全文获取类型
| 收费全文 | 1815篇 |
| 免费 | 216篇 |
| 国内免费 | 258篇 |
专业分类
| 测绘学 | 493篇 |
| 大气科学 | 224篇 |
| 地球物理 | 319篇 |
| 地质学 | 501篇 |
| 海洋学 | 185篇 |
| 天文学 | 23篇 |
| 综合类 | 177篇 |
| 自然地理 | 367篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 16篇 |
| 2022年 | 29篇 |
| 2021年 | 43篇 |
| 2020年 | 63篇 |
| 2019年 | 68篇 |
| 2018年 | 53篇 |
| 2017年 | 90篇 |
| 2016年 | 81篇 |
| 2015年 | 90篇 |
| 2014年 | 90篇 |
| 2013年 | 152篇 |
| 2012年 | 123篇 |
| 2011年 | 116篇 |
| 2010年 | 107篇 |
| 2009年 | 102篇 |
| 2008年 | 101篇 |
| 2007年 | 132篇 |
| 2006年 | 102篇 |
| 2005年 | 93篇 |
| 2004年 | 82篇 |
| 2003年 | 70篇 |
| 2002年 | 48篇 |
| 2001年 | 42篇 |
| 2000年 | 47篇 |
| 1999年 | 36篇 |
| 1998年 | 45篇 |
| 1997年 | 38篇 |
| 1996年 | 29篇 |
| 1995年 | 33篇 |
| 1994年 | 31篇 |
| 1993年 | 27篇 |
| 1992年 | 19篇 |
| 1991年 | 11篇 |
| 1990年 | 21篇 |
| 1989年 | 15篇 |
| 1988年 | 14篇 |
| 1987年 | 10篇 |
| 1986年 | 8篇 |
| 1985年 | 3篇 |
| 1984年 | 2篇 |
| 1982年 | 1篇 |
| 1981年 | 1篇 |
| 1980年 | 1篇 |
排序方式： 共有2289条查询结果，搜索用时 284 毫秒
51.
青藏高原气温分布的空间插值方法比较 总被引：66，自引：9，他引：66
使用反距离平方、趋势面、Kriging插值、Cokriging插值和综合方法对青藏高原1961-1990年30年平均1月气温进行空间插值比较研究，其中后两种方法能够把影响青藏高原气温分布的关键因素——高程置于插值算法之中。反距离平方和趋势面插值的结果都与实际情况相差较远；普通Kriging插值能够反映出青藏高原气温分布的一定的空间结构，但结果依然不好；由于考虑了高度变量，Cokriging插值表现出一定程度的性能改进，但因为台站海拔高度偏低，插值结果依然不理想。前4种方法效果不好的原因主要是因为青藏高原上气象台站稀少且高原西北地区无气象台站。综合方法把气温分解为结构化分量和随机分量，使用直减率把气温订正到同一海拔高度后，再对它们做Kriging插值分析，其结果较为正确地反映了青藏高原气温空间分布的特征，误差远小于其它方法。研究结果表明：样本本身的空间分布是影响插值精度的重要因素，合理的采样设计是必要的前提；对于台站稀少的地区，必须把随机插值方法和确定性方法结合起来估计气候变量的空间分布。同时给出了青藏高原1月的气温空间分布状况。 相似文献
52.
首先证明了加权最小范数平差一般解的一个重要性质，由此提出了利用附加的观测值来求解加权最小范数平差问题的方法，并证明了该法与利用一般解法等价性的充要条件。 相似文献
53.
54.
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
55.
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
56.
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
57.
58.
线性地质统计学的发展与地质工作逐渐由定性向定量、由人工劳动逐渐被计算机取代的过程密切相关的。对样品品位赋予一定的权值进行滑动加权平均，来估计中心块段平均品位的方法，使地质问题的计算能在计算机上实现开辟了新的途径。 相似文献
59.
静校正处理是消除近地层影响的一个重要地震资料数字处理方法，在近地表层纵，横向变化复杂，静校正量变化较大的地区，常规静校正方法和线性反演方法均不能解决问题，遗传算法是近年发展起来的一种模拟生物生化过程的较先进的非线性反演方法，对用遗传算法进行静校正的若干问题进行了讨论，说明它是可以解决表层情况复杂地区静校正问题的一个有效方法。 相似文献
60.