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   1990篇
  免费   367篇
  国内免费   455篇
测绘学   46篇
大气科学   266篇
地球物理   405篇
地质学   1180篇
海洋学   378篇
天文学   30篇
综合类   83篇
自然地理   424篇
  2024年   6篇
  2023年   31篇
  2022年   60篇
  2021年   76篇
  2020年   83篇
  2019年   111篇
  2018年   95篇
  2017年   81篇
  2016年   105篇
  2015年   98篇
  2014年   166篇
  2013年   188篇
  2012年   151篇
  2011年   132篇
  2010年   127篇
  2009年   149篇
  2008年   127篇
  2007年   121篇
  2006年   141篇
  2005年   93篇
  2004年   126篇
  2003年   86篇
  2002年   77篇
  2001年   59篇
  2000年   55篇
  1999年   38篇
  1998年   41篇
  1997年   34篇
  1996年   41篇
  1995年   17篇
  1994年   20篇
  1993年   9篇
  1992年   19篇
  1991年   6篇
  1990年   6篇
  1989年   3篇
  1988年   4篇
  1987年   6篇
  1986年   7篇
  1985年   6篇
  1984年   3篇
  1983年   3篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1971年   1篇
排序方式: 共有2812条查询结果,搜索用时 31 毫秒
51.
This paper presents the results of ground vibration measurements carried out in Hisarcik Boron open pit mine located on the west side of central Anatolia near Kütahya province in Turkey. Within the scope of this study to predict peak particle velocity (PPV) level for this site, ground vibration components were measured for 304 shots during bench blasting. In blasting operations, ANFO (blasting agent), gelatin dynamite (priming), and delay electric detonators (firing) were used as explosives. Parameters of scaled distance (charge quantity per delay and the distance between the source and the station) were recorded carefully and the ground vibration components were measured for all blast events using two different types of vibration monitors (one White Mini-Seis and one Instantel Minimate Plus Model). The absolute distances between shot points and monitor stations were determined using GPS. The equation of square root scaled distance extensively used in the literature was taken into consideration for the prediction of PPV. Then, the data pairs of scaled distance and particle velocity obtained from the 565 event records were analyzed statistically. At the end of statistical evaluation of the data pairs, an empirical relation which gives 50% prediction line with a reasonable correlation coefficient was established between PPV and scaled distance.  相似文献   
52.
Semi-Analytic Modelling of Subsidence   总被引:2,自引:0,他引:2  
This paper presents a forward model for subsidence prediction caused by extraction of hydrocarbons. The model uses combinations of analytic solutions to the visco-elastic equations, which approximate the boundary conditions. There are only a few unknown parameters to be estimated, and, consequently, calculations are very fast. The semi-analytic model is applicable to a uniform and layer-cake stratigraphy, with visco-elastic parameters changing per layer, and an arbitrary depletion pattern. By its capabilities to handle a multi-layered visco-elastic subsurface, the semi-analytic model fills the gap between the analytic single-layered elastic models available to date and the more elaborate numerical, e.g. finite element, models.  相似文献   
53.
After conducting many laboratory and field experiments, several key technical issues related to the construction of China’s Taklamakan Desert Highway have been satisfactorily resolved. In particular, considerable progress has been made on the dry compaction of a sand sub-base, road design parameters, the creation of a structure that combines a sub-grade and asphalt pavement, analysis of the stability of a sand sub-base strengthened with geotextiles, and on the development of a complete set of construction techniques. The achievements of this research were successfully applied for the first time in the Taklamakan Desert, where the environmental conditions are extremely harsh. The results suggest that the construction of this highway was economical and that the simple construction methods produced a reliable highway. The resulting highway is believed to be the world’s first long-distance graded highway running through a huge desert with migrating dunes.  相似文献   
54.
一次“高影响天气”的弱降雪过程的数值研究   总被引:11,自引:2,他引:11       下载免费PDF全文
该文对2001年12月7日下午北京的一次弱降雪天气过程进行了诊断分析和数值模拟研究。诊断结果表明:北京地区由于受500 hPa小槽东移及850 hPa东移小高压后部带来的来自东海和南海的偏南暖湿气流共同影响,造成降雪。数值模拟结果显示:MM5模式对这次降雪过程具有较好的模拟能力,不仅模拟出了北京地区的降雪量,而且对这次过程大尺度背景场的演变、触发机制和水汽源有很好的表述。利用诊断分析和数值模拟结果,对这次弱降雪过程引起交通大阻塞的可能因素进行了探讨,说明建立城市预警系统的迫切性。  相似文献   
55.
Terrestrial ecosystems provide a number of vital services for people and society, such as food, fibre, water resources, carbon sequestration, and recreation. The future capability of ecosystems to provide these services is determined by changes in socio-economic factors, land use, atmospheric composition, and climate. Most impact assessments do not quantify the vulnerability of ecosystems and ecosystem services under such environmental change. They cannot answer important policy-relevant questions such as 'Which are the main regions or sectors that are most vulnerable to global change?’ 'How do the vulnerabilities of two regions compare?’ 'Which scenario is the least harmful for a sector?’This paper describes a new approach to vulnerability assessment developed by the Advanced Terrestrial Ecosystem Analysis and Modelling (ATEAM) project. Different ecosystem models, covering biodiversity, agriculture, forestry, hydrology, and carbon sequestration are fed with the same Intergovernmental Panel on Climate Change (IPCC) scenarios based on the Special Report on Emissions Scenarios (SRES). Each model gives insights into specific ecosystems, as in traditional impact assessments. Moreover, by integrating the results in a vulnerability assessment, the policy-relevant questions listed above can also be addressed. A statistically derived European environmental stratification forms a key element in the vulnerability assessment. By linking it to other quantitative environmental stratifications, comparisons can be made using data from different assessments and spatial scales.  相似文献   
56.
近十年来我国气候变暖影响研究的若干进展   总被引:51,自引:12,他引:51       下载免费PDF全文
近年来,我国政府和科技界十分关注气候王馥棠变暖对我国经济发展可能影响的评估, 开展了许多重大项目和课题的研究。该文仅就气候变暖对我国自然植被、农业、森林、水资源、能源利用和区域海平面上升等领域影响评估研究的若干有意义的初步结果简要归纳和评述如下:取自不同GCM模型的未来气候变化情景下的影响评估模拟表明,我国的特征性自然植被类型将会发生明显的变化。同当前气候(1951~1980年)下的模拟分布相比,到2050年我国几乎所有地方的农业种植制度均将发生较大变化;气候变暖将导致复种指数增加和种植方式多样化,但降水与蒸散之间可能出现的负平衡和土壤水分胁迫的增加以及生育期的可能缩短,最终将导致我国主要作物的产量下降。气候变暖对我国水资源最明显的影响将会发生在黄淮海流域,这个区域的水资源供需短缺将大大提高。同时,气候变暖将改变我国室内取暖和降温的能源需求关系:北方冬季取暖的能源消耗将减少, 而南方夏季降温的能源消耗将会增加。海平面的上升将使我国三个主要沿海低洼脆弱区,即珠江三角洲、长江三角洲和黄河三角洲,面临部分遭受海水淹没的威胁。  相似文献   
57.
翁时瑞 《福建地质》2002,21(3):169-171
介绍了福州地热XG3深井采用气举反循环施工技术的实践过程和应用效果。  相似文献   
58.
FW-300型非开挖导向钻机给进机构的设计   总被引:4,自引:0,他引:4       下载免费PDF全文
指出了对非开挖钻机给进机构的基本要求,着重介绍了FW-300型非开挖导向钻机机构的设计思想及主要部件设计。  相似文献   
59.
The volcanological history of Campi Flegrei suggests that the most frequent eruptions are characterized by the emplacement of pyroclastic flow and surge deposits erupted from different vents scattered over a 150-km2 caldera. The evaluation of volcanic risk in volcanic fields is complex because of the lack of a central vent. To approach this problem, we subdivided the entire area of Campi Flegrei into a regular grid and evaluated the relative spatial probability of opening of vents based on geological, geophysical and geochemical data. We evaluated the volcanic risk caused by pyroclastic flows based on the formula proposed by UNESCO (1972), R=H×V×Va, where H is the hazard, V is the vulnerability and Va is the value of the elements at risk. The product H×V was obtained by performing simulations of type eruptions centered in each cell of the grid. The simulation is based on the energy cone scheme proposed by Sheridan and Malin [J. Volcanol. Geotherm. Res. 17 (1983) 187–202], hypothesizing a column collapse height of 100 m for eruptions of VEI=3 and 300 m for eruptions of VEI=4 with a slope angle of 6°. Each simulation has been given the relative probability value associated with the corresponding cell. We made use of the GIS software ArcView 3.2 to evaluate the intersection between the energy cone and the topography. The superposition of the areas invaded by pyroclastic flows (124 simulations for VEI=3 and 37 for VEI=4) was used to obtain the relative hazard map of the area. The relative volcanic risk map is obtained by superimposing the urbanization maps.  相似文献   
60.
In the Négron River catchment area (162 km2), surface‐sediment stores are composed of periglacial calcareous ‘grèze’ (5 × 106 t) and loess (21 × 106 t), and Holocene alluvium (12·6 × 106 t), peat (0·6 × 106 t) and colluvium (18·5 × 106 t). Seventy‐five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0·6 t km?2 a?1) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7·0 t km?2 a?1 from alluvium stores and 7·6 t km?2 a?1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18·7 t km?2 a?1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub‐basin to another (3·9 to 24·5 t km?2 a?1) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps (‘rideaux’). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one‐third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment. Copyright © 2002 John Wiley & Sons, Ltd.  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