全文获取类型
收费全文 | 4907篇 |
免费 | 889篇 |
国内免费 | 1380篇 |
专业分类
测绘学 | 656篇 |
大气科学 | 459篇 |
地球物理 | 918篇 |
地质学 | 3340篇 |
海洋学 | 736篇 |
天文学 | 25篇 |
综合类 | 477篇 |
自然地理 | 565篇 |
出版年
2024年 | 17篇 |
2023年 | 54篇 |
2022年 | 179篇 |
2021年 | 198篇 |
2020年 | 227篇 |
2019年 | 247篇 |
2018年 | 212篇 |
2017年 | 211篇 |
2016年 | 265篇 |
2015年 | 289篇 |
2014年 | 346篇 |
2013年 | 377篇 |
2012年 | 350篇 |
2011年 | 376篇 |
2010年 | 317篇 |
2009年 | 358篇 |
2008年 | 349篇 |
2007年 | 340篇 |
2006年 | 336篇 |
2005年 | 260篇 |
2004年 | 284篇 |
2003年 | 195篇 |
2002年 | 165篇 |
2001年 | 178篇 |
2000年 | 135篇 |
1999年 | 134篇 |
1998年 | 127篇 |
1997年 | 96篇 |
1996年 | 77篇 |
1995年 | 61篇 |
1994年 | 67篇 |
1993年 | 75篇 |
1992年 | 46篇 |
1991年 | 30篇 |
1990年 | 31篇 |
1989年 | 38篇 |
1988年 | 32篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 22篇 |
1984年 | 19篇 |
1983年 | 7篇 |
1982年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有7176条查询结果,搜索用时 15 毫秒
931.
强矿震地球物理过程及短临阶段预测的研究 总被引:5,自引:0,他引:5
基于中尺度地震实验场高密度数字地震和前兆台网对抚顺老虎台矿两次强矿震进行连续的现场观测,用地震学方法和小波工具分析强矿震孕震过程采集到的数据,提取短临阶段的异常信息;用区域强震震源机制解、强矿震的震源机制解、极近场震源调查、震源高精度定位、现场绝对地应力测量、三维有限差分数值试验方法,分析强矿震的孕震应力场环境和震源机制;通过定量观察采矿与矿震活动的相关性,分析强矿震的直接诱发原因.提出地质构造环境、地应力场和采矿活动共同作用诱发强矿震的机理和局部应力场在孕育该震过程起主导作用、卸荷重力应力场抑或耦合了高压瓦斯的膨胀起主要诱发作用的观点;发现煤炭深部开采条件下,矿震和瓦斯存在密切的相关性,有可能存在一种开采卸荷和高压瓦斯气体膨胀耦合作用诱发的新型矿震——卸胀耦合型矿震;提取到震前短临阶段存在的b值、η值、频次、波速比等可信的地震学异常和定点潮汐形变前兆异常,对异常信息的提取方法和强矿震短临阶段的预测进行了探讨. 相似文献
932.
933.
论我国矿业权市场主体缺位 总被引:2,自引:0,他引:2
国有地勘企业在矿业权市场中应占有绝对主体地位,应该是不争的事实.但在实际工作中国有地勘企业并没有真正进入矿业权市场中占有主体位置.造成矿业权市场主体缺位的原因是多方面的,主要是国有地勘企业体制问题,国家政策问题以及矿业权市场和国有地勘企业本身的问题.文章通过对矿业权市场主体缺位表现、原因和危害的分析,提出适合国有地勘企业发展的措施与建议. 相似文献
934.
An analysis of the radar backscattering from the ocean surface covered by oil spill is presented using a microwave scattering model and Monte-Carlo simulation. In the analysis, a one-dimensional rough sea surface is numerically generated with an ocean waveheight spectrum for a given wind velocity. A two-layered medium is then generated by adding a thin oil layer on the simulated rough sea surface. The electric fields backscattered from the sea surface with two-layered medium are computed with the method of moments (MoM), and the backscattering coefficients are statistically obtained with N independent samples for each oil-spilled surface using the Monte-Carlo technique for various conditions of surface roughness, oil-layer thickness, frequency, polarization and incidence angle. The numerical simulation results are compared with theoretical models for clean sea surfaces and SAR images of an oil-spilled sea surface caused by the Hebei (Hebei province, China) Spirit oil tanker in 2007. Further, conditions for better oil spill extraction are sought by the numerical simulation on the effects of wind speed and oil-layer thickness at different incidence angles on the backscattering coefficients. 相似文献
935.
Diapycnal Mixing (DM) within the upper 2000m of the global ocean is calculated by a fine-scale parameterization using the multiyear-mean density gridded product that created by employing all the Argo float observations to date through the recently published equation of seawater TEOS-10. The geographic distribution of Argo-derived DM derived in this study is spatial-dependent and varies with latitude and depth. The magnitude and pattern of DM is favorably validated by comparisons with previous studies. Furthermore, the mixing coefficient tensor K is calculated and analyzed. Components of the tensor fitting for the geopotential coordinate models are also presented. It is found that the tensor components in horizontal direction, Kxx and Kyy, have similar magnitude and distribution pattern. In the vertical, Kzz is enhanced over regions with rough topography and strong wind (e.g., Westerly region), suggesting agreement with previous estimates. This work presents a scheme to estimate the DM and mixing coefficient tensor using Argo observations, and offers a useful Argo-based mixing product for the purpose of promoting the study and modeling of ocean circulation and other processes. 相似文献
936.
Abstract Investigations aimed at obtaining knowledge about the environmental effects of mining aggregate from the sea bottom were carried out on the Stupsk Bank. Special attention was given to the magnitude and reach of changes of hydrologic conditions, in that to changes in concentration of matter suspended in sea water, and to dynamics of the sea bottom—change of bottom relief, resulting from both natural causes and from mining of the aggregate. 相似文献
937.
Omran Frihy Essam Deabes Wahid Moufaddal Adam El-Shahat 《Marine Georesources & Geotechnology》2013,31(5):408-418
Mineralogical and textural analyses of 45 undisturbed short cores and 80 grab sediment samples, collected from five frequently dredged navigational areas within harbors and water pathways of the Nile delta littoral system, were utilized for evaluation of these sediments as potential source of economic heavy minerals (EHMs). Results of mineralogical characterization indicate that the average total heavy mineral (HM) concentrations are as follows: Abu Qir Bay (1.7%), Rosetta estuary (3.1%), Burullus fishing port (4.5%), Damietta Harbor (2.9%), and El Gamil lagoon inlet (1.9%). Assessment of HM grades indicates predominance of magnetite, ilmenite, hematite, leucoxene, garnet, zircon, and rutile. Results of the feasibility analysis indicate that dredged sediments at these study areas are considered as a potential source of EHMs and economically promising to be mined for HMs. The present study suggests a practical operative plan of two successive phases for HMs recycling: (1) in-situ initial separation of HMs on the dredger deck using wet-gravity spirals, then (2) transportation of the recovered HM concentrates to an onshore processing plant to selectively separate individual HMs via wet and dry magnetic and electrostatic separators. Alternatively, dredged sediments can be directly pumped to a nearby onshore area as stockpile to be recycled afterward in inland processing plant. 相似文献
938.
Dr. Thomas Mathai K. N. Rajarama Satish Kumar M. Suresh Chandran M. R. Asoka Kumar 《Marine Georesources & Geotechnology》2013,31(2):180-193
Offshore geotechnical surveys form part of an integrated investigation to rejuvenate a decrepit minor port at Badagara, Kerala on the southwestern coast of India. The sediments typify a fluvio-marine milieu ranging from silty clay, sand, silty sand, sandy silt and clayey silt. Geotechnical and sedimentological studies of shallow cores reveal the geotechnical aspects besides the depositional history of the sediments. Downcore geotechnical variations and regressive coefficients based on their inter-relationships highlight diverse factorial inferences. X-Ray Diffraction data indicate the prominent clay type. A comparative evaluation of the geotechnical characteristics of clayey sediments off Badagara, with similar studies along various sectors of the Kerala coast, both on land as well as in the near shore, is broadly attempted. Geotechnical studies carried out earlier on the uplifted Cochin marine clays provide comparative data for evaluating the possible variations between present day marine clayey sediments occurring along the Kerala coast and uplifted marine clays which, besides their gross variations in levels with respect to the present sea-level, also obviously relate to a much older depositional environment and provenance during probable Holocene times. 相似文献
939.
ABSTRACTA heave compensation system combined with a vibration absorber and accumulator is proposed, with regard to the requirement for poly-metallic nodule mining systems at an ocean depth of 6,000?m in China. The principle of the compensation system was analyzed, its main parameters were designed, and its mathematical model was established. The system performance was simulated and analyzed in MATLAB, and the frequency domain characteristics and compensation rate of the system with different volume accumulators, and with or without vibration absorbers, were compared. A simulation test of the compensation system was conducted. The proposed compensation system had a better compensation effect for a random wave interference under level-6 sea conditions, and the compensation rate being more than 72% with optimal system parameters. 相似文献
940.
针对一个并联式涡轮基组合循环(Turbine Based Combined Cycle, TBCC)发动机排气系统的气动方案,对其在整个飞行包线范围内典型工作点上的流场进行了数值模拟研究,获得了飞行包线范围内排气系统相应的推力系数、升力、俯仰力矩随飞行马赫数的变化关系。计算结果显示,在整个飞行包线范围内,排气系统的轴向推力系数随着飞行马赫数先减小后增大,在跨声速飞行时降到最低 Ma=0.9,涡喷不加力时为0.562,加力时0.662),在设计点附近达到最大;升力和俯仰力矩性能在亚声速及跨声速飞行时较差,在超声速飞行时随着飞行马赫数增加逐渐好转。表明排气系统在跨声速飞行范围内工作时应采取措施以改善其性能。 相似文献