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   1417篇
  免费   301篇
  国内免费   421篇
测绘学   133篇
大气科学   321篇
地球物理   365篇
地质学   804篇
海洋学   176篇
天文学   37篇
综合类   143篇
自然地理   160篇
  2024年   2篇
  2023年   28篇
  2022年   46篇
  2021年   63篇
  2020年   63篇
  2019年   75篇
  2018年   86篇
  2017年   74篇
  2016年   76篇
  2015年   83篇
  2014年   98篇
  2013年   68篇
  2012年   99篇
  2011年   95篇
  2010年   89篇
  2009年   79篇
  2008年   87篇
  2007年   89篇
  2006年   69篇
  2005年   66篇
  2004年   49篇
  2003年   42篇
  2002年   45篇
  2001年   50篇
  2000年   55篇
  1999年   67篇
  1998年   51篇
  1997年   55篇
  1996年   62篇
  1995年   49篇
  1994年   29篇
  1993年   24篇
  1992年   28篇
  1991年   13篇
  1990年   23篇
  1989年   12篇
  1988年   7篇
  1987年   12篇
  1986年   5篇
  1985年   8篇
  1984年   3篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1980年   6篇
  1979年   4篇
  1958年   1篇
排序方式: 共有2139条查询结果,搜索用时 46 毫秒
961.
In order to improve the performance of precise point positioning (PPP), this paper presents a new data processing scheme to shorten the convergence time and the observation time required for a reliable ambiguity-fixing. In the new scheme, L1 and L2 raw observations are used and the slant ionospheric delays are treated as unknown parameters. The empirical spatial and temporal constraints and the ionospheric delays derived from a real-time available ionospheric model are all considered as pseudo-observations into the estimation for strengthening the solution. Furthermore, we develop a real-time computational procedure for generating uncalibrated phase delays (UPDs) on L1 and L2 frequencies. The PPP solution is first carried out on all reference stations based on the proposed scheme, undifferenced float ambiguities on L1 and L2 frequencies can be directly obtained from the new scheme. The L1 and L2 UPDs are then generated and broadcasted to users in real-time. This data product and also the performance of the new PPP scheme are evaluated. Our results indicate that the new processing scheme considering ionospheric characteristics can reduce the convergence time by about 30 % for float kinematic solutions. The observation time for a reliable ambiguity-fixing is shortened by 25 % compared to that of the traditional ambiguity-fixed kinematic solution. When the new method is used for static reference stations, the observation time for ambiguity-fixing is about 10 min in static mode and only 5  min if the coordinates are fixed to well-known values.  相似文献   
962.
摘 要:MERIS数据以其更为合理的水色波段设置和300m较高的空间分辨率,在内陆湖泊水环境遥感监测中有较大的应用潜力, 对其进行有效的大气校正则是水环境参数定量化反演的前提。以太湖为研究区, 研究基于氧气和水汽吸收波段的暗象元假设, 改进传统的近红外波段暗像元假设的大气校正方法。采用MERIS L2p数据辅助获取湖区气溶胶参数, 并利用2007年11月11日、2008年11月20日以及2009年4月25日三景MERIS影像进行方法验证。结果表明, 该方法能够快速、有效地完成MERIS影像的大气校正, 与地面准同步实测数据相比, 三次校正的RMSP都在25%以下; 与BEAM自带的二类水体大气校正算法、气溶胶厚度辅助的6S大气校正以及改进的暗象元算法进行精度比较, 表明该算法校正精度较高。由于该算法不需要同步实测气溶胶数据, 因此具有一定的适用性。  相似文献   
963.
Fractal is employed in this paper as a scale-based method for the identification of the scaling behavior of time series. Many spatial and temporal processes exhibiting complex multi(mono)-scaling behaviors are fractals. One of the important concepts in fractals is crossover time scale(s) that separates distinct regimes having different fractal scaling behaviors. A common method is multifractal detrended fluctuation analysis (MF-DFA). The detection of crossover time scale(s) is, however, relatively subjective since it has been made without rigorous statistical procedures and has generally been determined by eye balling or subjective observation. Crossover time scales such determined may be spurious and problematic. It may not reflect the genuine underlying scaling behavior of a time series. The purpose of this paper is to propose a statistical procedure to model complex fractal scaling behaviors and reliably identify the crossover time scales under MF-DFA. The scaling-identification regression model, grounded on a solid statistical foundation, is first proposed to describe multi-scaling behaviors of fractals. Through the regression analysis and statistical inference, we can (1) identify the crossover time scales that cannot be detected by eye-balling observation, (2) determine the number and locations of the genuine crossover time scales, (3) give confidence intervals for the crossover time scales, and (4) establish the statistically significant regression model depicting the underlying scaling behavior of a time series. To substantive our argument, the regression model is applied to analyze the multi-scaling behaviors of avian-influenza outbreaks, water consumption, daily mean temperature, and rainfall of Hong Kong. Through the proposed model, we can have a deeper understanding of fractals in general and a statistical approach to identify multi-scaling behavior under MF-DFA in particular.  相似文献   
964.
China is exposed to a wide range of natural hazards, and disaster losses have escalated over the past decade. Owing to the pressure from natural disasters, along with changes in climate, social conditions, and regional environment, assessment of social vulnerability (SV) to natural hazards has become increasingly urgent for risk management and sustainable development in China. This paper presents a new method for quantifying SV based on the projection pursuit cluster (PPC) model. A reference social vulnerability index (SVI) at the county level was created for the Yangtze River Delta area in China for 1995, 2000, 2005, and 2009. The result of social vulnerability assessment was validated using data of actual losses from natural disasters. The primary findings are as follows: (i) In the study area, the major factors that impact SVI are regional per capita GDP and per capita income. (ii) The study area was more vulnerable in 1995 than in later years. SV of the whole region had decreased over the study period. (iii) Most part of Shanghai and the southeast part of Jiangsu Province had been the least vulnerable within the region. From this least vulnerable zone to the periphery of the region, the situation deteriorated. The highest SVI values in all evaluated years were found in the northern, western, or southern tips of the Yangtze River Delta.  相似文献   
965.
The Changbai Mountains,located in the temperate monsoon climate zone of East Asia,is an ideal loca-tion for the research on timberline response to global changes.In this study,the topsoils were collected from different vertical vegetation zones on the northern slope of the Changbai Mountains,Northeast China in August 2009,and phytoliths in the soil samples were extracted by using wet oxidation method and identified with Motic 2.0 microscope in laboratory.The results show that phytoliths are abundant in the ...  相似文献   
966.
???????к????????????С???????????????С????????????????????????????????С??????????????????????????????????????A?????????????????????P??PX??P0????????LS??????TLS???????????????????????????????????????????????????????С????????????????????????????????  相似文献   
967.
城市化进程中城市水文系统受到严重影响,致使现阶段我国城市中自然水文循环系统和以供排水为重心的城市人工侧支水循环系统之间存在尖锐矛盾。如何协调两者之间的关系,实现流域内城市地区人工侧支水循环系统与自然水文循环系统之间的良性互动,从而促进城市系统中经济、社会和环境协调可持续发展成为亟待解决的问题。  相似文献   
968.
在实验室对5块储层砂岩进行了模拟地层压力条件下的超声波速度测试。砂岩样品采自WXS凹陷的W地层,覆盖了从低到高的孔隙度和渗透率范围。实验选用了卤水和4种不同密度油作为孔隙流体,结合温度变化,实现了对流体粘度引致的速度频散研究。对实验结果的分析表明:(1)对于高孔隙度和渗透率的样品,无论是哪种流体饱和,观察到的超声波速度测试值和零频率Gassmann预测值的差异较小(约2-3%),基本上可以用Biot模型解释;对于中等孔隙度和渗透率的样品,低粘度流体(<约3mP?S)的频散效应也可以用Biot模型得到合理解释;(2)对于低、中孔隙度和渗透率样品,当流体粘度增加时,喷射流机制起主导作用,导致严重的速度频散(可达8%)。对储层砂岩的微裂隙纵横比进行了估计并用于喷射流特征频率的计算,当高于该特征频率时,Gassmann理论的假设条件受到破坏,实验室测得的高频速度不能直接用于地震低频条件下的W地层砂岩的Gassmann流体替换研究。  相似文献   
969.
氡气测量法-高密度电法在断层定位中的应用   总被引:3,自引:0,他引:3       下载免费PDF全文
主要论述了氡气测量法与高密度电法相结合的综合物探方法在断层定位中的应用。通过已知断层在同一岩性、不同岩性的试验结果,同地质剖面对比分析,说明两种物探方法对断层定位是可行的。最后,将试验结果应用到工程地质推断断层,取得了较为理想的效果。  相似文献   
970.
塔木察格盆地塔南凹陷铜钵庙组火山碎屑岩中发育大量的片钠铝石自生矿物。通过偏光显微镜、扫描电镜及X射线衍射分析等,查明了火山碎屑岩中的胶结物与自生矿物类型和成岩共生序列。含片钠铝石火山碎屑岩的成岩共生序列为粘土包壳-方解石-一期微晶石英-一期高岭石-二期微晶石英、二期高岭石、片钠铝石-铁白云石;通过MAT253稳定同位素质谱仪确定了片钠铝石和铁白云石的稳定同位素组成为片钠铝石的δ13CPDB为-8.15‰~-2.81‰,与片钠铝石平衡的CO2δ13CCO2为-12.91‰~-7.55‰,通过与海拉尔盆地及BGS盆地系碳氧同位素数据的类比,表明片钠铝石中的碳主要为幔源岩浆来源,形成片钠铝石的钠和铝主要来源于安山岩岩屑、长石晶屑碎屑、玻屑和火山灰等;铁白云石的δ13CPDB为-6.76‰~-2.65‰,与片钠铝石的碳氧同位素数据相似,结合相关地质实例及地球化学模拟实验,可以推断铁白云石中的碳也是幔源岩浆来源,铁白云石中的铁离子主要来源于安山岩岩屑、玻屑、火山灰和蒙脱石等矿物。  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