全文获取类型
收费全文 | 3127篇 |
免费 | 404篇 |
国内免费 | 1189篇 |
专业分类
测绘学 | 204篇 |
大气科学 | 174篇 |
地球物理 | 352篇 |
地质学 | 3031篇 |
海洋学 | 384篇 |
天文学 | 61篇 |
综合类 | 168篇 |
自然地理 | 346篇 |
出版年
2024年 | 14篇 |
2023年 | 56篇 |
2022年 | 117篇 |
2021年 | 145篇 |
2020年 | 133篇 |
2019年 | 177篇 |
2018年 | 130篇 |
2017年 | 163篇 |
2016年 | 145篇 |
2015年 | 155篇 |
2014年 | 205篇 |
2013年 | 237篇 |
2012年 | 199篇 |
2011年 | 238篇 |
2010年 | 199篇 |
2009年 | 230篇 |
2008年 | 245篇 |
2007年 | 242篇 |
2006年 | 226篇 |
2005年 | 182篇 |
2004年 | 175篇 |
2003年 | 154篇 |
2002年 | 135篇 |
2001年 | 131篇 |
2000年 | 101篇 |
1999年 | 100篇 |
1998年 | 98篇 |
1997年 | 64篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 43篇 |
1993年 | 42篇 |
1992年 | 28篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 22篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有4720条查询结果,搜索用时 6 毫秒
91.
Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy 总被引:1,自引:0,他引:1
With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth element (REE) in the natural water systems, the ground and surface waters of the Ottana-Orani area (Central Sardinia, Italy) were sampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of 6.0-8.6, total dissolved solid (TDS) of between 0.1 and 0.6 g/l, and major cation composition dominated by Ca and Na, whereas predominant anions are Cl and/or HCO3.The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution. The concentrations of ∑REE+Y in the samples filtered at 0.4 μm vary between 140 and 1600 ng/l, with La of between 14 and 314 ng/l, and Yb of between <6 and 12 ng/l. A negative Ce anomaly, especially marked at high pH, is observed in the groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ce anomaly.Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO32− ligand are the dominant REE species at pH in the range of 6.7-8.6. The REE3+ ions dominate the speciation at pH <6.7 and only in the light REE (LREE).The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, when concentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REE (HREE) into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution. 相似文献
92.
Method of Data Reduction and Uncertainty Estimation for Platinum-Group Element Data Using Inductively Coupled Plasma-Mass Spectrometry 总被引:1,自引:0,他引:1
A data reduction method is described for determining platinum-group element (PGE) abundances by inductively coupled plasma-mass spectrometry (ICP-MS) using external calibration or the method of standard addition. Gravimetric measurement of volumes, the analysis of reference materials and the use of procedural blanks were all used to minimise systematic errors. Internal standards were used to correct for instrument drift. A linear least squares regression model was used to calculate concentrations from drift-corrected counts per second (cps). Furthermore, mathematical manipulations also contribute to the uncertainty estimates of a procedure. Typical uncertainty estimate calculations for ICP-MS data manipulations involve: (1) Carrying standard deviations from the raw cps through the data reduction or (2) calculating a standard deviation from multiple final concentration calculations. It is demonstrated that method 2 may underestimate the uncertainty estimate of the calculated data. Methods 1 and 2 do not typically include an uncertainty estimate component from a regression model. As such models contribute to the uncertainty estimates affecting the calculated data, an uncertainty estimate component from the regression must be included in any final error calculations. Confidence intervals are used to account for uncertainty estimates from the regression model. These confidence intervals are simpler to calculate than uncertainty estimates from method 1, for example. The data reduction and uncertainty estimation method described here addresses problems of reporting PGE data from an article in the literature and addresses both precision and accuracy. The method can be applied to any analytical technique where drift corrections or regression models are used. 相似文献
93.
Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons 总被引:1,自引:0,他引:1
B. Heisinger D. Lal A. J. T. Jull P. Kubik S. Ivy-Ochs K. Knie E. Nolte 《Earth and Planetary Science Letters》2002,200(3-4):357-369
We have determined the production yields for radionuclides in Al2O3, SiO2, S, Ar, K2SO4, CaCO3, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides 10Be, 14C and 26Al produced in quartz targets, 26Al in Al2O3 and S targets, 36Cl in K2SO4 and CaCO3 targets, and 53Mn in Fe2O3 targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ− capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ− capture, by fast muons and by neutron capture. 相似文献
94.
Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties 总被引:1,自引:0,他引:1
D.V. ChandrasekharD.C. Mishra G.V.S. Poornachandra RaoJ. Mallikharjuna Rao 《Earth and Planetary Science Letters》2002,201(2):277-292
The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10−5m/s2) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10−2 SI units with an average Koenigsberger ratio (Qn) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m3, respectively and susceptibility of 3.14×10−2 SI units with a Qn ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs. 相似文献
95.
96.
跨国公司地区总部职能与亚洲地区总部的区位研究 总被引:2,自引:0,他引:2
跨国公司建立地区总部制度是跨国公司应对经营“全球化”和“本地化”的要求而进行的一项组织变革。由于跨国公司地区总部承担着制定区域性经营战略和协调区域内众多子公司生产、销售、物流、研究与开发、资金筹供、人力资源培养等经营活动的职能,它的区位对东道国及其所在城市经济带来广泛影响。本文从跨国公司和地区总部职能特点入手,分析了该职能的区位因素,并探讨了跨国公司亚洲地区总部的区位特点,最后对中国大陆的国际商务职能建设提出了若干建议。 相似文献
97.
Yao Chun-xial Li Bao-sheng Jin He-ling David Dian Zhang Yan Man-cun Zhu Yi-zhi Li Hou-xin Zhang Yu-hong Luo Kai-li 《地理学报(英文版)》2002,12(1):65-71
The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley,
which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation
of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian
sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively
active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable
cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since
then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion
and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies
and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess
Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements
is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter
monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial
and interglacial periods since 150 ka BP. 相似文献
98.
99.
100.