全文获取类型
收费全文 | 1409篇 |
免费 | 293篇 |
国内免费 | 301篇 |
专业分类
测绘学 | 278篇 |
大气科学 | 333篇 |
地球物理 | 351篇 |
地质学 | 534篇 |
海洋学 | 224篇 |
天文学 | 48篇 |
综合类 | 126篇 |
自然地理 | 109篇 |
出版年
2024年 | 11篇 |
2023年 | 18篇 |
2022年 | 33篇 |
2021年 | 56篇 |
2020年 | 57篇 |
2019年 | 69篇 |
2018年 | 62篇 |
2017年 | 86篇 |
2016年 | 81篇 |
2015年 | 87篇 |
2014年 | 94篇 |
2013年 | 106篇 |
2012年 | 108篇 |
2011年 | 74篇 |
2010年 | 102篇 |
2009年 | 73篇 |
2008年 | 81篇 |
2007年 | 86篇 |
2006年 | 79篇 |
2005年 | 68篇 |
2004年 | 61篇 |
2003年 | 50篇 |
2002年 | 61篇 |
2001年 | 58篇 |
2000年 | 59篇 |
1999年 | 53篇 |
1998年 | 44篇 |
1997年 | 44篇 |
1996年 | 23篇 |
1995年 | 23篇 |
1994年 | 25篇 |
1993年 | 18篇 |
1992年 | 16篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2003条查询结果,搜索用时 0 毫秒
1.
2.
Martin Groenewegen 《Astrophysics and Space Science》1995,224(1-2):321-324
The relation between mass loss rate and pulsation period in carbon Miras is discussed. The dust mass loss rate is very low (about 2 × 10–10 M/yr) up to aboutP = 380 days, where there is a sudden increase. ForP > 400 days there is a linear relation between logM andP. The change in the mass loss rate near 380 days may be related to radiation pressure on dust becoming effective in driving the outflow. 相似文献
3.
We investigate the risk that supplemental feeding of white-tailed deer (Odocoileus virginianus) to improve trophy quality may degrade rangeland through overutilization of forage plants near the feeders. We divided a South Texas ranch into 6 areas (each 1100 ha). Supplemental feed was provided year-round in a centrally placed feeder in 3 areas, no supplement was provided in the 3 control areas. Radio-telemetry collars were fitted on 2 bucks and 2 does in each area. Spatial distribution of these deer was assessed using the CALHOME program. Feed supplementation did not alter home range size of does but did reduce their 50% core area. Bucks without supplemental feed showed 2 spatial strategies; sedentary bucks had home range distributions similar to supplemented bucks, while commuter bucks moved to supplemented areas during the breeding season. Browsing pressure near feeders and control sites was assessed by measuring consumption of seedlings set in arrays radiating from feeders and control points. Deer browsing was heavier near feeders than at the control sites, and the probability of a seedling being browsed increased with proximity to the feeder. We caution against long-term supplemental feeding in fixed locations because of the potential for localized range degradation around the feeders. 相似文献
4.
A generic network design in close range photogrammetry is one where optimal multi-ray intersection geometry is obtained with as few camera stations as practicable. Hyper redundancy is a concept whereby, once the generic network is in place, many additional images are recorded, with the beneficial impact upon object point precision being equivalent to the presence of multiple exposures at each camera position within the generic network. The effective number of images per station within a hyper redundant network might well be in the range of 10 to 20 or more. As is apparent when it is considered that a hyper redundant network may comprise hundreds of images, the concept is only applicable in practice to fully automatic vision metrology systems, where it proves to be a very effective means of enhancing measurement accuracy at the cost of minimal additional work in the image recording phase. This paper briefly reviews the network design and accuracy aspects of hyper redundancy and illustrates the technique by way of the photogrammetric measurement of surface deformation of a radio telescope of 26 m diameter. This project required an object point measurement accuracy of σ = 0·065 mm, or 1/400 000 of the diameter of the reflector. 相似文献
5.
6.
Habitat, occurrence and conservation of Saharo-Arabian-Turanian element Forsskaolea tenacissima L. in the Iberian Peninsula 总被引:1,自引:0,他引:1
Javier Cabello Domingo Alcaraz Francisco Gmez-Mercado Juan F. Mota Javier Navarro Julio Peas Esther Gimnez 《Journal of Arid Environments》2003,53(4):491-500
The aim of this study is to assess the Iberian populations of Forsskaolea tenacissima L. according to its biogeographical interest, habitat, geographical range and conservation status. Results point out that they are restricted to gravel wadis of Tabernas Desert (SE Spain), are scarcely included in protected areas and represent historically isolated populations with relict behaviour. We also describe a new association, Senecioni-Forsskaoleetum tenacissimae. Conservation status of species is cause for concern and two conservation actions must be carried out. Firstly, protected areas should house Forsskaolea populations and secondly, phytosociological characterization of a community allows inventorying its habitat and directing conservation efforts to community level. 相似文献
7.
Simulation of seismic response in a city-like environment 总被引:1,自引:0,他引:1
Jean-Philippe Groby Chrysoula Tsogka Armand Wirgin 《Soil Dynamics and Earthquake Engineering》2005,25(7-10):487-504
We study the seismic response of idealized 2D cities, constituted by non-equally spaced, non-equally sized homogenized blocks anchored in a soft layer overlying a hard half space. The blocks and soft layer are occupied by dissipative media. To simulate such response, we use an approximation of the viscoelastic modulus by a low-order rational function of frequency and incorporate this approximation into a first-order-in-time scheme. Our results display spatially variable, strong, long-duration responses inside the blocks and on the ground, which qualitatively match the responses observed in some earthquake-prone cities of Mexico, France, the USA, etc. 相似文献
8.
Climatology of Ozone, PAN, CO, and NMHC in the Free Troposphere Over the Southern North Atlantic 总被引:1,自引:0,他引:1
Free tropospheric measurements of ozone, peroxyacetylnitrate andprecursors (CO, NMHC) that were made within the framework of the EUROTRACsub-project TOR (Tropospheric Ozone Research) between 1990 and 1995 at theGAW station Izana, Tenerife (28°18N, 16°30W) arediscussed. The average annual cycles reveal the importance of transport fromnorthern mid-latitudes and the role of photo-chemistry. According toair-mass trajectories, which were supplied to us from AEROCE(Atmosphere/Ocean Chemistry Experiment), transport from northernmid-latitudes is associated with high precursor concentrations in winter,whereas ozone concentrations in winter are not much influenced by transportpatterns, suggesting a rather uniform distribution over the northern part ofthe Northern Hemisphere around mean value of 43 ± 5 ppb. In summer,high ozone concentrations of up to 90 ppb are often encountered duringtransport from north, while the levels of precursors are much lower than inwinter, because of photochemical destruction. Trajectories from southerlylatitudes and the Sahara usually have the lowest ozone concentrationsassociated with them. 相似文献
9.
10.
华北燕山地区雾迷山组疑源类化石组合及其特征 总被引:1,自引:0,他引:1
本文通过系统采样,利用化学浸泡法,对冀北坳陷凌源地区和宣龙坳陷北京延庆地区的雾迷山组疑源类进行了研究,并划分出四个化石组合,自下而上分别为:罗庄组合(Asperatopsophosphaera umishanensis-Oscillatori-opsis luozhuangensis)、磨盘峪组合(Stictosphaeridium pectinale-Micrhystridium pallidum)、二十里堡组合(Pale-amorpha punctulata-Orygmatosphaeridium rubignosum)和闪坡岭组合(Microconcentrica cymata-Baltisphaeridium cerinum).通过两个地区疑源类组合特征的对比,认为凌源地区和延庆地区雾迷山组微疑源类化石组合及特征具有可对比性. 相似文献