全文获取类型
收费全文 | 6887篇 |
免费 | 518篇 |
国内免费 | 479篇 |
专业分类
测绘学 | 3869篇 |
大气科学 | 329篇 |
地球物理 | 920篇 |
地质学 | 1073篇 |
海洋学 | 465篇 |
天文学 | 269篇 |
综合类 | 480篇 |
自然地理 | 479篇 |
出版年
2024年 | 23篇 |
2023年 | 47篇 |
2022年 | 172篇 |
2021年 | 227篇 |
2020年 | 212篇 |
2019年 | 289篇 |
2018年 | 153篇 |
2017年 | 318篇 |
2016年 | 317篇 |
2015年 | 342篇 |
2014年 | 333篇 |
2013年 | 480篇 |
2012年 | 405篇 |
2011年 | 364篇 |
2010年 | 312篇 |
2009年 | 355篇 |
2008年 | 418篇 |
2007年 | 398篇 |
2006年 | 371篇 |
2005年 | 363篇 |
2004年 | 321篇 |
2003年 | 281篇 |
2002年 | 214篇 |
2001年 | 235篇 |
2000年 | 167篇 |
1999年 | 127篇 |
1998年 | 126篇 |
1997年 | 93篇 |
1996年 | 81篇 |
1995年 | 62篇 |
1994年 | 49篇 |
1993年 | 55篇 |
1992年 | 52篇 |
1991年 | 26篇 |
1990年 | 24篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有7884条查询结果,搜索用时 65 毫秒
51.
52.
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
53.
Grain size and grain shape analysis of fault rocks 总被引:4,自引:0,他引:4
54.
Groundwater study using remote sensing and geographic information systems (GIS) in the central highlands of Eritrea 总被引:3,自引:1,他引:3
Remote sensing, evaluation of digital elevation models (DEM), geographic information systems (GIS) and fieldwork techniques were combined to study the groundwater conditions in Eritrea. Remote sensing data were interpreted to produce lithological and lineament maps. DEM was used for lineament and geomorphologic mapping. Field studies permitted the study of structures and correlated them with lineament interpretations. Hydrogeological setting of springs and wells were investigated in the field, from well logs and pumping test data. All thematic layers were integrated and analysed in a GIS. Results show that groundwater occurrence is controlled by lithology, structures and landforms. Highest yields occur in basaltic rocks and are due to primary and secondary porosities. High yielding wells and springs are often related to large lineaments, lineament intersections and corresponding structural features. In metamorphic and igneous intrusive rocks with rugged landforms, groundwater occurs mainly in drainage channels with valley fill deposits. Zones of very good groundwater potential are characteristic for basaltic layers overlying lateritized crystalline rocks, flat topography with dense lineaments and structurally controlled drainage channels with valley fill deposits. The overall results demonstrate that the use of remote sensing and GIS provide potentially powerful tools to study groundwater resources and design a suitable exploration plan.The online version of the original article can be found at 相似文献
55.
A Practical Procedure for the Measurement of Fragmentation by Blasting by Image Analysis 总被引:1,自引:0,他引:1
Summary. The operation of a digital image analysis system in a limestone quarry is described. The calibration of the system, required
in order to obtain moderately reliable fragmentation values, is done from muckpile sieving data by tuning the image analysis
software settings so that the fragmentation curve obtained matches as close as possible the sieving. The sieving data have
also been used to extend the fragment size distribution curves measured to sizes below the system’s optical resolution and
to process the results in terms of fragmented rock, discounting the material coming from a loose overburden (natural fines)
that is cast together with the fragmented rock. Automatic and manual operation modes of the image analysis are compared. The
total fragmentation of a blast is obtained from the analysis of twenty photographs; a criterion for the elimination of outlier
photographs has been adopted using a robust statistic. The limitations of the measurement system due to sampling, image processing
and fines corrections are discussed and the errors estimated whenever possible. An analysis of consistency of the results
based on the known amount of natural fines is made. Blasts with large differences in the amount of fines require a differentiated
treatment, as the fine sizes tend to be the more underestimated in the image analysis as they become more abundant; this has
been accomplished by means of a variable fines adjustment factor. Despite of the unavoidable errors and the large dispersion
always associated with large-scale rock blasting data, the system is sensitive to relative changes in fragmentation. 相似文献
56.
Traction image method for irregular free surface boundaries in finite difference seismic wave simulation 总被引:16,自引:1,他引:16
In this study, we propose a new numerical method, named as Traction Image method, to accurately and efficiently implement the traction-free boundary conditions in finite difference simulation in the presence of surface topography. In this algorithm, the computational domain is discretized by boundary-conforming grids, in which the irregular surface is transformed into a 'flat' surface in computational space. Thus, the artefact of staircase approximation to arbitrarily irregular surface can be avoided. Such boundary-conforming gridding is equivalent to a curvilinear coordinate system, in which the first-order partial differential velocity-stress equations are numerically updated by an optimized high-order non-staggered finite difference scheme, that is, DRP/opt MacCormack scheme. To satisfy the free surface boundary conditions, we extend the Stress Image method for planar surface to Traction Image method for arbitrarily irregular surface by antisymmetrically setting the values of normal traction on the grid points above the free surface. This Traction Image method can be efficiently implemented. To validate this new method, we perform numerical tests to several complex models by comparing our results with those computed by other independent accurate methods. Although some of the testing examples have extremely sloped topography, all tested results show an excellent agreement between our results and those from the reference solutions, confirming the validity of our method for modelling seismic waves in the heterogeneous media with arbitrary shape topography. Numerical tests also demonstrate the efficiency of this method. We find about 10 grid points per shortest wavelength is enough to maintain the global accuracy of the simulation. Although the current study is for 2-D P-SV problem, it can be easily extended to 3-D problem. 相似文献
57.
58.
59.
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
60.
本文主要对加密和身份认证技术进行了简要分析,并就如何利用证书解决WEB应用中 的信息对传输的保密性、完整性和不可否认性以及用户身份认证问题提出了安全解决 方案。 相似文献