全文获取类型
收费全文 | 4863篇 |
免费 | 666篇 |
国内免费 | 678篇 |
专业分类
测绘学 | 399篇 |
大气科学 | 395篇 |
地球物理 | 425篇 |
地质学 | 2058篇 |
海洋学 | 508篇 |
天文学 | 26篇 |
综合类 | 432篇 |
自然地理 | 1964篇 |
出版年
2024年 | 20篇 |
2023年 | 57篇 |
2022年 | 175篇 |
2021年 | 261篇 |
2020年 | 239篇 |
2019年 | 233篇 |
2018年 | 198篇 |
2017年 | 199篇 |
2016年 | 210篇 |
2015年 | 218篇 |
2014年 | 291篇 |
2013年 | 305篇 |
2012年 | 264篇 |
2011年 | 315篇 |
2010年 | 270篇 |
2009年 | 261篇 |
2008年 | 254篇 |
2007年 | 280篇 |
2006年 | 305篇 |
2005年 | 253篇 |
2004年 | 223篇 |
2003年 | 181篇 |
2002年 | 198篇 |
2001年 | 163篇 |
2000年 | 165篇 |
1999年 | 103篇 |
1998年 | 94篇 |
1997年 | 75篇 |
1996年 | 66篇 |
1995年 | 52篇 |
1994年 | 54篇 |
1993年 | 47篇 |
1992年 | 49篇 |
1991年 | 20篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1971年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有6207条查询结果,搜索用时 171 毫秒
181.
182.
《China Geology》2021,4(1):147-177
The Qinghai-Tibet Plateau (also referred to as the Plateau) has long received much attention from the community of geoscience due to its unique geographical location and rich mineral resources. This paper reviews the aeromagnetic surveys in the Plateau in the past 60 years and summarizes relevant research achievements, which mainly include the followings. (1) The boundaries between the Plateau and its surrounding regions have been clarified. In detail, its western boundary is restricted by West Kunlun-Altyn Tagh arc-shaped magnetic anomaly zone forming due to the arc-shaped connection of the Altyn Tagh and Kangxiwa faults and its eastern boundary consists of the boundaries among different magnetic fields along the Longnan (Wudu)-Kangding Fault. Meanwhile, the fault on the northern margin of the Northern Qilian Mountains serves as its northern boundary. (2) The Plateau is mainly composed of four orogens that were stitched together, namely East Kunlun-Qilian, Hoh-Xil-Songpan, Chamdo-Southwestern Sanjiang (Nujiang, Lancang, and Jinsha rivers in southeastern China), and Gangdese-Himalaya orogens. (3) The basement of the Plateau is dominated by weakly magnetic Proterozoic metamorphic rocks and lacks strongly magnetic Archean crystalline basement of stable continents such as the Tarim and Sichuan blocks. Therefore, it exhibits the characteristics of unstable orogenic basement. (4) The Yarlung-Zangbo suture zone forming due to continent-continent collisions since the Cenozoic shows double aeromagnetic anomaly zones. Therefore, it can be inferred that the Yarlung-Zangbo suture zone formed from the Indian Plate subducting towards and colliding with the Eurasian Plate twice. (5) A huge negative aeromagnetic anomaly in nearly SN trending has been discovered in the middle part of the Plateau, indicating a giant deep thermal-tectonic zone. (6) A dual-layer magnetic structure has been revealed in the Plateau. It consists of shallow magnetic anomaly zones in nearly EW and NW trending and deep magnetic anomaly zones in nearly SN trending. They overlap vertically and cross horizontally, showing the flyover-type geological structure of the Plateau. (7) A group of NW-trending faults occur in eastern Tibet, which is intersected rather than connected by the nearly EW trending that develop in middle-west Tibet. (8) As for the central uplift zone that occurs through the Qiangtang Basin, its metamorphic basement tends to gradually descend from west to east, showing the form of steps. The Qiangtang Basin is divided into the northern and southern part by the central uplift zone in it. The basement in the Qiangtang Basin is deep in the north and west and shallow in the south and west. The basement in the northern Qiangtang Basin is deep and relatively stable and thus is more favorable for the generation and preservation of oil and gas. Up to now, 19 favorable tectonic regions of oil and gas have been determined in the Qiangtang Basin. (9) A total of 21 prospecting areas of mineral resources have been delineated and thousands of ore-bearing (or mineralization) anomalies have been discovered. Additionally, the formation and uplift mechanism of the Plateau are briefly discussed in this paper.©2021 China Geology Editorial Office. 相似文献
183.
One of the main problems in reservoirs is sedimentation which reduces the operating life of dams if a proper plan and analysis method are not in place.The techniques to manage sediment in reservoirs include several sustainable management techniques that route sediment through or around the reservoir.One of the main economical methods in arid and semi-arid regions is pressurized flushing using moderate drawdown of the water level of the reservoir to evacuate sediment deposited behind dams.In the current study,the effect of a new structure called a dendritic bottomless extended(DBE)outlet structure at three angles of 30°,45°,and 60°on pressurized flushing efficiency was investigated.Consequently,45 experiments were designed for three discharge rates (Qo),three sediment levels(Hs),four types of structure,and a no-structure condition(reference test).The results indicated that the DBE structure with a 30°angle between the branches,a sedimentary dimensionless index of Hs/Do=4.59,and a flow dimensionless index of Qo=/√gD05=1:43(where g is the acceleration of gravity and Do is the diameter of the bottom outlet)lead to 10-fold increase in the sediment flushing cone dimensions and sediment removal efficiency compared to the results of the reference test.Finally,according to a statistical analysis of the results,a dimensionless equation for calculating the sediment flushing cone dimensions was developed for the tested sediment characteristics. 相似文献
184.
收缩城市建设用地利用效率时空分异及影响机制——以黑龙江省伊春市为例 总被引:1,自引:1,他引:0
建设用地是城镇经济社会活动的空间载体,其利用效率是调控建设用地扩张和配置的重要基础.论文以收缩型城市——黑龙江省伊春市为例,利用DEA模型测算了1995-2015年伊春市建设用地利用效率分析其时空演变规律,并运用地理探测器模型探究伊春市建设用地利用效率空间分异的影响机制,结论如下:①1995-2015年伊春市建设用地利... 相似文献
185.
186.
187.
中国北方磷矿成矿预测 总被引:1,自引:1,他引:0
在中朝准地台、塔里木准地台边缘深断裂附近和台中隆起带以及天山-兴安、祁连-秦岭、额尔古纳几个地槽褶皱区的中间地带,不同程度地具备中国北方三种主要类型磷矿的成矿条件。研究分析认为中国北方的内生磷矿具有良好的找矿前景,绿岩带型磷矿和古元古代沉积变质磷矿显示一定的找矿潜力,沉积型磷矿一般品位低、规模小。通过归纳,在中国北方划分出磷矿的二级城矿区4个、三级成矿带15个、四级成矿亚带19个。圈定成矿预测区A级2个,B级C级各23个。确定了21处可开展进一步工作的地区。 相似文献
188.
Biomass distribution and trophodynamics in the oceanic ecosystem in the Oyashio region are presented and analyzed, combining
the seasonal data for plankton and micronekton collected at Site H since 1996 with data for nekton and other animals at higher
trophic levels from various sources. The total biomass of biological components including bacteria, phytoplankton, microzooplankton,
mesozooplankton, micronekton, fishes/squids and marine birds/mammals was 23 g C m−2, among which the most dominant component was mesozooplankton (34% of the total), followed by phytoplankton (28%), bacteria
(15%) and microzooplankton (protozoans) (14%). The remainder (9%) was largely composed of micronekton and fish/squid. Marine
mammals/birds are only a small fraction (0.14%) of the total biomass. Large/medium grazing copepods (Neocalaus spp., Eucalanus bungii and Metridia spp.) accounted for 77% of the mesozooplankton biomass. Based on information about diet composition, predators were assigned
broadly into mean trophic level 3–4, and carbon flow through the grazing food chain was established based on the estimated
annual production/food consumption balance of each trophic level. From the food chain scheme, ecological efficiencies as high
as 24% were calculated for the primary/secondary production and 21% for the secondary/tertiary production. Biomass and production
of bacteria were estimated as 1/10 of the respective values for phytoplankton at Site H, but the role of the microbial food
chain remains unresolved in the present analysis. As keystone species in the oceanic Oyashio region, Neocalanus spp. are suggested as a vital link between primary production and production of pelagic fishes, mammals and birds. 相似文献
189.
190.
该文分析研究了彩山小流域的区域坡度和土壤特性,并根据区内土壤性质划分出4个小区域,即:自然坡度在20.~25°内为V1区,15°~20°内为V2区,10°~15°内为V3区,5°~10°内为V4区,分别反映了不同的土壤等级及土壤抗冲能力.同时,也研究出了一个系统化、理论化的科学依据i=V1035B,并对所有砂石山区水土保持治理工作具有借鉴性和实用性. 相似文献