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排序方式: 共有651条查询结果,搜索用时 31 毫秒
641.
Cristian Carli Anna Barbaro Mara Murri Maria Chiara Domeneghetti Antonio Langone Enrico Bruschini Alice Stephant Matteo Alvaro Stefania Stefani Tiberio Cuppone Martina Casalini Alessandra Migliorini Ted L. Roush Giovanni Pratesi 《Meteoritics & planetary science》2023,58(6):855-874
Al Huwaysah 010 is an ungrouped achondrite meteorite, recently referred to as a brachinite-like meteorite. This meteorite, showing a fine-grained assemblage of low-Ca pyroxene and opaque phases, is strongly reduced in comparison to other reduced brachinites. The occurrence of some tiny plates of graphite and oldhamite in this meteorite suggests that a partial melt residue has experienced a further reduction process. Olivine, the most abundant phase, is compositionally homogeneous (Fo83.3) as well as the clinopyroxene (En45.5Fs10.8Wo43.7) and the plagioclase (Ab69.5). Orthopyroxene (En85.4Fs13.9Wo0.7) also occurs but only in a fine intergrowth. Other accessory phases are Fe metal grains (Ni-free or Cr-bearing Fe-Ni alloy), troilite, chlorapatite, pentlandite (as inclusions in chromite). The sample shows two different closure temperatures: the highest (≈900°C) is determined via the olivine–chromite intercrystalline geothermometer and the lowest temperature (≈520°C) is determined via the pyroxene-based intracrystalline geothermometer. These temperatures may represent, respectively, the closure temperature associated with the formation and a subsequent impact event excavating the sample from the parental body. The visible to near-infrared (VNIR) reflectance spectra of Al Huwaysah 010 exhibit low reflectance, consistent with the presence of darkening components, and weak absorptions indicative of olivine and pyroxene. Comparing the spectral parameters of Al Huwaysah 010 to potential parent bodies characterized by olivine–pyroxene mineralogy, we find that it falls within the field previously attributed to the SIII type asteroids. These results lead us to classify the Al Huwaysah 010 meteorite as the most reduced brachinite, whose VNIR spectral features show strong affinities with those of SIII asteroids. 相似文献
642.
Zamponi Joaquin Giannetti Andrea Bovino Stefano Sabatini Giovanni Schleicher Dominik R. G. Krtgen Bastian Reissl Stefan Wolf Sebastian 《Astrophysics and Space Science》2022,367(6):1-9
Astrophysics and Space Science - Dynamical evolution of galaxies is a complex process, especially the centers. Gravitationally coupled gas and stellar discs have been observed to coexist in the... 相似文献
643.
A Bonded Particle Model Simulation of Shear Strength and Asperity Degradation for Rough Rock Fractures 总被引:3,自引:3,他引:0
Mohammad Sadegh Asadi Vamegh Rasouli Giovanni Barla 《Rock Mechanics and Rock Engineering》2012,45(5):649-675
Different failure modes during fracture shearing have been introduced including normal dilation or sliding, asperity cut-off and degradation. Attempts have been made to study these mechanisms using analytical, experimental and numerical methods. However, the majority of the existing models simplify the problem, which leads to unrealistic results. With this in mind, the aim of this paper is to simulate the mechanical behaviour of synthetic and rock fracture profiles during direct shear tests by using the two-dimensional particle flow computer code PFC2D. Correlations between the simulated peak shear strength and the fracture roughness parameter D R1 recently proposed by Rasouli and Harrison (2010) are developed. Shear test simulations are carried out with PFC2D and the effects of the geometrical features as well as the model micro-properties on the fracture shear behaviour are studied. The shear strength and asperity degradation processes of synthetic profiles including triangular, sinusoidal and randomly generated profiles are analysed. Different failure modes including asperity sliding, cut-off, and asperity degradation are explicitly observed and compared with the available models. The D R1 parameter is applied to the analysis of synthetic and rock fracture profiles. Accordingly, correlations are developed between D R1 and the peak shear strength obtained from simulations and by using analytical solutions. The results are shown to be in good agreement with the basic understanding of rock fracture shear behaviour and asperity contact degradation. 相似文献
644.
The renewed interest in chromite ore deposits is directly related to the increase in Cr price ruled by international market trends. Chromite, an accessory mineral in peridotites, is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel-bearing rock (Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993). However, the understanding of chromite alteration and metamorphic modification is still controversial (e.g. Evans and Frost in Geochim Cosmochim Acta 39:959–972, 1975; Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993; Oze et al. in Am J Sci 304:67–101, 2004). Metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study, we investigate the Vourinos complex chromitites (from the mines of Rizo, Aetoraches, Xerolivado and Potamia) with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation. The present study has been partially funded by the Aliakmon project in collaboration between the Public Power Corporation of Greece and Institute of Geology and Mineral Exploration of Kozani. 相似文献
645.
646.
647.
Terrestrial laser scanner and geomechanical surveys for the rapid evaluation of rock fall susceptibility scenarios 总被引:3,自引:1,他引:2
The primary objective of this paper is to present a semiautomatic procedure that, integrated with traditional methods, can be useful for a rapid definition of rock fall susceptibility scenarios with the purpose of civil protection. Due to its morphology (steep slopes and narrow valleys), regional seismicity, and rock mass characteristics, the Nera Valley (Valnerina, Umbria Region, Italy) is characterized by high rock fall risk. With the aim of covering a wide range of features and investigating the main advantages and drawbacks of the proposed approach, data collection (terrestrial laser scanning (TLS) and geomechanical surveys) was carried out at three different slopes. Detailed three-dimensional (3D) models were created to reconstruct the shape and volume of the most unstable blocks, to define the position of the main rock fall source areas, and to precisely distinguish the outcropping materials and the position of the elements at risk for reliable runout analyses. The proposed approach can be useful in supporting proper maintenance and land management programs both in ordinary and in emergency circumstances. 相似文献
648.
An integrating spectrometer based on a two-stage polyphase digital filter bank (PDFB) algorithm is described. The first PDFB operates on a time multiplexed wideband signal, with a bandwidth up to 1 GHz. The second PDFB is performed in parallel over couples of overlapping channels from the first filterbank. The design automatically deletes the unused portions of the first filterbank channels, providing a seamless, uniform channelization of the input band. The design has been implemented and tested on the VLBI DBBC platform, using a single Xilinx XC5VLX220 FPGA. 相似文献
649.
Generoso Aliasi Giovanni Mengali Alessandro A. Quarta 《Celestial Mechanics and Dynamical Astronomy》2011,110(4):343-368
This paper introduces a new approach to the study of artificial equilibrium points in the circular restricted three-body problem
for propulsion systems with continuous and purely radial thrust. The propulsion system is described by means of a general
mathematical model that encompasses the behavior of different systems like a solar sail, a magnetic sail and an electric sail.
The proposed model is based on the choice of a coefficient related to the propulsion type and a performance parameter that
quantifies the system technological complexity. The propulsion system is therefore referred to as generalized sail. The existence
of artificial equilibrium points for a generalized sail is investigated. It is shown that three different families of equilibrium
points exist, and their characteristic locus is described geometrically by varying the value of the performance parameter.
The linear stability of the artificial points is also discussed. 相似文献
650.
Rapid estimation of ground-shaking maps for seismic emergency management in the Campania Region of southern Italy 总被引:1,自引:1,他引:0
Vincenzo Convertito Raffaella De Matteis Luciana Cantore Aldo Zollo Giovanni Iannaccone Mauro Caccavale 《Natural Hazards》2010,52(1):97-115
Strong ground-shaking mapping soon after a moderate-to-large earthquake is crucial to recognize the areas that have suffered
the largest damage and losses. These maps have a fundamental role for emergency services, loss estimation and planning of
emergency actions by the Civil Protection Authorities. This is particularly important for areas with high seismic risk levels,
such as the Campania-Lucania Region in southern Italy. Taking advantage of the Irpinia Seismic Network (ISNet), a recently
installed dense and wide dynamic seismic network, we have developed a procedure for rapid estimation of ground-shaking maps
after moderate-to-large earthquakes (GRSmap). This uses an optimal data gridding scheme designed to account for bi-dimensional
features of strong ground-motion fields, such as directivity, radiation patterns and focal mechanisms, to which most damage
can be correlated. The basis of the mapping technique is a triangulation procedure to locally correct predicted data at the
triangle barycentres where their vertices correspond to seismic stations. The method has been tested off-line using a simulated
M 6.6 earthquake located at the centre of ISNet and applied to data of the 23 November 1980 Irpina M 6.9 earthquake recorded by a sparse network. This has highlighted its ability to predict peak ground-motion parameters of
large magnitude earthquakes with respect to the attenuation relationships. 相似文献