全文获取类型
| 收费全文 | 1934篇 |
| 免费 | 228篇 |
| 国内免费 | 157篇 |
专业分类
| 测绘学 | 243篇 |
| 大气科学 | 92篇 |
| 地球物理 | 387篇 |
| 地质学 | 736篇 |
| 海洋学 | 108篇 |
| 天文学 | 407篇 |
| 综合类 | 124篇 |
| 自然地理 | 222篇 |
出版年
| 2024年 | 2篇 |
| 2023年 | 7篇 |
| 2022年 | 25篇 |
| 2021年 | 33篇 |
| 2020年 | 47篇 |
| 2019年 | 74篇 |
| 2018年 | 41篇 |
| 2017年 | 47篇 |
| 2016年 | 48篇 |
| 2015年 | 46篇 |
| 2014年 | 76篇 |
| 2013年 | 81篇 |
| 2012年 | 98篇 |
| 2011年 | 95篇 |
| 2010年 | 69篇 |
| 2009年 | 90篇 |
| 2008年 | 110篇 |
| 2007年 | 123篇 |
| 2006年 | 136篇 |
| 2005年 | 110篇 |
| 2004年 | 132篇 |
| 2003年 | 109篇 |
| 2002年 | 97篇 |
| 2001年 | 84篇 |
| 2000年 | 78篇 |
| 1999年 | 68篇 |
| 1998年 | 64篇 |
| 1997年 | 55篇 |
| 1996年 | 49篇 |
| 1995年 | 38篇 |
| 1994年 | 38篇 |
| 1993年 | 36篇 |
| 1992年 | 24篇 |
| 1991年 | 30篇 |
| 1990年 | 30篇 |
| 1989年 | 12篇 |
| 1988年 | 8篇 |
| 1987年 | 6篇 |
| 1981年 | 1篇 |
| 1977年 | 1篇 |
| 1973年 | 1篇 |
排序方式： 共有2319条查询结果，搜索用时 31 毫秒
71.
72.
73.
Construction of Green's function to the external Dirichlet boundary-value problem for the Laplace equation on an ellipsoid of revolution 总被引：1，自引：0，他引：1
Green's function to the external Dirichlet boundary-value problem for the Laplace equation with data distributed on an ellipsoid
of revolution has been constructed in a closed form. The ellipsoidal Poisson kernel describing the effect of the ellipticity
of the boundary on the solution of the investigated boundary-value problem has been expressed as a finite sum of elementary
functions which describe analytically the behaviour of the ellipsoidal Poisson kernel at the singular point ψ = 0. We have
shown that the degree of singularity of the ellipsoidal Poisson kernel in the vicinity of its singular point is of the same
degree as that of the original spherical Poisson kernel.
Received: 4 June 1996 / Accepted: 7 April 1997 相似文献
74.
75.
76.
P-SH conversion is commonly observed in teleseismic P waves, and is often attributed to dipping interfaces beneath the receiver. Our modelling suggests an alternative explanation in terms of flat-layered anisotropy. We use reflectivity techniques to compute three-component synthetic seismograms in a 1-D anisotropic layered medium. For each layer of the medium, we prescribe values of seismic velocities and hexagonally symmetric anisotropy about a common symmetry axis of arbitrary orientation. A compressional wave in an anisotropic velocity structure suffers conversion to both SV -and SH -polarized shear waves, unless the axis of symmetry is everywhere vertical or the wave travels parallel to all symmetry axes. The P-SV conversion forms the basis of the widely used 'receiver function' technique. The P-SH conversion occurs at interfaces where one or both layers are anisotropic. A tilted axis of symmetry and a dipping interface in isotropic media produce similar amplitudes of both direct ( P ) and converted ( Ps ) phases, leaving the backazimuth variation of the P-Ps delay as the main discriminant. Seismic anisotropy with a tilted symmetry axis leads to complex synthetic seismograms in velocity models composed of just a few flat homogeneous layers. It is possible therefore to model observations of P coda with prominent transverse components with relatively simple 1-D velocity structures. Successful retrieval of salient model characteristics appears possible using multiple realizations of a genetic-algorithm (GA) inversion of P coda from several backazimuths. Using GA inversion, we determine that six P coda recorded at station ARU in central Russia are consistent with models that possess strong (> 10 per cent) anisotropy in the top 5 km and between 30 and 43 km depth. The symmetry axes are tilted, and appear aligned with the seismic anisotropy orientation in the mantle under ARU suggested by SKS splitting. 相似文献
77.
Time-domain electromagnetic (TDEM) migration is based on downward extrapolation of the observed field in reverse time. In fact, the migrated EM field is the solution of the boundary-value problem for the adjoint Maxwell's equations. The important question is how this imaging technique can be related to the solution of the geoelectrical inverse problem. In this paper we introduce a new formulation of the inverse problem, based on the minimization of the residual-field energy flow through the surface or profile of observations. We demonstrate that TDEM migration can be interpreted as the first step in the solution of this specially formulated TDEM inverse problem. However, in many practical situations this first step produces a very efficient approximation to the geoelectrical model, which makes electromagnetic migration so attractive for practical applications. We demonstrate the effectiveness of this approach in inverting synthetic and practical TDEM data. 相似文献
78.
79.
When inverting large matrices, iterative techniques are necessary because of their speed and low memory requirements, as opposed to singular value decomposition (SVD). Recently, there have been attempts to obtain information on the quality of the solutions calculated using conjugate gradient (CG) methods such as LSQR. The purpose of this note is to comment on the paper titled "Estimation of resolution and covariance for large matrix inversions' by Zhang & McMechan (1995), who extend Paige and Saunders' LSQR algorithm to obtain an orthonormal basis used to approximate resolution and covariance. We show that for larger problems, where the number of orthogonal vectors is several orders of magnitude smaller than the number of model parameters, the vectors obtained do not adequately span the range of the model space. We use a synthetic borehole experiment to illustrate the differences between the singular value spectrum obtained through the more complete method of SVD and the Ritz value spectrum that results from a simple extension of LSQR, We also present a trivial numerical example to illustrate the differences between Zhang & McMechan's approximate resolution matrix and the true resolution. 相似文献
80.