全文获取类型
收费全文 | 93251篇 |
免费 | 1811篇 |
国内免费 | 958篇 |
专业分类
测绘学 | 2339篇 |
大气科学 | 7100篇 |
地球物理 | 18973篇 |
地质学 | 33203篇 |
海洋学 | 7902篇 |
天文学 | 20696篇 |
综合类 | 264篇 |
自然地理 | 5543篇 |
出版年
2021年 | 802篇 |
2020年 | 892篇 |
2019年 | 969篇 |
2018年 | 2207篇 |
2017年 | 2069篇 |
2016年 | 2684篇 |
2015年 | 1608篇 |
2014年 | 2606篇 |
2013年 | 4766篇 |
2012年 | 2757篇 |
2011年 | 3787篇 |
2010年 | 3201篇 |
2009年 | 4287篇 |
2008年 | 3937篇 |
2007年 | 3629篇 |
2006年 | 3561篇 |
2005年 | 2922篇 |
2004年 | 2812篇 |
2003年 | 2685篇 |
2002年 | 2486篇 |
2001年 | 2287篇 |
2000年 | 2244篇 |
1999年 | 1827篇 |
1998年 | 1886篇 |
1997年 | 1835篇 |
1996年 | 1512篇 |
1995年 | 1520篇 |
1994年 | 1311篇 |
1993年 | 1196篇 |
1992年 | 1168篇 |
1991年 | 1087篇 |
1990年 | 1187篇 |
1989年 | 1028篇 |
1988年 | 957篇 |
1987年 | 1176篇 |
1986年 | 1021篇 |
1985年 | 1226篇 |
1984年 | 1370篇 |
1983年 | 1313篇 |
1982年 | 1245篇 |
1981年 | 1114篇 |
1980年 | 1041篇 |
1979年 | 978篇 |
1978年 | 982篇 |
1977年 | 894篇 |
1976年 | 838篇 |
1975年 | 798篇 |
1974年 | 850篇 |
1973年 | 807篇 |
1972年 | 523篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
An introduction to Maslov's asymptotic method 总被引:3,自引:0,他引:3
Summary. Familiar concepts such as asymptotic ray theory and geometrical spreading are now recognized as an asymptotic form of a more general asymptotic solution to the non-separable wave equation. In seismology, the name Maslov asymptotic theory has been attached to this solution. In its simplest form, it may be thought of as a justification of disc-ray theory and it can be reduced to the WKBJ seismogram. It is a uniformly valid asymptotic solution, though. The method involves properties of the wavefronts and ray paths of the wave equation which have been established for over a century. The integral operators which build on these properties have been investigated only comparatively recently. These operators are introduced very simply by appealing to the asymptotic Fourier transform of Ziolkowski & Deschamps. This leads quite naturally to the result that phase functions in different domains of the spatial Fourier transform are related by a Legendre transformation. The amplitude transformation can also be inferred by this method. Liouville's theorem (the incompressibility of a phase space of position and slowness) ensures that it is always possible to obtain a uniformly asymptotic solution. This theorem can be derived by methods familiar to seismologists and which do not rely on the traditional formalism of classical mechanics. It can also be derived from the sympletic property of the equations of geometrical spreading and canonical transformations in general. The symplectic property plays a central role in the theory of high-frequency beams in inhomogeneous media. 相似文献
72.
Thomas M. Tharp 《Geophysical Journal International》1985,80(2):419-437
Summary. Finite element models for shallow subduction produce realistic behaviour for a wide variety of mechanical strength and density distributions. Characteristic displacements are found to occur even without a discrete low-strength megathrust if there is a high-density subducted plate to localize lithospheric compression. A high-density plate is itself unnecessary in the presence of a low-strength megathrust and regional compression.
Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate. 相似文献
Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate. 相似文献
73.
Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 33, No. 2, pp. 271–281, September–October, 1990. 相似文献
74.
Reconnaissance seismic shot in 1971/72 showed a number of well defined seismic anomalies within the East Sengkang Basin which were interpreted as buried reefs. Subsequent fieldwork revealed that Upper Miocene reefs outcropped along the southern margin of the basin. A drilling programme in 1975 and 1976 proved the presence of shallow, gas-bearing, Upper Miocene reefs in the northern part of the basin. Seismic acquisition and drilling during 1981 confirmed the economic significance of these discoveries, with four separate accumulations containing about 750 × 109 cubic feet of dry gas in place at an average depth of 700 m. Kampung Baru is the largest field and contains over half the total, both reservoir quality and gas deliverability are excellent. Deposition in the East Sengkang Basin probably started during the Early Miocene. A sequence of Lower Miocene mudstones and limestones unconformably overlies acoustic basement which consists of Eocene volcanics. During the tectonically active Middle Miocene, deposition was interrupted by two periods of deformation and erosion. Carbonate deposition became established in the Late Miocene with widespread development of platform limestones throughout the East Sengkang Basin. Thick pinnacle reef complexes developed in the areas where reef growth could keep pace with the relative rise in sea level. Most reef growth ceased at the end of the Miocene and subsequent renewed clastic sedimentation covered the irregular limestone surface. Late Pliocene regression culminated in the Holocene with erosion. The Walanae fault zone, part of a major regional sinistral strike-slip system, separates the East and West Sengkang Basins. Both normal and reverse faulting are inferred from seismic data and post Late Pliocene reverse faulting is seen in outcrop. 相似文献
75.
Frans J.M. Rietmeijer 《Meteoritics & planetary science》1985,20(1):43-48
Poorly-graphitised carbon particles are formed during manufacture of sample substrates (holey carbon films) for Analytical Electron Microscopy studies of small particles. The particles form during heat treatment of cellulose acetobutyrate at about 975 °C and 1050 °C. In AEM studies of fine-grained carbonaceous extraterrestrial materials, these particles are easily recognised. 相似文献
76.
The SAS® computer software system, widely used and respected for its capabilities in statistical analysis and data base management, now includes a new set of graphic and cartographic procedures called SAS GRAPH?. We have used these cartographic procedures in research on mapping ethno-cultural census data from metropolitan areas in Ontario and in undergraduate and graduate courses in computer cartography. On the basis of that experience, we describe and evaluate SAS/GRAPH'S cartographic capabilities and illustrate with maps drawn by various devices. 相似文献
77.
B.K.Sharma A.M.Bhola 《《幕》》2004,27(2):119-124
The metasediments in the Chamba region experienced three phases of deformation: DF1, DF2 and DF3.Folded quartz veins are co-folded with the F2 crenulation folds. Their geometric and tectonic significance is studied by microstructures and shortening adjacent to the discrete crenulation cleavage, S2. Microstructures of folded vein segments, their geometric changes and truncation to cleavage (S2) are mainly due to pressure-solution phenomena and the estimated volume loss from reconstructed vein segments range from 16 to 25.5%,which is closely related to volume decrease (26%) calculated from the polydeformed slates of North Wales areas. 相似文献
78.
An empirical kinematic method developed by Hakamada and Akasofu (1982) is calibrated on the basis of a one-dimensional MHD solution. The calibrated results are used to simulate the stream-stream interaction and the background corotating structure in a simple situation and also during 22 November–6 December 1977. The solar wind disturbances caused by solar activities during this period are then introduced into the above background stream in simulating the heliospheric disturbance event which was observed by an aligned set of spacecraft at distances between 0.6 and 1.6 a.u. The observations and the simulated results are satisfactory, and a little more refinement in the simulation could reconstruct reasonably well the data by filling the data gaps in the solar wind speed, the density and the IMF magnitude. 相似文献
79.
80.
A new procedure to separate the instrumental and atmospheric components of stray light is presented. It is based on the dependence of the aureole's atmospheric component on the air mass and is applied to measurements taken with the Vacuum Newton Telescope (VNT) at the Observatorio del Teide (Tenerife). The resulting instrumental part is independent of the air mass.The variation of both components with wavelength is also studied. The instrumental component shows no dependence on wavelength, in contrast to the atmospheric one which is greater in the blue than in the red.It is concluded that observations with air masses larger than two will probably be strongly affected by stray light. 相似文献