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   248篇
  免费   107篇
  国内免费   52篇
测绘学   1篇
大气科学   7篇
地球物理   48篇
地质学   250篇
海洋学   65篇
天文学   6篇
综合类   15篇
自然地理   15篇
  2024年   1篇
  2023年   4篇
  2022年   12篇
  2021年   13篇
  2020年   14篇
  2019年   15篇
  2018年   17篇
  2017年   13篇
  2016年   16篇
  2015年   14篇
  2014年   25篇
  2013年   25篇
  2012年   23篇
  2011年   32篇
  2010年   21篇
  2009年   17篇
  2008年   19篇
  2007年   17篇
  2006年   9篇
  2005年   9篇
  2004年   13篇
  2003年   5篇
  2002年   12篇
  2001年   9篇
  2000年   7篇
  1999年   7篇
  1998年   21篇
  1997年   4篇
  1996年   5篇
  1995年   3篇
  1994年   2篇
  1992年   1篇
  1990年   1篇
  1985年   1篇
排序方式: 共有407条查询结果,搜索用时 468 毫秒
71.
如何实现地热资源合理开发、高效利用,实现地热资源采灌均衡可持续开发利用模式已成为地热资源开发利用地区重点研究内容。该文根据庆云县某典型小区(以下简称小区A)一个采暖季的开采、回灌监测数据,总结了水温、开采量、回灌量、水位及回灌率变化情况。通过数据分析及调查可知,庆云县回灌工程普遍存在地热尾水温度偏高、热利用效率偏低、地热尾水直接回灌引起管道腐蚀影响供暖效果等问题。针对地板辐射和暖气片采暖两种模式提出了换热技术和增加热泵机组的改进措施建议,既可提高热利用效率,又可保证供暖效果避免设备腐蚀,对庆云县地热供暖工程的升级改造具有一定的指导意义。  相似文献   
72.
Abstract

Aquatic plants and lake bottoms in optically shallow waters (OSWs) wield great influence on reflectance spectra, resulting in the inapplicability of most existing bio-optical models for water colour remote sensing in lakes. Based on radiative transfer theory and measured spectra from a campaign for Lake Taihu in October 2008, absorption and backscattering coefficients were used to simulate the remote-sensing reflectance, which are considered to be reliable if matched to their measured counterparts. Several cases of measured spectra at different depths, Secchi disk depth transparency, and aquatic plant height and coverage were analyzed thoroughly for spectral properties. The contribution of aquatic plants was evaluated and compared with the measured and simulated remote-sensing reflectance values. This is helpful for removing the influence of aquatic plants and lake bottoms from the spectra and for constructing an improved chlorophyll a retrieval model for OSWs, such as that for Lake Taihu, China.  相似文献   
73.
水源热泵系统中地下水流贯通及其对温度场的影响   总被引:3,自引:0,他引:3  
利用含水层储能的一般特点,提出流贯通;分析流贯通和热贯通的相互作用、相互影响及对工程实际的意义;并基于地下水水热运移的基本原理,建立地下水流动和热量输运的耦合数值模型。在此基础上对沈阳某场地水源热泵工程的运行进行流场及温度场的数值模拟分析,给出产生流贯通的依据,即利用水力坡度定量的判断流贯通发生与否;当水力坡度变化非常小、可忽略不计时,认为含水层出现流贯通;进而研究抽、灌量对流贯通的影响,发现抽、灌量越大,则出现流贯通时间越短;而大量的抽、灌量更容易发生流贯通,进而会引起热贯通的发生。在实际工程中可降低抽、灌水量,在条件许可的情况下可提高抽、灌井的间距,并可在抽、灌井运行一个周期后调整抽灌井位置,以减少热贯通的发生。  相似文献   
74.
姚克  方鹏  邵俊杰  李栋  张占强 《探矿工程》2016,43(10):165-169
针对现有定向长钻孔钻机在煤矿井下狭窄巷道及复杂地层条件应用存在的诸如结构体积偏大、功能结构与施工工艺匹配需改善、附属设备配套及操控性差等实际问题,研制了窄体型小定向钻机成套装备,通过现场工业性试验和长期推广应用验证了产品的性能与可靠性。阐明了该套设备的研制思路和必要性,以及解决的关键技术问题。  相似文献   
75.
Concentrations of nutrients (NO3, NO2, Si(OH)4, PO4 and dissolved inorganic carbon (DIC) were measured in a series of seawater samples collected over approximately 15 months in 2005 and 2006 by an automatic water sampler (Remote Access Sampler, RAS) in the Northwestern North Pacific. Seasonal variability and concentrations of NO3 + NO2 (NOx and Si(OH)4 were comparable to previous shipboard observations, although there were small errors associated with measurements of PO4 and DIC. Concentrations of these nutrients began to decrease in late April. After the end of June, NOx and Si(OH)4 decreased rapidly, with large fluctuations. After October, these nutrients increased again until late spring 2006. The ratio of the decrease of Si(OH)4 to that of NOx suggests that numbers of biogenic opal-producing creatures, such as diatoms, increased after the end of June. This conclusion was supported by a rapid increase in biogenic opal flux recorded in a sediment trap at 150 m. The relationship between NOx concentrations at the RAS depth of 35 m and NOx integrated over the upper 100 m was determined using previous shipboard hydrocast data. This relationship was used to estimate integrated mixed layer NOx concentration from RAS data. Estimated new production based on seasonal drawdown of integrated NOx averaged approximately 156 mg-C m−2day−1 annually, which agrees with previous estimates. Thus, an automatic seawater sampler that documents annual maximum and minimum nutrient concentrations and episodic events such as storms and spring blooms, which might be missed by an ordinary research vessel, will contribute to time-series observations of nutrients and, by extension, biological pump activity.  相似文献   
76.
Seasonal variations in coccolithophore abundance, chlorophyll, nutrients and production of particulate organic and inorganic carbon (POC and PIC) were determined along a coastal to oceanic east-west transect (Line P) culminating at Ocean Station Papa in the northeastern subarctic Pacific between 1998 and 2000. Offshore stations generally exhibited low seasonality in chlorophyll concentrations, with moderate seasonality in POC production. Near shelf stations showed a similar pattern to offshore stations, but were also characterized by sporadic events of higher POC productivity. During the 1998 El Niño, June was characterized by low chlorophyll and POC productivity along the transect, presumably as a result of depleted surface nitrate. In contrast, during the 1999 La Niña, and in 2000, higher POC productivity and surface nitrate occurred along the transect in June. Chlorophyll and POC productivity were similar in late summer in all 3 years. The coccolithophore population was usually numerically dominated by Emiliania huxleyi, particularly in June. Along the transect, abundance of coccolithophores was much higher in June during the 1998 El Niño (mean of 221 cells ml−1) than in the 1999 La Niña (mean of 40 cells ml−1), with their abundance in late summers of both years being very low. Abundances were even higher along the transect in June and the late summer of 2000 with sporadic ‘blooms’ of >1000 cells ml−1 at some stations (cruise averages 395 and 552 cell ml−1, respectively). Production rates of PIC did not consistently correlate with areas of high coccolithophore abundance. PIC production was high (100-250 mg C m−2 d−1) along the transect during June 1998, and low (1-40 mg C m−2 d−1) during both winters, June 1999 and during late summers of 1998 and 1999. The year 2000 was more complicated, with high rates of PIC production accompanying high abundance of coccolithophores in late summer, but lower rates of PIC production accompanying high coccolithophore numbers in June. Our data suggest that the abundance of coccolithophores and the production rates of PIC in the subarctic are higher than previously thought. Occasional PIC:POC production ratios of 1 or greater in 1998 and 2000 suggest that coccolithophores in this region could have a significant impact on the efficiency of the biological carbon pump.  相似文献   
77.
The distributions of CFC (chlorofluorocarbon) in the water column was determined twice in 2000 and 2001 in the northwestern Japan Sea. In 2000 the CFC-11 concentration decreased almost exponentially with depth from 6 pmol/kg at a few hundred m deep to 0.3 pmol/kg or less at the bottom of about 3400 m depth at three stations (40–41°N, 132–133°E) about 300 km off Vladivostok. In 2001 the CFC-11 concentration increased sharply up to 2 pmol/kg in the bottom water, while it did not increase at a station (42.0°N, 136.5°E) about 450 km away to the northeast. This is due to the renewal of the bottom water which is replaced by the surface water flowing down along the continental slope, as suggested by Tsunogai et al. (1999), who proposed the continental shelf pump. Furthermore, an increase in the CFC-11 concentration was observed throughout the entire water column above 3000 m depth, although the proportion of the increase was about 20%, which was one order of magnitude smaller than that in the bottom water. The increase in inventory is almost four times larger than that in the bottom water below 3000 m depth which is equivalent to about 1/6 of the total inventory found in 2000. The increase also means that 3% of the deep water was replaced by the recent surface water, or, if the turnover occurs every year, that the turnover time of the deep water to be about 30 years. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
78.
从理论上分析了转柱式散气增氧装置扩散帽的水动力性能和影响进气量、功率消耗等方面的因素。同时选择了两个系列4种技术参数的扩散帽装置进行对比试验,试验结果与理论分析情况基本吻合。通过理论分析与试验测试,最后选定出扩散帽的最佳技术参数。  相似文献   
79.
The lifting motor pump is the core equipment of the deep-sea hydraulic pipeline lifting system, and its research results are one of the important technical reserves for the research and development of marine mineral resources. It is an essential capacity for lifting motor pump to allow coarse particles pass through the flow passages, to realize this requirement, an increased flowrate method is employed to design a wide flow passage for an innovative six-stage lifting motor pump. Based on Computational Fluid Dynamics (CFD), the anti-blocking performance of the lifting motor pump designed by the conventional method and the increased flowrate method are compared, which show that the latter is better than the former. Numerical simulations of the internal two-phase flow inside the innovative pump are presented, and the flow parameters such as pressure, velocity and particle trajectory are obtained. According to the design method of the six-stage lifting motor pump, a two-stage lifting motor pump is manufactured, and the relevant performance experiments are performed. The results obtained by numerical simulation is in a good agreement with the experimental ones, thus the validity and accuracy of the numerical model and method is verified. The innovative two-stage lifting motor pump can operate smoothly with low noise and no blockage occurs, and its performance meets the design goals and requirements.  相似文献   
80.
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