全文获取类型
收费全文 | 84172篇 |
免费 | 1293篇 |
国内免费 | 1662篇 |
专业分类
测绘学 | 2855篇 |
大气科学 | 6625篇 |
地球物理 | 16567篇 |
地质学 | 32199篇 |
海洋学 | 6483篇 |
天文学 | 15058篇 |
综合类 | 2309篇 |
自然地理 | 5031篇 |
出版年
2021年 | 438篇 |
2020年 | 525篇 |
2019年 | 545篇 |
2018年 | 5738篇 |
2017年 | 5014篇 |
2016年 | 4013篇 |
2015年 | 1214篇 |
2014年 | 1586篇 |
2013年 | 3233篇 |
2012年 | 2695篇 |
2011年 | 4936篇 |
2010年 | 3955篇 |
2009年 | 4962篇 |
2008年 | 4262篇 |
2007年 | 4666篇 |
2006年 | 2386篇 |
2005年 | 2122篇 |
2004年 | 2237篇 |
2003年 | 2162篇 |
2002年 | 1937篇 |
2001年 | 1614篇 |
2000年 | 1598篇 |
1999年 | 1346篇 |
1998年 | 1272篇 |
1997年 | 1309篇 |
1996年 | 1100篇 |
1995年 | 1024篇 |
1994年 | 956篇 |
1993年 | 828篇 |
1992年 | 828篇 |
1991年 | 774篇 |
1990年 | 793篇 |
1989年 | 705篇 |
1988年 | 705篇 |
1987年 | 752篇 |
1986年 | 724篇 |
1985年 | 880篇 |
1984年 | 943篇 |
1983年 | 937篇 |
1982年 | 883篇 |
1981年 | 799篇 |
1980年 | 803篇 |
1979年 | 689篇 |
1978年 | 675篇 |
1977年 | 617篇 |
1976年 | 564篇 |
1975年 | 566篇 |
1974年 | 563篇 |
1973年 | 563篇 |
1972年 | 351篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
911.
912.
黄河口及渤海中南部沉积物工程特性及其机理 总被引:3,自引:0,他引:3
采用Kasten取样器从黄河口及渤海湾中南部海底沉积物中取样,对样品通过土工性质、微结构研究以及粘土矿物分析,阐明本研究区三维空间展布的海底沉积物形成机理及其工程地质规律,论证水动力作用和斜坡不稳定性对工程特性和微结构的重要影响. 相似文献
913.
A general method for representing the flow properties in the three-dimensional boundary layers around ship hulls of arbitrary shape is described. It makes use of an efficient two-point finite-diffirence schem to solve the boundary-layer equations and includes an algebraic eddy-viscosity representaion of the Reynolds-stress ternsor. The numericzal method contains novel and desirable features and allows the calculation of flows in which the circumferential velocity component contains regions of flow reversal across the boundary layer. The inviscid pressure distribution is determined with the Douglas-Neumann method which, if necessary, can conveniently allow for the boundary-layer displacement surface. To allow its application to ships, and particularly to those with double-elliptic and flat-bottomed hulls, a non-orthogonal coordinate system has been developed and is shown to be economical, precise and comparatively easy to use. Present calculations relate to zero Froude number but they can be extended to include the effects of a water wave and local regions of flow separation which may stem from bulbous-bow geometries. 相似文献
914.
Gassy sediments are detectable acoustically in subbottom profiles of Canada's Fraser Delta slope and prodelta areas. Interstitial gas is typically represented by zones of acoustic turbidity on high-resolution seismic profiles and by gas brightening on air-gun profiles. The top of the acoustically turbid zone is generally highest within the sediment column closest to the river mouths and lies within 10 ms (two-way travel time) of the sea floor in a nearly continuous area that covers over 530 km2. Most of the gas is believed to be biogenic in origin, although thermogenic gas derived from underlying Tertiary sedimentary rocks may be present in places. 相似文献
915.
L. D. Wright Z. -S. Yang B. D. Bornhold G. H. Keller D. B. Prior W. J. Wiseman Jr. Y. B. Fan Z. Su 《Geo-Marine Letters》1986,6(2):115-120
Internal waves with periods of about 5 minutes and trough to crest heights of up to 6.2 m were observed acoustically over the actively accreting delta front of the Huanghe (Yellow River) in the western portion of the Gulf of Bohai, Peoples Republic of China. The radian frequency of the internal waves was close to the locally-observed Brunt-Vaisala frequency. Through the relatively short (one hour) duration of an internal wave train, the amplitude was observed to decrease progressively with time. These internal waves may cause resuspension of delta-front sediments. 相似文献
916.
The process of Differential Roughness Secondary Flow (DRSF), hitherto little recognized, recently was demonstrated in a wind
tunnel and in a flume. It is suggested here the DRSF may, in the natural environment, play an important part in the persistence
and sharp definition of sand ribbons, previously initiated by other processes.
In 1980 Karl described a series of sand ribbons, alternating with exposed substrate, which he attributed to the effect of
Langmuir circulations. Although Langmuir Circulation Secondary Flow (LCSF) may indeed, in some cases, initiate sand ribbons,
the sharply-defined edges typical of these features are explained better by DRSF. 相似文献
917.
Adriano Mazzini D. Duranti R. Jonk J. Parnell B. T. Cronin A. Hurst M. Quine 《Geo-Marine Letters》2003,23(3-4):323-339
Petrographic and geochemical analyses performed on a North Sea core from the Gryphon Field reveal the presence of palaeo-degassing features surrounded by injected sandstones in the Eocene interval. The injected sandstones are oil-stained and poorly cemented by carbonate and quartz. 18O isotope analyses indicate that carbonate cementation occurred during shallow burial (likely less than about 300 m). Depleted 13C (around –30 V-PDB) carbonate cement suggests that bicarbonate was derived from the microbial oxidation of oil and gas. Late quartz overgrowths enclose oil present in the injected units. The tubular degassing conduits are composed of zoned cements and have 18O and 13C isotope values similar to the injected sandstones, indicating that oil and gas seepage induced the precipitation of authigenic carbonate in the shallow subsurface. Oil inclusions in inter- and intra-crystal cement sites in both injected sandstones and degassing conduits indicate that oil seepage was an ongoing feature at shallow burial. A proposed model involves oil and gas seepage and the formation of the degassing conduits, followed by a sand injection phase. It seems likely that oil and gas continued to leak towards the seabed by exploiting the network of permeable injected sandstones and the horizons of porous degassing features. 相似文献
918.
919.
J. M. Lort W. Q. Limond J. Segoufin Ph. Patriat J. R. Delteil B. Damotte 《Marine Geophysical Researches》1979,4(1):71-89
This paper presents seismic reflection and refraction data from the Mozambique Channel, collected between 1971 and 1973. A deep sedimentary basin (up to 5 km of sediments) opens southwards to the Mozambique Basin, and is bounded to the east by the Davie Ridge and beyond by the marginal plateau of Malagasy. A continuous reflector (C), possibly of Cretaceous age, is identified between layers having seismic interval velocities of 2.4–2.8 km/s and 3.1–3.4 km/s. The deepest sediments have velocities of 4.5–4.9 km/s and overlie a layer with velocity 5.5 km/s, which may be volcanic in the north-east of the Channel.The crust occupying most of the Channel is probably pre-Cretaceous in age, and may be largely continental in nature. This is supported by subdued magnetic anomalies and the possibility of a continuous Karroo sedimentary section across the northern Channel. The oceanic crust of the Mozambique Basin may extend as far north as 24°S, into the western Channel only. The problem of the origin of the Mozambique Channel remains unresolved, although a long sedimentary history indicates that Malagasy may have separated from Mainland Africa prior to Karroo times. The Davie Ridge may possibly represent a relict strike-slip fault, which permitted movement along a north-south line. 相似文献
920.
Seismic stratigraphic and main lithological features of the sedimentary cover overlying the basement of the Alboran Sea were established via the analysis of commercial multichannel seismic surveys, geophysical well logs, and well data. Six seismic stratigraphics units (VI to I), bounded by unconformities, form the marine sediments that range in age from early Miocene to Quaternary. They are dated by extrapolation of commercial drilling results from the northern Alboran Sea. Volcanic activity is recorded within sedimentary sequences of units V to II. Undercompaction features are detected in the two basal units. 相似文献