全文获取类型
收费全文 | 6132篇 |
免费 | 1666篇 |
国内免费 | 3193篇 |
专业分类
测绘学 | 62篇 |
大气科学 | 350篇 |
地球物理 | 1426篇 |
地质学 | 7703篇 |
海洋学 | 645篇 |
天文学 | 141篇 |
综合类 | 332篇 |
自然地理 | 332篇 |
出版年
2024年 | 21篇 |
2023年 | 122篇 |
2022年 | 242篇 |
2021年 | 209篇 |
2020年 | 295篇 |
2019年 | 337篇 |
2018年 | 340篇 |
2017年 | 322篇 |
2016年 | 364篇 |
2015年 | 412篇 |
2014年 | 459篇 |
2013年 | 468篇 |
2012年 | 508篇 |
2011年 | 503篇 |
2010年 | 423篇 |
2009年 | 530篇 |
2008年 | 422篇 |
2007年 | 513篇 |
2006年 | 524篇 |
2005年 | 436篇 |
2004年 | 428篇 |
2003年 | 411篇 |
2002年 | 325篇 |
2001年 | 309篇 |
2000年 | 312篇 |
1999年 | 291篇 |
1998年 | 267篇 |
1997年 | 210篇 |
1996年 | 188篇 |
1995年 | 176篇 |
1994年 | 158篇 |
1993年 | 136篇 |
1992年 | 90篇 |
1991年 | 49篇 |
1990年 | 56篇 |
1989年 | 30篇 |
1988年 | 30篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1979年 | 5篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
红砂是黄土高原荒漠植被带主要建群植物种之一,在植被恢复重建和生态建设中具有重要意义。本文以皋兰县40 a天然植被封育区为例,运用树木年轮学方法,对不同坡向红砂与当地水热条件变化的时空响应及分布格局进行了研究。研究结果表明,红砂具有明显的年轮特征,可以进行树木年轮方面的研究。虽然不同坡向水分条件和植被状况有很大差异,但红砂径向生长对区域水热变化具有非常一致的响应模式:同生长季降水呈正相关关系,尤以7月降水量最为显著;同生长季气温呈负相关关系,以6月最为显著。根据不同坡向红砂年龄分布格局分析结果,并考虑到水土流失的防治效果,黄土高原西部荒漠植被带的封育期限不宜少于10 a。 相似文献
123.
The present day morphology of the Zagros fold-thrust belt is dominated by magnificent exposures of NW–SE trending folds. These folds differ in their size and geometry and these differences are related mainly to the rheological profile of the cover rock. The cover rock succession of the Zagros consists of a sequence of competent and incompetent units which vary both along and across the belt. Field based study combined with the use of satellite images reveals that the thickness and facies distribution of the cover rock succession has a significant impact on the style of deformation. During the shortening linked to the current convergence of the Arabian and Iranian plates, the incompetent units act as detachment horizons which localise thrusting and which act as décollement above which detachment folds form. In addition, where these incompetent units are thick (e.g.> 1 km), they allow the deformation above and below them to become completely decoupled enabling disharmonic folding to occur. As a result the folds above and below the incompetent units in the central part of the Zagros Folded Belt, have significantly different geometries and wavelengths. As the Zagros folds host the majority of the hydrocarbon reserves in Iran and Iraq, an understanding of the processes that influence their geometry and spatial organization at different levels in the cover rock is crucial for the future exploration in the region. 相似文献
124.
Renato Moraes Reinhardt A. Fuck Mrcio Martins Pimentel Simone M.C.L. Gioia Maria H.B.M. de Hollanda Richard Armstrong 《Journal of South American Earth Sciences》2006,20(4):287-301
The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt. 相似文献
125.
E.P. Oliveira S.F. Toteu M.N.C. Araújo M.J. Carvalho R.S. Nascimento J.F. Bueno N. McNaughton G. Basilici 《Journal of African Earth Sciences》2006,44(4-5):470
The Yaoundé belt (Cameroon) and the Sergipano belt (NE Brazil) belonged to a major and continuous Neoproterozoic orogen at the northern margin of the ancient Congo-São Francisco craton. The Yaoundé belt comprises schists, quartzites, gneisses and migmatitic gneisses grouped into three domains; the low-grade Mbalmayo Group in south and the medium- to high-grade Yaoundé and Bafia Group in north. The Sergipano belt is divided into six domains, the three southernmost of which are mostly made up of clastic and chemical metasedimentary rocks whereas the others are more diverse with a migmatite–gneiss complex, and two metavolcanicplutonic complexes. In general, the two belts show structural vergence and decrease of metamorphic grade towards the craton; three main deformation phases are recognized in the Sergipano belt in contrast with two described in the Yaoundé belt. The minimum age of Pan-African-Brasiliano collision in the Sergipano belt is constrained at 628 ± 12 Ma on syn-collision granites, whereas in the Yaoundé belt collision took place between 620 and 610 Ma, i.e. the age of granulite facies metamorphism. Sm–Nd isotope geochemistry and U–Pb age dating indicate that most clastic metasedimentary rocks in both belts were derived from sources to the north and, to a lesser degree, from the cratons to the south. 相似文献
126.
A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran) 总被引:3,自引:0,他引:3
Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj–Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj–Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic–Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran, the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene. 相似文献
127.
The Ore-forming Fluid of the Gold Deposits of Muru Gold Belt in Eastern Shandong, China - a Case Study of Denggezhuang Gold Deposit 总被引:1,自引:0,他引:1
Qingdong Zeng Jianming Liu Hongtao Liu Ping Shen Lianchang Zhang 《Resource Geology》2006,56(4):375-384
Abstract. Denggezhuang gold deposit is an epithermal gold‐quartz vein deposit in northern Muru gold belt, eastern Shandong, China. The deposit occurs in the NNE‐striking faults within the Mesozoic granite. The deposit consists of four major veins with a general NNE‐strike. Based on crosscutting relationships and mineral parageneses, the veins appear to have been formed during the same mineralization epochs, and are further divided into three stages: (1) massive barren quartz veins; (2) quartz‐sulfides veins; (3) late, pure quartz or calcite veinlets. Most gold mineralization is associated with the second stage. The early stage is characterized by quartz, and small amounts of ore minerals (pyrite), the second stage is characterized by large amounts of ore minerals. Fluid inclusions in vein quartz contain C‐H‐O fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two‐phase, aqueous vapor and an aqueous liquid phase (L+V); type II, aqueous‐carbonic inclusions, a CC2‐liquid with/without vapor and aqueous liquid (LCO2+VCC2+Laq.); type III, mono‐phase aqueous liquid (Laq.). Data from fluid inclusion distribution, microthermometry, and gas analysis indicate that fluids associated with Au mineralized quartz veins (stage 2) have moderate salinity ranging from 1.91 to 16.43 wt% NaCl equivalent (modeled salinity around 8–10 wt% NaCl equiv.). These veins formatted at temperatures from 80d? to 280d?C. Fluids associated with barren quartz veins (stage 3) have a low salinity of about 1.91 to 2.57 wt% NaCl equivalent and lower temperature. There is evidence of fluid immiscibility and boiling in ore‐forming stages. Stable isotope analyses of quartz indicate that the veins were deposited by waters with δO and δD values ranging from those of magmatic water to typical meteoric water. The gold metallogenesis of Muru gold belt has no relationship with the granite, and formed during the late stage of the crust thinning of North China. 相似文献
128.
Rock fracturing by explosive energy: review of state-of-the-art 总被引:1,自引:0,他引:1
M. R. Saharan H. S. Mitri J. L. Jethwa 《Fragblast: International Journal for Blasting and Fragmentation》2006,10(1):61-81
A study of the dynamic rock fracture initiation and propagation due to explosive energy is presented through a detailed state-of-the-art review. Explosive energy dissipation in crushing and fracturing is examined and the various means to enhance the explosive energy utilization for dynamic rock fracturing are reviewed. The study highlights the need for a better understanding of the dynamic fracturing process particularly in the presence of in situ stresses in the rock mass. 相似文献
129.
The stochastic nature of the cyclic swelling behavior of mudrock and its dependence on a large number of interdependent parameters was modeled using Time Delay Neural Networks (TDNNs). This method has facilitated predicting cyclic swelling pressure with an acceptable level of accuracy where developing a general mathematical model is almost impossible. A number of total pressure cells between shotcrete and concrete walls of the powerhouse cavern at Masjed–Soleiman Hydroelectric Powerhouse Project, South of Iran, where mudrock outcrops, confirmed a cyclic swelling pressure on the lining since 1999. In several locations, small cracks are generated which has raised doubts about long term stability of the powerhouse structure. This necessitated a study for predicting future swelling pressure. Considering the complexity of the interdependent parameters in this problem, TDNNs proved to be a powerful tool. The results of this modeling are presented in this paper. 相似文献
130.
Compressibility and swelling characteristics of Al-Khobar Palygorskite, eastern Saudi Arabia 总被引:1,自引:0,他引:1
Expansive soils are found in different locations in eastern Saudi Arabia. The area is arid with high temperatures, highly variable humidity and an excessive rate of evaporation compared to the low precipitation. This resulted in the formation of water sensitive soils. In the present investigation, line valve buildings for a sweet water feeder (1118 mm in diameter) were constructed on a highly expansive material consisting mainly of brown palygorskite and gray palygorskite with thin sheets of gypsum and limestone. Block samples from both palygorskites were brought to the laboratory and cores as well as remolded samples were obtained from the blocks. The two palygorskites were found to be highly plastic and have a very high swelling potential. The liquid limit (LL) and plastic limit (PL) values for the brown palygorskite are 261% and 140%, respectively. The gray palygorskite has a LL of a 285% and a PL of 123%. The oedometer free swell tests for the two palygorskites produced an expansion ranging between 31.8% and 42.5% for the remolded samples. However, the expansion for cores ranges between 8.3% and 19.3%. The constant volume pressure tests produced a stress in excess of 4240 kPa. The swell potential reached a steady state after four days while the swelling pressure reached a steady state in about 3 h. The paper addresses the geology of the area, the characterization of the geomaterial including mineralogical composition using X-ray diffraction and SEM techniques and the swelling characteristics of the material. 相似文献