全文获取类型
收费全文 | 39021篇 |
免费 | 747篇 |
国内免费 | 245篇 |
专业分类
测绘学 | 758篇 |
大气科学 | 2613篇 |
地球物理 | 7709篇 |
地质学 | 14175篇 |
海洋学 | 3580篇 |
天文学 | 8752篇 |
综合类 | 87篇 |
自然地理 | 2339篇 |
出版年
2022年 | 270篇 |
2021年 | 443篇 |
2020年 | 511篇 |
2019年 | 586篇 |
2018年 | 1017篇 |
2017年 | 1029篇 |
2016年 | 1101篇 |
2015年 | 593篇 |
2014年 | 1037篇 |
2013年 | 1865篇 |
2012年 | 1207篇 |
2011年 | 1656篇 |
2010年 | 1452篇 |
2009年 | 1814篇 |
2008年 | 1644篇 |
2007年 | 1710篇 |
2006年 | 1573篇 |
2005年 | 1069篇 |
2004年 | 1087篇 |
2003年 | 1120篇 |
2002年 | 1006篇 |
2001年 | 875篇 |
2000年 | 796篇 |
1999年 | 726篇 |
1998年 | 722篇 |
1997年 | 726篇 |
1996年 | 590篇 |
1995年 | 573篇 |
1994年 | 500篇 |
1993年 | 454篇 |
1992年 | 409篇 |
1991年 | 424篇 |
1990年 | 439篇 |
1989年 | 393篇 |
1988年 | 369篇 |
1987年 | 398篇 |
1986年 | 418篇 |
1985年 | 507篇 |
1984年 | 546篇 |
1983年 | 542篇 |
1982年 | 498篇 |
1981年 | 454篇 |
1980年 | 429篇 |
1979年 | 408篇 |
1978年 | 375篇 |
1977年 | 384篇 |
1976年 | 344篇 |
1975年 | 353篇 |
1974年 | 340篇 |
1973年 | 370篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
881.
Study of geothermal water intrusion due to groundwater exploitation in the Puebla Valley aquifer system, Mexico 总被引:1,自引:1,他引:1
E. Leticia Flores-Márquez Gabriel Jiménez-Suárez Raymundo G. Martínez-Serrano René E. Chávez Daniel Silva Pérez 《Hydrogeology Journal》2006,14(7):1216-1230
Significant intrusion of geothermal water into fresh groundwater takes place in the Puebla Valley aquifer system, Mexico. The decline in the potentiometric surface due to the overexploitation of the groundwater induces this intrusion. This hydrological system comprises three aquifers located in Plio-Quaternary volcanic sediments and Mesozoic calcareous rocks. The hydraulic balance of the aquifer shows that the annual output exceeds the natural inputs by 12 million m3. Between 1973 and 2002, a drop in the potentiometric surface, with an 80 m cone of depression, was identified in a 5-km-wide area located southwest of the city of Puebla. Chemical analyses performed on water samples since 1990 have shown an increase in total dissolved solids (TDS) of more than 500 mg/L, coinciding with the region showing a cone of depression in the potentiometric surface. A three-dimensional flow and transport model, based on the hydrogeological and geophysical studies, was computed by using the MODFLOW and MT3D software. This model reproduces the evolution of the aquifer system during the last 30 years and predicts for 2010 an additional drawdown in the potentiometric surface of 15 m, and an increase in the geothermal water intrusion. 相似文献
882.
A. F. Kholtygin G. A. Galazutdinov T. E. Burlakova G. G. Valyavin S. N. Fabrika B. -C. Lee 《Astronomy Reports》2006,50(3):220-231
We present the results of a search for and analysis of line-profile variations in the spectrum of the star ι Her. The observations were acquired with the 1.8 m telescope of the Bohyunsan Optical Astronomy Observatory (Republic of Korea) in May–June 2004. We obtained 69 spectra of the star with signal-to-noise ratios ≈300 and a time resolution of 5–7 min. Profile variability was revealed for six lines of HI, HeI, and SiIII, in the central parts of the lines. The variability amplitude is ≈(1–2)% in units of the intensity of the adjacent continuum. Evidence was found for cyclic variations of the lines, with periods from ≈7h to ≈2.9d. We conclude that ι Her belongs to the group of slowly pulsating stars. 相似文献
883.
884.
885.
L. E. Levin 《Geotectonics》2006,40(5):357-366
The lithosphere and asthenosphere make up a common geodynamic system but are characterized by different physical parameters. The former has a temperature of 1200–1300°C, a density of 3.3 g/cm3, and a viscosity of 1022 poise, while the latter has a density of 3.23 g/cm3, a viscosity in the range 1021-1018–19 poise, and a temperature from 1200–1300°C to 1600–1700°C. The asthenosphere is distinguished by a great variability of its physical state in the lateral and vertical directions. This circumstance necessitates the recognition of the different types of the asthenosphere: seismic (LVZ zone), electrical, thermal, and seismological. The structure and the physical state of the thermal asthenosphere is considered in this paper on the basis of P-T parameters. Its state normally fits viscous Newtonian liquid beneath the continents and provides partial (5–20%) melting in spreading zones and along continental margins. No partial melting is detected beneath the main portion of the continents. The interaction between the asthenosphere and lithosphere is characterized by spatiotemporal migration of partial melting zones and asthenosphere upwelling, and such interaction determines the entire range of geodynamic processes from spreading and rifting to collision and vertical motions of different senses. 相似文献
886.
887.
888.
889.
890.
The tectonic evolution of the Por’ya Guba segment of the White Sea Rift System began in the late Paleoproterozoic, i.e., soon
after completion of the Svecofennian collision. The fracture system that controlled localization of the lamproite dike complex
was formed under conditions of horizontal compression combined with shear. Subsequently, this system predetermined the location
of a rift-graben segment that formed as a result of simple shear. The reactivation of the rift system in the Middle Paleozoic
proceeded in two stages. The first stage, when strike-slip movements along previously formed faults predominated, resulted
in formation of quartz-carbonate veins bearing base-metal mineralization. The veins that filled the shear fractures opened
owing to local reorientation of the stress field. The second stage fitted the transtension conditions, and the Late Devonian
alkaline ultramafic dikes of this stage introded into the already existing fracture system, which was oriented at a roughly
right angle to the predominant stress orientation. 相似文献