全文获取类型
收费全文 | 1470篇 |
免费 | 302篇 |
国内免费 | 354篇 |
专业分类
测绘学 | 79篇 |
大气科学 | 372篇 |
地球物理 | 395篇 |
地质学 | 724篇 |
海洋学 | 136篇 |
天文学 | 67篇 |
综合类 | 137篇 |
自然地理 | 216篇 |
出版年
2024年 | 5篇 |
2023年 | 22篇 |
2022年 | 60篇 |
2021年 | 50篇 |
2020年 | 73篇 |
2019年 | 64篇 |
2018年 | 80篇 |
2017年 | 64篇 |
2016年 | 73篇 |
2015年 | 67篇 |
2014年 | 78篇 |
2013年 | 68篇 |
2012年 | 75篇 |
2011年 | 83篇 |
2010年 | 96篇 |
2009年 | 74篇 |
2008年 | 73篇 |
2007年 | 83篇 |
2006年 | 86篇 |
2005年 | 92篇 |
2004年 | 49篇 |
2003年 | 73篇 |
2002年 | 62篇 |
2001年 | 52篇 |
2000年 | 58篇 |
1999年 | 89篇 |
1998年 | 56篇 |
1997年 | 43篇 |
1996年 | 33篇 |
1995年 | 50篇 |
1994年 | 32篇 |
1993年 | 40篇 |
1992年 | 25篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1958年 | 8篇 |
排序方式: 共有2126条查询结果,搜索用时 187 毫秒
991.
992.
秦岭地区秋季降水的气候特征分析 总被引:5,自引:1,他引:4
利用1961-2005年观测资料和NCEP/NCAR再分析资料, 对秦岭邻近地区秋季(9-10月) 降水的气候特征及其与大气环流关系进行分析,结果表明:秦岭地区秋季降水具有明显年代际变化特征, 1960s后期、1970s和1980s初秋季多雨,而1950s和1960s初前期、1980s中后期和1990s秋少雨, 2000年以后秋季降水略有增多.秦岭地区秋季雨涝年环流特征是高原北侧高空西风急流偏强,乌拉尔山长波脊稳定,西大平洋副热带高压位置偏西,强度偏强,对流层低层东南风气流和西南风气流为秦岭地区输送了大量水汽;而少雨干旱年则高空高原北侧西风急流偏弱,秦岭地区受青藏高压控制或西北气流控制,与东亚冬季风相联系的偏北气流阻断了偏南气流的水汽输送. 相似文献
993.
994.
小麦生长中后期易受晚霜冻害,造成小麦黄叶、干枯、矮化、不抽穗、穗畸形、不孕穗或穗粒少等多种症状,对产量影响极大.为减少产量损失,2007年4月3日嵩县小麦冻害发生后,对不同类型的冻麦田进行了6次连续监测和详细调查,在综合分析冻害成因的基础上,制定相应的防治措施. 相似文献
995.
This paper reports on the compressive strength and microstructure of unfired clay masonry bricks. Blended binders comprising of lime-activated Ground Granulated Blastfurnace Slag (GGBS) and Portland Cement (PC)-activated GGBS were used to stabilise Lower Oxford Clay (LOC) for unfired masonry brick production. The compressive strength of the stabilised bricks incorporating lime–GGBS–LOC was higher than that of PC–GGBS–LOC. Scanning Electron Microscopy (SEM) with a Solid-state Backscattered Detector (SBD) and Energy Dispersive X-ray (EDX) analysis was employed to obtain a view of the microstructure and to conduct an analysis on the morphology and composition of the dried unfired clay brick samples, after 28 days of moist curing. The analytical results together with the physical observations have shown the formation of Calcium Silicate Hydrate (C-S-H) gel and additional pozzolanic (C-S-H) gel. The quantification of the compound content of the unfired bricks showed the presence of Calcite (CaCO3), Quartz (SiO2), Alumina (Al2O3) and Wollastonite (CaSiO8) crystals. Traces of other crystals were also detected. 相似文献
996.
综合利用岩心、测井、录井和生产测试等资料,对海拉尔盆地布达特群潜山的储集特征进行了深入研究.结果表明:研究区潜山的储集类型可划分为溶蚀风化壳型和裂缝破碎带型2种基本类型,前者主要受古暴露期大气水淋滤作用的控制,溶蚀作用随与不整合面距离增大而减弱;裂缝破碎带型储层主要受断层性质的控制.古地貌恢复结果表明:贝尔凹陷布达特群在兴安岭群沉积之前具有东高西低的总趋势,与现今潜山顶面构造图比较,有占高今低、古今均高、古低今高和古今均低4种情形,前两种情况更有利于形成溶蚀风化壳型储层.布达特群潜山发育多期裂缝,裂缝发育程度和产状与断层有很好的相关性,通过钻井资料预测,苏德尔特潜山带为裂缝发育区. 相似文献
997.
The relations between land use and karst rocky desertification in a typical karst area,China 总被引:13,自引:0,他引:13
Karst rocky desertification (KRD) is one major type of desertification which is under irrational human impact on the vulnerable
eco-geo-environment. In this study, Zhudong fengcong rocky desertification landscape was selected to investigate the land
use types distribution law in different rock types and the KRD land in different land use types. KRD data was determined by
user–computer interactive interpreting method from Aster images in 2004, according to the exposed ratio of rock, the coverage
of vegetation and soil in 0.2 km2 unit, integrating with land utilizing present situation map, hydrogeology map, relief map, vegetation map, soil distribution
map, as well as the practical investigation and the population census datum in 2002. Results showed that the occurrence ratio
of KRD land is different in land use types and rocky assemblages obviously; land cover has a strong impact upon KRD. The sloping
cropland distributed in homogenous limestone has a higher occurrence ratio of light KRD, secondly, is in the limestone interbedded
with clastic rock. Light KRD land was dominated by shrubland, and sloping cropland accounts for 11.67% of it, moderate KRD
land was dominated by moderate coverage grass slopes, strong and extremely strong KRD was dominated by rocky dry land which
is difficult to use. 相似文献
998.
Sulfur isotope composition of carbonate-associated sulfate (δ34SCAS) and carbon isotope composition of carbonate (δ13Ccarb) were jointly investigated on the Late Permian rocks at Shangsi Section, Guanyuan, Northeast Sichuan, South China. Both δ34SCAS and δ13Ccarb show gradual decline trends in Late Permian strata, inferring the occurrence of the long-term variation of marine environmental
conditions. Associated with the long-term variation are the two coincident negative shifts in δ34SCAS and δ13Ccarb, with one occurring at the boundary between Middle Permian Maokou Formation and Late Permian Wujiaping Formation and another
at Middle Dalong Formation. Of significance is the second shift which clearly predates the regression and the biotic crisis
at the end of Permian at Shangsi Section, providing evidence that a catastrophic event occurred prior to the biotic crisis.
The frequent volcanisms indicated by the volcanic rocks or fragments, and the upwelling are proposed to cause the second negative
excursion. An abrupt extreme negative δ34SCAS (ca. −20‰) associated with a low relative concentration of CAS and total organic carbon without large change in δ13Ccarb is found at the end of the second shift, which might arise from the short-term oxygenation of bottom waters and sediments
that resulted from the abrupt sea level drop. 相似文献
999.
Mafic intrusions and dykes are well preserved in the Yinmin and Lanniping districts, located within the western margin of the Yangtze Block, SW China. Although these mafic rocks from the two areas formed during different periods, they share similar ranges of PGE concentration. Most of the Yinmin gabbroic dykes contain relatively high PGE concentrations (PGEs = 13.9–87.0 ppb) and low S contents (0.003 %–0.020 %), higher than the maximum PGE concentrations of mafic magmas melting from the mantle. Two exceptional Yinmin samples are characterized by relatively low PGE (PGEs = 0.31–0.37 ppb) and high S (0.114 %–0.257 %) contents. In contrast, most samples from the Lanniping gabbroic intrusion have low PGE concentrations (PGEs = 0.12–1.02 ppb) and high S contents (0.130 %–0.360 %), except that the three samples exhibit relatively high PGE (PGEs = 16.3–34.8 ppb) and low S concentrations (0.014 %–0.070 %). All the Yinmin and Lanniping samples are characterized by the enrichment of PPGE relative to IPGE in the primitive-mantle normalized diagrams, and the high-PGE samples exhibit obvious Ru anomalies. This study suggests that during the ascent of the parental magma, removal of Os–Ir–Ru alloys and/or chromite/spinel leads to high Pd/Ir ratios and Ru anomalies for the Yinmin high-PGE samples and relatively lower Pd/Ir ratios and Ru anomalies for the Lanniping low-PGE samples. We propose that the magmas parental to the Yinmin gabbroic dykes are initially S-unsaturated, and subsequently, minor evolved magma reached sulfur saturation and led to sulfide segregation. Although the Lanniping parental magmas are originally not saturated in S, the high Cu/Pd ratios (3.8 × 104 to 3.2 × 106) for most of the Lanniping samples indicate the S-saturated state and sulfide segregation. A calculation shows that the PGE-poor magmas might have experienced 0.01 %–0.1 % sulfide segregation in the magma chamber. Therefore, our study provides a possible opportunity to discover PGE-enriched sulfide mineralization somewhere near or within the Lanniping mafic intrusion. 相似文献
1000.
Employing decoupling index and industrial structure characteristic bias index methods, this study analyzed the spatial-temporal characteristics of industrial structure transformations and their resulting carbon emissions in the Xuzhou Metropolitan Area from 2000 to 2014, with a focus on their relationships and driving factors. Our research indicates that carbon emission intensity from industrial structures in the Xuzhou Metropolitan Area at first showed an increasing trend, which then decreased. Furthermore, the relationship between emissions and industrial economic growth has been trending toward absolute decoupling. From the perspective of the center-periphery, the Xuzhou Metropolitan Area formed a concentric pattern, where both progress towards low emissions and the level of technological advancement gradually diminished from the center to the periphery. In terms of variation across provinces, the ISCB index in the eastern Henan has decreased the slowest, followed by the southern Shandong and the northern Anhui, with the northern Jiangsu ranking last. During this period, resource- and labor- intensive industries were the primary growth industries in the northern Anhui and the eastern Henan, while labor-intensive industries dominated the southern Shandong and capital-intensive industries dominated the northern Jiangsu. In terms of city types, the spatial pattern for industrial structure indicates that recession resource-based cities had higher carbon emission intensities than mature resource-based cities, followed by non-resource-based cities and regenerative resource-based cities. Generally, the industrial structure in the Xuzhou Metropolitan Area has transformed from being resource-intensive to capital-intensive, and has been trending toward technology-intensive as resource availability has been exploited to exhaustion and then been regenerated. Industrial structure has been the leading factor causing heterogeneity of carbon emission intensities between metropolitan cities. Therefore, the key to optimizing the industrial structure and layout of metropolitan areas is to promote industrial structure transformation and improve the system controlling collaborative industrial development between cities. 相似文献