pond fraction (fp) from satellite synthetic aperture radar (SAR). In this study, 5.5 GHz (C-band) dual co- (HH + VV – horizontal transmit and horizontal receive + vertical transmit and vertical receive) and cross-polarisation (HV + HH – horizontal transmit and vertical receive + horizontal transmit and horizontal receive) radar scatterometer measurements of melt-pond-covered FYI are combined with ice and pond properties to analyse the effects of in situ physical and morphological changes on backscatter parameters. Surface roughness statistics of ice and ponds are characterised and compared to the validity domains of the Bragg and integral equation model (IEM) scattering models. Experimental and model results are used to outline the potential and limitations of the co-polarisation ratio (VV / HH) for retrieving melt pond information, including fp, at large incidence angles (≥35°). Despite high variability in cross-polarisation ratio (HV / HH) magnitudes, increases at small incidence angles (<30°) are attributed to the formation of ice lids on ponds. Implications of the results for pond information retrievals from satellite C-, L- and P-band SARs are discussed.

Spectral and polarisation sensitivity were compared among juvenile (parr) rainbow trout (Oncorhynchus mykiss), steelhead (O. mykiss), cutthroat trout (O. clarki clarki), kokanee (O. nerka) and brook char (Salvelinus fontinalis) using multi-unit recording from the optic nerve. Although reared under the same conditions, differences in photopic spectral sensitivity were evident. Specifically, ON-responses were co-dominated by L- and M-cone mechanisms in all fish except O. nerka, consistent with an M-cone mechanism sensitivity. The sensitivity of OFF-responses was dominated by the M-cone mechanism for all fish, but O. mykiss appeared to show an additional contribution from the L-cone mechanism. Using chromatic adaptation, an independent ultraviolet-sensitive mechanism is described for the first time for the salmonid genus Salvelinus. In addition, this ultraviolet-cone mechanism was present in the members of the genus Oncorhynchus that were examined. Thus, ultraviolet sensitivity appears to be common to the major extant clades of the subfamily Salmoninae. All species showed differential sensitivity to both vertical and horizontal linearly polarised light. This sensitivity differed between ON- and OFF-responses. The ON-responses were maximally sensitive to both vertically and horizontally polarised light, whereas the OFF-responses displayed maximal sensitivity to horizontally polarised light in all species, with reduced sensitivity to vertically polarised light compared with ON-responses. Because of the similarity in the physiological characteristics of polarisation sensitivity among the salmonid species examined, no relationship between the degree of migratory tendency and the ability to detect polarised light could be identified.

SUMMARYButterflies of the genus Papilio have polarisation-sensitive photoreceptors in all regions of the eye, and different spectral types of receptor are sensitive to different e-vector orientations. We have studied the consequences of this eye design for colour vision in behavioural tests and find that Papilio spp. see false colours due to the polarisation of light. They discriminate between vertically and horizontally polarised light of the same colour in the contexts of oviposition and feeding. The discrimination depends on the spectral composition of the stimuli. In the blue and probably in the green range, discrimination does not depend on intensity. However, colour discrimination is influenced by polarisation. Thus, colour and polarisation processing are not separated in the visual system of Papilio spp. From these results, we propose hypotheses about which photoreceptors contribute to colour vision in Papilio spp. and what adaptational value such a system might have for the butterflies. Finally, we give examples for other eyes that have a similar structure.

Abstract. The microwave interferometric radiometer of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission measures at a frequency of 1.4 GHz in the L-band. In contrast to other microwave satellites, low frequency measurements in L-band have a large penetration depth in sea ice and thus contain information on the ice thickness. Previous ice thickness retrievals have neglected a snow layer on top of the ice. Here, we implement a snow layer in our emission model and investigate how snow influences L-band brightness temperatures and whether it is possible to retrieve snow thickness over thick Arctic sea ice from SMOS data. We find that the brightness temperatures above snow-covered sea ice are higher than above bare sea ice and that horizontal polarisation is more affected by the snow layer than vertical polarisation. In accordance with our theoretical investigations, the root mean square deviation between simulated and observed horizontally polarised brightness temperatures decreases from 20.9 K to 4.7 K, when we include the snow layer in the simulations. Although dry snow is almost transparent in L-band, we find brightness temperatures to increase with increasing snow thickness under cold Arctic conditions. The brightness temperatures' dependence on snow thickness can be explained by the thermal insulation of snow and its dependence on the snow layer thickness. This temperature effect allows us to retrieve snow thickness over thick sea ice. For the best simulation scenario and snow thicknesses up to 35 cm, the average snow thickness retrieved from horizontally polarised SMOS brightness temperatures agrees within 0.1 cm with the average snow thickness measured during the IceBridge flight campaign in the Arctic in spring 2012. The corresponding root mean square deviation is 5.5 cm, and the coefficient of determination is r2 = 0.58.

L'objectif de ce travail est d'étudier et de réaliser de nouveaux concepts de systèmes antennaires à faible encombrement, permettant de mesurer des signaux de polarisations diverses pour des applications de radiogoniométrie dans les bandes V/UHF. Une première partie a été consacrée à l'étude théorique des performances de radiogoniométrie de différents systèmes antennaires à partir de la borne de Cramer-Rao « réaliste » des directions d'arrivées de signaux. Le deuxième point majeur de cette étude concerne les capteurs élémentaires dont le choix a été fortement influencé par l'application finale du système : la radiogoniométrie sur porteur mobile. Ces antennes ont ensuite été associées en réseau de manière à obtenir une couverture radioélectrique omnidirectionnelle. Une troisième partie a été consacrée à l'installation, à la mise en œuvre et à l'estimation des performances des deux systèmes antennaires dans leur environnement réel, à savoir placés sur le toit d'un véhicule
The goal of this work is to study and realize new small antenna systems to measure diversely polarized signals for radio direction finding applications in the V/UHF bands. The first part was focused on the theoretical study of radio direction finding performances of two différent systems using the "realistic" Cramer-Rao lower bound of the signal's directions of arrival. The second point of the study concern the antenna's choice and design, influence by the final application: radio direction finding on vehicle. Then, these antennas have been associated to realize a array with an omnidirectionnal coverage. The final part of this work concerns the antenna installation and the radio direction finding performances in real environment : on the roof of a vehicle

Cette thèse, menée en collaboration avec le groupe RADIALL, est consacrée à l’étude d’antennes de station de base conçues à partir de matériaux à bande interdite électromagnétique (BIE) présentant un diagramme de rayonnement omnidirectionnel en azimut et à fort gain. Les structures envisagées sont au nombre de deux : l’antenne cylindrique et l’antenne coaxiale possédant une âme centrale métallique. Après avoir présenté les sources d’excitation ainsi que les matériaux BIE diélectriques et métalliques, des abaques ont été proposés pour faciliter la conception. Des techniques d’amélioration des performances ont permis d’augmenter la directivité, d’élargir la bande passante en rayonnement et enfin d’adapter les antennes. Deux prototypes d’antenne ont été réalisés. Les performances obtenues ont alors été comparées à une technologie concurrente : les réseaux de dipôles. Enfin, d’autres polarisations d’antennes omnidirectionnelles ont été étudiées. Une antenne en polarisation horizontale a été réalisée et a permis de confirmer avec succès le fonctionnement théorique. La double polarisation et la polarisation circulaire ont simplement été simulées compte tenu de la complexité de la structure
This thesis, managed with the RADIALL group, is dedicated to the study of base station antennas conceived with electromagnetic band gap (EBG) materials. These structures boast an omnidirectional radiation pattern in the azimuth plane with a high gain. Two structures have been considered: the cylindrical antenna and the coaxial antenna with a metallic center core. After having presented the sources of excitation as well as dielectric and metallic EBG materials, abacuses have been purposed to facilitate the design. Performances improvement techniques made it possible to increase the directivity, to widen the radiation bandwidth and finally to match the antennas. Two antenna prototypes have been realized. The experimental performances have then been compared to a competitive technology: dipole arrays. Lastly, other polarizations have been studied. An antenna in horizontal polarization has been realized which measurement successfully confirmed the theoretical functioning. Double polarization and circular polarization have only been simulated due to the structure complexity

Cette thèse contribue à l'analyse quantitative des fusions horizontales (concentrations). Elle propose des outils quantitatifs innovateurs pour étudier les gains des fusions, les déterminants de leurs gains de synergie, et la puissance prédictive des tests de pouvoir de marché utilisés par des autorités de concurrence pour bloquer ou autoriser une concentration. Dans le premier chapitre, nous analysons une large liste de différents motifs de fusions-acquisitions qui ont été proposés dans les littératures d’organisation industrielle et de la gouvernance de l’entreprise. Nous les classons selon les bénéficiaires de l’opération, à savoir les propriétaires ou les dirigeants des sociétés qui fusionnent. Nous analysons également les différentes méthodes empiriques qui ont été proposées pour étudier les motifs, les gains et les effets des fusions. Le deuxième chapitre propose un outil analytique et empirique pour examiner les sources de gains d'efficacité des fusions. Pour ceci, nous distinguons explicitement les gains d'efficacité technique des synergies, les premiers étant ceux qui peuvent être obtenus par d'autres moyens que la fusion et les seconds étant spécifiques à la fusion. Nous trouvons que les gains de synergie sont associés à la réorganisation des biens intangibles. Dans le troisième chapitre, nous proposons une méthodologie pour estimer les gains des fusions, tout en contestant le paradoxe de la fusion (échec de fusions) et les problèmes d'identification rencontrés dans des méthodes sous forme réduite. Avec une base de données contenant les caractéristiques des firmes qui fusionnent et les prix payés pour la firme acquise, nous mettons en œuvre une approche structurale pour estimer les gains des acquéreurs en interprétant une fusion comme une enchère. Nous estimons donc les valeurs privées que les enchérisseurs attribuent aux cibles ainsi que leurs rentes informationnelles. Nous trouvons des gains positifs et significatifs et nous confortons l'hypothèse de synergie, à savoir que les gains sont déterminés par la complémentarité des actifs spécifiques aux firmes fusionnées. Dans le quatrième chapitre nous étudions l'exactitude individuelle et relative de deux tests de pouvoir de marché: l'Index de Herfindahl Hirschman (l'lHH) et le modèle de simulation des effets unilatéraux. Nous proposons une méthodologie pour comparer leur puissance prédictive en les appliquant dans une économie hypothétique. Nous constatons que, quand rapproche économétrique structural manque (le test des effets unilatéraux), l'approche de concentration (l' IHH) est biaisée vers le haut. En outre, le test des effets unilatéraux est influencé par la taille du marché, ou plus précisément par la taille du marché du bien extérieur. En particulier, quand celle-ci est grande, une décision basée sur l’lHH n'aurait rien à voir avec une décision basée sur des effets unilatéraux. En général, le test des effets unilatéraux est plus performant, en capturant la vraie situation du marché, que le test de l’IHH
This PhD thesis aims at contributing to the empirical analysis of horizontal mergers. It proposes innovative quantitative tools to investigate for: the source of synergy gains in mergers, merger gains and the predictive power of the post-merger market power tests employed by competition agencies to block or clear a merger. In the first chapter we analyze and summarize a broad list of different merger motives that has been proposed in the literature. We propose a categorization of such motives based on the residual claimant of the mergers gains, namely the owners or the managers of the merging firms, and on welfare effects. We also review the different empirical methods that have been proposed to investigate for merger motives, gains and effects. Based on the dissatisfaction of the existing empirical methods and the lack of evidence about merger efficiencies, the second chapter proposes an analytical and empirical tool to investigate for the sources of efficiencies in mergers. For this, we explicitly distinguish technical efficiencies from synergies where the former are those that can be obtained by other means than merger and the latter are merger-specific. We find evidence that synergetic gains are associated with the reorganization of the non-tradable intangible assets. In the third chapter, we propose a methodology for accurately estimating merger gains and solve for the merger paradox (mergers fail) and the misspecification problems encountered in previously proposed reduced-form methods. With a database rich in merging firms' characteristics and final bids we implement a structural approach to infer acquirers' gains by interpreting a merger as an auction. Using nonparametric methods, we estimate bidders - private values for targets and thus informational rents. We provide evidence of significant and positive merger gains as well as support for the synergy hypothesis as the merger rational. In the fourth chapter we investigate for the individual and relative accuracy of two major market power tests: the Helfindahl Hirschman Index (HHI), and the merger simulation model of unilateral effects. We propose a methodology to compare the predictive and screening power of both tests by implementing them in a hypothetical economy. We find that when a structural econometric approach is missed (substantive test) the concentration approach (dominance test) is biased upwards. We also find that the substantive test is influenced by the market size or more precisely the market size of the outside good. In particular, when it is large a decision based on the HHI test would have nothing to do with a decision based on the unilateral effects test. Still, in the overall, the substantive test performs better in capturing the true situation of the market compared to the dominance test of concentration