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   515篇
  免费   48篇
  国内免费   114篇
测绘学   27篇
大气科学   17篇
地球物理   114篇
地质学   269篇
海洋学   100篇
天文学   70篇
综合类   15篇
自然地理   65篇
  2024年   2篇
  2023年   8篇
  2022年   5篇
  2021年   20篇
  2020年   18篇
  2019年   21篇
  2018年   24篇
  2017年   23篇
  2016年   22篇
  2015年   20篇
  2014年   25篇
  2013年   30篇
  2012年   27篇
  2011年   31篇
  2010年   23篇
  2009年   33篇
  2008年   21篇
  2007年   24篇
  2006年   45篇
  2005年   41篇
  2004年   27篇
  2003年   24篇
  2002年   13篇
  2001年   16篇
  2000年   15篇
  1999年   24篇
  1998年   19篇
  1997年   16篇
  1996年   7篇
  1995年   8篇
  1994年   10篇
  1993年   4篇
  1992年   5篇
  1991年   6篇
  1990年   3篇
  1989年   6篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1982年   2篇
  1972年   1篇
排序方式: 共有677条查询结果,搜索用时 15 毫秒
71.
分析评价西北区域参加实验室比对的试验结果,及时发现问题并进行纠正,总结实施实验间比对工作的经验,提高实验室检定/校准能力和质量管理水平。参加比对的计量检定实验室分别对气压、温度、湿度、风速比对样品进行比对试验,用归一化偏差(En)进行统计分析。2011年西北区域气象计量检定的温度、湿度、气压实验室比对结果为满意,风速实验室比对结果为较满意。该次比对的实验数据客观、真实、可信,是对各参加比对实验室检定/校准能力的综合评价,较为客观地反映了各技术机构检定装置的测量能力、检定人员的技术水平以及西北区域气象计量标准装置的现状。  相似文献   
72.
Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out: oxygenated VOC, aromatic VOC, biogenic VOC, short-lived intermediates, such as alkoxy and alkylperoxy radicals.At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.  相似文献   
73.
为建立适用于遥感相机定标的实验室,需要有几何标定的算法和软件对摄影图像进行计算.使用近景摄影测量中基于3维控制场的定标方法,对单CCD四波段遥感相机进行了几何定标,研究并自主编程实现了直接线性变换DLT(Direct Linear Transformation)算法,与基于空间后方交会算法的定标结果进行了精度对比.结论...  相似文献   
74.
In this study we assess the feasibility of remotely measuring canopy biochemistry, and thus the potential for conducting large-scale mapping of habitat quality. A number of studies have found nutrient composition of eucalypt foliage to be a major determinant of the distribution of folivorous marsupials. More recently it has been demonstrated that a specific group of secondary plant chemicals, the diformylphloroglucinols (DFPs), are the most important feeding deterrents, and are thus vital determinants of habitat quality. We report on the use of laboratory spectroscopy to attempt to identify one such DFP, sideroxylonal-A, in the foliage of Eucalyptus melliodora, one of the few eucalypt species browsed by folivorous marsupials. Reflectance spectra were obtained for freeze-dried, ground leaves using near infrared spectroscopy (NIRS) and for both oven-dried and fresh whole leaves using a laboratory-based (FieldSpec) spectroradiometer. Modified partial least squares (MPLS) regression was used to develop calibration equations for sideroxylonal-A concentration based on the reflectance spectra transformed as both the first and second difference of absorbance (Log 1/R). The predictive ability of the calibration equations was assessed using the standard error of calibration statistic (SECV). Coefficients of determination (r2) were highest for the ground leaf spectra (0.98), followed by the fresh leaf and dry leaf spectra (0.94 and 0.87, respectively). When applied to independent validation sub-sets, sideroxylonal-A was most accurately predicted from the ground leaf spectra (r2 = 0.94), followed by the dry leaf and fresh leaf spectra (0.72 and 0.53, respectively). Two spectral regions, centred on 674 nm and 1394 nm, were found to be highly correlated with sideroxylonal-A concentration for each of the three spectral data sets studied. Results from this study suggest that calibration equations derived from modified partial least squares regression may be used to predict sideroxylonal-A concentration, and hence leaf palatability, of Eucalyptus melliodora trees, thereby indicating that the remote estimation of habitat quality of eucalypt forests for marsupial folivores is feasible.  相似文献   
75.
在分析了当前测绘仪器管理存在问题的基础上,从仪器在实验室的管理体制、模式方面做了有益探讨和分析,提出了测绘仪器实行动态管理和开放使用的模式。  相似文献   
76.
面向市场需求的GIS专业本科教学探讨   总被引:1,自引:0,他引:1  
在分析目前我国GIS专业人才就业前景和就业现状的基础上,指出高校GIS教学存在的突出问题,并从专业培养目标、课程设置、教师队伍建设和开放性实验建设等方面提出GIS专业本科教学改革的方向。  相似文献   
77.
Vertical 2D slice laboratory experiments were carried out in homogenous and layered sand tanks to elucidate the effects of a highly permeable (coarse‐grained sand) interlayer on seawater intrusion and transport of contaminants to a coastal sea. Tidal fluctuations produced oscillations in the seawater–freshwater transition zone, fluctuations of the contaminant infiltration rate and a zigzag contaminant plume outline. The seawater wedge became discontinuous at the (vertical) edges of the interlayer because of increased lateral movement of the seawater–freshwater interface within the interlayer. The contaminant plume formed a tail within the interlayer depending on the tidal stage, and similar to the wedge, its movement was accentuated. A simple analytical model that neglected vertical flow reliably predicted steady‐state seawater intrusion into the coastal aquifer. Numerical modeling was used to gain insight into the groundwater hydrodynamics and contaminant migration. The numerical results confirmed the experimental findings, i.e. that a highly permeable interlayer can provide a rapid transit path for contaminants to reach the seaward boundary and that the interlayer amplifies the effects of tidal fluctuations, resulting in wider transition zones for the seawater wedge and contaminant plume. Numerical simulations further showed that, with increasing interlayer hydraulic conductivity, the maximum seawater intrusion distance inside the interlayer increases approximately linearly. For the fixed‐head contaminant injection condition used, the model showed that contaminant infiltration increases approximately logarithmically with increasing interlayer hydraulic conductivity (other factors held fixed). Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
78.
深地实验室具有电磁干扰小的"超净"地磁观测优势,已成为地磁观测的新平台和研究热点.为了评估淮南深地实验室地磁环境、认识地磁总场时变特征及地下与地面耦合性,我们于2022年在地下巷道(-848 m)和地表(+22 m)同步实施了地磁总场长期连续观测,对观测数据进行了功率谱、小波谱、地磁总场变化特征的分析及与附近的蒙城地磁台比对.研究结果表明:(1)地下无(或弱)磁干扰点位的地磁环境较优越,与蒙城地磁台相媲美,适用于地磁场变化的高精度、长期连续观测;(2)地下地磁观测可有效过滤地表上由工业与人类活动等的电磁干扰,其记录的地磁总场变化与蒙城地磁台具有较好的耦合性;(3)地下磁测可清晰记录地磁总场的平静变化、不同周期磁暴、地磁脉冲等丰富的时变信息,可为地球系统科学研究提供地磁学依据.  相似文献   
79.
Abstract This paper provides untilted paleomagnetic data obtained from the early Miocene strata around the Kanazawa‐Iozen area, in the eastern part of south‐west Japan. A thick pile of volcaniclastics and marine transgressive sediments underlie the area; they were deposited in the early stage of the Japan Sea opening event. Progressive thermal demagnetization tests isolated stable primary magnetic vectors from eight sites in the upper part of the Iozen Formation. Overall, the tilt‐corrected mean direction of this unit is D = 36.4°, I = 51.6° and α95 = 12.1. Together with a published paleomagnetic and chronological database, the present results suggest that clockwise rotation of south‐west Japan, linked to the back‐arc opening, commenced in the early Miocene and accelerated at the same time as rapid subsidence along the Japan Sea coast. Post‐opening, differential rotation within the eastern part of south‐west Japan is assumed, based on selected paleomagnetic data from the latest Early Miocene.  相似文献   
80.
Laboratory simulation of powder snow avalanches serves for the validation of numerical models and as a tool in its own right for consulting and research purposes. Two-phase gravity currents are simulated in water with quartz powder or glass spheres as the particle phase. Particle-phase velocity profiles and particle-phase volume fraction profiles are measured with an ultrasonic device. The experimental setup and the current state of the measurement technique are described and preliminary results are presented.  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