全文获取类型
收费全文 | 3045篇 |
免费 | 62篇 |
国内免费 | 100篇 |
专业分类
测绘学 | 149篇 |
大气科学 | 463篇 |
地球物理 | 373篇 |
地质学 | 871篇 |
海洋学 | 327篇 |
天文学 | 870篇 |
综合类 | 22篇 |
自然地理 | 132篇 |
出版年
2022年 | 23篇 |
2021年 | 26篇 |
2020年 | 43篇 |
2019年 | 40篇 |
2018年 | 48篇 |
2017年 | 159篇 |
2016年 | 116篇 |
2015年 | 190篇 |
2014年 | 182篇 |
2013年 | 260篇 |
2012年 | 115篇 |
2011年 | 139篇 |
2010年 | 154篇 |
2009年 | 194篇 |
2008年 | 101篇 |
2007年 | 140篇 |
2006年 | 108篇 |
2005年 | 99篇 |
2004年 | 74篇 |
2003年 | 80篇 |
2002年 | 78篇 |
2001年 | 47篇 |
2000年 | 45篇 |
1999年 | 73篇 |
1998年 | 31篇 |
1997年 | 31篇 |
1996年 | 23篇 |
1995年 | 17篇 |
1994年 | 22篇 |
1993年 | 20篇 |
1992年 | 15篇 |
1991年 | 21篇 |
1990年 | 26篇 |
1989年 | 17篇 |
1988年 | 26篇 |
1987年 | 44篇 |
1986年 | 19篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1983年 | 38篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 20篇 |
1977年 | 21篇 |
1976年 | 23篇 |
1975年 | 15篇 |
1974年 | 26篇 |
1973年 | 22篇 |
排序方式: 共有3207条查询结果,搜索用时 156 毫秒
101.
Currently, there is limited literature dealing with the length of stay of circular migrants in urban areas (LSCMU), although many studies have discussed the phenomenon of migration and the determinants of migration. This study attempts to fill that gap using survey data from the mountainous areas of Chongqing, Southwest China. From a comparative perspective, this study divided workers into two groups (a young group aged between 16 and 35 years and an old group aged between 36 and 65 years). The average LSCMU values for the young and old groups were 225 days and 174 days, respectively. Two multi-regression models were used to estimate the determinants of LSCMU in the two groups. The results showed that LSCMU was closely related to individual factors in both groups, including gender, age and job training. Family and community factors, including household size, arable land per capita and the distance from market, had much weaker effects on the dependent variable, especially in the older group. It was noticeable that job training had significant positive effects on LSCMU in both groups; these findings have special implications for the urbanization process in China. 相似文献
102.
Gregory B. Baecher 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2016,10(2):92-108
The character and importance of uncertainty in dam safety risk analysis drives how risk assessments are used in practice. The current interpretation of uncertainty is that, in addition to the aleatory risk which arises from presumed uncertainty in the world, it comprises the epistemic aspects of irresolution in a model or forecast, specifically model and parameter uncertainty. This is true in part but it is not all there is to uncertainty in risk analysis. The physics of hazards and of failure may be poorly understood, which goes beyond uncertainty in its conventional sense. There may be alternative scenarios of future conditions, for example non-stationarity in the environment, which cannot easily be forecast. There may also be deep uncertainties of the type associated with climate change. These are situations in which analysts do not know or do not agree on the system characterisation relating actions to consequences or on the probability distributions for key parameters. All of these facets are part of the uncertainty in risk analysis with which we must deal. 相似文献
103.
104.
Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the off-shore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability. 相似文献
105.
In order to study the origin of the spatial structure of the Northern Hemisphere Annular Mode (NAM),a linear stochastic model is constructed empirically from the output of a GCM run.Optimal stochastic forcing in terms of the maximum variance contribution,which may be potentially related to the maintenance of the NAM,is investigated.Theoretical analysis on the dominant non-modal response to the stochastic forcing shows that this dominance is jointly decided by the properties of forcing and the non-modal grow... 相似文献
106.
Bas van Geel Daniel C. Fisher Adam N. Rountrey Jan van Arkel Joost F. Duivenvoorden Aline M. Nieman Guido B.A. van Reenen Alexei N. Tikhonov Bernard Buigues Barbara Gravendeel 《Quaternary Science Reviews》2011,30(27-28):3935-3946
Intestinal samples from the one-month-old Siberian mammoth calf ‘Lyuba’ were studied using light microscopy and ancient DNA to reconstruct its palaeo-environment and diet. The palynological record indicates a ‘mammoth steppe’. At least some pollen of arboreal taxa was reworked, and thus the presence of trees on the landscape is uncertain. In addition to visual comparison of 11 microfossil spectra, a PCA analysis contributed to diet reconstruction. This yielded two clusters: one of samples from the small intestine and the other of large-intestine samples, indicating compositional differences in food remains along the intestinal tract, possibly reflecting different episodes of ingestion. Based on observed morphological damage we conclude that the cyperaceous plant remains and some remains of dwarf willows were originally eaten by a mature mammoth, most likely Lyuba’s mother. The mammoth calf probably unintentionally swallowed well-preserved mosses and mineral particles while eating fecal material deposited on a soil surface covered with mosses. Coprophagy may have been a common habit for mammoths, and we therefore propose that fecal material should not be used to infer season of death of mammoths. DNA sequences of trnL and rbcL genes amplified from ancient DNA extracted from intestinal samples confirmed and supplemented plant identifications based on microfossils and macro-remains. Results from different extraction methods and barcoding markers complemented each other and show the value of longer protocols in addition to fast and commercially available extraction kits. 相似文献
107.
I summarise observations within a continuous-discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and deformation approximating simple shear. The Chrystalls Beach Complex, New Zealand, comprises phacoids formed from dismembered beds by layer-parallel extension, enclosed within a relatively incompetent matrix. Local extension is generally subparallel to the regional direction of shortening, and overall it appears that layer-parallel extension is a geometrical necessity in low angle shear zones where significant flattening occurs in response to simple shear accompanied by volume loss.Preferential stress loading of phacoids is predicted by fibre-loading theory, and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix. The principal stress orientations in a phacoid are likely rotated relative to the matrix, and either parallel or perpendicular to the phacoid-matrix interface. As preferential loading of phacoids decreases the stress level in the matrix, an increased volume fraction of phacoids increases the strength of the shear zone as a whole. However, only small matrix volume fractions are required for the composite to act nearly as weak as the matrix. 相似文献
108.
Xenoliths of quartz‐absent Fe‐rich aluminous metapelite are common within the platinum group element‐rich mafic/ultramafic magmatic rocks of the Platreef. Relative to well‐characterized protoliths, the xenoliths are strongly depleted in K2O and H2O, and have lost a substantial amount of melt (>50 vol.%). Mineral equilibria calculations in the NCKFMASHTO system yield results that are consistent with observations in natural samples. Lower‐grade rocks that lack staurolite constrain peak pressures to ~2.5 kbar in the southern Platreef. Smaller xenoliths and the margins of larger xenoliths comprise micro‐diatexite rich in coarse acicular corundum and spinel, which record evidence for the metastable persistence of lower‐grade hydrous phases and rapid melting consequent on a temperature overstep of several hundred degrees following their incorporation in the mafic/ultramafic magmas. In the cores of larger xenoliths, temperatures increased more slowly enabling progressive metamorphism by continuous prograde equilibration and the loss of H2O by subsolidus dehydration; the H2O migrated to xenolith margins where it may have promoted increased melting. According to variations in the original compositional layering, layers became aluminosilicate‐ and/or cordierite‐rich, commonly with spinel but only rarely with corundum. The differing mineralogical and microstructural evolution of the xenoliths depends on heating rates (governed by their size and, therefore, proximity to the Platreef magmas) and the pre‐intrusive metamorphic grade of the protoliths. The presence or absence of certain phases, particularly corundum, is strongly influenced by the degree of metastable retention of lower‐grade hydrates in otherwise identical protolith bulk compositions. The preservation of fine‐scale compositional layering that is inferred to be relict bedding in xenolith cores implies that melt loss by compaction was extremely efficient. 相似文献
109.
David L. Finnegan Janet P. Kotra Diane M. Hermann William H. Zoller 《Bulletin of Volcanology》1989,51(2):83-87
A simple system was designed using7LiOH-impregnated filters to collect acidic gases from ambient air and from highly concentrated volcanic plumes or gas streams. These filters were developed for analysis using instrumental neutron activation analysis, but other analytical techniques could be used as well. The sampling system was designed to use a series of 1–3 M7LiOH-impregnated filters to collect high concentrations of acidic gases found in gas plumes of active volcanoes. The filters are quantitative for SO2 and the halide acids when sufficient base is present to neutralize the acidic species. Extremely high concentrations of SO2 may not be collected quantitatively since SO2 is a relatively weak Lewis acid compared to the halide acids. The acidic oxides of Sb, As, and Se were also collected quantitatively. A particle filter preceded the impregnated filters in order to remove particles from the fumes. This system has proven effective under difficult sampling conditions and, since it is portable and light weight, it could be used for many volcanological applications where high levels of acidic gaseous phase species need to be collected. 相似文献
110.
Summary Annual and seasonal changes of the urban heat island magnitude in Washington, D.C. and a close-in suburb were analyzed for the period of 1945 through 1979. Monthly and annual mean temperature data, corrected for elevation, from three stations (National Airport, D.C.—downtown, Silver Spring, MD—suburban, and Beltsville, MD—rural) were evaluated by testing the statistical significance of temperature differences between the different data sets.The analysis indicated that the absolute heat island magnitudes have increased at the rate of 0.020 and 0.019 C/yr at Washington National Airport and Silver Spring, respectively during the study period. The differences between these two rates were not statistically significant at the 0.95 confidence level. However, the increase of the heat island magnitude from 1945 to 1979 at the suburban station was 20% greater than that at the downtown station. Those results indicate that as urbanization progresses, the suburban site, Silver Spring, has become incorporated into the Washington, D.C. urban heat island. The data also indicate that the magnitude of the heat island near the city center has continued to increase despite a significant population decrease since 1950.With 5 Figures 相似文献