全文获取类型
收费全文 | 26985篇 |
免费 | 4610篇 |
国内免费 | 6681篇 |
专业分类
测绘学 | 5054篇 |
大气科学 | 4782篇 |
地球物理 | 6502篇 |
地质学 | 11654篇 |
海洋学 | 3753篇 |
天文学 | 305篇 |
综合类 | 2424篇 |
自然地理 | 3802篇 |
出版年
2024年 | 176篇 |
2023年 | 471篇 |
2022年 | 946篇 |
2021年 | 1155篇 |
2020年 | 1313篇 |
2019年 | 1457篇 |
2018年 | 1217篇 |
2017年 | 1396篇 |
2016年 | 1533篇 |
2015年 | 1655篇 |
2014年 | 1742篇 |
2013年 | 1960篇 |
2012年 | 1814篇 |
2011年 | 1799篇 |
2010年 | 1448篇 |
2009年 | 1649篇 |
2008年 | 1672篇 |
2007年 | 1763篇 |
2006年 | 1655篇 |
2005年 | 1464篇 |
2004年 | 1292篇 |
2003年 | 1112篇 |
2002年 | 1023篇 |
2001年 | 883篇 |
2000年 | 810篇 |
1999年 | 727篇 |
1998年 | 689篇 |
1997年 | 605篇 |
1996年 | 536篇 |
1995年 | 481篇 |
1994年 | 450篇 |
1993年 | 366篇 |
1992年 | 230篇 |
1991年 | 196篇 |
1990年 | 140篇 |
1989年 | 120篇 |
1988年 | 104篇 |
1987年 | 72篇 |
1986年 | 38篇 |
1985年 | 32篇 |
1984年 | 16篇 |
1983年 | 2篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Tidal mixing plays an important role in the modification of dense water masses around the Antarctic continent. In addition to the vertical (diapycnal) mixing in the near-bottom layers, lateral mixing can also be of relevance in some areas. A numerical tide simulation shows that lateral tidal mixing is not uniformly distributed along the shelf break. In particular, strong mixing occurs all along the Ross Sea and Southern Weddell Sea shelf breaks, while other regions (e.g., the western Weddell Sea) are relatively quiet. The latter regions correspond surprisingly well to areas where indications for cross-shelf exchange of dense water masses have been found. The results suggest that lateral tidal mixing may account for the relatively small contribution of Ross Sea dense water masses to Antarctic Bottom Water. 相似文献
62.
The objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics (spacing and size). To isolate the effects of sediment availability and specific gravity on cluster evolution, mono‐sized spheres were used to simulate the cluster evolutionary cycle. Overall, twelve experimental runs were carried out in the laboratory flume. Six of these tests were performed by using glass spheres (specific gravity, SG = 2·58) and the other six by employing an equal combination of glass and Teflon spheres (SG = 2·12) of the same diameter to evaluate the role of specific gravity on cluster evolution. The three sediment availability conditions that were investigated here simulated isolated gravel elements, pool–riffle sequences and densely packed gravel‐bed. An advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area. The results of this study showed that: (1) sediment availability affects the architecture and size of cluster microforms; and (2) clusters consisting of mono‐sized sediments start disintegrating at twice the incipient conditions. By performing complementary tests for the isolated gravel elements case, it was found that the evolutionary cycle of individual clusters could be described as follows, in order of increasing stress: no cluster→two particle cluster→comet→triangle→rhomboid→break up. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
63.
64.
65.
Most geostatistical studies consider multiple-related variables. These relationships often show complex features such as nonlinearity, heteroscedasticity, and mineralogical or other constraints. These features are not handled by the well-established Gaussian simulation techniques. Earth science variables are rarely Gaussian. Transformation or anamorphosis techniques make each variable univariate Gaussian, but do not enforce bivariate or higher order Gaussianity. The stepwise conditional transformation technique is proposed to transform multiple variables to be univariate Gaussian and multivariate Gaussian with no cross correlation. This makes it remarkably easy to simulate multiple variables with arbitrarily complex relationships: (1) transform the multiple variables, (2) perform independent Gaussian simulation on the transformed variables, and (3) back transform to the original variables. The back transformation enforces reproduction of the original complex features. The methodology and underlying assumptions are explained. Several petroleum and mining examples are used to show features of the transformation and implementation details. 相似文献
66.
Xavier Emery 《Mathematical Geology》2003,35(6):699-718
This paper presents a methodology for assessing local probability distributions by disjunctive kriging when the available data set contains some imprecise measurements, like noisy or soft information or interval constraints. The basic idea consists in replacing the set of imprecise data by a set of pseudohard data simulated from their posterior distribution; an iterative algorithm based on the Gibbs sampler is proposed to achieve such a simulation step. The whole procedure is repeated many times and the final result is the average of the disjunctive kriging estimates computed from each simulated data set. Being data-independent, the kriging weights need to be calculated only once, which enables fast computing. The simulation procedure requires encoding each datum as a pre-posterior distribution and assuming a Markov property to allow the updating of pre-posterior distributions into posterior ones. Although it suffers some imperfections, disjunctive kriging turns out to be a much more flexible approach than conditional expectation, because of the vast class of models that allows its computation, namely isofactorial models. 相似文献
67.
The basic concepts of spectral and multiscale selective reconstruction of (geophysically relevant) vector fields on the sphere from error-affected data is outlined in detail. The reconstruction mechanism is formulated under the assumption that spectral as well as multiscale approximation is well-representable in terms of only a certain number of expansion coefficients at the various resolution levels. It is shown that spectral denoising by means of orthogonal expansions in terms of vector spherical harmonics reflects global a priori information of the noise (e.g., in form of a covariance tensor field), whereas multiscale signal-to-noise thresholding can be performed under locally dependent noise information within a multiresolution analysis in terms of spherical vector wavelets. An application of the multiscale formalism to Earth's magnetic field determination is presented. 相似文献
68.
69.
We compare flux and concentration footprint estimates of athree-dimensional Lagrangian stochastic dispersion modelapplying backward trajectories with the results of ananalytical footprint model by Kormann and Meixner.The comparison is performed for varying stability regimesof the surface layer as well as for different measurementheights. In general, excellent correspondence is found. 相似文献
70.
The spacetime gravitational field of a deformable body 总被引:3,自引:0,他引:3
The high-resolution analysis of orbit perturbations of terrestrial artificial satellites has documented that the eigengravitation
of a massive body like the Earth changes in time, namely with periodic and aperiodic constituents. For the space-time variation
of the gravitational field the action of internal and external volume as well as surface forces on a deformable massive body
are responsible. Free of any assumption on the symmetry of the constitution of the deformable body we review the incremental
spatial (“Eulerian”) and material (“Lagrangean”) gravitational field equations, in particular the source terms (two constituents:
the divergence of the displacement field as well as the projection of the displacement field onto the gradient of the reference
mass density function) and the `jump conditions' at the boundary surface of the body as well as at internal interfaces both
in linear approximation. A spherical harmonic expansion in terms of multipoles of the incremental Eulerian gravitational potential
is presented. Three types of spherical multipoles are identified, namely the dilatation multipoles, the transport displacement
multipoles and those multipoles which are generated by mass condensation onto the boundary reference surface or internal interfaces.
The degree-one term has been identified as non-zero, thus as a “dipole moment” being responsible for the varying position
of the deformable body's mass centre. Finally, for those deformable bodies which enjoy a spherically symmetric constitution,
emphasis is on the functional relation between Green functions, namely between Fourier-/ Laplace-transformed volume versus
surface Love-Shida functions (h(r),l(r) versus h
′(r),l
′(r)) and Love functions k(r) versus k
′(r). The functional relation is numerically tested for an active tidal force/potential and an active loading force/potential,
proving an excellent agreement with experimental results.
Received: December 1995 / Accepted: 1 February 1997 相似文献