全文获取类型
收费全文 | 2861篇 |
免费 | 445篇 |
国内免费 | 642篇 |
专业分类
测绘学 | 190篇 |
大气科学 | 536篇 |
地球物理 | 677篇 |
地质学 | 1527篇 |
海洋学 | 329篇 |
天文学 | 157篇 |
综合类 | 285篇 |
自然地理 | 247篇 |
出版年
2024年 | 12篇 |
2023年 | 36篇 |
2022年 | 113篇 |
2021年 | 135篇 |
2020年 | 103篇 |
2019年 | 109篇 |
2018年 | 157篇 |
2017年 | 118篇 |
2016年 | 149篇 |
2015年 | 137篇 |
2014年 | 160篇 |
2013年 | 127篇 |
2012年 | 144篇 |
2011年 | 149篇 |
2010年 | 156篇 |
2009年 | 132篇 |
2008年 | 138篇 |
2007年 | 139篇 |
2006年 | 92篇 |
2005年 | 115篇 |
2004年 | 85篇 |
2003年 | 112篇 |
2002年 | 116篇 |
2001年 | 107篇 |
2000年 | 109篇 |
1999年 | 155篇 |
1998年 | 91篇 |
1997年 | 108篇 |
1996年 | 110篇 |
1995年 | 87篇 |
1994年 | 69篇 |
1993年 | 77篇 |
1992年 | 64篇 |
1991年 | 41篇 |
1990年 | 42篇 |
1989年 | 32篇 |
1988年 | 24篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1966年 | 1篇 |
1964年 | 2篇 |
1958年 | 3篇 |
排序方式: 共有3948条查询结果,搜索用时 15 毫秒
131.
132.
133.
Shallow water depth retrieval from space-borne SAR imagery 总被引:1,自引:1,他引:0
Kaiguo Fan Weigen Huang Hui Lin Jiayi Pan Bin Fu Yanzhen Gu 《Journal of Oceanography》2011,67(4):405-413
Based on shallow water bathymetry synthetic aperture radar (SAR) imaging mechanism and the microwave scattering imaging model
for oceanic surface features, we developed a new method for shallow water depth retrieval from space-borne SAR images. The
first guess of surface currents and winds are estimated from the normalized radar crossing section (NRCS) profile of shallow
water bathymetry SAR imagery, according to the linear theory and geophysical model function. The NRCS profile is then simulated
by the microwave scattering imaging model. Both the surface currents and winds are adjusted by using the dichotomy method
step by step to make the M4S-simulated NRCS profiles approach those observed by SAR. Then, the surface currents and the wind
speeds are retrieved when a best fit between simulated signals and the SAR image appears. Finally, water depths are derived
using the Navier–Stokes equation and finite difference method with the best estimated currents and the surface winds. The
method is tested on two SAR images of the Taiwan Shoal. Results show that the simulated shallow water NRCS profile is in good
agreement with those measured by SAR with the correlation coefficient as high as 85%. In addition, when water depths retrieved
from the SAR image are compared with in situ measurements, both the root mean square and relative error are less than 3.0 m
and 6.5%, respectively, indicating that SAR images are useful for shallow water depth retrieval and suggesting that the proposed
method in this paper is convergent and applicable. 相似文献
134.
Assessing adaptability of planted trees using leaf traits: A case study with Robinia pseudoacacia L. in the Loess Plateau, China 总被引:1,自引:0,他引:1
Leaf trait patterns and their variations with climate are interpreted as an adaptive adjustment to environment.This study assessed the adaptability of planted black locust (Robinia pseudoacacia L.) based on the analysis of leaf traits and the comparison of its leaf traits with inter-specific ones existing in the same area.We measured some water and N use related leaf traits: leaf dry mass per unit area (LMA) and N,P and K concentrations based on both leaf area (Narea,Parea and Karea) and leaf mass (Nmass,Pm... 相似文献
135.
Monitoring Muztagh Kuksai glacier surface velocity with L-band SAR data in southwestern Xinjiang, China 总被引:1,自引:1,他引:0
This study presents the results of measuring surface velocity fields of alpine glaciers with multi-temporal ALOS/PALSAR synthetic aperture radar (SAR) data using the normalized cross-correlation technique. The main limitations of spaceborne optical data and interferometric SAR for retrieving reliable interferometric product are firstly discussed. Next, an efficient method of deriving surface velocity with spaceborne SAR system using the intensity-tracking method based on the cross-correlation optimization is expounded. This method is an invaluable alternative way of estimating the glacier surface field when the differential SAR interferometry is limited by the loss of coherence and can also overcome the drawback of the lack of available optical data in alpine glacier areas. The method was successfully applied in the region of the Muztagh Kuksai Glacier using ALOS/PALSAR L-band data with 44 days temporal separation. The glacier surface velocity fields were obtained for two different time intervals (14 January to 11 March 2009 and 1 September to 17 October 2009). The accuracy of the method was estimated from the residual value in the non-glacier area, which is often used to evaluate the performance of the approach for monitoring the glacier surface velocity. Finally the distribution maps of the surface velocity fields on the Muztagh Kuksai Glacier were retrieved with an accuracy of 0.3 m (SD 0.2 m) using L-band spaceborne SAR data. The results show that overall average velocity for 1 September to 17 October in 2009 is about 11 % faster than that for 14 January to 11 March in 2009. It is concluded that the potential of monitoring the long-term evolution of glaciers with SAR data should be emphasized. 相似文献
136.
泥质烃源岩密度分级分离与有机碳分配 总被引:4,自引:0,他引:4
选取东营凹陷第三系11 块泥质烃源岩样品,粉碎后按小于1.6 g/cm3,1.6~2.2 g/cm3 和大于2.2 g/cm3 的密度,分离成1,2,3 三个组分,希望可以把烃源岩中游离的有机质、无机矿物与有机黏土复合体分别提取出来。对各组分进行质量、有机碳及扫描电镜分析,探讨不同密度组分中有机质赋存形式及有机碳分配方式的差异性。密度分组后各组分的质量、有机碳量回收率基本上达到100%,表明密度分离方法可信,且对烃源岩的破坏较弱。通过扫描电镜观察发现,1 组分中存在大量的生物碎屑有机质,经盐酸处理后,有机质颗粒表面干净,而2,3 组分中只见有大量的矿物颗粒,但能谱分析显示了C 元素的存在。这些特征表明1 组分为生物碎屑有机质,而2,3 组分中有机质与无机矿物相混合。有机碳分析表明,1 组分有机碳含量平均高达14.24%,但其仅占全岩有机碳量的1.11%;2,3 组分中有机碳含量分别为3.26%,2.77%,两者占全岩有机碳量的90% 以上。由此看来与无机矿物相结合是烃源岩中有机质较为重要的赋存方式,这部分有机质对全岩生烃量的贡献不容忽视。 相似文献
137.
Jian Hua Li Zheng Ping Zhang Rui Huan Jing Chong Fu Huang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(4):485-492
In this paper, we introduce a value chain approach, on the view of the value chain analytic in business management, to be
the framework of a crisis response system serving for crisis management strategy in identifying risk sources, responding to
unexpected events, and recovering from a shock. This system is not only a computer system but also a system of governmental
emergency response mechanism and it is tremendous and complex. The system integrates and coordinates the correlative resources
of emergency response units of the city. We consider the efficiency of the system. In addition, the run cost of the system
also is taken into account. Based on this framework, the Qingdao’s practical situation is analyzed. As a result, a design
proposal of the Qingdao’s crisis response system is put forward. 相似文献
138.
139.
140.
Xiao-Bing?Li Dongfang?Wang Qing-Chang?Lu Zhong-Ren?PengEmail author Qingyan?Fu Xiao-Ming?Hu Juntao?Huo Guangli?Xiu Bai?Li Chao?Li Dong-Sheng?Wang Hanyu?Wang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(5):1189-1203
A lightweight unmanned aerial vehicle (UAV) and a tethered balloon platform were jointly used to investigate three-dimensional distributions of ozone and PM2.5 concentrations within the lower troposphere (1000 m) at a localized coastal area in Shanghai, China. Eight tethered balloon soundings and three UAV flights were conducted on May 25, 2016. Generalized additive models (GAMs) were used to quantitatively describe the relationships between air pollutants and other obtained parameters. Field observations showed that large variations were captured both in the vertical and horizontal distributions of ozone and PM2.5 concentrations. Significant stratified layers of ozone and PM2.5 concentrations as well as wind directions were observed throughout the day. Estimated bulk Richardson numbers indicate that the vertical mixing of air masses within the lower troposphere were heavily suppressed throughout the day, leading to much higher concentrations of ozone and PM2.5 in the planetary boundary layer (PBL). The NO and NO2 concentrations in the experimental field were much lower than that in the urban area of Shanghai and demonstrated totally different vertical distribution patterns from that of ozone and PM2.5. This indicates that aged air masses of different sources were transported to the experimental field at different heights. Results derived from the GAMs showed that the aggregate impact of the selected variables for the vertical variations can explain 94.3% of the variance in ozone and 94.5% in PM2.5. Air temperature, relative humidity and atmospheric pressure had the strongest effects on the variations of ozone and PM2.5. As for the horizontal variations, the GAMs can explain 56.3% of the variance in ozone and 57.6% in PM2.5. The strongest effect on ozone was related to air temperature, while PM2.5 was related to relative humidity. The output of GAMs also implied that fine aerosol particles were in the stage of growth in the experimental field, which is different from ozone (aged air parcels of ozone). Geographical parameters influenced the horizontal variations of ozone and PM2.5 concentrations by changing underlying surface types. The differences of thermodynamic properties between land and sea resulted in quick changes of PBL height, air temperature and dew point over the coastal area, which was linked to the extent of vertical mixing at different locations. The results of GAMs can be used to analyze the sources and formation mechanisms of ozone and PM2.5 pollutions at a localized area. 相似文献