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71.
Marco Nardini Gabriele Ghisellini Giancarlo Ghirlanda 《Monthly notices of the Royal Astronomical Society》2008,383(3):1049-1057
The luminosities of the optical afterglows of gamma-ray bursts, 12 h (rest-frame time) after the trigger, show a surprising clustering, with a minority of events being at a significantly smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre- Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic burst is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflect a real bimodality: either the optical luminosity distribution of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength-independent extinction. This population of intrinsically weak or grey-absorbed events can be associated with dark bursts. 相似文献
72.
A numerical estimation is presented on the effects induced in an existing tunnel by the development of a fault from the deep
bedrock during a seismic event. The spreading of the fault within the alluvial deposit hosting the tunnel, and the consequent
effects on its permanent liner, are studied in static conditions through a series of elastic-plastic, plane strain finite
element analyses. They account for the reduction of the shear strength and stiffness characteristics of the faulting zone
with increasing irreversible strains. Even though the calculations require only “standard”, e.g. peak and residual, material
parameters it is shown that these properties can hardly be obtained for the alluvial deposit at hand. To overcome this drawback
a relatively large scale in-situ tests could be performed and its results could be interpreted through a suitable back analysis.
This would permit characterizing the numerical model to be subsequently adopted for the analysis of the faulting process. 相似文献
73.
A finite element analysis is presented of some direct shear tests carried out on samples of a stiff, heavily overconsolidated clay. The effects of softening are accounted for through a simple material model which views this phenomenon as a consequence of an intrinsic loss of strength of the material taking place when plastic strains increase. After illustrating the main characteristics of the finite element approach, with particular reference to the correct determination of the non-reversible strains, its use in the interpretation of the mentioned shear tests is discussed. Some comments are also presented on the mesh dependence of the numerical results, and on the use of a relation between material parameters and element size which reduces this negative effect. Finally, the solution procedure is applied to the simulation of a compression test in order to investigate the possibility of numerically detecting the ‘spontaneous’ (i.e. not directly induced by the boundary conditions) formation of shear zones within the specimen. 相似文献
74.
75.
Fabio Bellatreccia Giancarlo Della Ventura Luisa Ottolini Eugen Libowitzky Anton Beran 《Physics and Chemistry of Minerals》2005,32(1):65-76
A well-characterized suite of vesuvianite samples from the volcanic ejecta (skarn or syenites) from Latium (Italy) was studied by single-crystal, polarized radiation, Fourier-transform infrared (FTIR) spectroscopy and secondary-ion mass-spectrometry (SIMS). OH-stretching FTIR spectra consist of a rather well-defined triplet of broad bands at higher-frequency (3,700–3,300 cm–1) and a very broad composite absorption below 3,300 cm–1. Measurements with E//c or Ec show that all bands are strongly polarized with maximum absorption for E//c. They are in agreement with previous band assignments (Groat et al. Can Mineral 33:609, 1995) to the two O(11)–H(1) and O(10)–H(2) groups in the structure. Pleochroic measurements with changing direction of the E vector of the incident radiation show that the orientation of the O(11)–H(1) dipole is OHc~35°, in excellent agreement with the neutron data of Lager et al. (Can Mineral 37:763, 1999). A SIMS-based calibration curve at ~10% rel. accuracy has been worked out and used as reference for the quantitative analysis of H2O in vesuvianite by FTIR. Based on previous SIMS results for silicate minerals (Ottolini and Hawthorne in J Anal At Spectrom 16:1266, 2001; Ottolini et al. in Am Mineral 87:1477, 2002) the SiO2 and FeO content of the matrix were assumed as the major factors to be considered at a first approximation in the selection of the standards for H. The lack of vesuvianite standards for quantitative SIMS analysis of H2O has been here overcome by selecting low-silica elbaite crystals (Ottolini et al. in Am Mineral 87:1477, 2002). The resulting integrated molar absorption FTIR coefficient for vesuvianite is i=100.000±2.000 l mol–1 cm–2. SIMS data for Li, B, F, Sr, Y, Be, Ba REE, U and Th are also provided in the paper. 相似文献
76.
A survey is presented of some recent developments of the numerical techniques for back analysis in the field of geomechanics, with particular reference to tunnelling problems. In the spirit of Terzaghi's observational design method, these techniques are seen as practical tools for interpreting the available field measurements, in order to reduce the uncertainties that in many instances affect the parameters governing the solution of complex geomechanics problems. Both deterministic and probabilistic viewpoints are considered and some significant applications to practical problems are illustrated. 相似文献
77.
In the coupled finite element analysis of the two-phase, saturated soils the pore pressure could decrease below the atmospheric pressure in particular when the plastic dilation of the skeleton is accounted for. The consequent reduction of the bulk modulus of the liquid phase is here modelled in a simplified manner through a tension cut off on the pore pressure. Two alternative approaches are outlined in the framework of the finite element method. The first one introduces a unilateral constraint on the nodal pore pressures and involves manipulations on matrices of the assembled mesh, whilst the second approach requires calculations solely at the element level. The effects of the pore pressure tension cut off are illustrated through some undrained test examples in elastic–plastic regime. 相似文献
78.
The expected fraction of buildings lost in the event of an earthquake of a given intensity can be considered as a useful indicator of the seismic vulnerability of a small urban nucleus and can be used for the evaluation of seismic risk on a regional scale. This work describes a procedure for quantifying this indicator through data obtained from the buildings which constitute the nucleus during rapid on-site surveys. The knowledge on which the procedure is based is that given by the observed vulnerability of old Italian centres. 相似文献
79.
Giancarlo Bortolami Jean-Charles Fontes Costanzo Panichi 《Earth and Planetary Science Letters》1973,19(2):154-167
L'enfoncement de la plaine de Venise, quel'on peut estimeràprès de 1 cm/an sous le site historique, est liéàla surexploitation des nappes artésiennes profondes. L'investigation hydrogéologique est rendue délicate par la nature de l'aquifère composéde plusieurs centaines de mètres de dépo?ts quaternaires continentaux et marins. Au sein de ce complexe les nappes se superposent en un système multicouche artésien. L'étude du régime de ces nappes aétéentrepris par les moyens géochimiques et isotopiques (oxygène 18, deuterium, tritium, carbone 13 et carbone 14).Les teneurs en isotopes stables montrent l'existence de deux familles d'eaxu profondes attribuablesàune recharge “froide” (δ18O= ?10 et ?12‰). Ces eaux sont nettements différentes de celles des nappes phréatiques (δ18O ? ?8‰). Les apports profonds peuvente?tre respectivement liés aux bassins des rivières préalpines Brenta et Piave (δ18O ? ?10‰) d'une part et au paléocours de l'Adige, fleuve alpin d'autre part. Les eaux alpines ont un gisement limitéàl'aplomb de la lagune etàla partie sud-ouest de la plaine. Les eaux de type préalpin occupent le reste de la plaine, c'est-à-dire plus grande partie.La détermination des vitesses d'écoulement pose le problème de la validitédes temps estimésàl'aide des teneurs en14C.Les teneurs en13Célevées des eaux alpines montrent clairement que les activités en14C de celles-ci ne sont pas significatives par suite d'unéchange avec l'aquifère ou d'une surcharge en carbone ancien dissous.Pour les eaux qui viennent du domaine préalpin, on observe dans les zones de recharge de fortes teneurs en3H: 85à120 UT alliéesàdes activités relativement faibles en14C (60à70%). Les teneurs en13C indiquent que le carbone dissous n'a pas subi d'échange avec l'aquifère. La discussion de l'ensemble des données suggère que la circulation est rapide dans la zone de recharge oùl'alimentation en eaux d'altitude se faitàpartir d'infiltrations en provenance du réseau hydrographique ou (et) du système karstique des préalpes. Dans les parties basses de la plaine les vitesses sont faibles et décroissent, vers l'Est ou en profondeur, de 3 m/anà1 m/an. Il est possible qu'une rupture dans l'intensitédes vitesses d'écoulement interviennent dans la partie occidentale de la plaine sous l'effet d'une discontinuitésédimentologique majeure (limite occidentale des dépo?ts marins). 相似文献
80.
Giancarlo Lucarelli Raffaele Picone Salvatore Troisi 《Celestial Mechanics and Dynamical Astronomy》1998,70(1):59-73
In special cases in which satellite constellations can be described by a limited set of parameters, a correlation between
the latter and the constellation characteristics (i.e. shape and kinematics of the satellite configurations) has been investigated.
Such cases may be a useful starting point for designing more complex constellations in order to achieve better the desired
performances.
Therefore, this proves to be one way to fix an initial set of parameters once the desired behaviour of the constellation has
been established on the basis of the operational purposes required.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献