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   3040篇
  免费   577篇
  国内免费   1319篇
测绘学   63篇
大气科学   19篇
地球物理   171篇
地质学   4270篇
海洋学   52篇
天文学   1篇
综合类   241篇
自然地理   119篇
  2024年   9篇
  2023年   33篇
  2022年   75篇
  2021年   96篇
  2020年   83篇
  2019年   116篇
  2018年   93篇
  2017年   66篇
  2016年   123篇
  2015年   144篇
  2014年   264篇
  2013年   171篇
  2012年   211篇
  2011年   246篇
  2010年   224篇
  2009年   245篇
  2008年   271篇
  2007年   263篇
  2006年   249篇
  2005年   193篇
  2004年   194篇
  2003年   177篇
  2002年   165篇
  2001年   181篇
  2000年   142篇
  1999年   177篇
  1998年   154篇
  1997年   121篇
  1996年   98篇
  1995年   57篇
  1994年   43篇
  1993年   54篇
  1992年   50篇
  1991年   24篇
  1990年   25篇
  1989年   19篇
  1988年   19篇
  1987年   14篇
  1986年   12篇
  1985年   14篇
  1984年   11篇
  1983年   3篇
  1982年   2篇
  1981年   2篇
  1979年   1篇
  1976年   1篇
  1954年   1篇
排序方式: 共有4936条查询结果,搜索用时 15 毫秒
91.
济宁二号矿孤岛工作面冲击矿压危险及其控制   总被引:2,自引:0,他引:2  
孤岛工作面及其周围巷道附近应力集中程度高,顶板运动剧烈,再加上地质构造的影响,采深较大时,冲击危险程度就高,很容易引发冲击矿压。《煤矿安全规程》要求孤岛工作面按冲击矿压危险工作面来管理。孤岛工作面冲击矿压危险检测预报及控制的技术是先分析冲击危险程度,确定冲击危险指数,提出早期预报;采用电磁辐射和钻屑法进行及时预报;采用卸压爆破进行处理,并采用电磁辐射和钻屑法检验防治措施的效果。实践证明,这套技术安全、可靠、有效,能够保证工作面的安全高效生产。  相似文献   
92.
煤层上覆地层含水不均匀性电法探测的可能性   总被引:5,自引:4,他引:5  
煤田上覆岩层中含水直接对煤层的开采构成威胁,本文先从理论上阐述了电阻率勘探可以探测煤层上覆地层含水不均匀性的可能性,然后用简单的断层型薄层含水模型的CSAMT正演模拟结果展示出:地下900m深处,尽管只有20m厚的的含水岩层,但在电阻率剖面上该薄层的反映具有非常明显的特征,从理论上和数值模拟实例说明,一定深度内一定厚度的薄层含水性的电阻率法探测是可能的。  相似文献   
93.
An unusual liptinite coal component has been reported in the Chinese literature over the past sixty years. It has been described as a maceral in the Chinese National Standard (1991), but it has not been named internationally. In Chinese literature it is called “barkinite”, on the basis of its morphological features and because it is believed to have originated as bark tissue.“Barkinite” occurs in Late Permian, marine-influenced coals and is best represented in the Changguang, Leping and Shuicheng Basins of southern China.The material originates from plant periderm or the bark of higher plants. However, “bark” contains a variety of substances, including resin and suberin, which are recognised as the precursors of the resinite and suberinite macerals. “Barkinite” is distinguished by (i) its thickness; individual pieces can be more than ten cells thick and several centimetres long and (ii) it fluoresces strongly at 0.6% vitrinite reflectance and loses its fluorescence at about 1.1% vitrinite reflectance.The reporting of “barkinite” from only Chinese coals may be due to its origin from Lepidodendron and Psaronius flora, which was common in the Northern Hemisphere during the Carboniferous, but which was isolated to China by the Late Permian. It is proposed that the remnant flora evolved into unique forms in China by the Late Permian. Lepidodendron and Psaronius remains, coupled with a strongly marine-influenced, peat-forming environment have given rise to “barkinite” and to its restricted distribution.  相似文献   
94.
The Harlem coal is postulated to have been deposited in raised-bog settings. It was initially deposited as low-sulfur peat, but groundwater of a later origin altered portions of it to high-sulfur coal. The total sulfur content of 172 samples from 31 locations on a moisture- and ash-free basis have a quadrimodal distribution with two major modes at 0.75% S and 2.0%S. Multiple sampling and analyses at specific mine sites indicate that the addition of sulfur of a later origin occurred either laterally adjacent to sandstone channels or vertically adjacent to faults. Significant sulfur addition also occurred above clay partings at these sites. The original low-sulfur nature is preserved where the coal bed is isolated from descending groundwater by a thick impervious layer. Trace-element analyses show high concentrations of mobile elements such as Na and Mn in the high-sulfur coals.  相似文献   
95.
The origin and distribution of twelve potentially Hazardous Air Pollutants (HAPs; As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, and U) identified in the 1990 Clean Air Act Amendments were examined in relation to the maceral composition of the A1 bed (Paleocene, Calvert Bluff Formation, Wilcox Group) of the Calvert mine in east-central Texas. The 3.2 m-thick A1 bed was divided into nine incremental channel samples (7 lignite samples and 2 shaley coal samples) on the basis of megascopic characteristics. Results indicate that As, Cd, Cr, Ni, Pb, Sb, and U are strongly correlated with ash yield and are enriched in the shaley coal samples. We infer that these elements are associated with inorganic constituents in the coal bed and may be derived from a penecontemporaneous stream channel located several kilometers southeast of the mining block. Of the HAPs elements studied, Mn and Hg are the most poorly correlated to ash yield. We infer an organic association for Mn; Hg may be associated with pyrite. The rest of the trace elements (Be, Co, and Se) are weakly correlated with ash yield. Further analytical work is necessary to determine the mode of occurrence for these elements. Overall, concentrations of the HAPs elements are generally similar to or less than those reported in previous studies of lignites of the Wilcox Group, east-central region, Texas. Petrographic analysis indicates the following ranges in composition for the seven lignite samples: liptinites (5–8%), huminites (88–95%), and inertinites (trace amounts to 7%). Samples from the middle portion of the A1 bed contain abundant crypto-eugelinite compared to the rest of the samples; this relationship suggests that the degradation of plant material was an important process during the development of the peat mire. With the exception of Hg and Mn, relatively low levels of the HAPs elements studied are found in the samples containing abundant crypto-eugelinite. We infer that the peat-forming environment for this portion of the coal bed was very wet with minimal detrital input. Relatively high concentrations of crypto-humotelinite were found in samples from the top and base of the coal bed. The presence of abundant crypto-humotefinite in this part of the coal bed suggests the accumulation of wood-rich peat under conditions conducive to a high degree of tissue preservation in the peat mire. Although several of the trace elements (Be, Co, Ni, and Sb) exhibit enrichment in these samples, they are not necessarily chemically associated with humotelinite. We infer that these elements, with the exception of Be, are possibly associated with deposition of the roof and floor rock of the coal bed; however, further analytical work would be necessary to confirm this hypothesis. Beryllium may have an organic origin.  相似文献   
96.
对煤中锗矿化若干问题的思考——以临沧锗矿为例   总被引:8,自引:0,他引:8  
胡瑞忠  毕献武 《矿物学报》1997,17(4):364-368
本文在综述煤中锗矿化国内外研究现状的基础上,以临沧锗矿为例,深入讨论了在煤系地层中形成超大型独立锗矿床的特殊条件。结果表明,煤是锗成矿极有利的围岩,煤能否富集锗主要取决于有无富锗流体与煤相互作用。  相似文献   
97.
扬子地块西缘灯影组层控铅锌矿床成因的地球化学论证   总被引:5,自引:0,他引:5  
扬子地块西缘灯影组层控铅锌矿床成因的地球化学论证邵世才(有色总公司北京矿产地质研究所,北京100012)李朝阳(中国科学院地球化学研究所,贵阳550002)关键词层控铅锌矿床地球化学矿床成因扬子地块西缘1矿床地质特征扬子地块西缘灯影组铅锌矿床呈南北向...  相似文献   
98.
深成煤化作用是川南地区煤化作用的主导因素,本文着重阐明了构造对煤类分布的影响。  相似文献   
99.
淄博地区本溪期—石千峰期沉积环境经历了障壁岛—泻湖—台地环境,三角洲环境,曲流河环境体系的演替。相应的泥炭形成环境有不稳定的局限泻湖泥炭坪,较开阔泻湖泥炭坪,中等稳定的开阔泻湖泥炭坪,三角洲平原分流河道两侧的泥炭沼泽,洪泛平原上的泥炭沼泽。成煤植物上,本溪期以Cordaites,Lepidodendrales,Neuropteris为主,太原期华夏植物群发展,Lepidodendron,Cordaites继续发育,真蕨纲和种子蕨纲占越来越重要的地位。山西期以Odontopterissp.,Annulariaorientalis,Pecopterisorientalis,P.norini为主。自煤系形成至今,本区经历了海西印支期的普遍沉降埋藏、燕山期的普遍抬升局部断陷、喜山期的剥蚀、新构造运动期的差异沉降等阶段。古构造、古气候对本区成煤环境有重要的控制作用。  相似文献   
100.
恢复烃源岩古地层压力和对于研究其生留排烃过程具有重要意义。Nakayama[1]等在开展盆地模拟研究时应用Rubey(1959)和Chapman(197)等人提出的数学地质模型对欠压实地层的古地层压力进行恢复。本文通过对松辽盆地欠压实地层的研究发现欠压实因子随地层厚度、埋深、围岩渗透性等地质条件的不同而改变,在此基础上提出了欠压实地层压力恢复的统计模拟法,克服了目前Nakayama[1]等将地层埋藏过程中的欠压实因子视为常数的不足,从而使计算结果更接近实际。统计模拟法在松辽盆地的应用结果表明了这一方法在实用中的可行性和有效性。  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