全文获取类型
收费全文 | 90篇 |
免费 | 7篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 2篇 |
地球物理 | 33篇 |
地质学 | 32篇 |
海洋学 | 3篇 |
天文学 | 17篇 |
自然地理 | 4篇 |
出版年
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 8篇 |
2015年 | 4篇 |
2014年 | 6篇 |
2013年 | 7篇 |
2012年 | 6篇 |
2011年 | 4篇 |
2010年 | 4篇 |
2009年 | 6篇 |
2008年 | 1篇 |
2006年 | 2篇 |
2005年 | 2篇 |
2004年 | 2篇 |
2003年 | 2篇 |
2002年 | 1篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1995年 | 1篇 |
1992年 | 1篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1970年 | 3篇 |
1949年 | 1篇 |
排序方式: 共有98条查询结果,搜索用时 368 毫秒
71.
A structure's health or level of damage can be monitored by identifying changes in structural or modal parameters. This research directly identifies changes in structural stiffness due to modelling error or damage for a post‐tensioned pre‐cast reinforced concrete frame building with rocking beam column connections and added damping and stiffness (ADAS) elements. A structural health monitoring (SHM) method based on adaptive least mean squares (LMS) filtering theory is presented that identifies changes from a simple baseline model of the structure. This method is able to track changes in the stiffness matrix, identifying when the building is (1) rocking, (2) moving in a hybrid rocking–elastic regime, or (3) responding linearly. Results are compared for two different LMS‐based SHM methods using an L 2 error norm metric. In addition, two baseline models of the structure, one using tangential stiffness and the second a more accurate bi‐linear stiffness model, are employed. The impact of baseline model complexity is then delineated. The LMS‐based methods are able to track the non‐linearity of the system to within 15% using this metric, with the error due primarily to filter convergence rates as the structural response changes regimes while undergoing the El Centro ground motion record. The use of a bi‐linear baseline model for the SHM problem is shown to result in error metrics that are at least 50% lower than those for the tangential baseline model. Errors of 5–15% with this L 2 error norm are fairly stringent compared to the greater than 2 × changes in stiffness undergone by the structure, however, in practice the usefulness of the results is dependent on the resolution required by the user. The impact of sampling rate is shown to be negligible over the range of 200–1000Hz, along with the choice of LMS‐based SHM method. The choice of baseline model and its level of knowledge about the actual structure is seen to be the dominant factor in achieving good results. The methods presented require 2.8–14.0 Mcycles of computation and therefore could easily be implemented in real time. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
72.
K-A ages from a variety of alkaline rocks, carbonatites (sövites) and alnöites from the Alnö complex range between 370 and 605 m.y. but the most meaningful ages concentrate between 546 and 578 m.y. Most nephelinites, phonolites and alkali trachytes that occur in a dike complex surrounding the main intrusion give a middle Cambrian RbSr whole-rock isochron age of 553±6 m.y. (2σ). Some samples (deleted from the age regression) were contaminated by radiogenic strontium. Plutonic pyroxenites, ijolites and nepheline syenites that form most of the core of the intrusion formed at about the same time and from the same parental magma as the dike rocks. Sövites and alnöites that crosscut the dike and core rocks did not intrude significantly later and may have formed from the same parent magma or mantle source. Many samples show evidence of either mixing or isotopic exchange of the magma with the country rocks during intrusion (fenitisation?) or open system behavior after crystallization. These processes resulted in some scatter of the data points about the best-fit RbSr isochron lines and in some anomalous K-A ages. Fenites have higher Rb/Sr and 87Sr/86Sr ratios than the alkaline rocks, making it unlikely that the latter were the remobilized products of extreme fenitization, as suggested by von Eckermann (1948). The Alnö intrusion is about the same age as the Fen complex of southern Norway, but is significantly older or younger than many other apparently similar intrusions in Scandinavia. 相似文献
73.
Claus-Dieter Hillenbrand Dieter K. Fütterer Hannes Grobe Thomas Frederichs 《Geo-Marine Letters》2002,22(2):51-59
Records of glaciomarine deposition recovered from the West Antarctic continental margin in the Amundsen Sea allow the reconstruction of the behaviour of the West Antarctic Ice Sheet (WAIS) in response to the natural climatic changes of the last 1.8 million years. Contents of gravel-sized and lithogenic components represent the input and redeposition of glaciogenic debris, whereas variations in the proportions of the calcareous sediment fraction reflect palaeoproductivity changes. All proxies, which are regarded as sensitive to a WAIS collapse, changed markedly during the global climatic cycles, but do not confirm a complete disintegration of the WAIS during the Pleistocene. 相似文献
74.
75.
Ronny Boch Xianfeng Wang Tobias Kluge Albrecht Leis Ke Lin Hannes Pluch Florian Mittermayr Andre Baldermann Michael E. Bttcher Martin Dietzel 《Sedimentology》2019,66(2):604-635
The well‐known Erzberg site represents the largest siderite (FeCO3) deposit in the world. It consists of various carbonates accounting for the formation of prominent CaCO3 (dominantly aragonite) precipitates filling vertical fractures of different width (centimetres to decimetres) and length (tens of metres). These commonly laminated precipitates are known as ‘erzbergite’. This study focuses on the growth dynamics and environmental dependencies of these vein fillings. Samples recovered on‐site and from mineral collections were analyzed, and these analyses were further complemented by modern water analyses from different Erzberg sections. Isotopic signatures support meteoric water infiltration and sulphide oxidation as the principal hydrogeochemical mechanism of (Ca, Mg and Fe) carbonate host rock dissolution, mobilization and vein mineralization. Clumped isotope measurements revealed cool formation temperatures of ca 0 to 10°C for the aragonite, i.e. reflecting the elevated altitude Alpine setting, but unexpectedly low for aragonite nucleation. The 238U–234U–230Th dating yielded ages from 285·1 ± 3·9 to 1·03 ± 0·04 kyr bp and all samples collected on‐site formed after the Last Glacial Maximum. The observed CaCO3 polymorphism is primarily controlled by the high aqueous Mg/Ca ratios resulting from dissolution of Mg‐rich host rocks, with Mg/Ca further evolving during prior CaCO3 precipitation and CO2 outgassing in the fissured aquifer. Aragonite represents the ‘normal’ mode of erzbergite formation and most of the calcite is of diagenetic (replacing aragonite) origin. The characteristic lamination (millimetre‐scale) is an original growth feature and mostly associated with the deposition of stained (Fe‐rich) detrital particle layers. Broader zonations (centimetre‐scale) are commonly of diagenetic origin. Petrographic observations and radiometric dating support an irregular nature for most of the layering. Open fractures resulting from fault tectonics or gravitational mass movements provide water flow routes and fresh chemical reaction surfaces of the host rock carbonates and accessory sulphides. If these prerequisites are considered, including the hydrogeochemical mechanism, modern water compositions, young U‐Th ages and calculated precipitation rates, it seems unlikely that the fractures had stayed open over extended time intervals. Therefore, it is most likely that they are geologically young. 相似文献
76.
Bjarne S. G. Almqvist Sonja A. Bosshard Ann M. Hirt Hannes B. Mattsson Gy?rgy Hetényi 《Bulletin of Volcanology》2012,74(7):1667-1681
The anisotropy of magnetic susceptibility (AMS) and rock magnetic properties were measured on specimens from a basalt plate that was cut from a vertical section of a basalt column from Hrepphólar, Iceland. Macroscopic structures are clearly distinguishable in the plate, including banding inferred to represent viscous fingering parallel to the vertical axis of the column. Rock magnetic experiments indicate that the dominant ferromagnetic (sensu lato) mineral is titanomagnetite, Fe 3?x Ti x O4, with a Ti-composition of x?=?~0.6. Magnetic properties are related to the position within the plate and reveal a dominant volume fraction of single domain titanomagnetite in the center of the basalt column, with multidomain titanomagnetite away from the center. The AMS determined by low-field measurements shows an inconclusive relationship with the visual structures, which arises from variation of the grain size (i.e., single domain versus multidomain) across the column. In contrast, the AMS measured with a high-field torsion magnetometer avoids the complication of magnetic domain state, as is demonstrated in this contribution, and additionally allows for the separation of ferrimagnetic from paramagnetic sub-fabrics. Both sub-fabrics display a clear relationship with the macroscopic structures and support the hypothesis that vertical flow of melt took place during development of the Hrepphólar columnar basalt. Maximum susceptibility axes of the ferrimagnetic sub-fabric are grouped near the vertical axis of the column. The paramagnetic sub-fabric varies systematically across the column in coincidence with internal structure. The shape of the magnetic susceptibility ellipsoid varies across the basalt column, showing an increasingly prolate fabric toward its center. 相似文献
77.
78.
Hans Burchard Frank Janssen Karsten Bolding Lars Umlauf Hannes Rennau 《Continental Shelf Research》2009
Only recently, medium intensity inflow events into the Baltic Sea have gained more awareness because of their potential to ventilate intermediate layers in the Southern Baltic Sea basins. With the present high-resolution model study of the Western Baltic Sea a first attempt is made to obtain model based realistic estimates of turbulent mixing in this area where dense bottom currents resulting from medium intensity inflow events are weakened by turbulent entrainment. The numerical model simulation which is carried out using the General Estuarine Transport Model (GETM) during nine months in 2003 and 2004 is first validated by means of three automatic stations at the Drogden and Darss Sills and in the Arkona Sea. In order to obtain good agreement between observations and model results, the 0.5×0.5 nautical mile bathymetry had to be adjusted in order to account for the fact that even at that scale many relevant topographic features are not resolved. Current velocity, salinity and turbulence observations during a medium intensity inflow event through the Øresund are then compared to the model results. Given the general problems of point to point comparisons between observations and model simulations, the agreement is fairly good with the characteristic features of the inflow event well represented by the model simulations. Two different bulk measures for mixing activity are then introduced, the vertically integrated decay of salinity variance, which is equal to the production of micro-scale salinity variance, and the vertically integrated turbulent salt flux, which is related to an increase of potential energy due to vertical mixing of stably stratified flow. Both measures give qualitatively similar results and identify the Drogden and Darss Sills as well as the Bornholm Channel as mixing hot spots. Further regions of strong mixing are the dense bottom current pathways from these sills into the Arkona Sea, areas around Kriegers Flak (a shoal in the western Arkona Sea) and north–west of the island of Rügen. 相似文献
79.
Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence
of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to
cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at
1-atm confining pressure and in a temperature range between 630°C and 300°C. The experiments were set up to characterize the
chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes
occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization
of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630°C, followed by nyerereite
at 595°C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e.,
Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes
in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with
the changes in melt composition in as little time as <11 min. We therefore conclude that crystallization and diffusion are
extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo
Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly
crystallized natrocarbonatitic lavas at Oldoinyo Lengai. 相似文献
80.
Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure 下载免费PDF全文
Xiaoming Xu Thomas Kenkmann Zhiyong Xiao Sebastian Sturm Nicolai Metzger Yu Yang Daniela Weimer Hannes Krietsch Meng‐Hua Zhu 《Meteoritics & planetary science》2017,52(9):1822-1842
The Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin‐section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area. 相似文献