全文获取类型
收费全文 | 9271篇 |
免费 | 734篇 |
国内免费 | 1425篇 |
专业分类
测绘学 | 251篇 |
大气科学 | 248篇 |
地球物理 | 650篇 |
地质学 | 3538篇 |
海洋学 | 861篇 |
天文学 | 5071篇 |
综合类 | 326篇 |
自然地理 | 485篇 |
出版年
2024年 | 20篇 |
2023年 | 59篇 |
2022年 | 184篇 |
2021年 | 188篇 |
2020年 | 217篇 |
2019年 | 269篇 |
2018年 | 213篇 |
2017年 | 183篇 |
2016年 | 215篇 |
2015年 | 246篇 |
2014年 | 460篇 |
2013年 | 498篇 |
2012年 | 528篇 |
2011年 | 640篇 |
2010年 | 702篇 |
2009年 | 957篇 |
2008年 | 893篇 |
2007年 | 785篇 |
2006年 | 732篇 |
2005年 | 643篇 |
2004年 | 528篇 |
2003年 | 444篇 |
2002年 | 351篇 |
2001年 | 314篇 |
2000年 | 269篇 |
1999年 | 249篇 |
1998年 | 165篇 |
1997年 | 51篇 |
1996年 | 49篇 |
1995年 | 39篇 |
1994年 | 41篇 |
1993年 | 60篇 |
1992年 | 16篇 |
1991年 | 23篇 |
1990年 | 26篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 5篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 15篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
131.
Soil nails have been widely used to stabilize slopes and earth retaining structures in many countries and regions, especially, in Hong Kong. The analysis of the interaction between a soil nail and the surrounding soil is of great interests to both design engineers and researchers. In this paper, authors present a simple mathematical model for the interaction analysis of a soil nail and the surrounding soil considering a few key factors which are soil dilation, bending of the soil nail, vertical pressure, and non-linear subgrade reaction stiffness. The lateral subgrade reaction between the soil and the soil nail is assumed to obey a hyperbolic relation. Reported test data in the literature are used to verify the present model. The contributions of the soil-nail bending on the pull-out resistance are evaluated in two case studies. 相似文献
132.
Conventional understanding of the nitrogen cycle in marine sediments has changed in recent years with the discovery of an alternative pathway for ammonia oxidation via the reduction of manganese oxides (during anoxic nitrification). In anoxic sediments, the potential for manganese oxides to serve as oxidant for nitrification may be considerable yet previous work on manganese-rich sediments has suggested anoxic nitrification may not be significant. In this study, the potential for anoxic nitrification in a range of sediment types was investigated. Laboratory incubation of sediment from three sites on the Humber Estuary, a microbially diverse environment, showed anoxic accumulation of nitrate, nitrite and dinitrogen gas, with and without the addition of synthetic manganese oxides. Incubation experiments confirmed anoxic nitrification as microbially mediated, with heat-killed controls yielding negative results. The anoxic nitrification reaction significantly depleted ammonia concentrations, and occurred simultaneously with manganese-, iron- and sulphate reduction, and methanogenesis. Taken in conjunction with other studies, results suggest anoxic nitrification may not only be dependent on total manganese concentrations but on manganese dynamics. Anoxic nitrification may be explained as a non-steady state reaction, dependent on the recent stability of a sediment system. Physical perturbation of sediments may cause the redistribution and/or introduction of manganese oxides and promote anoxic nitrification. The significance and persistence of anoxic nitrification is likely to depend on the frequency and magnitude of sediment perturbation, which explains why the reaction varies so widely across studied sites, and why it may not occur in some manganese-rich sediment. 相似文献
133.
M. Draoui J. Vias B. Andreo K. Targuisti J. Stitou El Messari 《Environmental Geology》2008,54(3):455-463
This paper presents the results of a comparative study relating to the application of four vulnerability mapping methods,
GOD, AVI, DRASTIC and SINTACS, in a pilot detritic aquifer situated in NW Morocco, known as the Martil–Alila aquifer. The
principal objective of this work is to determine the most suitable such methods for this aquifer type within a Mediterranean
context, and to show the effect of the rainfall variations that are characteristic of the Mediterranean climate on the degree
of vulnerability. The methods applied distinguish five classes of vulnerability, these being irregularly divided up in space,
with the division varying according to the method in question. The vulnerability maps obtained by the different methods strongly
suggest that the eastern half of the aquifer is more vulnerable to contamination than the western half, for all hydrological
situations. The effect of climatic conditions on the degree of vulnerability is well represented by the DRASTIC, according
to which the aquifer is moderately to strongly vulnerable during humid hydrological years and weakly to moderately vulnerable
during dry ones. For the other methods, this climatic effect is limited to the area occupied by the two predominant classes
(“High” and “Low” for GOD and “High” and “Moderate” for SINTACS) while it is null for AVI. In conclusion, DRASTIC appears
the most suitable for mapping the vulnerability to contamination of Mediterranean coastal detritic aquifers such as the Martil–Alila
aquifer. 相似文献
134.
This paper explores the links between a strategic policy, urban consolidation, and house prices by examining the changes in
the mix of housing and in house price for the period 1991–2004. We contend that urban consolidation could be seen as a source
of additional supply, (which might be expected to be felt in lower prices and so contribute to a local policy objective) but
also as a stimulus to demand (by developers who could bid up the price of lots where it was understood more housing could
be built). Analyses were carried out at the metropolitan and sub-regional scales using correlation tests. The research finds
very weak statistical connections, and concludes that this policy has not been associated with price changes. 相似文献
135.
在山东省莒南地质公园内发现6个小型恐龙足迹化石,被归入足迹属Minisauripus。化石产于下白垩统大盛群田家楼组.时代为早白垩世巴列姆(Barremian)-阿普特期(Apdan)。5个较大,长约6cm,产于下部“主”层面上,其中4个组成2节行迹:1个较小,长约3cm,位于“主”层面30cm之上的上部层面上。山东Minisauripus的特征是:足迹个体小(长3.1~5.6cm,宽2.0~3.7cm),三趾型,略不对称;足迹纵长,各趾近平行,趾垫较清晰;趾末端较钝,但爪迹较尖。Ⅲ趾比Ⅳ趾略长,而Ⅳ趾比Ⅱ趾略长且窄。此外,步幅较长,足长与步长之比约为10:1。与四川、韩国Minisau却淞不同之处是:足迹个体较大,长约是它们的2倍。继中国四川和韩国之后,山东是Minisau而淞在全球的第三个发现点。 相似文献
136.
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
137.
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
138.
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
140.