全文获取类型
收费全文 | 48435篇 |
免费 | 8086篇 |
国内免费 | 10170篇 |
专业分类
测绘学 | 5730篇 |
大气科学 | 8403篇 |
地球物理 | 11989篇 |
地质学 | 21012篇 |
海洋学 | 5927篇 |
天文学 | 1535篇 |
综合类 | 3434篇 |
自然地理 | 8661篇 |
出版年
2024年 | 154篇 |
2023年 | 522篇 |
2022年 | 1382篇 |
2021年 | 1848篇 |
2020年 | 2009篇 |
2019年 | 2540篇 |
2018年 | 1860篇 |
2017年 | 2388篇 |
2016年 | 2357篇 |
2015年 | 2550篇 |
2014年 | 3168篇 |
2013年 | 3674篇 |
2012年 | 3105篇 |
2011年 | 3334篇 |
2010年 | 2733篇 |
2009年 | 3292篇 |
2008年 | 3222篇 |
2007年 | 3414篇 |
2006年 | 3249篇 |
2005年 | 2699篇 |
2004年 | 2418篇 |
2003年 | 2073篇 |
2002年 | 1639篇 |
2001年 | 1402篇 |
2000年 | 1331篇 |
1999年 | 1234篇 |
1998年 | 1182篇 |
1997年 | 1075篇 |
1996年 | 837篇 |
1995年 | 754篇 |
1994年 | 668篇 |
1993年 | 565篇 |
1992年 | 445篇 |
1991年 | 325篇 |
1990年 | 247篇 |
1989年 | 262篇 |
1988年 | 180篇 |
1987年 | 113篇 |
1986年 | 87篇 |
1985年 | 74篇 |
1984年 | 36篇 |
1983年 | 22篇 |
1982年 | 29篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 15篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1973年 | 17篇 |
1954年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
本文介绍了对时序垂直角观测值进行谱分析的方法和所得到的结论。使以往对于大气折射影响及其变化规律的定性认识得以定量化。 相似文献
132.
133.
134.
Results of a single group participating in an international experiment are analyzed. The experiment served to verify computational predictions of the ground-motion variations due to near-surface geological effects at a site established for that purpose by the California Department of Conservation. Based on an acceleration record at a rock location, and geotechnical model of medium, records at the other locations of a nearby sedimentary deposit were predicted. A 2-D finite-difference sensitivity analysis suggested that the lateral wave-propagation effects are negligibly small, and locally 1-D computations are sufficient for the present site. Those computations are compared with observations not available to the authors during the blind prediction. Peak accelerations, peak velocities and RMS accelerations were predicted with errors less than 159%, 114% and 62%, respectively. Maxima of the response spectra were fitted within a factor of 2. The predicted and observed Husid's plots (i.e., the normalized cumulative plots of the acceleration squared) have the correlation coefficients 0.98. The detected misfits do not show any simple relation to the instrument location, component, frequency, or time. 相似文献
135.
Six anchor stations in the St. Lawrence River from the outflow of Lake Ontario to Quebec City, were occupied for ca. 24 hours in June 1987 during low flow conditions. Samples of water and suspended particulate matter were separated by continuous-flow centrifugation, and were collected every two hours.During this sampling period, fluxes of dissolved forms of zinc, copper and nickel increased in Lac St. François and downstream relative to the sum of the fluxes for the two upstream stations at the outflow of Lake Ontario. Increases in the flux of dissolved zinc and copper were pronounced below Montreal and above Lac St. Pierre. For particulate forms of metals, all five metals show that there are significant inputs in the section of the St. Lawrence River between Lac St. François and the station just above the entrance to Lac St. Pierre.The average concentrations of dissolved cadmium, lead, zinc, copper and nickel ranged from 7–23 ng/l; 9–35 ng/l; 0.434–0.939 g/l; 0.15–0.89 g/l and 0.58–1.12 g/l respectively.Regression analysis of the dissolved and particulate metal concentrations suggests that the concentration of dissolved cadmium, lead, zinc and nickel can be predicted from the regression equation and the determination of particulate metal concentration. This prediction appears to be independent of the suspended particulate matter concentration which varies from ca. 1 to 10 mg/l from the outflow of Lake Ontario to Québec City. 相似文献
136.
On the practice of estimating fractal dimension 总被引:11,自引:0,他引:11
Coastlines epitomize deterministic fractals and fractal (Hausdorff-Besicovitch) dimensions; a divider [compass] method can be used to calculate fractal dimensions for these features. Noise models are used to develop another notion of fractals, a stochastic one. Spectral and variogram methods are used to estimate fractal dimensions for stochastic fractals. When estimating fractal dimension, the objective of the analysis must be consistent with the method chosen for fractal dimension calculation. Spectal and variogram methods yield fractal dimensions which indicate the similarity of the feature under study to noise (e.g., Brownian noise). A divider measurement method yields a fractal dimension which is a measure of complexity of shape. 相似文献
137.
A new multidimensional scaling (MS) technique, referred to as the Pijk model, is formulated on the basis of associations among triple objects (samples or variables), instead of pairs of objects as used in the usual MS methods, such as factor analysis. The computational scheme provided for this method is the reduction of an original problem to a standard eigenvalue-eigenvector problem. The major goal of the technique is simplification and reduction of data structures and the rescaling of original objects into a new and reduced space, so that patterns and relations of the original objects can be conventiently examined in two-dimensional factor plots. The Pïjk method is illustrated and tested by using a set of geochemical data related to the epithermal gold and silver vein deposits in the Walker Lake quadrangle of Nevada and California. The characteristics of element associations suggested in the Pijk analysis are consistent with field observations. A preliminary comparison between the new method and the ordinary factor analysis also is made on the basis of the same data set. Results are encouraging in that analysis by the Pijk model captures triple-object associations that might be missed by the ordinary factor analysis which considers only pair-variable correlations 相似文献
138.
139.
The true potential energy curves for the electronic ground states of astrophysically important AlH and CaH molecules are constructed by the Rydberg-Klein-Rees method. Empirical potential functions, of three-parameters by Lippincott, of five-parameters by Hulburt and Hirsch-felder and, of electronegativity by Szöke and Baitz, are examined for the adequacy to represent the true curve. From the best-fitting function, the dissociation energiesD 0 0 of AlH and CaH molecules are estimated to be 2.99 ± 0.08 and 2.72 ± 0.06 eV respectively. The force constants indicate that these values are of correct magnitude. 相似文献
140.
The main structural characteristics of the Caggiano and Polla faults, exposed in the epicentral area of the 1561 earthquake (Mw = 6.4), southern Italy, have been investigated in detail to assess their spatial and temporal properties, and to evaluate their seismogenic potential. These right stepping normal faults show an overlap of about 7 km and an across strike separation of about 4 km. The geometric relationships between the Caggiano and Polla faults, but also the displacement distribution along each fault, demonstrate that they have been strongly interacting throughout the Pleistocene. Nevertheless, geological evidence of Holocene tectonic activity was mainly recognized along the Caggiano Fault (faulted late glacial deposits) and in the southernmost part of the Polla Fault (faulted deposits of probably Late Pleistocene age). This suggests that the Caggiano Fault can be considered as the most tectonically active fault in the Vallo di Diano Fault System. By calculating Coulomb stress changes, we have constrained modes of mechanical interactions between the two faults in a scenario compatible with the 1561 earthquake. This approach allows us to argue that both the Caggiano and the Polla Faults are probably linked at depth, and part of the same seismogenic structure which may be potentially responsible for composite ruptures with magnitude ≥ 6.5. 相似文献