全文获取类型
收费全文 | 81659篇 |
免费 | 1478篇 |
国内免费 | 1340篇 |
专业分类
测绘学 | 2207篇 |
大气科学 | 6299篇 |
地球物理 | 16131篇 |
地质学 | 28800篇 |
海洋学 | 7042篇 |
天文学 | 18361篇 |
综合类 | 363篇 |
自然地理 | 5274篇 |
出版年
2021年 | 701篇 |
2020年 | 742篇 |
2019年 | 776篇 |
2018年 | 1908篇 |
2017年 | 1818篇 |
2016年 | 2045篇 |
2015年 | 1352篇 |
2014年 | 2096篇 |
2013年 | 3976篇 |
2012年 | 2387篇 |
2011年 | 3131篇 |
2010年 | 2763篇 |
2009年 | 3581篇 |
2008年 | 3197篇 |
2007年 | 3209篇 |
2006年 | 3081篇 |
2005年 | 2870篇 |
2004年 | 2924篇 |
2003年 | 2562篇 |
2002年 | 2137篇 |
2001年 | 1863篇 |
2000年 | 1782篇 |
1999年 | 1610篇 |
1998年 | 1605篇 |
1997年 | 1567篇 |
1996年 | 1271篇 |
1995年 | 1254篇 |
1994年 | 1168篇 |
1993年 | 1069篇 |
1992年 | 1038篇 |
1991年 | 992篇 |
1990年 | 1129篇 |
1989年 | 966篇 |
1988年 | 922篇 |
1987年 | 1075篇 |
1986年 | 906篇 |
1985年 | 1163篇 |
1984年 | 1364篇 |
1983年 | 1262篇 |
1982年 | 1210篇 |
1981年 | 1154篇 |
1980年 | 1042篇 |
1979年 | 988篇 |
1978年 | 961篇 |
1977年 | 915篇 |
1976年 | 836篇 |
1975年 | 759篇 |
1974年 | 827篇 |
1973年 | 845篇 |
1972年 | 514篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
72.
Quinn B.G. Barrett R.F. Kootsookos P.J. Searle S.J. 《Oceanic Engineering, IEEE Journal of》1993,18(4):557-564
A hidden Markov model (HMM) technique for the estimation of the shape of a towed array is presented. It is assumed that there is a far-field source radiating sound containing possibly weak spectral lines. The technique uses either the Fourier coefficients at a given frequency computed from a single time block or the maximal eigenvector of a sample spectral covariance matrix. The technique is illustrated using several simulations. The results of these simulations indicate that the HMM technique yields shape and bearing estimates more accurate than those provided by a maximum-likelihood array shape estimation technique 相似文献
73.
Gerald R. Webster 《The Professional geographer》1991,43(1):49-60
Did Congressional influence play a role in the distribution of federal outlays between FY81 and FY86? This study hypothesizes that states with majority party House and Senate delegations and with seniority in the delegations experienced the largest increases in federal expenditures. These two hypotheses are tested for fifteen divisions of federal outlays using a stepwise regression model. The results provide only very modest evidence of any systematic Congressional influence over changes in the distribution of federal outlays during the study period. 相似文献
74.
This paper considers the transient response of a pressurized long cylindrical cavity in an infinite poroelastic medium. To obtain transient solutions, Biot's equations for poroelastodynamics are specialized for this problem. A set of exact general solutions for radial displacement, stresses, pore pressure and discharge are derived in the Laplace transform space by using analytical techniques. Solutions are presented for three different types of prescribed transient radial pressures acting on the surface of a permeable as well as an impermeable cavity surface. Time domain solutions are obtained by inverting Laplace domain solutions using a reliable numerical scheme. A detailed parametric study is presented to illustrate the influence of poroelastic material parameters and hydraulic boundary conditions on the response of the medium. Comparisons are also presented with the corresponding ideal elastic solutions to portray the poroelastic effects. It is noted that the maximum radial displacement and hoop stress at the cavity surface are substantially higher than the classical static solutions and differ considerably from the transient elastic solutions. Time histories and radial variations of displacement, hoop stress, pore pressure and fluid discharge corresponding to a cavity in two representative poroelastic materials are also presented. 相似文献
75.
Elastic–plastic deformations in pavements consisting of layers of different frictional materials are investigated. The upper bound, kinematic shakedown theorem is used to obtain estimates of the critical shakedown loads. Fully general three-dimensional deformations are considered. The influence of the loading distribution, interactions between loads, and the effect of varying the thickness, stiffness and strength of the layers are explored. Consequences of this investigation for particular existing designs of flexible pavements are investigated. In particular, it is found that the strength of the subgrade has no effect on the magnitude of the critical shakedown load. 相似文献
76.
F. Goodarzi L.R. Snowdon P.R. Gunther W.A.M. Jenkins 《Marine and Petroleum Geology》1985,2(3):254-259
Samples of lower Palaeozoic bedrock from the Grand Banks of Newfoundland were examined using reflected light microscopy and Rock-Eval pyrolysis. These samples contained organic material which included bitumen (structureless organic material), acritarchs, chitinozoa, scolecodonts and graptolites. The reflectance of the organic material increased from acritarchs towards graptolites with bitumen showing a wide range of reflectance. Multiple phases of oil migration through one sample were inferred by examination of the bitumens: the lowest reflecting being more recent than the highest reflecting bitumen. Combined reflectance and Rock-Eval analysis indicate that these samples are mature to overmature. The two oldest samples (Arenig-Llanvirn, Ordovician) are assessed as mature and as having potential for generation of liquid hydrocarbons, but must be considered as mainly gas prone. 相似文献
77.
Berry Head, a limestone headland in Torbay, southwest England, exhibits a series of subaerial marine erosion platforms and raised beaches spanning an altitude range of 97 m. Solution caves on the headland show preferred horizontal development at elevations that are correlated with the marine erosion platforms, and developed in a marine/freshwater mixing zone whose position was controlled by high sea-level still-stands. Corbridge Cave in Berry Head Quarry lies below the raised beaches in Torbay, and contains evidence of three marine transgressions in the form of fine-grained marine ponding deposits with a marine microfauna. Uranium-series dating of intercalated speleothems indicates that a transgression during oxygen isotope stage 5e reached an elevation of 5.8 m OD, while an earlier transgression (probably during stage 7) reached at least 7.2 m OD. These findings are used to constrain possible interpretations of the aminostratigraphy of raised beaches in Southwest Britain, and a correlation of the Unnamed Stage of Bowen, Sykes, Reeves, Miller, Andrews, Brew and Hare with oxygen isotope stage 5e is proposed. 相似文献
78.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
79.
R. K. Thakur 《Astrophysics and Space Science》1993,199(1):159-164
It is suggested that a collapsing supermassive object, which acts as an ultra-high energy particle accelerator, is the precursor of an active galactic nucleus and that the gravitational energy released during the collapse of the object is locked in the quark-gluon plasma permeated by leptons into which the entire matter in the core of the object is converted as a result of the collapse. It is also pointed out that the collapse of the object to a space-time singularity is inhibited by Pauli's exclusion principle as well as by Heisenberg's uncertainty principle and that the object explodes, before it could collapse to a singularity, thereby releasing the enormous amount of energy locked in the quark-gluon plasma. 相似文献
80.
L. I. Matveenko 《Astronomy Letters》2001,27(11):686-690
The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths. 相似文献