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   2024篇
  免费   492篇
  国内免费   581篇
测绘学   29篇
大气科学   35篇
地球物理   460篇
地质学   1670篇
海洋学   437篇
天文学   1篇
综合类   100篇
自然地理   365篇
  2024年   21篇
  2023年   41篇
  2022年   65篇
  2021年   82篇
  2020年   88篇
  2019年   90篇
  2018年   85篇
  2017年   78篇
  2016年   95篇
  2015年   104篇
  2014年   167篇
  2013年   125篇
  2012年   119篇
  2011年   132篇
  2010年   132篇
  2009年   167篇
  2008年   137篇
  2007年   109篇
  2006年   138篇
  2005年   109篇
  2004年   111篇
  2003年   84篇
  2002年   82篇
  2001年   87篇
  2000年   86篇
  1999年   85篇
  1998年   67篇
  1997年   62篇
  1996年   60篇
  1995年   56篇
  1994年   43篇
  1993年   37篇
  1992年   28篇
  1991年   29篇
  1990年   19篇
  1989年   6篇
  1988年   8篇
  1987年   5篇
  1986年   3篇
  1985年   13篇
  1984年   7篇
  1983年   11篇
  1982年   7篇
  1981年   15篇
  1978年   1篇
  1971年   1篇
排序方式: 共有3097条查询结果,搜索用时 0 毫秒
91.
煤层气选区和评价的若干问题探讨   总被引:2,自引:1,他引:2  
根据近几年从事煤层气评价和勘探工作的实践和思考,对比较流行的选区思路和评价方法作提出了一些疑问,休克有定选区的目标和从哪些方面进行评价进行了探讨,并提出了选区的思路要“实际”,在评价方法上要“实用”的观点。  相似文献   
92.
连立祥 《铀矿地质》1995,11(5):284-290
红层(盆)内外存在着工业金矿化现象,与红层(盆)有关的金矿化包括:次火山岩型、卡林型、穆龙套型、不整合脉型、风化壳型、热泉型以及砂金等众多类型。它们的形成过程及分布与红层(盆)关系密切。本文总结了与红层(盆)有关金矿化的成矿模式及找矿方向。  相似文献   
93.
94.
在充分分析米脂—绥德岩盐田地质特征的基础上,对位于其中部的米脂姬家峁矿区O2m65亚段盐矿层赋存特征进行了研究,预测了矿区O2m65亚段岩盐资源储量,并对其开采条件进行了评价。  相似文献   
95.
Trials conducted at Dry Creek, a solar saltfield north of Adelaide, Australia, indicate that manganese, as manganous sulphate, can increase both the purity and strength of salt crystals grown at an ambient or neutral-slightly acid pH range resulting in a salt suitable for high purity uses. The improved load bearing strength of the crystals indicates that manganous sulphate could also be useful in salt floor maintenance. However, the impact of an apparently slight rise in pH to 8.5 was significant. Crystals produced at the higher pH from the manganese enriched brine were opaque with many tiny, almost colloidal crystals. The salt did not drain well, had low load bearing strength and was notably impure.  相似文献   
96.
多途径解决我国钾盐资源紧缺的对策探讨   总被引:6,自引:0,他引:6  
探讨了钾盐矿产的应用,钾盐矿产在世界及我国的分布状况,提出了解决我国钾盐矿产资源紧缺问题的途径。  相似文献   
97.
卤水中的化学成分因受多种因素影响,使其在时间序列上的变化波动强烈而规律难循。灰色—马尔科夫过程预测是预测事物在时间序列上发展趋势的一种良好有效的统计分析手段,它将灰色系统预测法和马尔科夫过程预测法有效结合,既弥补了两种方法各自的缺点,又能充分考虑数据给予的信息,可以大大提高随机波动较大数据序列的预测精度。本文用灰色—马尔科夫过程预测法对吉兰泰盐湖卤水中几种主要离子浓度的变化进行了趋势分析,分析结果发现几种离子浓度的变化均有一定的规律性,而且这些离子在预测数据的变化规律同原始实测数据变化规律有很好的一致性。  相似文献   
98.
A large data set on ripples was collected and examined. A set of new formulas for the prediction of the ripple characteristics is proposed with an emphasis on the disappearance of the ripples. The ripple wavelength was observed to be proportional to the bottom wave excursion but also to be a function of the grain-related Shields parameter and wave period parameter introduced by Mogridge et al. (1994). The ripple steepness was found to be nearly constant for orbital ripples, and with a sharp decrease for suborbital ripples. Two empirical functions are added including the effects of the critical Shields parameters (inception of transport and inception of sheet flow), i.e. giving the boundaries for the ripple existence's domain. The proposed formulas yield better prediction capabilities compared to the previously published formulas, especially when ripples are washed out. The effect of the ripple characteristics on the roughness height and the calculation of the bed shear stress is also discussed. It appeared that the bed shear stress calculation is more sensitive to the empirical coefficient ar introduced in the estimation of the ripple-induced roughness height or to the limits of existence of the ripples than the ripple characteristics themselves.  相似文献   
99.
邴慧  何平  杨成松 《冰川冻土》2004,26(Z1):44-49
As to salty soil, salt migrates with water in freezing soils, assembles and crystallizes continuously. Consequently the swelling of the soil volume leads to the phenomenon of salt heaving. It has a practical significance for solving salt-heaving and frost-heaving damage in engineering to deepen the understanding of salt heaving mechanism. In this paper a general overview about new research results at this aspect was presented. And then the study of salt migration and salt heaving mechanism and present salt heaving models were summarized. For further researching the field of salt transfer it is urgent to continually strengthen the salt migration and the numerical simulation study of salt heaving mechanism to expect perfecting the general evaluation of salt heaving prediction models so as to have a better service for engineering.  相似文献   
100.
Numerical modeling of nonlinear water waves over heterogeneous porous beds   总被引:1,自引:0,他引:1  
Eric C. Cruz  Qin Chen   《Ocean Engineering》2007,34(8-9):1303-1321
The transformation of nonlinear water waves over porous beds is studied by applying a numerical model based on Chen's [2006. Fully nonlinear Boussinesq-type equations for waves and currents over porous beds. Journal of Engineering Mechanics, 132:2, 220–230] Boussinesq-type equations for highly nonlinear waves on permeable beds. The numerical model uses a high-order time-marching solution and fourth-order finite-difference schemes for discretization of first-order spatial derivatives to obtain a computational accuracy consistent with the model equations. By forcing the wave celerity and spatial porous-damping rate of the linearized model to match the exact linear theory for horizontal porous bed over a prescribed range of relative depths, the values of the model parameters are optimally determined. Numerical simulations of the damped wave propagation over finite-thickness porous layer demonstrate the accuracy of both the numerical model and governing equations, which have been shown by prior theoretical analyses to be accurate for both nominal and thick porous layers. These simulations also elucidate on the significance of the higher-order porous-damping terms and the influence of the hydraulic parameters. Application of the model to the simulation of the wave field around a laboratory-scale submerged porous mound provides a measure of its capability, as well as useful insight into the scaling of the porous-resistance coefficients. For application to heterogeneous porous beds, the assumption of weak spatial variation of the porous resistance is examined using truncated forms of the governing equations. The results indicate that the complete set of Boussinesq-type equations is applicable to porous beds of nonhomogeneous makeup.  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