全文获取类型
收费全文 | 98483篇 |
免费 | 5190篇 |
国内免费 | 5716篇 |
专业分类
测绘学 | 3095篇 |
大气科学 | 10189篇 |
地球物理 | 20462篇 |
地质学 | 38519篇 |
海洋学 | 9179篇 |
天文学 | 18622篇 |
综合类 | 2655篇 |
自然地理 | 6668篇 |
出版年
2022年 | 1319篇 |
2021年 | 1796篇 |
2020年 | 1695篇 |
2019年 | 1846篇 |
2018年 | 3111篇 |
2017年 | 2906篇 |
2016年 | 3610篇 |
2015年 | 2239篇 |
2014年 | 3479篇 |
2013年 | 5179篇 |
2012年 | 3502篇 |
2011年 | 4266篇 |
2010年 | 3840篇 |
2009年 | 4690篇 |
2008年 | 4058篇 |
2007年 | 3981篇 |
2006年 | 3681篇 |
2005年 | 3046篇 |
2004年 | 2836篇 |
2003年 | 2794篇 |
2002年 | 2710篇 |
2001年 | 2419篇 |
2000年 | 2480篇 |
1999年 | 2468篇 |
1998年 | 2311篇 |
1997年 | 2334篇 |
1996年 | 2104篇 |
1995年 | 1899篇 |
1994年 | 1638篇 |
1993年 | 1466篇 |
1992年 | 1338篇 |
1991年 | 1219篇 |
1990年 | 1216篇 |
1989年 | 1014篇 |
1988年 | 1019篇 |
1987年 | 1056篇 |
1986年 | 962篇 |
1985年 | 1096篇 |
1984年 | 1200篇 |
1983年 | 1120篇 |
1982年 | 1038篇 |
1981年 | 946篇 |
1980年 | 908篇 |
1979年 | 884篇 |
1978年 | 870篇 |
1977年 | 735篇 |
1976年 | 679篇 |
1975年 | 694篇 |
1974年 | 679篇 |
1973年 | 708篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
852.
Song H. Kuperman W.A. Hodgkiss W.S. Gerstoft P. Jea Soo Kim 《Oceanic Engineering, IEEE Journal of》2003,28(2):250-261
Adaptive-array beamforming achieves high resolution and sidelobe suppression by producing sharp s in the adaptive beampattern. Large-aperture sonar arrays with many elements have small resolution cells; interferers may move through many resolution cells in the time required for accumulating a full-rank sample covariance matrix. This leads to "snapshot-deficient" processing. In this paper, the -broadening technique originally developed for an ideal stationary problem is extended to the snapshot-deficient problem combined with white-noise constraint (WNC) adaptive processing. Null broadening allows the strong interferers to move through resolution cells and increases the number of degrees of freedom, thereby improving the detection of weak stationary signals. 相似文献
853.
本文给出了吉尼系数的一般计算公式 ,推导出正态分布的吉尼系数渐进无偏估计 ,并以实例对比了传统的吉尼系数的算法——万分法与本文中提出的估计方法。 相似文献
854.
The relationship between the RMS amplitudes of the wind wave spectral components and the wind speed has been studied at ten frequencies in the band of 0.65–23 Hz. To measure the parameters of the high-frequenci waves, a resistance elevation wave gauge was operated, which was deployed in the Black See on an oceanographic platform near Katsively. The correlation between the wave amplitudes and the wind velocity at high frequencies of 5–23 Hz, corresponding to gravitation-capillary ripples, was found to reach a value of 0.8. At lower frequencies of 0.65–4.3 Hz, corresponding to short gravity waves, it dropped to 0.5–0.7. The response of spectral components to the wind speed variations in the gravity-capillary range is higher than in the range of short gravity waves. The results obtained differ from Phillips' idea about a saturated range for the frequency form of the spectrum of high-frequency gravity waves, since a linear dependence of the spectral amplitudes on the wind speed is established at a wind of force 1–8.Translated by Mikhail M. Trufanov. 相似文献
855.
To estimate the source and diagenetic state of organic matter reaching bottom sediments, fatty acids and sterols were measured in unconsolidated surface material (flocs) at 12 sites ranging from 600 to 2000 m across the mid-Atlantic continental slope off Cape Hatteras, North Carolina. Total free and esterefied fatty acids were similar in distribution and concentration to other coastal systems, with values ranging from 0.64 to 46.52 μg mg−1 organic carbon (1.10–68.85 μg g−1 dry sediment). Although shallow (600 m) stations contained significantly greater fatty acid concentrations than deep (> 1400m) stations, high variability observed at mid-depth (800 m) collections precluded a consistent relationship between total fatty acid concentration and station depth. At three sites where underlying sediments were also collected, decreases in total fatty acids, reduced amounts of polyenoic acids and significant presence of bacterial fatty acid suggest rapid reworking of labile organic material that reaches the sediment surface. The distribution of sterols was remarkably consistent among all sites even though there were large variations in concentrations (1.8–20.7 μg mg−1 organic carbon). Sterol composition indicated phytoplankton, principally diatoms and dinoflagellates, as the principal source of labile organic matter to sediments, together with a significant input of cholest-5-en-3β-ol typical of zooplankton and their feeding activity. A minor but widespread terrigenous input was also evident based upon significant concentrations of sterols dominant in vascular plants. 相似文献
856.
857.
Alkalinity, dissolved inorganic carbon (DIC), and 13C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative 13C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the13C-depletions are attributed to the release of CO2 by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The 13C composition of the migrating CO2 is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and 13C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory. 相似文献
858.
Trends in biomass, density and diversity of North Sea macrofauna 总被引:6,自引:2,他引:6
859.
860.
This article describes the impact of satellite altimeter data on the simulations of sea level variability (SLV) by a nonlinear reduced gravity model of the entire Indian Ocean. The model has been forced by 6-hourly analyzed wind stress data containing SSM/I observations and has been able to produce realistic circulation features. However, SLV values observed by Topex/Poseidon altimeter do not fit these simulations because of imperfect initial data. Hence an attempt has been made to initialize the model using altimeter data. The initialized model-generated SLVvalues have been compared with SLV derived by altimeter for monsoon as well as nonmonsoon months of 1996. Experimental runs have been performed for 10 days, 20 days, and one month. It has been found that the initialized model results on the final day of these experiments are in very good agreement with altimeter data of the same day. It is thus possible, in principle, to hindcast and forecast sea level variations in the time scale of 10 days to one month with the availability of good quality wind data for forcing the model and altimeter observations of sea level for initializing it. 相似文献