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   55298篇
  免费   5935篇
  国内免费   8545篇
测绘学   3866篇
大气科学   8563篇
地球物理   12119篇
地质学   26895篇
海洋学   4842篇
天文学   2967篇
综合类   5256篇
自然地理   5270篇
  2024年   175篇
  2023年   571篇
  2022年   1481篇
  2021年   1751篇
  2020年   1389篇
  2019年   1651篇
  2018年   6082篇
  2017年   5252篇
  2016年   4152篇
  2015年   1892篇
  2014年   2142篇
  2013年   1946篇
  2012年   2811篇
  2011年   4520篇
  2010年   3904篇
  2009年   4072篇
  2008年   3493篇
  2007年   3829篇
  2006年   1355篇
  2005年   1411篇
  2004年   1291篇
  2003年   1299篇
  2002年   1137篇
  2001年   979篇
  2000年   1124篇
  1999年   1488篇
  1998年   1204篇
  1997年   1124篇
  1996年   1037篇
  1995年   908篇
  1994年   787篇
  1993年   712篇
  1992年   563篇
  1991年   451篇
  1990年   330篇
  1989年   310篇
  1988年   270篇
  1987年   150篇
  1986年   143篇
  1985年   95篇
  1984年   99篇
  1983年   75篇
  1982年   65篇
  1981年   79篇
  1980年   60篇
  1979年   34篇
  1978年   11篇
  1977年   11篇
  1976年   12篇
  1958年   30篇
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
891.
选取河南省7家大型煤炭企业为研究对象,实证分析环境管制对其竞争力的影响。结果发现,严格的环境管制不但不会影响企业竞争力,反而有助于企业竞争力的提升。但与核心技术、产品差异化和基础设施相比,环境管制对企业竞争力的提升作用还不够显著。结合劳动成本对其竞争力的负面影响,河南省煤炭企业消化环境管理成本的能力还比较薄弱,仍然停留在低成本的比较优势阶段,环境技术创新尚未成为企业竞争优势的重要来源。  相似文献   
892.
Strokes are products of a higher-level aggregation of street segments that can reflect functional importance and perceptual significance that is associated with them in human spatial mental conceptualizations, which is of vital importance for network analysis, street selection, and map generalization. Street properties (e.g., street names) and angles between street segments are the two main elements used for generating street strokes according to the continuity principle of perceptual grouping into networks. However, it is difficult to automatically generate strokes with good continuity from street networks with multiple lanes such as dual carriageways or complex street junctions. This article proposes a method for generating street strokes that maintain good continuity across multiple lanes and complex street junctions. The proposed method first detects dual carriageways and complex junctions in street networks and then generates strokes according to the continuity principle of perceptual grouping. Finally, it groups the generated street strokes across the dual carriageways and complex street junctions to maintain good continuity. Moreover, the generated strokes are hierarchically ranked based on stroke length and centrality measurements. Experimental studies demonstrate the validity and effectiveness of the proposed method. The result shows that the generated street strokes maintain good continuity and reflect well the hierarchical structure of the street networks.  相似文献   
893.
在资料收集的基础上,利用遥感(RS)和地理信息系统(GIS)技术,结合经济社会因子和数学模型,以浙江沿海地区1990年、2000年和2010年的土地利用数据为基础,对20年间土地利用变化动态进行研究。利用灰色动态系统建立模型分析浙江沿海地区各类土地利用的变化趋势,在此基础上探讨其变化驱动力,揭示其变化的主要方式、原因及发展方向,预测未来土地利用的发展趋势,以期能为浙江省沿海地区土地利用规划提供科学依据。  相似文献   
894.
The association of iodine with organic matter in sedimentary basins is well documented. High iodine concentration in soils overlying oil and gas fields and areas with hydrocarbon microseepage has been observed and used as a geochemical exploratory tool for hydrocarbons in a few studies. In this study, we measure iodine concentration in soil samples collected from parts of Deccan Syneclise in the west central India to investigate its potential application as a geochemical indicator for hydrocarbons. The Deccan Syneclise consists of rifted depositional sites with Gondwana–Mesozoic sediments up to 3.5 km concealed under the Deccan Traps and is considered prospective for hydrocarbons. The concentration of iodine in soil samples is determined using ICP-MS and the values range between 1.1 and 19.3 ppm. High iodine values are characteristic of the northern part of the sampled region. The total organic carbon (TOC) content of the soil samples range between 0.1 and 1.3%. The TOC correlates poorly with the soil iodine (r 2 < 1), indicating a lack of association of iodine with the surficial organic matter and the possibility of interaction between the seeping hydrocarbons and soil iodine. Further, the distribution pattern of iodine compares well with two surface geochemical indicators: the adsorbed light gaseous hydrocarbons (methane through butane) and the propane-oxidizing bacterial populations in the soil. The integration of geochemical observations show the occurrence of elevated values in the northern part of the study area, which is also coincident with the presence of exposed dyke swarms that probably serve as conduits for hydrocarbon microseepage. The corroboration of iodine with existing geological, geophysical, and geochemical data suggests its efficacy as one of the potential tool in surface geochemical exploration of hydrocarbons. Our study supports Deccan Syneclise to be promising in terms of its hydrocarbon prospects.  相似文献   
895.
改革开放以来中国农业政策效果的时空计量分析   总被引:1,自引:0,他引:1  
Since reform and opening up, how much contribution has China’s implementation of new agricultural policy made to agricultural output? This paper is trying to establish an agricultural policy output econometric model for doing a quantitative analysis of China’s new agricultural policy. The results show that China’s agricultural policies on agricultural output have an average contribution rate of about 7% since 1978, which is consistent with the OECD’s basic forecast. There are obvious temporal and spatial differences. Generally speaking, we can divide the contribution of agricultural policy into three periods, which are the start-up phase from 1978 to 1991 (14 years), the stationary phase from 1992 to 2002 (11 years) and the rising phase from 2003 to 2008 (6 years). In space, the contribution of agricultural policy underwent a process from the all-low in the start-up phase, the gradual increase in the stationary phase to the all-high in the rising phase. Northern and western regions are more sensitive to policies. There are three major factors that can affect the contribution of regional agricultural policies, which are the process of national industrialization strategy, terrain and the level of local finance.  相似文献   
896.
大城市边缘区农村居民点用地存在用地粗放、布局散乱、难于规范管理等问题。以大城市边缘区农村居民点用地整理为契机,优化空间布局不仅有利于形成农村经济新增长点,还可促进农村居民点土地集约节约利用。在分析当地农村居民点用地现状及其分布状况的基础上,以典型地区——上海南汇地区为例,对其空间分布的随机性进行了检验,对农村居民点用地按标准偏差分类,并进一步按欧氏空间距离对大于2.25标准偏差的地块进行聚类分析。结果显示:南汇地区农村居民点用地的空间分布并不是随机分布,可分为远离中心类和集聚中心类两种类型。依据结果,提出了"远离中心方案"和"集聚中心方案"两种不同的农村居民点用地整理方案。  相似文献   
897.
Unlike other branches of geosciences, exploratory drilling has not been investigated within the framework of an information system; so, the expression “value of exploratory drilling information” (despite its common usage) is vague. This article presents a model for the evaluation of value of the information gathered from exploratory drilling after studying different mineral exploration and exploratory drilling systems within the framework of an “information system.” Although this model does not present the economic value of information, it is a suitable tool for comparing different drilling patterns. The model was verified on the basis of drilling data for the Gol-Gohar XIIA anomaly.  相似文献   
898.
899.
The Haji-Gak iron deposit of eastern Bamyan Province, eastern Afghanistan, was studied extensively and resource calculations were made in the 1960s by Afghan and Russian geologists. Recalculation of the resource estimates verifies the original estimates for categories A (in-place resources known in detail), B (in-place resources known in moderate detail), and C1 (in-place resources estimated on sparse data), totaling 110.8 Mt, or about 6% of the resources as being supportable for the methods used in the 1960s. C2 (based on a loose exploration grid with little data) resources are based on one ore grade from one drill hole, and P2 (prognosis) resources are based on field observations, field measurements, and an ore grade derived from averaging grades from three better sampled ore bodies. C2 and P2 resources are 1,659.1 Mt or about 94% of the total resources in the deposit. The vast P2 resources have not been drilled or sampled to confirm their extent or quality. The purpose of this article is to independently evaluate the resources of the Haji-Gak iron deposit by using the available geologic and mineral resource information including geologic maps and cross sections, sampling data, and the analog-estimating techniques of the 1960s to determine the size and tenor of the deposit.  相似文献   
900.
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