全文获取类型
收费全文 | 1497篇 |
免费 | 494篇 |
国内免费 | 361篇 |
专业分类
测绘学 | 129篇 |
大气科学 | 24篇 |
地球物理 | 615篇 |
地质学 | 1248篇 |
海洋学 | 139篇 |
天文学 | 22篇 |
综合类 | 50篇 |
自然地理 | 125篇 |
出版年
2024年 | 11篇 |
2023年 | 19篇 |
2022年 | 38篇 |
2021年 | 54篇 |
2020年 | 96篇 |
2019年 | 86篇 |
2018年 | 99篇 |
2017年 | 92篇 |
2016年 | 107篇 |
2015年 | 98篇 |
2014年 | 112篇 |
2013年 | 146篇 |
2012年 | 100篇 |
2011年 | 95篇 |
2010年 | 93篇 |
2009年 | 83篇 |
2008年 | 95篇 |
2007年 | 85篇 |
2006年 | 109篇 |
2005年 | 114篇 |
2004年 | 70篇 |
2003年 | 68篇 |
2002年 | 67篇 |
2001年 | 47篇 |
2000年 | 50篇 |
1999年 | 45篇 |
1998年 | 42篇 |
1997年 | 35篇 |
1996年 | 37篇 |
1995年 | 24篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有2352条查询结果,搜索用时 15 毫秒
21.
黄河下游断面形态与水沙输移关系及数学模拟方法 总被引:4,自引:0,他引:4
以前人对黄河下游的实测资料分析为基础,本文讨论断面形态与来水来沙关系、断面形态对输水输沙的影响,并根据实测资料,提出黄河下游弯段与直段断面冲淤变化的两种模式,建立了主流摆动及坍岸影响断面形态变化以及纵向冲淤量在断面的分布两种数学模拟方法。 相似文献
22.
Risk and uncertainty assessments for waste containment systems employing clay barriers often include spatial variability in
the hydraulic conductivity as part of the analysis. The two-parameter log-normal distribution is often used to describe the
spatial variability, but for compacted clays the three-parameter form can be more appropriate. A statistical hypothesis test
was developed that can be used to determine if the two- or three-parameter form of the log-normal distribution is more appropriate.
The test is based on a likelihood ratio, comparing likelihood functions for the two- and three-parameter forms at their maximums.
Likelihood functions are used assuming that a data set can be segregated into a set of distinct groups of hydraulic conductivities.
A step-by-step calculation procedure is described and the test is applied to data collected from 45 sites. 相似文献
23.
24.
Abdullah Al-Khanbashi Shihab W. Abdalla 《Geotechnical and Geological Engineering》2006,24(6):1603-1625
This study reports on waterborne polymer emulsion systems that were used for structural modification of sandy soils. The first
emulsion used was a styrene-acrylic, copolymer emulsion; the second and third were vinyl-acrylic, copolymer-based emulsions.
Specimens were prepared by adding different polymer emulsions to the sand and water in prescribed amounts. The performance
of the different emulsion systems as sand stabilizers was estimated through measurement of the hydraulic conductivity and
compressive strength. Measurements of hydraulic conductivity were conducted in a flexible membrane test apparatus. The hydraulic
conductivity for the three emulsions decreased at different rates, with an increase in polymer content up to 3 wt. (i.e.,
% by weight). Additional increase of polymer content did not introduce significant decrease in hydraulic conductivity, and,
at 5 wt.%, the three emulsions had essentially the same effect. Measurements made on dry cylindrical specimens disclosed remarkable
enhancement in the mechanical behavior of the system. The compressive strength and modulus of elasticity of the examined emulsions
increased with a higher concentration of polymer. Scanning electron microscope (SEM) revealed that the reduction in the permeability
and the improved mechanical properties can be attributed to three dominant structural changes: the development of interconnecting
ties between the sand particles, the development of adhesion between neighboring sand particles in contact, and the covering
of the sand particles with a thin polymer film. 相似文献
25.
1 IntroductionFracturing plays an important role in increasingthe production and injection and enhancing the final re-covery of oil and gas. At present, the use of ahydroxypropyl guar gum, borate-crosslinked, hydraulicfracturing fluid systemhas become ext… 相似文献
26.
27.
28.
Semi‐active stiffness damper (SASD) is one of many semi‐active control systems with the capability to mitigate the dynamic response using only a small amount of external power. The system consists of a hydraulic damper connected to the bracing frame in a selected story unit. In this paper, study of a SASD in two building models of five‐stories under four benchmark earthquake records is reported. The purpose of this study is to evaluate the effectiveness of the control system against structure type and varying earthquake inputs. Various control laws are chosen to work with SASD, such as: resetting control, switching control, linear quadratic regulator (LQR) and modified LQR, and the results are compared with no control and passive control cases. Numerical results show that the use of a SASD is effective in reducing seismic responses. Control effectiveness is dependent on the type of structure and earthquake excitation. Passive control is less effective than other control cases as expected. Resetting control, switching control and LQR generally perform similarly in response reduction. While modified LQR is more efficient and robust compared with other control algorithms. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
29.
耦合的质量传输和动力学限制的同位素交换模型综合考虑了平流、扩散、热液弥散 ,以及岩石与水之间的不平衡同位素交换等项因素。把耦合模型应用于 2个构造环境下的古热液系统 ,进而解释稳定同位素数据。对于造成环绕浅成侵入岩体分布的18O亏损环带的流体的流动几何学 ,以及浅部正断层流体流动的几何学 ,耦合模型提供了不同于单一同位素交换模型的解释。耦合模型也提供了关于同位素相对交换速率和同位素交换机制的信息。结果表明 ,断层带的动态重结晶促进了表面反应 ,进而便利了同位素交换 ;在化学不反应性和未变形的矿物中 ,同位素交换可能受制于固态条件下的扩散。 相似文献
30.
强水动力湖泊夏季分层期氮的生物地球化学循环初步研究:以贵州红枫湖南湖为例 总被引:9,自引:1,他引:9
湖泊在夏季由于藻类生长而消耗大量硝酸盐,水体硝酸盐含量一般要低于春季。而红枫湖南湖水体硝酸盐含量却高于春季(比平均含量高0.83mg/L),说明尚有其他重要的硝酸盐来源。据估算,南湖水体硝酸盐含量升高0.83mg/L约需要1.66×105kg硝酸盐,另外有约10.1×105kg硝酸盐随下泻水输出南湖,再加上夏季藻类生长(生产的chla量约为640kg)所消耗的硝酸盐3.52×105kg,共消耗硝酸盐15.28×105kg。扣除河流输入的4.42×105kg硝酸盐,南湖尚存在约10.86×105kg硝酸盐的亏空。利用氮稳定同位素示踪技术,结合硝酸盐及叶绿素a(chla)含量、溶解氧(DO)等的变化,认为这部分硝酸盐来自湖泊中下部(斜温层)有机质的大量矿化(硝化),是水动力驱动高DO的上部水体下沉从而引起下部有机质(硝化)的结果。南湖这种强水动力湖泊整个夏季分层期氮的生物地球化学循环是斜温层有机质矿化(硝化)释放硝酸盐和变温层藻类生长同化硝酸盐为有机质同时发生的特殊类型。 相似文献