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   13966篇
  免费   1771篇
  国内免费   1269篇
测绘学   609篇
大气科学   1824篇
地球物理   3058篇
地质学   6225篇
海洋学   1257篇
天文学   2371篇
综合类   615篇
自然地理   1047篇
  2024年   115篇
  2023年   219篇
  2022年   314篇
  2021年   377篇
  2020年   347篇
  2019年   349篇
  2018年   596篇
  2017年   510篇
  2016年   650篇
  2015年   445篇
  2014年   642篇
  2013年   781篇
  2012年   558篇
  2011年   692篇
  2010年   697篇
  2009年   789篇
  2008年   642篇
  2007年   657篇
  2006年   603篇
  2005年   519篇
  2004年   461篇
  2003年   437篇
  2002年   374篇
  2001年   382篇
  2000年   381篇
  1999年   386篇
  1998年   382篇
  1997年   394篇
  1996年   329篇
  1995年   274篇
  1994年   232篇
  1993年   223篇
  1992年   183篇
  1991年   179篇
  1990年   137篇
  1989年   147篇
  1988年   127篇
  1987年   141篇
  1986年   125篇
  1985年   94篇
  1984年   97篇
  1983年   99篇
  1982年   101篇
  1981年   90篇
  1980年   77篇
  1979年   76篇
  1977年   58篇
  1976年   59篇
  1975年   53篇
  1973年   53篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
X-ray fluorescent spectroscope analysis easily finds ratios of geochemical elements in soil. Applying the method of ratio matching to measured ratios, the classification of geological layers, and the flow directions of groundwater can be determined. This method is applied to three hill slopes in Japan and the results are in good agreement with the observations and measurements of soil cores obtained by drilling. The classification of geological layers is found to be quantitatively connected with the rates of organic matter in soil cores. Received: 5 February 1999 · Accepted: 7 September 1999  相似文献   
42.
Assuming homogeneity in alluvial aquifers is convenient, but limits our ability to accurately predict stream‐aquifer interactions. Research is needed on (i) identifying the presence of focused, as opposed to diffuse, groundwater discharge/recharge to streams and (ii) the magnitude and role of large‐scale bank and transient storage in alluvial floodplains relative to changes in stream stage. The objective of this research was to document and quantify the effect of stage‐dependent aquifer heterogeneity and bank storage relative to changes in stream stage using groundwater flow divergence and direction. Monitoring was performed in alluvial floodplains adjacent to the Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping, observation wells were installed in high and low electrical resistivity subsoils. Water levels in the wells were recorded real time using pressure transducers (August to October 2009). Divergence was used to quantify heterogeneity (i.e. variation in hydraulic conductivity, porosity, and/or aquifer thickness), and flow direction was used to assess the potential for large‐scale (100 m) bank or transient storage. Areas of localized heterogeneity appeared to act as divergence zones allowing stream water to quickly enter the groundwater system, or as flow convergence zones draining a large groundwater area. Maximum divergence or convergence occurred with maximum rates of change in flow rates or stream stage. Flow directions in the groundwater changed considerably between base and high flows, suggesting that the floodplains acted as large‐scale bank storage zones, rapidly storing and releasing water during passage of a storm hydrograph. During storm events at both sites, the average groundwater direction changed by at least 90° from the average groundwater direction during baseflow. Aquifer heterogeneity in floodplains yields hyporheic flows that are more responsive and spatially and temporally complex than would be expected compared to more common assumptions of homogeneity. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
43.
44.
Landslide risk assessment and management: an overview   总被引:29,自引:0,他引:29  
Landslides can result in enormous casualties and huge economic losses in mountainous regions. In order to mitigate landslide hazard effectively, new methodologies are required to develop a better understanding of landslide hazard and to make rational decisions on the allocation of funds for management of landslide risk. Recent advances in risk analysis and risk assessment are beginning to provide systematic and rigorous processes to enhance slope management. In recent years, risk analysis and assessment has become an important tool in addressing uncertainty inherent in landslide hazards.This article reviews recent advances in landslide risk assessment and management, and discusses the applicability of a variety of approaches to assessing landslide risk. Firstly, a framework for landslide risk assessment and management by which landslide risk can be reduced is proposed. This is followed by a critical review of the current state of research on assessing the probability of landsliding, runout behavior, and vulnerability. Effective management strategies for reducing economic and social losses due to landslides are described. Problems in landslide risk assessment and management are also examined.  相似文献   
45.
46.
Landslides generate enormous volumes of sediment in mountainous watersheds; however, quantifying the downstream transport of landslide‐derived sediment remains a challenge. Landslide erosion and sediment delivery to the Shihmen Reservoir watershed in Taiwan was estimated using empirical landslide frequency–area and volume–area relationships, empirical landslide runout models, and the Hydrological Simulation Program‐ FORTRAN (HSPF). Landslide erosion rates ranged from 0.4 mm yr‐1 to 2.2 mm yr‐1 during the period 1986–2003, but increased to 7.9 mm yr‐1 following Typhoon Aere in 2004. The percentage of landslide sediment delivered to streams decreased from 78% during the period 1986–1997 to 55% in 2004. Although the delivery ratio was lower, the volume of landslide sediment delivered to streams was 2.81 × 106 Mg yr‐1 in 1986–1997 and 8.60 × 106 Mg yr‐1 in 2004. Model simulations indicate that only a small proportion of the landslide material was delivered downstream. An average of 13% of the landslide material delivered to rivers was moved downstream during the period 1986–1997. In 2004, the period including Typhoon Aere, the annual fluvial sediment yield accounted for approximately 23% of the landslide material delivered to streams. In general, the transfer of sediment in the fluvial system in the Shihmen Reservoir watershed is dominantly transport limited. The imbalance between sediment supply and transport capacity has resulted in a considerable quantity of landslide material remaining in the upper‐stream regions of the watershed. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
47.
针对鹤岗地震台在实际工作中遇到的各种问题,结合台站实际情况,分别从主机系统、电子记录系统等方面,提出仪器运行中经常出现的问题.并总结DSQ水管仪在日常问题中系统维护的具体解决办法,以此提高观测质量和工作效率.  相似文献   
48.
To study the deep dynamic mechanism leading to the difference in rifting pattern and basin structure from shelf to oceanic basin in passive continental margin,we constructed long geological sections across the shelf,slope and oceanic basin using new seismic data.Integrated gravity-magnetic inversion and interpretation of these sections were made with the advanced dissection method.Results show that the basement composition changes from intermediate-acid intrusive rocks in the sheff to intermediate-basic rocks in the slope.The Moho surface shoals gradually from 31 km in the sheff to 22.5 km in the uplift and then 19 km in the slope and finally to 13 km in the oceanic basin.The crust thickness also decreases gradually from 30 km in the northern fault belt to 9 km in the oceanic basin.The crustal stretching factor increases from the shelf toward the oceanic basin,with the strongest extension under the sags and the oceanic basin.The intensity of mantle upwelling controlled the style of basin structures from sheff to oceanic basin.In the Zhu 1 depression on the shelf,the crust is nearly normal,the brittle and cold upper crust mainly controlled the fault development;so the combinative grabens with single symmetric graben are characteristic.In the slope,the crust thinned with a large stretching factor,affected by the mantle upwelling.The ductile deformation controlled the faults,so there developed an asymmetric complex graben in the Baiyun (白云) sag.  相似文献   
49.
The decontamination of buried wastes of the Tyrnyauz Tungsten–Molybdenum Plant is complicated by the geochemical features of the waste composition: low sulfide and high carbonate content, polyelemental composition, and considerable amounts of technogenic admixtures (kerosene, oils, soda, and soluble glasses). These circumstances result in sufficient complication of the suggested technology of waste treatment, including the sulfuric-acid leaching and separate sorption recovery of hazardous and useful elements from the working solution.  相似文献   
50.
One of the Pre-Siwalik foreland basin sedimentary units, the Dumri Formation, is tectonically covered by the Lesser Himalayan Crystalline nappe and the Kuncha-Naudanda thrust sheet. It is narrowly distributed in the eastern margin of the Karnali klippe along the NNE–SSW trending Chakure Fault. The whole sequence of the fluvial Dumri Formation attaining 1500 m in thickness is weakly metamorphosed to muscovite phyllite and foliated phyllitic sandstone. The metamorphic grade decreases stratigraphically downward and underlying Nummulitic limestone of the middle Eocene Bhainskati Formation is converted into a slaty limestone. No metamorphic mica is detected from the late Cretaceous to Paleocene Amile Formation below the Bhainskati Formation. These facts indicate that the Tansen Group has undergone inverted metamorphism.A 40Ar/39Ar plateau age of 25.69±0.13 Ma was obtained from garnetiferous biotite gneiss in the lower part of the crystalline nappe. Another 40Ar/39Ar age spectrum from muscovite phyllite of the Dumri Formation suggests that metamorphism occurred at 16–17 Ma. The origin of the inverted metamorphism limited to the uppermost part of the Lesser Himalayan autochthon can be attributed to heat from the hot crystalline nappe and shearing along the sole thrust of the Kuncha-Naudanda thrust sheet. The depositional age of the Dumri Formation is estimated to be 26–17 Ma.Provenance of the Dumri Formation is considered to be from the Naudanda Quartzite, the Kuncha Formation and the Tibetan Tethys sediments, because the sandstone contains orthoquartzite pebbles, phyllitic lithic fragments and a sparry calcite cement. The sedimentary facies indicates deposition by meandering rivers on flood-plains in the distal part of the foreland basin. No proximal facies, such as alluvial fan and pebbly braided river deposits, could be detected from the formation, though it is near the Main Central Thrust (MCT). The northern continuation of the foreland basin sediments must be concealed beneath the Higher Himalayan Crystalline. Judging from the present distribution of the Dumri Formation from the south of the Main Boundary Thrust (MBT) to near the MCT and from the shortening of the Lesser Himalayan sediments by thrusts and folds, the width of the foreland basin where the Dumri Formation was deposited is estimated to have been more than 300 km.  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