首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2287篇
  免费   337篇
  国内免费   548篇
测绘学   402篇
大气科学   213篇
地球物理   479篇
地质学   1412篇
海洋学   162篇
天文学   42篇
综合类   289篇
自然地理   173篇
  2024年   8篇
  2023年   43篇
  2022年   75篇
  2021年   95篇
  2020年   103篇
  2019年   101篇
  2018年   63篇
  2017年   91篇
  2016年   115篇
  2015年   108篇
  2014年   134篇
  2013年   155篇
  2012年   123篇
  2011年   116篇
  2010年   107篇
  2009年   117篇
  2008年   145篇
  2007年   114篇
  2006年   113篇
  2005年   132篇
  2004年   103篇
  2003年   97篇
  2002年   88篇
  2001年   81篇
  2000年   79篇
  1999年   99篇
  1998年   88篇
  1997年   65篇
  1996年   72篇
  1995年   65篇
  1994年   53篇
  1993年   51篇
  1992年   46篇
  1991年   32篇
  1990年   31篇
  1989年   14篇
  1988年   11篇
  1987年   11篇
  1986年   2篇
  1985年   8篇
  1984年   2篇
  1983年   3篇
  1982年   3篇
  1980年   1篇
  1979年   2篇
  1976年   5篇
  1973年   1篇
  1971年   1篇
排序方式: 共有3172条查询结果,搜索用时 93 毫秒
981.
铜石金矿田主要由隐爆角砾岩型(归来庄式)、碳酸盐岩层状微细浸染型(磨坊沟式)、夕卡岩叠加型、斑岩型和蚀变岩型等多种类型的矿床(点)组成,矿种以金为主,伴有银、铜、铁矿化,构成一个较完整的成矿亚系列.成矿作用与铜石富钾碱性次火山杂岩关系密切,岩浆活动为矿床的形成提供了热液和热源.泰山岩群为主要矿源层;寒武纪朱砂洞组上灰岩段之白云质灰岩、灰质白云岩为磨坊沟式金矿的有利含矿层位;寒武系上部-奥陶系下部的白云质灰岩、白云岩等是归来庄式金矿的有利围岩.杂岩体构成的次火山穹窿及伴生的环形放射状构造,控制着矿体的分布.  相似文献   
982.
Small bodies of the solar system are now the targets of space exploration. Many of these bodies have elongated, non-spherical shapes, and the usual spherical harmonic expansions of their gravity fields are not well suited for the modelling of spacecraft orbits around these bodies. An elegant remedy is to use ellipsoidal harmonic expansions instead of the usual spherical ones. In this paper, we present their mathematical theory as well as a real application: the simulation of a landing on the surface of a kilometer-sized comet. We show that with an ellipsoidal harmonic expansion up to degree 5, the error on the landing position is at the meter level, while the corresponding error for the spherical harmonic expansion can reach tens of meters.This revised version was published online in October 2005 with corrections to the Cover Date.  相似文献   
983.
时间序列处理的新插值方法   总被引:10,自引:3,他引:7  
通过对GPS和地震形变时间序列的研究,提出了一种新的插值方法——多点3次样条方法。该方法可以在一定条件下解决时间序列处理中较多数据的缺失问题。对该方法与其它插值方法进行了对比分析,并给出了该方法的定量分析结果。  相似文献   
984.
根据外汇汇率数据的特点,尤其是数据之间的依赖关系这一重要特性,并结合汇率预测要求快且准的原则,以数理统计学为基础,采用时间序列分析中的ARMA方法。通过数据的平稳性分析、模型的识别、参数估计、模型诊断以及最后预测精度的分析,建立了英镑兑美元汇率的ARMA预测模型并验证了其有效性。  相似文献   
985.
According to the classical theory of equilibrium figures, surfaces of equal density, potential and pressure concur (let us call them isobars). Isobars can be represented by means of Liapunov power series in small parameter q, up to the first approximation coinciding with the centrifugal to gravitational force ratio at the equator. Liapunov has proved the existence of the universal convergence domain: the above mentioned series converge for all bodies (satisfying a natural condition that the density ρ decreases from the center to the surface) if |q| < q*. Using Liapunov’s algorithm and symbolic manipulation tools, we have found q*= 0.000370916. Evidently, the convergence radius q* may be much greater in common situations. To comfirm it it is reasonable to consider two limiting and one or two intermediate cases for the density behaviour: ρ is a constant, the Dirac’s δ-function, linear function of the distance from the center, etc. And indeed, in the previous paper we find a three orders of magnitude greater value for homogeneous figures. In the present paper we find that in the opposite case of Huygens-Roche figures (a point-mass surrounded by a weightless atmosphere) the convergence radius is unexpectedly large and coincides with the well-known biggest possible value q*= 0.541115598 for such a class of figures. To ascertain it we ought to use numerical calculations, so our main result is demonstrated by means of a computer assisted proof. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
986.
本文从形态功能分析的角度对三叶虫的运动习性及取食过程作了叙述讨论,进而分析了造成遗迹图案多姿多态的主要原因,并对有图可循的遗迹属“塔丝漫提”、“露宿非窟”、“双形移迹”、“二叶石迹”“双痕趄趔”等运用一物多态原理作了推断解释。遗迹属的中文名称一律采用泰氏的四字命名法。  相似文献   
987.
A challenging computational problem arises when a discrete structure (e.g. foundation) interacts with an unbounded medium (e.g. deep soil deposit), particularly if general loading conditions and non‐linear material behaviour is assumed. In this paper, a novel method for dealing with such a problem is formulated by combining conventional three‐dimensional finite‐elements with the recently developed scaled boundary finite‐element method. The scaled boundary finite‐element method is a semi‐analytical technique based on finite‐elements that obtains a symmetric stiffness matrix with respect to degrees of freedom on a discretized boundary. The method is particularly well suited to modelling unbounded domains as analytical solutions are found in a radial co‐ordinate direction, but, unlike the boundary‐element method, no complex fundamental solution is required. A technique for coupling the stiffness matrix of bounded three‐dimensional finite‐element domain with the stiffness matrix of the unbounded scaled boundary finite‐element domain, which uses a Fourier series to model the variation of displacement in the circumferential direction of the cylindrical co‐ordinate system, is described. The accuracy and computational efficiency of the new formulation is demonstrated through the linear elastic analysis of rigid circular and square footings. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
988.
The purpose of this paper is the semi-analytical computation of the bounded orbits of Hill’s equations, describing the relative motion of two particles in their Keplerian motion around a central body. We have considered the case in which one of the particles moves along a circular reference orbit. The solutions obtained are the generalisation of the usual periodic orbits obtained for the linearised equations and also of the third-order solution computed by D.L. Richardson and J.W. Mitchell (2003). With the algorithm presented, those orbits can be computed in a fast and efficient way up to a high-order in the in-plane and out-of-plane amplitudes.  相似文献   
989.
Analysis of high-frequency multipath in 1-Hz GPS kinematic solutions   总被引:1,自引:1,他引:0  
High-frequency multipath would be problematic for studies at seismic or antenna dynamical frequencies as one could mistakenly interpret them as signals. A simple procedure to identify high-frequency multipath from global positioning system (GPS) time series records is presented. For this purpose, data from four GPS base stations are analyzed using spectral analyses techniques. Additional data, such as TEQC report files of L1 pseudorange multipath, are also used to analyze the high-frequency multipath and confirmation of the high-frequency multipath inferred from the phase records. Results show that this simple procedure is effective in identification of high-frequency multipath. The inferred information can aid interpretation of multipath at the GPS site, and is important for a number of reasons. For example, the information can be used to study GPS site selections and/or installations.
Clement OgajaEmail:
  相似文献   
990.
基于ICEEMDAN算法无需先验信息即可准确分离和提取低频信号与趋势信息的特性,以及SSA具有较好的信号重构优势,提出基于ICEEMDAN和SSA的联合重构方法。该方法将弱周期信号利用ICEEMDAN方法进行提取与重构,可弥补SSA方法中当弱周期信号对应的Hankel矩阵的奇异值和噪声Hankel矩阵的奇异值接近时容易被噪声掩盖而难以提取的不足。通过模拟实验和真实站点数据验证该算法分解和重构精度,并与奇异谱分析法、小波分解法、滑动最小二乘法进行比较。实验结果表明,ICEEMDAN-SSA联合算法相对于已有方法具有更好的重构精度。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号