全文获取类型
收费全文 | 1256篇 |
免费 | 249篇 |
国内免费 | 284篇 |
专业分类
测绘学 | 298篇 |
大气科学 | 168篇 |
地球物理 | 171篇 |
地质学 | 732篇 |
海洋学 | 190篇 |
天文学 | 13篇 |
综合类 | 89篇 |
自然地理 | 128篇 |
出版年
2024年 | 6篇 |
2023年 | 15篇 |
2022年 | 39篇 |
2021年 | 43篇 |
2020年 | 62篇 |
2019年 | 63篇 |
2018年 | 52篇 |
2017年 | 81篇 |
2016年 | 78篇 |
2015年 | 85篇 |
2014年 | 111篇 |
2013年 | 109篇 |
2012年 | 86篇 |
2011年 | 113篇 |
2010年 | 102篇 |
2009年 | 96篇 |
2008年 | 76篇 |
2007年 | 104篇 |
2006年 | 77篇 |
2005年 | 81篇 |
2004年 | 51篇 |
2003年 | 45篇 |
2002年 | 39篇 |
2001年 | 27篇 |
2000年 | 30篇 |
1999年 | 22篇 |
1998年 | 14篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1789条查询结果,搜索用时 15 毫秒
31.
大跨度切眼锚网锚索支护研究 总被引:5,自引:0,他引:5
全煤大跨度切眼传统的支护方式是钢棚支护,为解决无轨胶轮车直接进入工作面,提高工作面设备安装效率,课题组进行锚网锚索支护试验与研究。该项目的试验成功,为矿井高产高效创造了条件。 相似文献
32.
库区塌岸空间信息管理系统构建 总被引:3,自引:1,他引:3
以ArcGIS为平台,扩展其专业功能,建立一套集库区塌岸预测与防治有关的基本信息采集、信息管理、预测评价,以及塌岸防治工程决策于一体的“库区塌岸信息管理系统及防治决策支持系统”。建立起库区各数据库和各库岸段的塌岸模式数据库,结合典型塌岸防治实例、专家经验、专家知识,建立塌岸防治决策支持系统,为库区塌岸防治工程的宏观决策提供技术支撑,同时也可对重大灾情进行及时预警预报提供数据支撑。 相似文献
33.
In underground coal mining any increase in coal recovery rate is dependent on a decrease in pillar size. Backfilling is one way of reducing the required size of pillars and hence the volume of coal left underground. Therefore any comparisons made between a self-supported mine layout and backfill supported mine layout are based directly on pillar design. The most effective way to examine the effect of backfill on pillar support, and subsequently the rate of recovery, would be to incorporate the mechanisms of backfill support directly into the current design procedure for coal pillars. This paper presents a review of the mechanics of backfill support, a method of estimating the magnitude of that support based on earth pressure theory, and an example that incorporates backfill support into current coal pillar design. 相似文献
34.
35.
Christian Brannstrom 《Geoforum》2005,36(2):257-271
The expansion of modern agriculture in developing countries presents numerous challenges for environmental policy makers. Environmental policies for agriculture in north-eastern Brazil’s soybean belt are analyzed, with emphasis on the role of a non-state actor in leading policy reforms. An organization representing large farmers is leading policy reforms to reduce the environmental impacts of modern agriculture. By contrast, state agencies are relatively weak and ineffective. The analysis situates this question conceptually in literatures stressing the political and structural causes of corporate environmentalism and literatures explaining the increasingly strong role of non-state actors in environmental governance. The case study focuses on the content of reforms, reasons why the non-state actor is so prominent, and the implications of policy reforms. State agencies face major challenges in environment-agriculture policy debates in places where environmental subsidies are unlikely, environmental information is poor, and organized private interests are influential. 相似文献
36.
In this study, the climate teleconnections with meteorological droughts are analysed and used to develop ensemble drought prediction models using a support vector machine (SVM)–copula approach over Western Rajasthan (India). The meteorological droughts are identified using the Standardized Precipitation Index (SPI). In the analysis of large‐scale climate forcing represented by climate indices such as El Niño Southern Oscillation, Indian Ocean Dipole Mode and Atlantic Multidecadal Oscillation on regional droughts, it is found that regional droughts exhibits interannual as well as interdecadal variability. On the basis of potential teleconnections between regional droughts and climate indices, SPI‐based drought forecasting models are developed with up to 3 months' lead time. As traditional statistical forecast models are unable to capture nonlinearity and nonstationarity associated with drought forecasts, a machine learning technique, namely, support vector regression (SVR), is adopted to forecast the drought index, and the copula method is used to model the joint distribution of observed and predicted drought index. The copula‐based conditional distribution of an observed drought index conditioned on predicted drought index is utilized to simulate ensembles of drought forecasts. Two variants of drought forecast models are developed, namely a single model for all the periods in a year and separate models for each of the four seasons in a year. The performance of developed models is validated for predicting drought time series for 10 years' data. Improvement in ensemble prediction of drought indices is observed for combined seasonal model over the single model without seasonal partitions. The results show that the proposed SVM–copula approach improves the drought prediction capability and provides estimation of uncertainty associated with drought predictions. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
37.
Statistical learning algorithms provide a viable framework for geotechnical engineering modeling. This paper describes two statistical learning algorithms applied for site characterization modeling based on standard penetration test (SPT) data. More than 2700 field SPT values (N) have been collected from 766 boreholes spread over an area of 220 sqkm area in Bangalore. To get N corrected value (Nc), N values have been corrected (Nc) for different parameters such as overburden stress, size of borehole, type of sampler, length of connecting rod, etc. In three‐dimensional site characterization model, the function Nc=Nc (X, Y, Z), where X, Y and Z are the coordinates of a point corresponding to Nc value, is to be approximated in which Nc value at any half‐space point in Bangalore can be determined. The first algorithm uses least‐square support vector machine (LSSVM), which is related to a ridge regression type of support vector machine. The second algorithm uses relevance vector machine (RVM), which combines the strengths of kernel‐based methods and Bayesian theory to establish the relationships between a set of input vectors and a desired output. The paper also presents the comparative study between the developed LSSVM and RVM model for site characterization. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
38.
城市规划领域越来越需要三维建模技术的支持。CityEngine作为一种新型的三维建模软件,不仅可以批量生成规划模型,还可以用来辅助三维城市规划设计。本文基于CityEngine规则建模方法,实现了规划方案对比、建筑体量调整、规划指标计算等功能,通过对某小区进行实际建模验证,证明了CityEngine可以为城市规划者们提供新的思路,为城市设计和城市管理提供决策支持。 相似文献
39.
《Geoforum》2016
We develop the outlines of a new approach to study the role of nonhumans in constituting ‘implementation’ and calculative–discursive practices in development projects and programs. Developing a framework around the concept of friction (material resistance or recalcitrance encountered in processes of transformation), we analyze an Energy Self-sufficient Village program in Indonesia. Focusing on specific projects and episodes within this program, we identify multiple distinctive instances of friction. These were driven by nonhumans’ (and humans’) resistance, as remolding of development beneficiaries’ practices was attempted by project administrators, government officials, entrepreneurs and by the (scientific) calculations embedded in their policies, strategies and models. In concluding, we distill four ways in which nonhumans relationally shape development practices: (a) by resisting representations and calculations produced by human actors, (b) by re-directing planned/expected courses of action, (c) through biophysical change to their weight or textures as they move in space and time, and (d) by mediating competition for resources. Overall, nonhumans play a central role in making and unmaking asymmetric relations of power in practice and by constituting practices that diverge from prior expectations, problematize linear understandings of ‘policy implementation’. Their material and discursive agency is multiple, manifesting differently in different relational settings, which highlights the importance of broadening the range of spokespersons who speak on behalf of nonhumans and whose voices can be considered reliable and true. Our study thus provides support to calls for pluralizing and democratizing development ‘expertise’ beyond the usual suspects in science, government and civil society. 相似文献
40.
三稀矿产资源是战略性新兴产业发展的基础原料,广泛应用在国防、航天、高端电子设备、核工业、发光材料等高精尖技术领域。本文在系统收集国内外三稀矿产资源生产、消费领域、价格以及进出口资料的基础上,提出了促进三稀矿产资源新兴产业发展的政策建议 相似文献