全文获取类型
收费全文 | 6736篇 |
免费 | 376篇 |
国内免费 | 321篇 |
专业分类
测绘学 | 3839篇 |
大气科学 | 144篇 |
地球物理 | 837篇 |
地质学 | 1103篇 |
海洋学 | 293篇 |
天文学 | 54篇 |
综合类 | 598篇 |
自然地理 | 565篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 111篇 |
2021年 | 154篇 |
2020年 | 173篇 |
2019年 | 131篇 |
2018年 | 112篇 |
2017年 | 294篇 |
2016年 | 265篇 |
2015年 | 282篇 |
2014年 | 338篇 |
2013年 | 532篇 |
2012年 | 494篇 |
2011年 | 440篇 |
2010年 | 298篇 |
2009年 | 335篇 |
2008年 | 458篇 |
2007年 | 461篇 |
2006年 | 441篇 |
2005年 | 335篇 |
2004年 | 333篇 |
2003年 | 249篇 |
2002年 | 215篇 |
2001年 | 193篇 |
2000年 | 155篇 |
1999年 | 117篇 |
1998年 | 87篇 |
1997年 | 66篇 |
1996年 | 48篇 |
1995年 | 58篇 |
1994年 | 43篇 |
1993年 | 41篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 16篇 |
1989年 | 28篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有7433条查询结果,搜索用时 409 毫秒
81.
Summary The newly developed light sectioning method has been used to investigate some of the causes and costs of overbreak and underbreak. Investigations at the Aquamilpa Hydroelectric Project in Mexico have shown decreased overbreak and increased underbreak as a result of increased rock quality and decreased explosive energy. A new measure of explosive energy, the perimeter powder factor (PPF), has been defined and shown to be useful in the context of tunnel-wall rock damage. Tentative results indicate that explosive energy (PPF) may be a more important factor in producing underbreak, whereas rock quality may be a greater factor in producing overbreak. A site-specific equation is given for predicting overbreak or underbreak as a function of rock quality and explosive energy, with an evaluation of the cost of underbreak and overbreak. 相似文献
82.
Stefan M. Luthi 《Mathematical Geology》1994,26(2):181-196
The texture of digital rock images, as recorded, for instance, with borehole imaging devices, is shown to reflect different bedding types. Textural segmentation of borehole images, therefore, subdivides the recorded sequence into bedding units. We show that a textural segmentation algorithm based on the concept of texture energy achieves good results when compared with synthetic as well as real data in which petroleum geologists have performed zonations on cores. Texture energy involves filtering of the original image with a set of texture sensitive masks. The filtering is done as a finite convolution over the size of the masks. On the resulting images the variance is computed over a relatively large sliding window, which, in its practical implementation, covers the full width of the image. The resulting nine one-dimensional curves are then clustered hierarchically into a user-determined number of image texture or lithological bedding classes. Principal component analysis previous to clustering can be used to reduce redundancy in the data. A recurring and relatively ill-defined problem in this field are macro-textures, i.e., the cyclic interbedding of two or more bedding types. We show that sliding Fourier transforms and variable mask scale can successfully address the zonation of macro-textures. In general, the method gives best results with mask sizes equivalent to 2–4 centimeters, reflecting the length scale at which the investigated geological bedding seems to have its highest variation. 相似文献
83.
84.
Rudolf Brázdil Hubert Valáek Kateina Chromá 《Geografiska Annaler: Series A, Physical Geography》2006,88(2):79-86
This paper deals with documentary evidence of an economic character as a proxy for direct study of meteorological and hydrological extremes. Taxation records and reports of those who administrated domains and estates are described with respect to information about meteorological and hydrological extremes. Based on data from eight domains or estates from Moravia (in the Czech Republic), frequency series of floods and convective storms (including hailstorms) were developed for the period 1650–1849. One example of disastrous weather, which took place on 10 August 1694 in the Pern?tejn domain, is used to demonstrate the potential for such studies of the intensity of extremes and their impact on human activities. The importance of economic evidence in the instrumental period is shown through tax rebate data contingent upon hailstorm damage in Moravia (1896–1906). The benefits of employing documentary economic evidence for historical climatology and the study of the impact of meteorological and hydrological extremes on human activities are discussed. 相似文献
85.
基于微电子系统(MEMS)加速度地震勘探三分量数字检波器简介 总被引:4,自引:0,他引:4
基于微电子机械系统(MEMS)制成的微加速度三分量数字地震检波器。集数据采集、处理及传输功能干一体,在结构模型、工作原理、性能指标控制等方面不同于常规检波器,其信号幅值在低于自然频率范围内没有衰减。因其独特的制造技术,MEMS检波器具有超低噪声,大动态范围和极高的向量保真度性能。 相似文献
86.
介绍了一种用三次插值样条对公开出版的旅游图进行纠正,制作汽车导航电子地图的方法,分析了其中存在的问题,提出了解决的办法。 相似文献
87.
提出快速获得航空影像并及时制作DOM、DLG的一种手段.介绍遥控飞艇数字摄影系统的工作原理,探讨存在的问题. 相似文献
88.
89.
90.