全文获取类型
收费全文 | 1987篇 |
免费 | 416篇 |
国内免费 | 561篇 |
专业分类
测绘学 | 189篇 |
大气科学 | 330篇 |
地球物理 | 487篇 |
地质学 | 1151篇 |
海洋学 | 335篇 |
天文学 | 53篇 |
综合类 | 211篇 |
自然地理 | 208篇 |
出版年
2024年 | 17篇 |
2023年 | 64篇 |
2022年 | 116篇 |
2021年 | 131篇 |
2020年 | 115篇 |
2019年 | 120篇 |
2018年 | 117篇 |
2017年 | 110篇 |
2016年 | 123篇 |
2015年 | 118篇 |
2014年 | 126篇 |
2013年 | 126篇 |
2012年 | 148篇 |
2011年 | 118篇 |
2010年 | 118篇 |
2009年 | 124篇 |
2008年 | 126篇 |
2007年 | 113篇 |
2006年 | 96篇 |
2005年 | 74篇 |
2004年 | 84篇 |
2003年 | 64篇 |
2002年 | 104篇 |
2001年 | 107篇 |
2000年 | 49篇 |
1999年 | 70篇 |
1998年 | 32篇 |
1997年 | 47篇 |
1996年 | 26篇 |
1995年 | 31篇 |
1994年 | 30篇 |
1993年 | 21篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1978年 | 1篇 |
1958年 | 1篇 |
1954年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有2964条查询结果,搜索用时 13 毫秒
131.
132.
In this paper, we perform an inverse method to simultaneously estimate aquifer parameters, initial condition, and boundary conditions in groundwater modelling. The parameter estimation is extended to a complete inverse problem that makes the calibrated groundwater flow model more realistic. The adjoint state method, the gradient search method, and the least square error algorithm are combined to build the optimization procedure. Horizontal two‐dimensional groundwater flow in a confined aquifer is exemplified to demonstrate the correlation between unknowns, the contribution of observation, as well as the suitability of applying the inverse method. The correlation analysis shows the connection between storage coefficient and initial condition. Besides, transmissivity and boundary conditions are also highly correlated. More observations at different location and time are necessary to provide sufficient information. A time series of unsteady head is requested for estimation of storage coefficient and initial condition. Observation near boundary is very effective for boundary condition estimation. The observation at pumping well mostly contributes to the estimation of transmissivity. According to all observations, it is possible to identify parameters, initial condition, and boundary condition simultaneously. Furthermore, the results not only illustrate the traditional assumption of known boundary condition but also initial condition, which may cause an incorrect estimation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
133.
根据苏北地区大地电磁勘探环境特点进行了电极距长度、磁场位置和采集时长的试验工作,确定了大地电磁测深勘探技术参数设置下限,电极距长度不能小于50m,采集时间要大于8h,磁距应控制在2km范围内。 相似文献
134.
Wei-yue Li Chun Liu Yang Hong Xin-hua Zhang Zhan-ming Wan Manabendra Saharia Wei-wei Sun Dong-jing Yao Wen Chen Sheng Chen Xiu-qin Yang Yue Yue 《山地科学学报》2016,13(7):1275-1285
Landslide inventory plays an important role in recording landslide events and showing their temporal-spatial distribution. This paper describes the development, visualization, and analysis of a China's Landslide Inventory Database (CsLID) by utilizing Google’s public cloud computing platform. Firstly, CsLID (Landslide Inventory Database) compiles a total of 1221 historical landslide events spanning the years 1949-2011 from relevant data sources. Secondly, the CsLID is further broken down into six zones for characterizing landslide cause-effect, spatiotemporal distribution, fatalities, and socioeconomic impacts based on the geological environment and terrain. The results show that among all the six zones, zone V, located in Qinba and Southwest Mountainous Area is the most active landslide hotspot with the highest landslide hazard in China. Additionally, the Google public cloud computing platform enables the CsLID to be easily accessible, visually interactive, and with the capability of allowing new data input to dynamically augment the database. This work developed a cyber-landslide inventory and used it to analyze the landslide temporal-spatial distribution in China. 相似文献
135.
本文在对微细浸染型金矿床的地质及地球化学特征和成矿条件进行综合分析的基础上 ,对省内该类型金矿床的特点和成矿地质条件进行了分析和对比 ,指出了省内寻找该类型金矿床的找矿方向。 相似文献
136.
Historically, Crescent City is one of the most vulnerable communities impacted by tsunamis along the west coast of the United States, largely attributed to its offshore geography. Trans-ocean tsunamis usually produce large wave runup at Crescent Harbor resulting in catastrophic damages, property loss and human death. How to determine the return values of tsunami height using relatively short-term observation data is of great significance to assess the tsunami hazards and improve engineering design along the coast of Crescent City. In the present study, the extreme tsunami heights observed along the coast of Crescent City from 1938 to 2015 are fitted using six different probabilistic distributions, namely, the Gumbel distribution, the Weibull distribution, the maximum entropy distribution, the lognormal distribution, the generalized extreme value distribution and the generalized Pareto distribution. The maximum likelihood method is applied to estimate the parameters of all above distributions. Both Kolmogorov-Smirnov test and root mean square error method are utilized for goodness-of-fit test and the better fitting distribution is selected. Assuming that the occurrence frequency of tsunami in each year follows the Poisson distribution, the Poisson compound extreme value distribution can be used to fit the annual maximum tsunami amplitude, and then the point and interval estimations of return tsunami heights are calculated for structural design. The results show that the Poisson compound extreme value distribution fits tsunami heights very well and is suitable to determine the return tsunami heights for coastal disaster prevention. 相似文献
137.
The gastric shield and underlying stomach epithelium of Chlymas ffarreri were examined at the light and ultrastructural levels.The results showed that the gastric shield consisted of two different size lobes joined together by a narrow middle piece,the thicker lobe was shaped like a funnel,but unclosed at the lateral side;the other lobe was irregularly triangular-shaped.The transverse section of the thicker lobe was obviously laminated and gradually decreased in thickness from the peak to the margins of the shield.The underlying epithelium bore numerous about 3μm diameter spherical processes formed by the apical plasmalemma of the epithelial cells becoming blunt pseudopodia.Microvilli and some interspersed cilia were present in the areas among the spherical processes regions where only microvilli existed.Rough endoplasmic reticulum,Golgi apparatus,different-sized electron-dense secretory granules and electron-lucent vacuoles as well las abundant mitochondria were present in the underlying epithelial cells.Fused droplets of the secretion from the underlying epithelial cells formed the gastric shield. 相似文献
138.
GIS的体系结构与全面社会化应用发展分析 总被引:1,自引:0,他引:1
近年来GIS取得了巨大的发展,其动力主要来自于应用需求的牵引和相关的IT技术的推动。本文主要从应用需求的变化对GIS体系结构的影响,以及包括Interbet、移动通信与定位、软件开发、软件工程等IT技术的进展对GIS的推动两个方面,对近年来GIS发展的内因和外因进行分析和探讨。 相似文献
139.
China is a mountainous country,and Southwest mountain areas cover the most mountain areas in China and have the most serious problems.Taking Zhaotong city as the study area,based on 902 rural household questionnaires of 11 villages in 2 counties and Tobit model,this paper analyzes the geographical differences and influencing factors of energy consumption for non-production purposes of rural households living in different terrain conditions.This research finds that:(1) Coal takes up the main part of energy consumption in valley areas and coal consumption is mainly affected by per capita cultivated land area,household income,proportion of rural household energy expenditure in total expenditure,coal price,and family population size.Firewood takes up the main part of energy consumption in high mountain areas and firewood consumption is mainly affected by per capita firewood forest area,distance to purchase coal,household income,electricity price,and coal price.(2) Only when the distance is greater than 20 kilometers,that is the average distance of rural households living in middle mountain areas(1,600m~1,800m) to purchase coal,the transportation condition has a significant impact on coal consumption.(3) In high mountain areas,prices of coal and electricity are the main factors influencing energy consumption choice of rural households.Too high prices of coal and electricity would to some extent lead rural households to choose firewood as the main energy consumption type.Compared to coal,rural households prefer to choose electricity. 相似文献
140.