全文获取类型
收费全文 | 3112篇 |
免费 | 458篇 |
国内免费 | 463篇 |
专业分类
测绘学 | 590篇 |
大气科学 | 199篇 |
地球物理 | 510篇 |
地质学 | 1311篇 |
海洋学 | 501篇 |
天文学 | 433篇 |
综合类 | 257篇 |
自然地理 | 232篇 |
出版年
2024年 | 6篇 |
2023年 | 25篇 |
2022年 | 107篇 |
2021年 | 133篇 |
2020年 | 126篇 |
2019年 | 151篇 |
2018年 | 102篇 |
2017年 | 145篇 |
2016年 | 148篇 |
2015年 | 162篇 |
2014年 | 187篇 |
2013年 | 195篇 |
2012年 | 190篇 |
2011年 | 202篇 |
2010年 | 139篇 |
2009年 | 196篇 |
2008年 | 190篇 |
2007年 | 199篇 |
2006年 | 154篇 |
2005年 | 161篇 |
2004年 | 167篇 |
2003年 | 143篇 |
2002年 | 127篇 |
2001年 | 82篇 |
2000年 | 106篇 |
1999年 | 63篇 |
1998年 | 69篇 |
1997年 | 74篇 |
1996年 | 48篇 |
1995年 | 46篇 |
1994年 | 38篇 |
1993年 | 26篇 |
1992年 | 23篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1978年 | 2篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
1882年 | 2篇 |
1880年 | 2篇 |
1877年 | 7篇 |
1875年 | 3篇 |
排序方式: 共有4033条查询结果,搜索用时 15 毫秒
991.
A series of long-term 1D oedometer test on compacted loess has been conducted using a high pressure consolidometer. It is found that the compacted loess has remarkable creep deformation and the proportion of the creep deformation in total deformation can come up to 6%-23%. The creep deformation proportion goes higher as the water content of the sample increases and the compaction degree reduces. And the creep deformation proportion decreases with the increase of stress level. A nonlinear creep model, which can well describe the rules of post-construction settlement of compacted loess, is proposed. The validity of model is verified by comparing the results from proposed model with those from the layer-wise summation method. The proposed model is employed to examine the variation law of the post-construction settlement under the conditions of the different water contents and compaction degrees. It is found that there exist logarithmic relationships between the post-construction settlement and the compaction degree/water contents. Supposing the velocity of the post-construction settlement v<0.1 mm/d is the stabilized standard, the high embankment settlement trends to be stable in 200 to 650 days after construction. The higher the compaction degree is, the lower the water content is, the longer the time up to stable required is. 相似文献
992.
K.G. Strassmeier I. Ilyin A. Jrvinen M. Weber M. Woche S. I. Barnes S.‐M. Bauer E. Beckert W. Bittner R. Bredthauer T. A. Carroll C. Denker F. Dionies I. DiVarano D. Dscher T. Fechner D. Feuerstein T. Granzer T. Hahn G. Harnisch A. Hofmann M. Lesser J. Paschke S. Pankratow V. Plank D. Plüschke E. Popow D. Sablowski J. Storm 《Astronomische Nachrichten》2015,336(4):324-361
PEPSI is the bench‐mounted, two‐arm, fibre‐fed and stabilized Potsdam Echelle Polarimetric and Spectroscopic Instrument for the 2×8.4 m Large Binocular Telescope (LBT). Three spectral resolutions of either 43 000, 120 000 or 270 000 can cover the entire optical/red wavelength range from 383 to 907 nm in three exposures. Two 10.3k×10.3k CCDs with 9‐µm pixels and peak quantum efficiencies of 94–96 % record a total of 92 échelle orders. We introduce a new variant of a wave‐guide image slicer with 3, 5, and 7 slices and peak efficiencies between 92–96 %. A total of six cross dispersers cover the six wavelength settings of the spectrograph, two of them always simultaneously. These are made of a VPH‐grating sandwiched by two prisms. The peak efficiency of the system, including the telescope, is 15 % at 650 nm, and still 11 % and 10 % at 390 nm and 900 nm, respectively. In combination with the 110 m2 light‐collecting capability of the LBT, we expect a limiting magnitude of ≈20th mag in V in the low‐resolution mode. The R = 120 000 mode can also be used with two, dual‐beam Stokes IQUV polarimeters. The 270 000‐mode is made possible with the 7‐slice image slicer and a 100‐µm fibre through a projected sky aperture of 0.74″, comparable to the median seeing of the LBT site. The 43 000‐mode with 12‐pixel sampling per resolution element is our bad seeing or faint‐object mode. Any of the three resolution modes can either be used with sky fibers for simultaneous sky exposures or with light from a stabilized Fabry‐Pérot étalon for ultra‐precise radial velocities. CCD‐image processing is performed with the dedicated data‐reduction and analysis package PEPSI‐S4S. Its full error propagation through all image‐processing steps allows an adaptive selection of parameters by using statistical inferences and robust estimators. A solar feed makes use of PEPSI during day time and a 500‐m feed from the 1.8 m VATT can be used when the LBT is busy otherwise. In this paper, we present the basic instrument design, its realization, and its characteristics. Some pre‐commissioning first‐light spectra shall demonstrate the basic functionality. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
993.
994.
基于MODIS数据的湖北省油菜种植分布信息提取 总被引:1,自引:0,他引:1
MODIS归一化差值植被指数(MODIS-normalized difference vegetation index,MODIS-NDVI)时间序列产品能够连续反映植被的覆盖情况,是农作物遥感测量的重要数据源。为研究基于MODIS数据的油菜种植分布信息提取技术,选取湖北省为研究区,利用2008—2013年75个时相的MODIS-NDVI时序数据,结合农作物物候和地面调查样本等辅助资料,通过建立油菜种植面积提取模型,采用多次阈值比较方法提取了2009—2013年湖北省油菜种植分布信息,与统计数据比较,总体提取精度为85%左右。最后利用环境小卫星HJ-1A CCD数据进行精度验证,证明了MODIS-NDVI时序数据及本文方法在油菜种植面积提取中的可靠性,对掌握油菜种植面积和产量信息、加强农业生产管理、调整农业结构及辅助政府有关部门制定科学合理的农业政策具有重要意义。 相似文献
995.
996.
利用暗目标法从高分一号卫星16 m相机数据反演气溶胶光学厚度 总被引:5,自引:3,他引:2
针对高分一号卫星(GF-1)的16 m宽覆盖相机数据,探讨了暗目标法的应用。首先,利用地面观测的植被光谱数据,结合模拟计算,发现利用红蓝波段线性关系能更好地去除地表影响,而利用反演的气溶胶光学厚度AOD进行大气校正能很好地去除伪暗目标;然后,以天津地区和北京地区为试验区进行了反演试验。结果表明,利用本算法能较好地观测气溶胶分布,与地面观测结果均有较好的相关性(R0.8),但反演结果整体偏高,可能是云像元的影响。误差分析表明,整景图像采用统一的观测天顶角会带来较大误差,最大误差为0.3;绝对辐射定标精度在3%以下,反演精度能控制在10%,城市型气溶胶会对反演带来较大误差。 相似文献
997.
遥感卫星在空间环境运转过程中,星敏感器的姿态角、成像相机的镜头和CCD等几何参数会发生不可预估的变化,从而对卫星影像的定位精度产生影响。因此,对卫星影像进行自检校区域网平差处理是实现卫星影像精确定位的一项关键技术。本文以"天绘一号"卫星为研究对象,首先分析了"天绘一号"卫星三线阵立体测绘相机的镜头和CCD几何形变,并提出了适用于"天绘一号"卫星三线阵CCD影像的自检校模型;然后建立"天绘一号"卫星三线阵CCD影像的自检校区域网平差模型,对外方位元素和自检校参数进行整体平差,消除存在于外方位元素观测值和自检校标定值中的系统误差,以提高卫星图像定位精度。最后利用嵩山实验场对"天绘一号"卫星三线阵CCD影像进行了自检校区域网平差处理,验证了自检校模型和自检校区域网平差模型的正确性和有效性,并分析研究了不同数目的控制点条件对自检校区域网平差的影响。实验结果证实采用自检校区域网平差技术可以有效地消除系统定位误差,显著提高定位精度。 相似文献
998.
999.
In this paper, seismic records of Taiwan LSST array and SMART-1 array were selected to calculate the S-wave and surface wave coherence coefficients at different station distances. And then the coherence function model proposed by Loh was used to fit the calculation results. After comparison and analysis, we found that when the distance d < 50 m, the coherency coefficients of surface wave and S-waves are basically the same; when the distance d = 50 m , the coherency coefficients of surface wave is smaller than that of S-wave, and as the distance increases, the differences gradually increase. When the distance d > 500 m, the spatial coherency of the surface wave hardly exists, so no further consideration is needed. Finally, the surface wave coherency model parameters were given in this paper, which can be used as a reference for the synthetic ground motion field in the seismic analysis for long and large structures in large basins. 相似文献
1000.
Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta
Horst Uwe Keller Sonia Fornasier Stubbe F. Hviid Jörg Knollenberg Miriam Rengel Gabriele Cremonese Detlef Koschny Ekkehard Kührt Holger Sierks Cesare Barbieri Hans Rickman Michael F. A'Hearn Maria-Antonella Barucci Vania da Deppo Björn J.R. Davidsson Stefano Debei Fritz Gliem José J. Lopez Moreno Giampiero Naletto Angel Sanz Andrés 《Icarus》2007,187(1):87-103
The OSIRIS cameras on the Rosetta spacecraft observed Comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impact projectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed through filters sensitive to emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showed regular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness of Tempel 1 decreased by about 10% during the OSIRIS observations. The analysis of the impact ejecta shows that no new permanent coma structures were created by the impact. Most of the material moved with . Much of it left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet ( of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations of the density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain. 相似文献