全文获取类型
收费全文 | 49640篇 |
免费 | 640篇 |
国内免费 | 328篇 |
专业分类
测绘学 | 1334篇 |
大气科学 | 3872篇 |
地球物理 | 9377篇 |
地质学 | 16376篇 |
海洋学 | 4173篇 |
天文学 | 12164篇 |
综合类 | 108篇 |
自然地理 | 3204篇 |
出版年
2020年 | 339篇 |
2019年 | 337篇 |
2018年 | 828篇 |
2017年 | 810篇 |
2016年 | 1052篇 |
2015年 | 696篇 |
2014年 | 1170篇 |
2013年 | 2443篇 |
2012年 | 1110篇 |
2011年 | 1543篇 |
2010年 | 1437篇 |
2009年 | 1931篇 |
2008年 | 1775篇 |
2007年 | 1777篇 |
2006年 | 1680篇 |
2005年 | 1545篇 |
2004年 | 1509篇 |
2003年 | 1413篇 |
2002年 | 1359篇 |
2001年 | 1231篇 |
2000年 | 1155篇 |
1999年 | 1121篇 |
1998年 | 1056篇 |
1997年 | 1062篇 |
1996年 | 844篇 |
1995年 | 836篇 |
1994年 | 796篇 |
1993年 | 740篇 |
1992年 | 711篇 |
1991年 | 682篇 |
1990年 | 782篇 |
1989年 | 678篇 |
1988年 | 655篇 |
1987年 | 748篇 |
1986年 | 625篇 |
1985年 | 829篇 |
1984年 | 959篇 |
1983年 | 918篇 |
1982年 | 870篇 |
1981年 | 834篇 |
1980年 | 726篇 |
1979年 | 708篇 |
1978年 | 704篇 |
1977年 | 636篇 |
1976年 | 608篇 |
1975年 | 523篇 |
1974年 | 599篇 |
1973年 | 594篇 |
1972年 | 368篇 |
1971年 | 342篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data. 相似文献
72.
R. A. Howland 《Celestial Mechanics and Dynamical Astronomy》1988,44(3):209-226
The librational motion of the Ideal Resonance Problem (Garfinkel, 1966, Jupp, 1969) is treated through an initialnon-canonical transformation which, however, leaves the equations of motion in a quasi-canonical form, with Hamiltonian expressed in standard trigonometric functions amenable to traditional averaging techniques. The perturbed solutions, similarly expressed intrigonometric near-identity transformations, and their frequencies can be found to arbitrary order, with the elliptic integrals expected of the system introduced only in a final explicit quadrature for a Kepler-type equation in the angular variable. The specific transformations and resulting equations of motion are introduced, and explicit solutions for the original variables are found to second order, with mean motion accurate to fifth. Limitation of the present solution to the librational region, the extension of that solution to higher order, and observations on the form of the associated Hamiltonian are also discussed. 相似文献
73.
Using plane wave theory and assuming a given input wavelet the shape of the reflected (or transmitted) wavelet from a layered boundary is derived. Several types of boundaries are considered, among them the weathered layer and a wedge shaped intermediate layer. Different angles of incidence and all internal multiples are taken into account. The examples shown in the figures can be used for a direct comparison between theoretical and observed shapes of reflected (or transmitted) wavelets from special boundaries. 相似文献
74.
75.
Detailed studies of the daytime E-region critical frequency at Aberystwyth (geomagnetic latitude +56°) show clear evidence for changes associated with both the axially-symmetric (Dst) and asymmetric (DS) components of the disturbance magnetic field. Comparison of the sensitivity of the E-region peak density to these two influences shows that the changes cannot entirely (if at all) be ascribed to the influence of electric currents in the region. It is suggested that a major role is played by dynamical influences associated with the neutral air “storm circulation” which distributes the energy fed into the auroral region to lower latitudes. 相似文献
76.
77.
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ~5° for light, and ~7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ~14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements. 相似文献
78.
I. R. Plimer 《Mineralogy and Petrology》2006,88(3-4):443-478
Summary The Palaeoproterozoic Broken Hill Pb–Zn–Ag stratiform orebody is intimately associated with manganoan garnet-bearing rocks.
On stratigraphic and chemical grounds it is argued that garnet-rich metasediments below, equivalent to and above massive sulphide
were hydrothermal precipitates. Other manganoan garnet rocks formed during pre-metamorphic hydrothermal alteration, syn-metamorphic
dehydration and reaction of manganese with prograde pelitic rocks, reaction between cataclastic manganese-bearing sulphide
rocks injected along axial planes, shears and faults and pelitic wall rocks and reaction between dolerite dykes and sulphide
rocks. 相似文献
79.
80.
Uniform models for the Earth–ionosphere cavity are considered with particular attention to the physical properties of the ionosphere for the extremely low frequency (ELF) range. Two consistent features have long been recognized for the range: the presence of two distinct altitude layers of maximum energy dissipation within the lower ionosphere, and a “knee”-like change in the vertical conductivity profile representing a transition in dominance from ion-dominated to electron-dominated conductivity. A simplified two-exponential version of the Greifinger and Greifinger (1978) technique widely used in ELF work identifies two slopes in the conductivity profile and, providing accurate results in the ELF communication band (45–75 Hz), simulates too flat a frequency dependence of the quality factor within the Schumann resonance frequency range (5–40 Hz). The problem is traced to the upward migration, with frequency increasing, of the lower dissipation layer through the “knee” region resulting in a pronounced decrease of the effective scale height for conductivity. To overcome this shortcoming of the two-exponential approximation and still retain valuable model analyticity, a more general approach (but still based on the Greifinger and Greifinger formalism) is presented in the form of a “knee” model whose predictions for the modal frequencies, the wave phase velocities and the quality factors reasonably represent observations in the Schumann resonance frequency range. 相似文献