全文获取类型
收费全文 | 4296篇 |
免费 | 546篇 |
国内免费 | 779篇 |
专业分类
测绘学 | 221篇 |
大气科学 | 951篇 |
地球物理 | 1051篇 |
地质学 | 1721篇 |
海洋学 | 426篇 |
天文学 | 584篇 |
综合类 | 282篇 |
自然地理 | 385篇 |
出版年
2024年 | 10篇 |
2023年 | 47篇 |
2022年 | 122篇 |
2021年 | 121篇 |
2020年 | 114篇 |
2019年 | 120篇 |
2018年 | 165篇 |
2017年 | 179篇 |
2016年 | 206篇 |
2015年 | 166篇 |
2014年 | 201篇 |
2013年 | 246篇 |
2012年 | 213篇 |
2011年 | 287篇 |
2010年 | 211篇 |
2009年 | 291篇 |
2008年 | 227篇 |
2007年 | 222篇 |
2006年 | 219篇 |
2005年 | 175篇 |
2004年 | 154篇 |
2003年 | 170篇 |
2002年 | 146篇 |
2001年 | 145篇 |
2000年 | 154篇 |
1999年 | 145篇 |
1998年 | 170篇 |
1997年 | 152篇 |
1996年 | 144篇 |
1995年 | 101篇 |
1994年 | 114篇 |
1993年 | 103篇 |
1992年 | 68篇 |
1991年 | 54篇 |
1990年 | 36篇 |
1989年 | 32篇 |
1988年 | 39篇 |
1987年 | 26篇 |
1986年 | 20篇 |
1985年 | 14篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 13篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1973年 | 2篇 |
1972年 | 3篇 |
排序方式: 共有5621条查询结果,搜索用时 15 毫秒
971.
金家庄超基性岩型金矿围岩蚀变地球化学研究 总被引:3,自引:0,他引:3
金家庄金矿近围岩蚀变为蛇纹石化、滑石化、碳酸盐化和硅化。岩石化学与微量元素研究表明,从未蚀变-强蚀变透辉岩至矿化蚀变岩,SiO2、CaO、Al2O3含量逐渐降低,CO2、H2O、S和Au、Ag、Cu含量逐渐增高。蚀变岩石同位素研究揭示,碳同位素较低(δ^18O=12.10‰~22.3‰)远高于超基性岩正常范围。围岩蚀变热液主要源于大气降水、碳、硫和成矿元素主要是在围岩蚀变过程中从超基性岩中萃取。 相似文献
972.
C. Covey A. Abe-Ouchi G. J. Boer B. A. Boville U. Cubasch L. Fairhead G. M. Flato H. Gordon E. Guilyardi X. Jiang T. C. Johns H. Le Treut G. Madec G. A. Meehl R. Miller A. Noda S. B. Power E. Roeckner G. Russell E. K. Schneider R. J. Stouffer L. Terray J.-S. von Storch 《Climate Dynamics》2000,16(10-11):775-787
We examine the seasonal cycle of near-surface air temperature simulated by 17 coupled ocean-atmosphere general circulation models participating in the Coupled Model Intercomparison Project (CMIP). Nine of the models use ad hoc “flux adjustment” at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux-adjusted models separately and examine the behavior of each class. When averaged over all of the flux-adjusted model simulations, near-surface air temperature falls within 2?K of observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ~6?K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ~5?K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January to measure seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ~30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ~5?K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle is a necessary condition for confidence in their prediction of long-term climatic changes (such as global warming), but it is not a sufficient condition unless the seasonal cycle and long-term changes involve similar climatic processes. To test this possible connection, we compare seasonal cycle amplitude with equilibrium warming under doubled atmospheric carbon dioxide for the models in our data base. A small but positive correlation exists between these two quantities. This result is predicted by a simple conceptual model of the climate system, and it is consistent with other modeling experience, which indicates that the seasonal cycle depends only weakly on climate sensitivity. 相似文献
973.
Analysis of Impact Hammer Rebound to Estimate Rock Drillability 总被引:6,自引:0,他引:6
Summary ?In this paper, the piston rebound common to both the Schmidt Impact Hammer and down-hole hammer drills has been analyzed
and calculated by means of stress wave theory and the energy conservation law. A quantitative relationship between the amount
of rebound of the piston and the impact resistance index, or hardness of the rock, has been established. Those analytical
results will not only be of benefit in acquiring a deep understanding of the usable range and condition of the Schmidt Impact
Hammer, but also provide a definite answer to the feasibility of using the hammer rebound in drills with down-hole hammer
tool to carry out a real time measurement of the nature of the formation under the bit. 相似文献
974.
Evaluation of heavy metal contamination and implication of multiple sources from Hunchun basin, northeastern China 总被引:1,自引:0,他引:1
J.-W. Moon H.-S. Moon N.-C. Woo J.-S. Hahn J.-S. Won Y. Song X. Lin Y. Zhao 《Environmental Geology》2000,39(9):1039-1052
Present concentrations and distributions of heavy metals through profiles, surface soil, and stream sediment samples in the
Hunchun area, north-eastern China, were investigated to determine the elemental background values. This study also aims to
characterize potentially toxic materials such as pulverized fly ash (PFA) from power stations or ash and slag from coal used
domestically in urban areas, agrochemicals applied inappropriately, and urban sewage sludges from Hunchun City, as well as
to ascertain the possibility of natural enrichment through site characterization by mineralogical and geochemical investigation.
The distribution of contaminants in the alluvial soils (fluvisol) of this area has been influenced by several interacting
factors. The parent alluvial materials from weathered products of amphiboles have made coatings such as ferrihydrite, goethite,
and hematite. This natural inheritance factor is supported by the fact that the concentrations of weak acid-extractable (plant-available)
heavy metals are very low, except for Fe and Mn. However, in agricultural soils and adjacent stream sediments, an anthropogenic
input of Cd, Pb, Ni and Cr by agrochemicals is strongly suggested. Also, F contamination by coal combustion and the dissolution
of F-bearing minerals could cause some future problems. Wide distribution and significantly high concentrations of Cd, Fe,
Mn, and F in soils throughout the combination of pollutants originating from lithogenic and the anthropogenic sources pose
potential problems in utilizing water resources.
Received: 14 June 1999 · Accepted: 27 December 1999 相似文献
975.
An Eocene magmatic belt across central Tibet: mantle subduction triggered by the Indian collision? 总被引:2,自引:0,他引:2
F. Roger P. Tapponnier N. Arnaud U. Schärer M. Brunel X. Zhiqin & Y. Jingsui 《地学学报》2000,12(3):102-108
Mountain domes rising to ≈ 6600 m along Tibet's Tanggula range herald the Eocene intrusion of calcalkaline granites into terranes accreted much earlier. Together with coeval, cogenetic volcanics, such intrusives, which have similar crystallization and cooling ages, may be part of an ESE-trending belt cutting central Tibet in half. This magmatic belt may have marked a former northern boundary of the plateau, testifying to S-directed subduction of the Asian mantle. Such subduction would have developed soon after India's collision but long before the rise of the plateau's present rims, along one of central Tibet's Mesozoic sutures. 相似文献
976.
土层地震反应传递函数的模拟 总被引:3,自引:0,他引:3
探讨了用单一土层模型替换复杂场地求解土层场地传递函数的方法,并用SMART-1台阵地震记录对这一方法的可行性进行了研究和讨论,结果表明,只要选取适当的地震波入射角Ψ和土介质的滞回阻尼比d,简化模型的传递函数与实际传递函数能符合得较好,这一方法可用来近似估计缺少详细土质勘测资料地区的土层地震反应的传递函数。 相似文献
977.
皖北奥陶系碳酸盐岩稀土元素地球化学特征及其古岩溶意义 总被引:1,自引:0,他引:1
依据安徽北部刘桥二矿奥陶系碳酸盐岩隐伏剖面 (下奥陶统顶部 ) 17个碳酸盐岩样品的稀土元素 (15个项目 )的测试资料 ,针对不同岩性 ,分类阐述了它们的REE总量和配分模式、LREE和HREE、δEu和δCe的地球化学特征及其剖面变化特点。在此基础上 ,详细讨论了碳酸盐岩稀土地球化学特征成因机理与古岩溶发育环境的关系 ,为在岩溶岩地球化学特征与其形成环境及溶蚀程度间建立定量识别标准提供了依据。 相似文献
978.
在海原断裂带内部发现一个第三纪拉分盆地 ,命名为老龙湾拉分盆地。根据卫片解译结果和野外地质调查 ,对该盆地内的地层沉积序列、地层分布、相关断裂等特征进行了研究。结果表明 ,老龙湾拉分盆地发育于海原断裂内的最大斜列部位 ,盆地沉积受边界断裂控制 ;盆地内部沉积了巨砾岩、杂砾岩及紫红色 -灰绿色泥岩、桔红色角砾岩等地层 ,最大沉积厚度约 4 6 0 0m。老龙湾盆地内部地层不整合于不同的外围地层之上 ,根据对盆地内部沉积序列及外围第三系的区域对比 ,认为盆地沉积开始于中新世中期。由以上特征确定老龙湾盆地为海原断裂带内的第三纪拉分盆地。它为青藏高原东北缘第三纪的走滑断层活动的研究提供了地质证据 相似文献
979.
探讨了CFZ - 1 50 0型冲击反循环钻机研制中的几个关键技术问题 :采用曲柄摇杆自动冲击机构 ;采用潜水砂石泵实现反循环 ;采用同步卷筒解决 2根钢丝绳的受力平衡 ;采用棘轮式可旋转冲击反循环钻头。 相似文献
980.