全文获取类型
收费全文 | 12066篇 |
免费 | 1052篇 |
国内免费 | 1809篇 |
专业分类
测绘学 | 1503篇 |
大气科学 | 542篇 |
地球物理 | 1178篇 |
地质学 | 4101篇 |
海洋学 | 1188篇 |
天文学 | 5182篇 |
综合类 | 608篇 |
自然地理 | 625篇 |
出版年
2024年 | 35篇 |
2023年 | 90篇 |
2022年 | 299篇 |
2021年 | 337篇 |
2020年 | 363篇 |
2019年 | 413篇 |
2018年 | 329篇 |
2017年 | 373篇 |
2016年 | 356篇 |
2015年 | 410篇 |
2014年 | 653篇 |
2013年 | 730篇 |
2012年 | 769篇 |
2011年 | 834篇 |
2010年 | 806篇 |
2009年 | 1113篇 |
2008年 | 1039篇 |
2007年 | 951篇 |
2006年 | 862篇 |
2005年 | 765篇 |
2004年 | 629篇 |
2003年 | 550篇 |
2002年 | 401篇 |
2001年 | 370篇 |
2000年 | 309篇 |
1999年 | 288篇 |
1998年 | 192篇 |
1997年 | 90篇 |
1996年 | 77篇 |
1995年 | 66篇 |
1994年 | 60篇 |
1993年 | 78篇 |
1992年 | 26篇 |
1991年 | 33篇 |
1990年 | 30篇 |
1989年 | 25篇 |
1988年 | 26篇 |
1987年 | 9篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 7篇 |
1980年 | 13篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 15篇 |
1976年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
长82亚油层组是甘肃庆城地区庄19井区上三叠统延长组中储集砂岩相对富集的层位,但砂岩低渗透性的特点显著,成为影响该区石油储产量增长的主要地质因素。结合前人的相关工作,通过钻井岩心观察、测井曲线分析、储层岩石实验测试等工作,详细地分析了庄19井区长82亚油层组低渗透储层的地质特征,认为沉积微相和压实作用、胶结作用是控制低渗透性储集砂岩发育和分布的主要地质因素,寻找以水下分流河道微相为代表的有利储集相带砂岩体是油气勘探的重要方向。 相似文献
152.
提出一种基于BP神经网络的结构破损诊断方法,该方法以结构破损前后柔度的变化作为破损诊断网络输入,为了解决由于系统响应样本数据空间分布不均匀对网络收敛速度及网络诊断影响问题,对网络训练样本采用广义空间格点进行了交换,模拟算例及应用实例均表明,本文方法能准确诊断结构破损位置与破坏程度,是一种有效的结构破损诊断方法. 相似文献
153.
Management of a shallow temperate estuary to control eutrophication: The effect of hydrodynamics on the system’s nutrient loading 总被引:1,自引:0,他引:1
A.I. Lilleb J.M. Neto I. Martins T. Verdelhos S. Leston P.G. Cardoso S.M. Ferreira J.C. Marques M.A. Pardal 《Estuarine, Coastal and Shelf Science》2005,65(4):697-707
The Mondego estuary, a shallow warm-temperate intertidal system located on the west coast of Portugal, has for some decades been under severe ecological stress, mainly caused by eutrophication. Water circulation in this system was, until 1998, mainly dependent on tides and on the freshwater input of a small tributary artificially controlled by a sluice. After 1998, the sluice opening was effectively minimised to reduce the nutrient loading, and the system hydrodynamics improved due to engineering work in the upstream areas. The objective of the present study was to evaluate the effect of the mitigation measures implemented in 1998. Changes to the hydrodynamics of the system were assessed using precipitation and salinity data in relation to the concentrations of dissolved inorganic nutrients, as well as the linkage between dissolved N:P ratios and the biological parameters (phytoplankton chlorophyll a concentrations, green macroalgal biomass and seagrass biomass). Two distinctive periods were compared, over a ten year period: from January 1993 to January 1997 and from January 1999 until January 2003. The effective reduction in the dissolved N:P atomic ratio from 37.7 to 13.2 after 1998 is a result of lowered ammonia, but not the oxidised forms of nitrogen (nitrate plus nitrite), or increased concentrations of dissolved inorganic phosphorus. Results suggest that the phytoplankton is not nutrient limited, yet maximum and mean biomass of green macroalgae was reduced by one order of magnitude after the mitigation measures. This suggests that besides lowering the water residence time of the system, macroalgal growth became nitrogen limited. In parallel to these changes the seagrass-covered area and biomass of Zostera noltii showed signs of recovery. 相似文献
154.
H. Jonsson K. Viken Sandnes D. Schiedek R. Schneider B. E. Grsvik A. Goksyr 《Marine environmental research》2004,58(2-5):655
In an attempt to learn more about the cytochrome P450 (CYP) system of mussels, we used protein databases and alignment software to extract highly conserved CYP sequences. From these alignments synthetic peptides were produced and used for rabbit immunisation, which yielded polyclonal antibodies against the CYP families 2 and 4. The antibodies were evaluated with Western Blot and ELISA assays, using digestive gland microsomal samples from the mussel Mytilus edulis. Western Blots revealed immunoreactions for both antibodies. The anti-CYP2 sequence rendered one major immunopositive protein of ≈49 kDa size, and weak signals for proteins of ≈41 and 56 kDa size. The anti-CYP4 sequence rendered two major bands of ≈56 and 59 kDa size, and also a weak immunoreaction with a protein of ≈43 kDa size. ELISA rendered only weak signals even with a 1:50 dilution of IgG-purified serum. A 10-day exposure to Aroclor 1254 did not appear to affect any of the immunopositive proteins, while total PCBs in soft bodies increased from 14–40 ng/g DW in controls to 373–638 ng/g DW in exposed mussels. 相似文献
155.
1 .IntroductionNondestructiveinspection (NDI)isveryimportantforensuringthereliabilityofoffshorestructuresintheirservicelives (Lauraetal.,1 996 ) .Itiswellknownthatdetectionofflawsinvolvesconsider ablestatisticaluncertainties.Asaresult,theprobabilityofdetection (POD)forallflawsofagivensizehasbeenusedintheliteraturetodefinethecapabilityofaparticularNDItechniqueinagivenen vironment.SincethedataofPODusuallyscatterlargely ,itisdifficulttodeterminewhichmodelfitstheavailabledatabest.Thismodelun… 相似文献
156.
158.
159.
160.