全文获取类型
收费全文 | 38700篇 |
免费 | 18849篇 |
国内免费 | 43823篇 |
专业分类
测绘学 | 3570篇 |
大气科学 | 21792篇 |
地球物理 | 13276篇 |
地质学 | 37396篇 |
海洋学 | 16676篇 |
天文学 | 975篇 |
综合类 | 4815篇 |
自然地理 | 2872篇 |
出版年
2024年 | 211篇 |
2023年 | 526篇 |
2022年 | 956篇 |
2021年 | 1276篇 |
2020年 | 2346篇 |
2019年 | 5707篇 |
2018年 | 5930篇 |
2017年 | 5642篇 |
2016年 | 5697篇 |
2015年 | 5050篇 |
2014年 | 4601篇 |
2013年 | 5127篇 |
2012年 | 4843篇 |
2011年 | 4602篇 |
2010年 | 4529篇 |
2009年 | 3817篇 |
2008年 | 3096篇 |
2007年 | 3157篇 |
2006年 | 2700篇 |
2005年 | 2581篇 |
2004年 | 2853篇 |
2003年 | 2619篇 |
2002年 | 2499篇 |
2001年 | 2196篇 |
2000年 | 1791篇 |
1999年 | 1809篇 |
1998年 | 1849篇 |
1997年 | 1841篇 |
1996年 | 1455篇 |
1995年 | 1414篇 |
1994年 | 1278篇 |
1993年 | 1200篇 |
1992年 | 1030篇 |
1991年 | 766篇 |
1990年 | 726篇 |
1989年 | 602篇 |
1988年 | 514篇 |
1987年 | 400篇 |
1986年 | 326篇 |
1985年 | 259篇 |
1984年 | 281篇 |
1983年 | 164篇 |
1982年 | 208篇 |
1981年 | 147篇 |
1980年 | 108篇 |
1979年 | 106篇 |
1978年 | 50篇 |
1977年 | 45篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Deformation Mechanism and Stability of a Rocky Slope 总被引:1,自引:1,他引:1
A high slope is located on the side of the spillway at a hydropower station in Southwest China, which has some weak inter-layers inclining outwards. Parts of the slope show heavy weathering and unloading. There appeared deformation and tensile crack either on the surface or on the afteredge of the slope during excavation, and under a platform (elev. 488 m), two levels of slopes collapsed on the downriver side. Based on the investigation in situ and the analysis of the geological structure, the conceptual model of deformation and failure mechanism was erected for this slope. Furthermore, the deformation characteristics were studied with FLAC^3D numerical simulation. Comprehensive analysis shows that the whole deformation of the slope is unloading rebound in certain depth scope and the whole body does not slide along any weak interlayer. In addition, two parts with prominent local deformation in the shallow layer of the slope show the models of "creep sfiding-tensile cracking" and "slidlng-tensile cracking", respectively. Based on the above analysis, the corresponding project of support and reinforcement is proposed to make the slope more stable. 相似文献
992.
Detailed geological and petrological-geochemical study of rocks of the lava complex of Young Shiveluch volcano made it possible
to evaluate the lava volumes, the relative sequence in which the volcanic edifice was formed, and the minimum age of the onset
of eruptive activity. The lavas of Young Shiveluch are predominantly magnesian andesites and basaltic andesites of a mildly
potassic calc-alkaline series (SiO2 = 55.0–63.5 wt %, Mg# = 55.5–68.9). Geologic relations and data on the mineralogy and geochemistry of rocks composing the
lava complex led us to conclude that the magnesian andesites of Young Shiveluch volcano are of hybrid genesis and are a mixture
of silicic derivatives and a highly magnesian magma that was periodically replenished in the shallow-depth magmatic chamber.
The fractional crystallization of plagioclase and hornblende at the incomplete segregation of plagioclase crystals from the
fractionating magmas resulted in adakitic geochemical parameters (Sr/Y = 50–71, Y < 18 ppm) of the most evolved rock varieties.
Our results explain the genesis of the rock series of Young Shiveluch volcano without invoking a model of the melting of the
subducting Pacific slab at its edge. 相似文献
993.
A new method to estimate strain and competence contrast from natural fold shapes is developed and verified by analogue and numerical experiments. Strain is estimated relative to the nucleation amplitude, AN, which is the fold amplitude when the amplification velocities caused by kinematic layer thickening and dynamic folding are identical. AN is defined as the initial amplitude corresponding to zero strain because folding at amplitudes smaller than AN is dominantly by kinematic layer thickening. For amplitudes larger than AN, estimates of strain and competence contrast are contoured in thickness-to-wavelength (H/λ) and amplitude-to-wavelength (A/λ) space. These quantities can be measured for any observed fold shape. Contour maps are constructed using existing linear theories of folding, a new nonlinear theory of folding and numerical simulations, all for single-layer folding. The method represents a significant improvement to the arc length method. The strain estimation method is applied to folds in viscous (Newtonian), power-law (non-Newtonian) and viscoelastic layers. Also, strain partitioning in fold trains is investigated. Strain partitioning refers to the difference in strain accommodated by individual folds in the fold train and by the whole fold train. Fold trains within layers exhibiting viscous and viscoelastic rheology show different characteristic strain partitioning patterns. Strain partitioning patterns of natural fold trains can be used to assess the rheological behaviour during fold initiation. 相似文献
994.
A radiotracer technique, employing 27Mg, is used to determine the Mg released by ammonium exchange on undis-solved humic acid in a seawater medium. This new method allows for the measurement of exchangeable Mg on the solid phase surface, which eliminates the problem caused by the high-Mg background in the seawater matrix. The precision calculated from the counting statistics is better than ±2%; the reproducibility among repeated counts ranged from ±1% to ±3%. The higher sensitivity of the method allows for monitoring the Mg---NH4 exchange at concentrations as low as 30 mM NH4. This is a major improvement relative to the data obtained with the analytical methods used so far, which allow detection of exchangeable Mg only at NH4 concentrations higher than 1 M. The lower experimental concentrations are more in accordance with the natural ammonium levels found in anoxic marine sediments. For the undissolved humic acid used in this experiment, the amount of exchangeable Mg in apparent equilibrium with an ammonium-free seawater matrix was found to be 96.6 ± 0.4 meq/100g. The Mg---NH4 exchange on humic acid in seawater comes to a steady-state value in < 18 min. The conditional equilibrium constant obtained for this reaction, Kcond = 0.039 ± 0.001 M−1. The technique can be expanded to other geochemical solid phases in seawater and it can be modified to study the behavior of the major cations by using 24Na, 42K and 49Ca. 相似文献
995.
CHI Zhao-li WEI Xu-guang LIU Li-qin WU Xiu-li) Nanjing Institute of Geology Palaeontology Chinese Academy of Sciences Nanjing ) Algeria Branch of International Oil Development Company SINOPEK Dongying 《地层学杂志》2007,(Z2)
对阿尔及利亚Zarzaitine油田储层岩芯进行观察与描述,在沉积特征综合分析的基础上,结合岩芯的分析化验资料、测井和区域地质背景资料,根据沉积微相和电相之间的对应关系研究,划分出了各小层沉积微相。从沉积微相的分布特点,分析了该区储层(F4)沉积微相的空间演化规律,进而建立了该油田储层(F4)的三维地质模型。 相似文献
996.
哀牢山-金沙江岩带透辉石花岗岩锆石SHRIMP U-Pb年龄及地质意义--以玉召块、马头湾和十里村岩体为例 总被引:9,自引:2,他引:9
位于哀牢山—金沙江新生代钾质碱性岩浆岩带南段的玉召块、马头湾和十里村透辉石花岗岩锆石SHRIMP定年结果为 38Ma、34Ma和 37Ma,相当于渐新世,属于喜马拉雅期,结合岩石化学、微量元素、稀土元素和Sr、Nd、Pb同位素组成等,对控岩机制、岩石的属性和成因以及锆石的成因进行探讨。 相似文献
997.
W. J. Zhang M. Zhang F. Xiao L. P. Fang D. S. Wang 《International Journal of Environmental Science and Technology》2014,11(3):731-738
Pretreatment of waste emulsions with high organic content by a combined process of vibratory shear enhanced process and Fenton’s oxidation prior to biological treatment was investigated. Vibrating membrane had shown good performance in chemical oxygen demand and oil removals and the mitigation of concentration polarization. However, the permeate after filtration processing still contained high content of organics. Thus, additional Fenton oxidation was applied to reduce the organic loading, and improve the biodegradability of the wastewater. The optimal molar ratio of ferrous iron to hydrogen peroxide was 0.05 obtained from the jar-test experiments. Removal of organics was enhanced by increasing hydrogen peroxide dosage, while efficiency of hydrogen peroxide reached maximum of 1.11(w/w) at the hydrogen peroxide dosage of 6.8 g/L. Furthermore, the biological experiments indicated that the high concentration of organics could inhibit microbial activity, which decreased the chemical oxygen demand degradation rates. The adaptive period of the microbe was greatly shortened using Fenton’s reagent at the low dosages. The improvement of the biodegradability could be explained by partial mineralization and chemical transformation of parent organic compounds after Fenton oxidation. 相似文献
998.
V. I. Ivashchenko M. Valkama K. Sundblad A. I. Golubev V. Yu. Alekseev 《Doklady Earth Sciences》2011,440(1):1307-1311
A wide application of modern precision research techniques to the studies of Pitkyaranta ores allowed us to find increased
contents of indium (to 0.33%), silver (447 g/t), gold (0.2–0.4 g/t), and palladium (0.2 g/t). A series of rare minerals previously
not found here was also discovered. Among ore minerals, these are roquesite, zavartskite, electrum, stutzite, altaite, bismite,
glaucodot, cervelleite, hedleyite, pavonite, cannonite, plantnerite, lindkvistite, ashoverite, etc. The discovery of roquesite
and electrum is the most important in terms of metallogeny. Roquesite (indium sulfide) is found in Karelia for the first time.
The highest indium contents in direct correlation to those of zinc are characteristic for polymetallic ores of the Pitkyaranta
ore fields with sphalerite as the concentrating mineral (to 0.5% of In). The predicted zinc resources are evaluated to ∼2.5
million t for the Pitkyaranta group of ore deposits, and to 400 000 t for the Hopunvaara region. Respectively, the resources
of indium amount to ∼2400 t (total) and 600 t for the Hopunvaara region. 相似文献
999.
研究了新显色剂 5 -(4-溴苯偶氮 ) -8-羟基喹啉 (BAHQ)与钴 (Ⅱ )形成配合物的条件。试验表明 ,该显色剂的离解常数pKa1 ,pKa2 分别为 1.80 ,7.90。BAHQ与钴 (Ⅱ )形成配合物的最大吸收峰在 483nm处。钴 (Ⅱ )浓度在 0~ 16 μg/ 2 5mL范围内符合比耳定律 ,表观摩尔吸光系数ε483 =5 .10× 10 4L·mol- 1 ·cm- 1 ,对比度△λ =88nm。考查了二十多种离子的干扰情况 ,用于测定维生素B1 2 药品中的钴 ,结果满意 相似文献
1000.
Rare—Earth Element Geochemistry of Elogites from the Ultra—High Pressure Metamorphic Belt in Central China 总被引:1,自引:1,他引:1
Based on their REE contents and REE patterns,eclogites from the ultra-high pressure metamorphic belt in central China may be roughly divided into xis types including LREE-rich.LREE-rich positive Eu anomaly,LREE-rich negative Eu anomaly,REE pattern-smooth,MREE-rich and HREE-rich.The LREE_rich,LREE-rich positive Eu anomaly and LREE-rich negative Eu anomaly types of eclogites are dominant .REE types of eclogites in different areas can be compared and the REE feactures of the same REE type of eclogites in different areas are similar.The results of reconstruction of the primary rocks show that the primary rocks of eclogites possibly are dominated by continental tholeiites which are the product of partial melting of relatively fertile mantle and the rocks of tholeiite crystallization-differentiation.There is perfect evolution relationship among the primary rocks of the LREE-rich, LREE-rich positive Eu anomaly and LREE-rich negative Eu anomaly types of eclogites and among those of the REE pattern smooth and MREE-rich types of eclogites,the former three types were deried from continental settings and the latter two from nearly oceanic settings.Meanwhile,it is concluded that the mantle sources of primary rocks of the eclogites are inhomogeneous and the primary rocks of eclogites in this area appear to have undergone varying degree of crustal contamination. 相似文献