Showing papers by "University of Rennes published in 1987"

Application of spheroidal sequences to array processing

Abstract: Eigenvector methods for narrow-band source location are based upon the key assumption that the spatial correlation matrix of the noise field is known. Unfortunately, these methods are very sensitive to errors upon it. This paper describes a method which performs a spatial filtering using discrete spheroidal sequences, before applying eigenvector methods. Thus, narrow spatial sectors are selected in which the noise field is easily modeled. In case of large arrays, the proposed method yields resolution properties far superior to the MUSIC technique with known noise correlation.

Site selection spectroscopy in Eu3+-doped fluorozirconate glass

Abstract: Optical absorption and emission spectra are presented for Eu3+ ions in fluorozirconate (ZBL) glass at room temperature. The energy levels, responsible for the various transitions, are assigned. The point symmetry of the Eu3+ probes is investigated using site selection spectroscopy techniques. Emission and excitation spectra at 4.4 K are presented and two major types of sites are observed. Comparison of the 5D0 → 7F1 and 5D0 → 7F2 intensities suggests that the Eu3+ ion sites have a high symmetry. This is confirmed by decay-time measurements which have been carried out for both glassy and crystalline hosts. Agreement with previous structural results is discussed and a model is proposed.

Crustal accretion and metamorphism in Taiwan, a post-Palaeozoic mobile belt*

Abstract: Taiwan consists chiefly of Permo-Mesozoic basement unconformably overlain by Cainozoic cover strata; on the east, this complex is juxtaposed along the Longitudinal Valley against the Neogene Coastal Range, the northern extension of the Luzon calc-alkaline arc and intervening fore-arc basin. Accumulation of well-ordered sandstones, shales, limestones and intercalated basaltic units, and outboard argillite melange along the eastern margin of Asia in Permian and early to mid-Mesozoic time was terminated by a major late Cretaceous (85—90 Ma) dynamothermal event that produced the composite basement complex of Taiwan, and also affected much of southeastern China. This basement consists of a westerly Tailuko belt, separated from the easterly Yuli belt by a major fault. The miogeoclinal Tailuko belt is characterized by chloride and biotitic greenschist facies low-pressure assemblages, except in the north where amphibolite facies assemblages are associated with emplacement of remobilized granitic rocks; metamorphic grade increases gradually eastward. The eugeoclinal Yuli belt lacks marble layers and granitic intrusions, and instead contains serpentinite bodies and associated rare highpressure epidote-bearing barroisitic amphibolite tectonic lenses. Tailuko quartzofeldspathic rocks contain mineralogic and geochronologic evidence of Palaeogene and Neogene reheating. Mafic tectonic blocks in the Yuli belt show partial conversion to glaucophanic assemblages; radiometric ages for the blueschist metamorphism are 8-14 Ma. The Cainozoic slate sequence was deposited on the basement complex following renewed Palaeogene rifting. It consists largely of sedimentary strata (and intercalated basalts) laid down on the Asiatic passive margin and seaward in the South China Sea as continental slope deposits. An accretionary wedge was constructed adjacent to the approaching Neogene Luzon arc, marking the non-subducted western margin of the Philippine Sea plate. During Plio-Pleistocene collision of the Luzon arc with the Chinese continental margin, the landward Cainozoic shelf and slope units were imbricated and thrust westward; increased pressure ( P ) and temperature ( T ) during this loading evidently promoted recrystallization of the basement and passive margin cover. Metamorphism ranged from diagenetic and zeolite facies in the Western Foothills to upper greenschist and locally amphibolite facies in the basement complex. No metamorphic hiatus between Mesozoic basement and Cainozoic cover is recognized. The accretionary terrane lying east of the Longitudinal Valley is virtually unmetamorphosed. However, the East Taiwan Ophiolite, occurring as clastic debris and slide blocks in the unmetamorphosed olistostromal, largely Pliocene, Lichi Melange of the Coastal Range fore-arc basin, carries the effects of a mid-Miocene oceanic-ridge recrystallization of actinolite hornfels facies, overprinted by a late Miocene ocean-floor zeolitization. Large portions of the sialic crust making up Taiwan evidently formed approximately in situ , then were deformed and thrust landward during subsequent tectonic events, but far-travelled terranes and allochthonous fragments of oceanic material played an important role in accretion. Recognized and suspected exotics include. (1) Lower or mid-Mesozoic meta-ophiolites, now greenstones, amphibolites and minor serpentinites, previously obducted into the Upper Mesozoic Tailuko belt; amphibolites also occur as enclaves in granitic intrusives 85—90 Ma old; (2) the entire Upper Mesozoic Yuli belt; (3) Mio-Pliocene tectonic blocks of blueschistic meta-ophiolite, emplaced in the east-central part of the Yuli terrane; (4) olistostromal ophiolitic debris (preexisting Miocene oceanic crust of the South China Sea) within the Pliocene Lichi Melange of the Coastal Range; and (5) the Neogene calc-alkaline Luzon arc, which began to collide with Asiatic continental crust in Plio-Pleistocene time. The Cainozoic slate series represents an additional, parautochthonous terrane, which was deposited as the Tertiary miogeoclinal cover along the Asiatic passive margin, and was thrust westward during Plio-Pleistocene arc collision. Among the suspected allochthonous units, Mesozoic amphibolites of both Tailuko and Yuli belts have been identified as of chiefly normal more affinites from their chemical and Nd and Sr isotopic compositions. The clearly oceanic East Taiwan Ophiolite apparently formed along a transform-interrupted spreading centre of the South China Sea marginal basin, based on mineralogic, chemical and isotopic evidence.

Macrostructure and Diamagnetic Response of YBa2Cu3O7-x

Abstract: The experimental low-frequency magnetic response of Y-Ba-Cu-O sintered samples is astonishingly similar to that of model systems made of weakly coupled classical superconducting grains. A quantitative description of this behaviour in the framework of the coherence transition is given. It allows the extraction of important physical information, like the penetration depth λ(T), and provides explanations for the observed low-field sensitivity and temperature dependence of λ.

Uniaxial Compression of 2d Packings of Cylinders. Effects of Weak Disorder

Abstract: We study uniaxial compression of 2d-ordered packings of parallel cylinders, which present weak geometrical defects responsible for a heterogeneity of the spatial distribution of intergranular stresses. The strongest stresses form links which generate an irregular network. By creating different types of vacancies, we show the importance of that network.

A structural model for prototypical fluorozirconate glass

Abstract: Existing interpretations of structural data and computer simulation results for binary BaF2-ZrF4 glasses are reviewed. Detailed comparisons of X-ray scattering results and new simulation results are made to determine the important structural features that a suitable model should be able to reproduce. A bipolyhedral module-based model for the glass which satisfies these constraints is then described and proposed as an idealization of the BaF2-2ZrF4 glass structure. Generalizations to other compositions and densities are considered. Key relationships to, and differences from, the bipolyhedral Zr2F124− module-based layer lattice recently determined for β-BaZr2 F10 are examined. The model is used as a basis for discussion of optical properties and crystallization kinetics.

TRM deviations in anisotropic assemblages of multidomain magnetite

Abstract: Anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility (AMS), which in turn induces deviations of thermo-remanent magnetization (TRM) from the field direction. From the theories of multidomain TRM acquisition, it is shown that the TRM anisotropy tensor has its eigenvalue ratios (Ti) related to the principal weak-field susceptibility ratios (Pi) by the order of magnitude Ti≃P2i. This relation has been experimentally verified on two sets of highly anisotropic rock samples. The exponent has been determined to be 1.94 in the samples from a Peruvian gabbro, and 1.81 in those from the granite of Flamanville (NW France). Accounting for experimental difficulties in determining the TRM anisotropy tensors, these exponents are judged to agree well with the expected one. It is therefore stressed that AMS measurements provide a good means of evaluating the magnetic field direction from deviated TRM directions, providing magnetic carriers are mainly multidomain magnetites.

Unstraining paleomagnetic vectors: The current state of debate

Abstract: One of the main goals of paleomagnetic studies is to produce evidence for ancient continent displacements and thus produce paleogeographical reconstructions of the earth in the geological past. To achieve this goal, paleomagnetists use the direction of remanent vectors as measured in rocks to derive latitudinal and azimuthal data, according to the model of the Earth magnetic field (EMF). One of the many difficulties faced by these researchers is the determination of whether the rock deformation may have altered the original paleomagnetic direction. If so, the paleomagnetists must correct the effect of such a deformation. Obviously, this aspect is of secondary importance in cratonic areas where beds lie flat or are gently folded, and in earlier times, most paleomagnetic studies were conducted in such areas. However, it now appears that we cannot avoid work within orogenic belts if we want to resolve geodynamic problems [Van der Voo and Channel, 1980]. It therefore became necessary to examine more carefully the interaction between magnetizations and deformation. During the last decade, several teams have conducted research efforts in this direction.

Line shape in the low-frequency wing of self-broadened CO 2 lines

Abstract: The shape of the far wings of self-broadened CO2 lines has been investigated in the spectral region (2140–2250 cm−1), i.e., on the low wavenumber side of the lines of the most intense band (ν3 of 12C16O2). The experimental results have been compared to calculated values based on the AFGL 1982 compilation. From this comparison it appears that the wings of the lines on the low wavenumber side of the centers decay much more rapidly than do Lorentz lines, by about the same amount as has been previously shown for the high wavenumber side. However, an asymmetrical correcting line shape factor is needed to obtain good agreement between experimental and synthetic spectra.

The effects of demagnetizing fields on thermoremanent magnetization acquired by parallel‐sided baked clay blocks

Abstract: Summary. The archaeomagnetic study of displaced objects allows dating of homogeneous groups of tiles or bricks generally recovered as fragments during archaeological excavations or fieldwalking surveys. Its principle is based on the property of baked clay in acquiring. during cooling after firing. a thermorenianent magnetization (TRM) parallel to the terrestrial magnetic field (TMF) present at the time and place of cooling. In taking together certain hypotheses verified u posteriori on the positions of the objects during firing. nieasurement of the direction of the TRM permits the inclination of the TMF to be obtained, and thus the date of firing through the use of reference curves of secular variation. However, theoretical study of the properties of the magnetic induction field shows that this parallelism may be disturbed by the action of a demagnetizing field. known as the magnetic refraction or shape effect, in the case of materials which are strongly magnetized and of unusually flat shape. Theoretical calculations made for parallelepipeds of various dimensions and susceptibility placed in a uniform magnetic field. allow quantification of the effects of this demagnetizing field and the construction of charts for correcting the effect. Thus. for a parallelepiped of relative dimensions of thickness/ length I / 15 and width/leiigth 1 1 /IS, placed upright perpendicular to the magnetic meridian of an exterior uniform field of 65" inclination. the theoretical difference at the centre of the object between the inclination of the TRM and that of the external field would be t 3" for a susceptibility of 0.2 SI. and + 6" for a susceptibility of 0.4.51. Some examples are given for groups of Roniano-Gallic tiles, so-called Tegulue. As it happens, this material presents very high voluniic susceptibilities which reach and sometimes exceed 0.3 SI. producing important disturbances in the direction of the TRM. Using the values of inclination and deviation measured in this material it is possible to obtain the initial direction of the TMF at the time of firing. The correction model proposed thus allows an

The Mycoplasma hyopneumoniae plasma membrane as a vaccine against porcine enzootic pneumonia

Abstract: Summary From an economical standpoint, the colonization of the respiratory epithelium by Mycoplasma hyopneumoniae, the mollicute responsible for enzootic pneumonia in pigs which leads to a reduction in the weight gain/feed ratio, has serious consequences in very young animals; we show here that its is possible to efficiently protect these animals with the colostral antibodies raised in sows immunized during their gestation with M. hyopneumoniae BQ14 membranes. In an experiments using 9 sows and 75 piglets, the vaccinal preparation was found to protect 70 to 100 % of the piglets (depending upon the presentation of the immunogen) as well as being totally innocuous. The best results were obtained with an adjuvant made of hydroxide aluminium gel. Of the piglets born to non-immunized sows, 8.3 % exhibited typical pneumonic lesions. The vaccination procedure was found to enhance the elimination of M. hyopneumoniae from the respiratory tract of the piglets, as in 95 % of the protected animals, the microorganisms could no longer be isolated. Conversely, M. hyopneumoniae was isolated from 89 % of the piglets born to unvaccinated sows. A correlation was found between the protective effect and the rise in the titre of circulating antibodies directed against M. hyopneumoniae surface antigens. These results show that the plasma membrane of M. hyopneumoniae is a potential vaccine against porcine enzootic pneumonia. In particular, passively transmitted immunity via the colostrum provides the most precocious means of protection for the animals.