全文获取类型
收费全文 | 21194篇 |
免费 | 4078篇 |
国内免费 | 5476篇 |
专业分类
测绘学 | 1520篇 |
大气科学 | 4160篇 |
地球物理 | 5571篇 |
地质学 | 10909篇 |
海洋学 | 3129篇 |
天文学 | 789篇 |
综合类 | 2001篇 |
自然地理 | 2669篇 |
出版年
2024年 | 109篇 |
2023年 | 375篇 |
2022年 | 940篇 |
2021年 | 1103篇 |
2020年 | 875篇 |
2019年 | 1032篇 |
2018年 | 1128篇 |
2017年 | 1050篇 |
2016年 | 1200篇 |
2015年 | 1027篇 |
2014年 | 1182篇 |
2013年 | 1379篇 |
2012年 | 1281篇 |
2011年 | 1294篇 |
2010年 | 1371篇 |
2009年 | 1266篇 |
2008年 | 1148篇 |
2007年 | 1187篇 |
2006年 | 920篇 |
2005年 | 863篇 |
2004年 | 616篇 |
2003年 | 619篇 |
2002年 | 609篇 |
2001年 | 619篇 |
2000年 | 693篇 |
1999年 | 942篇 |
1998年 | 764篇 |
1997年 | 766篇 |
1996年 | 729篇 |
1995年 | 648篇 |
1994年 | 571篇 |
1993年 | 488篇 |
1992年 | 414篇 |
1991年 | 285篇 |
1990年 | 230篇 |
1989年 | 189篇 |
1988年 | 182篇 |
1987年 | 105篇 |
1986年 | 108篇 |
1985年 | 70篇 |
1984年 | 61篇 |
1983年 | 55篇 |
1982年 | 56篇 |
1981年 | 39篇 |
1980年 | 41篇 |
1979年 | 33篇 |
1978年 | 15篇 |
1977年 | 10篇 |
1975年 | 10篇 |
1958年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
311.
As a new method, the ultraviolet spectrum technique is applied to studying the connectivity of biode-gradable heavy oil reservoirs. The similarity of crude oils can be judged according to the extinction coefficient (E) because aromatic hydrocarbons and non-hydrocarbons have conjugated bonds and obvious absorption in the ultraviolet range, and different materials have different characteristics and additives. The relationship diagram is made in terms of the extinction coefficients (E) of the samples by taking E as the Y-axis and wavelength as the X-axis. The connectivity of oil reservoirs is estimated according to the curve positions and characteristic fingerprints of the sampies. The connectivity of part of the reservoirs in the western part of the QHD32-6 oilfield was studied with this method. The results showed that the connectivity of samples from wells F7 and F8 in the Nm-2 oil reservoir zone is good, that of samples from wells F17 and F20 in the Nm-1 oil reservoir zone also is good, and that of samples from wells F17, F19, and F20 is poor. 相似文献
312.
LIU Luofu HUO Hong GUO Yongqiang CHEN Lixin LI Shuangwen ZHAO Yande LI Yan WANG Ping CHEN Zhijun 《中国地球化学学报》2008,27(1):1-8
The results presented in this paper indicated that the carbazole-type compounds have high thermal stability and also show stability in oxidation and bio-degradation. This kind of compounds still has high concentrations and a complete distribution in the analyzed dry asphalt samples, showing that they are particularly useful in the study of hydrocarbon migration of the paleo-pools. The difference in the contents of nitrogen compounds in the Silurian dry asphalts from the Awati, Tabei and Tazhong areas is attributed to the difference in the extent of oxidation and (or) bio-degradation for the areas; the Awati and Tabei areas underwent relatively strong oxidation and bio-degradation. During the first stage of hydrocarbon pool formation in the Silurian system in the Tazhong and Tabei areas of the Tarim Basin (at the end of the Silurian period) and at the second stage in the Awati area (in Permian), the hydrocarbons experienced a long-distance migration. 相似文献
313.
Jianqiang Ren Zhongxin Chen Qingbo Zhou Huajun Tang 《International Journal of Applied Earth Observation and Geoinformation》2008,10(4):403
The significance of crop yield estimation is well known in agricultural management and policy development at regional and national levels. The primary objective of this study was to test the suitability of the method, depending on predicted crop production, to estimate crop yield with a MODIS-NDVI-based model on a regional scale. In this paper, MODIS-NDVI data, with a 250 m resolution, was used to estimate the winter wheat (Triticum aestivum L.) yield in one of the main winter-wheat-growing regions. Our study region is located in Jining, Shandong Province. In order to improve the quality of remote sensing data and the accuracy of yield prediction, especially to eliminate the cloud-contaminated data and abnormal data in the MODIS-NDVI series, the Savitzky–Golay filter was applied to smooth the 10-day NDVI data. The spatial accumulation of NDVI at the county level was used to test its relationship with winter wheat production in the study area. A linear regressive relationship between the spatial accumulation of NDVI and the production of winter wheat was established using a stepwise regression method. The average yield was derived from predicted production divided by the growing acreage of winter wheat on a county level. Finally, the results were validated by the ground survey data, and the errors were compared with the errors of agro-climate models. The results showed that the relative errors of the predicted yield using MODIS-NDVI are between −4.62% and 5.40% and that whole RMSE was 214.16 kg ha−1 lower than the RMSE (233.35 kg ha−1) of agro-climate models in this study region. A good predicted yield data of winter wheat could be got about 40 days ahead of harvest time, i.e. at the booting-heading stage of winter wheat. The method suggested in this paper was good for predicting regional winter wheat production and yield estimation. 相似文献
314.
315.
福鼎山后尖玄武岩位于福鼎一福清北东向断裂带北东段,呈岩筒状产于石帽山群下组下段火山沉积岩中,属钠质碱性玄武岩系列,由玄武质火山角砾岩、碧玄岩、橄榄辉长一玄武岩组合,经历火山爆发、喷溢、次火山侵入3个阶段而形成。 相似文献
316.
通过对粤东地区地层和岩石中成矿元素含量和区域地质背景的分析,认为粤东地区铜铅-锌-银成矿作用与燕山期的构造-岩浆作用关系密切,成矿物质中银-铜-锌以下地壳来源为主,铅则以上地壳及沉积地层来源为主。 相似文献
317.
318.
319.
320.