全文获取类型
收费全文 | 649篇 |
免费 | 20篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 46篇 |
地球物理 | 158篇 |
地质学 | 248篇 |
海洋学 | 66篇 |
天文学 | 62篇 |
综合类 | 2篇 |
自然地理 | 75篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2021年 | 2篇 |
2020年 | 12篇 |
2019年 | 10篇 |
2018年 | 21篇 |
2017年 | 16篇 |
2016年 | 21篇 |
2015年 | 18篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 23篇 |
2011年 | 33篇 |
2010年 | 37篇 |
2009年 | 49篇 |
2008年 | 42篇 |
2007年 | 35篇 |
2006年 | 20篇 |
2005年 | 33篇 |
2004年 | 28篇 |
2003年 | 23篇 |
2002年 | 22篇 |
2001年 | 15篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 17篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有673条查询结果,搜索用时 15 毫秒
81.
Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary.Seven samples of quartz diorites and granodiorites from the Ilo and Toquepala intrusive complexes, and seven samples of Toquepala Group subaerial volcanics were analyzed for Pb isotopic compositions. The plutonic rocks range in age from Jurassic to Eocene; the volcanic rocks are all Late Cretaceous to Eocene. With one exception, the Pb isotopic ratios are in the ranges 206Pb/204Pb= 18.52–18.75, 207Pb/204Pb= 15.58–15.65, and 208Pb/204Pb= 38.53–38.74. The data reflect very little or no interaction with old continental material of the Arequipa massif type.Lead from four Miocene Huaylillas Formation ash-flow tuffs, two Pliocene Capillune Formation andesites and five Quaternary Barroso Group andesites has lower 206Pb/204Pb than that in the pre-Miocene rocks, but relatively high 207Pb/204Pb and 208Pb/204Pb (206Pb/204Pb= 18.16–18.30, 207Pb/204Pb= 15.55–15.63, 208Pb/204Pb= 38.45–38.90). Tilton and Barreiro [9] have shown that contamination by Arequipa massif granulites can explain the isotopic composition of the Barosso Group lavas, and the new data demonstrate that this effect is evident, to varying degrees, in all the analysed Neogene volcanic rocks. The initial incorporation of such basement material into the magma coincided with the Early Miocene uplift of this segment of the Cordillera Occidental [32], and thus with the creation of a thick crustal root. The data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area, but a rigorous relationship is not yet established. 相似文献
82.
Characterizing the properties of dissolved organic matter isolated by XAD and C-18 solid phase extraction and ultrafiltration 总被引:1,自引:0,他引:1
83.
Consumption of aquifer Fe(III) during biodegradation of ground water contaminants may result in expansion of a contaminant plume, changing the outlook for monitored natural attenuation. Data from two research sites contaminated with petroleum hydrocarbons show that toluene and xylenes degrade under methanogenic conditions, but the benzene and ethylbenzene plumes grow as aquifer Fe(III) supplies are depleted. By considering a one-dimensional reaction front in a constant unidirectional flow field, it is possible to derive a simple expression for the growth rate of a benzene plume. The method balances the mass flux of benzene with the Fe(III) content of the aquifer, assuming that the biodegradation reaction is instantaneous. The resulting expression shows that the benzene front migration is retarded relative to the ground water velocity by a factor that depends on the concentrations of hydrocarbon and bioavailable Fe(III). The method provides good agreement with benzene plumes at a crude oil study site in Minnesota and a gasoline site in South Carolina. Compared to the South Carolina site, the Minnesota site has 25% higher benzene flux but eight times the Fe(III), leading to about one-sixth the expansion rate. Although it was developed for benzene, toluene, ethylbenzene, and xylenes, the growth-rate estimation method may have applications to contaminant plumes from other persistent contaminant sources. 相似文献
84.
Maria Alp Barbara L. Peckarsky Stefano M. Bernasconi Christopher T. Robinson 《Aquatic Sciences - Research Across Boundaries》2013,75(4):595-606
Stable isotopes (13C and 15N) are widely applied in studies of trophic links. We used this method to investigate the contribution of aquatic and terrestrial prey to the diet of riparian predatory arthropods in two mountain headwater streams in Colorado, USA. Aquatic and terrestrial prey and riparian predators were collected during summer 2009. To evaluate the reliability of conclusions based on stable isotope ratios, we compared the isotopic signatures of aquatic larval and terrestrial adult stages of three abundant stream insect species and assessed variation in mixing model estimates for spider diet composition under varying assumptions for trophic fractionation. Adult isotopic signatures of some aquatic prey species were indistinguishable from those of prey species with exclusively terrestrial life histories (stoneflies: 13C and 15N, chironomids: 13C). Other prey had distinctly aquatic isotopic signatures as both larvae and adults (a mayfly and a caddisfly). There was no evidence that prey with aquatic isotopic signatures contributed to the diet of the spiders near one stream. For the other stream, mixing model analysis suggested that chironomids were included in the diets of lycosid, linyphiid and liocranid spiders. Reliable estimates of the contributions of prey sources were compromised by the sensitivity of mixing models to assumptions on trophic fractionation and the presence of “isotopically cryptic” prey. This study emphasizes the importance of supporting isotope-based studies on cross-boundary trophic links with data on isotopic shifts in prey with complex life cycles and assessment of fractionation rates specific to the study system. 相似文献
85.
The structure, function and dynamics of reed-belts from three ‘Bornhoeved-Lakes’ was investigated by using methods from vegetation science and molecular biology. Between 1992 and 1994 the morphometry and production of Phragmites was measured at twelve locations, differing in sedimentology and hydrochemistry. The development and production of the plants, as well as the nutrient uptake and relocation, were similar throughout the years within each of the twelve plots. Differences in stand-structure and amounts of nutrients occurred between the habitats. Nitrogen content in the plant material varied between 5 and 30 g N m−2 and phosphorus between 0.3 and 1.2 g PO4-P m−2.80 natural reed habitats were measured for Restriction Fragment Length Polymorphisms (RFLPs) in order to identify the genetic diversity of Phragmites within the lakes. A cluster analysis exhibited groups with high, as well as groups with low genetic similarity. Even if genetically similar, the habitats were not necessarily directly neighboured at the shoreline. A discriminance analysis showed 40% correspondence of the genetic groups with their phenotype and habitat parameters. 相似文献
86.
Barbara G. Simpson Ph.D. 《地震工程与结构动力学》2020,49(14):1406-1427
Strongback-braced frames employ an essentially elastic steel truss, or strongback, that distributes demands more uniformly to delay or prevent story mechanisms. Because inertial forces are no longer limited by the formation of a story mechanism, strongback-braced frames can exhibit large elastic force demands, particularly in the higher modes. This paper characterizes the higher-mode force response of strongback-braced frames. Four-story archetypes were designed using nonlinear dynamic analyses to incorporate higher-mode force demands into the design process. The response of the archetypes was compared with that of reference buckling-restrained braced frames that were allowed to form story mechanisms. The force demands in the strongback were then described using equivalent-static forces to represent the inertial forces induced by the higher modes. Force demands in the strongback arise from a yielding first-mode ‘pivoting’ and elastic higher-mode ‘bending’ response. These higher-mode force demands are elastic, ill-constrained by the strength of the yield mechanism, and depend significantly on the choice of ground motion record used for the analysis. In remaining elastic in the higher modes, the strongback distributes demands more uniformly and mitigates the formation of story mechanisms. Consequently, design and analysis methods for strongback-braced frames need to include estimates for these near-elastic higher-mode force demands. 相似文献
87.
Crowd-based hydrological observations can supplement existing monitoring networks and allow data collection in regions where otherwise no data would be available. In the citizen science project CrowdWater, repeated water level observations using a virtual staff gauge approach result in time series of water level classes (WL-classes). To investigate the quality of these observations, we compared the WL-class data with “real” (i.e., measured) water levels from the same stream at a nearby gauging station. We did this for nine locations where citizen scientists reported multiple observations using a smartphone app and at 12 locations where signposts were set up to ask citizens to record observations on a paper form that could be left in a letterbox. The results indicate that the quality of the data collected with the app was better than for the forms. A possible explanation is that for each app location, a single person submitted the vast majority of the observations, whereas at the locations of the forms almost every observation was made by a different person. On average, there were more contributions between May and September than during the other months. Observations were submitted for a range of flow conditions, with a higher fraction of high flow observations for the locations were data were collected with the app. Overall, the results are encouraging for citizen science approaches in hydrology and demonstrate that the smartphone application and the virtual staff gauge are a promising approach for crowd-based water level class observations. 相似文献
88.
Xiu Luo Hiroshi Haya Tomoaki Inaba Tomoki Shiotani 《Soil Dynamics and Earthquake Engineering》2006,26(12):1101-1110
One difficult task for the seismic diagnosis of existing structures is how to nondestructively evaluate the damage degree of invisible substructures, such as embedded foundations. To diagnose substructures efficiently, a method for nondestructive inspection is developed by applying acoustic emission (AE) technique. As a newly proposed method, characteristics of secondary AE induced by train operations were investigated, and experiments using model piles and in-situ AE monitoring of in-service railway bridges conducted under railroad traffic, from which it was demonstrated that the proposed method is practicable enough to detect invisible defects in structures. A new index, known as RTRI (ratio of Repeated Train load at the onset of AE activity to Relative maximum load for Inspection period) is proposed for structural damage qualification based on the results of in-situ AE monitoring. 相似文献
89.
Catches of perch have strongly decreased in Lake of Geneva from 1975 to 1981. This decrease was attributed to the impact of
overfishing, enhanced by the accelerated growth of perch. Therefore, too many perches were captured before their first reproduction
and the stocks of reproducers were strongly reduced. Furthermore, adverse meteorological conditions occurred probably during
several periods of reproduction. The accelerated growth of perch was attributed to the combined effects of increasing eutrophication
(more food available) and of overfishing (decreased perch density in the shoals).
相似文献
90.