全文获取类型
收费全文 | 5948篇 |
免费 | 565篇 |
国内免费 | 162篇 |
专业分类
测绘学 | 304篇 |
大气科学 | 630篇 |
地球物理 | 2037篇 |
地质学 | 2410篇 |
海洋学 | 287篇 |
天文学 | 552篇 |
综合类 | 190篇 |
自然地理 | 265篇 |
出版年
2022年 | 13篇 |
2021年 | 18篇 |
2020年 | 18篇 |
2019年 | 17篇 |
2018年 | 482篇 |
2017年 | 413篇 |
2016年 | 302篇 |
2015年 | 173篇 |
2014年 | 156篇 |
2013年 | 152篇 |
2012年 | 684篇 |
2011年 | 449篇 |
2010年 | 146篇 |
2009年 | 169篇 |
2008年 | 146篇 |
2007年 | 133篇 |
2006年 | 145篇 |
2005年 | 842篇 |
2004年 | 883篇 |
2003年 | 658篇 |
2002年 | 177篇 |
2001年 | 77篇 |
2000年 | 49篇 |
1999年 | 23篇 |
1998年 | 15篇 |
1997年 | 24篇 |
1996年 | 17篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 25篇 |
1990年 | 17篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 26篇 |
1986年 | 17篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1976年 | 6篇 |
1975年 | 11篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 4篇 |
1971年 | 5篇 |
排序方式: 共有6675条查询结果,搜索用时 31 毫秒
61.
The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T2). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T1) and the present-day flood plain (terrace-T0). Terrace-T1 shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T0 and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T2 show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka. 相似文献
62.
本文对松辽盆地东南部后五家户气田划分出22种沉积微相,建立7种相层序,分析了扇三角洲及三角洲沉积环境对天然气成藏的控制作用。指出了扇三角洲前缘砂体和三角洲前缘砂体较为发育,沉积物源来自气田东北部和东南部两个方向。后五家户气田多为构造—岩性气藏和岩性气藏。 相似文献
63.
Introduction The Western Kunlun Mountain Pass M=8.1 earthquake occurred on November 14, 2001 is the other M=8 earthquake occurred 50 years after Dangxiong, Tibet M=8.0 earthquake in Chinese mainland. The earthquake has caused the attention of the seismologists in the following aspects: 1) The fracture length is more than 400 km, which is far away from the estimated length by the statistic empirical function between the magnitude and the fracture length (WANG, et al, 2002); 2) The aftersh… 相似文献
64.
65.
Andesitic pillow lavas containing biogenic, solid bitumen (SB) are a constituent of a Neoproterozoic volcanosedimentary sequence (Teplá-Barrandian unit, Bohemian Massif) in the Mítov area of the Czech Republic. A black shale formation that is crosscut by these andesitic basalts is 565 Ma old. Carbon disulfide extracts of two powdered samples of SB contain 0.2 and 0.3 ppm of C60, respectively, as determined by high-pressure liquid chromatography. The peak assignment based on retention time is fully supported by high-resolution electron ionization mass spectrometry (EI-MS). No C70 was detected, nor was C60 found in two other SB samples from this locality. Other investigated carbonaceous samples from Bohemia (coals and anthracites of Upper Paleozoic age and anthraxolite, graphitoids, and graphite of Upper Proterozoic age) did not contain fullerenes at concentrations above the detection limit of 0.01 ppm. The absence of C60 in these samples was confirmed by EI-MS. The proposed mechanism of fullerene formation involves a primary algal phase, generation of a hydrocarbonaceous mixture in the course of thermal evolution of the sedimentary series, and their high-temperature transformation related to the extrusion of basalt. An important feature for fullerene conservation was the enclosure of fullerenes in SB with a structure similar to glasslike carbon, where the fullerene was protected against oxidation. 相似文献
66.
Dilip K Mukhopadhyay Bidyut K Bhadra Tamal K Ghosh Deepak C Srivastava 《Journal of Earth System Science》1996,105(2):157-171
Numerous peraluminous and porphyritic granitic bodies and augen gneisses of granitic compositions occur in the nappe sequences
of the Lower Himalaya. They are Proterozoic-to-lower Paleozoic in age and have been grouped into the ‘Lesser Himalaya granite
belt’. The mode of emplacement and tectonic significance of these granites are as yet uncertain but they are generally considered
to be sheet-like intrusions into the surrounding rocks. The small and isolated granite body (the Chur granite) that crops
out around the Chur peak in the Himachal Himalaya is one of the more famous of these granites. Several lines of evidence have
been adduced to show that the Chur granite has a thrust (the Chur thrust) contact with the underlying metasedimentary sequence
(locally called the Jutogh Group). The Chur granite with restricted occurrence at the highest topographic and structural levels
represents an erosional remnant of a much larger sub-horizontal thrust sheet. The contact relations between the country rocks
and many of the other granite and granitic augen gneisses in the Lesser Himalaya belt are apparently similar to that of the
Chur granite suggesting that at least some of them may also represent thrust sheets. 相似文献
67.
68.
Satellite altimetry can be used to infer subsurface geological structures analogous to gravity anomaly maps generated through
ship-borne survey. The Eastern offshore was taken up for analysis using Geosat Exact Repeat Mission (ERM) altimeter data.
A methodology is developed to use altimeter data as an aid to offshore hydrocarbon exploration. Processing of altimeter data
involves corrections for various atmospheric and oceanographic effects, stacking and averaging of repeat passes, cross-over
correction, removal of deeper earth and bathymetric effects, spectral analysis and conversion into free-air gravity anomaly.
The final processed results were derived for Eastern offshore in the form of prospecting geoid and gravity anomaly maps and
their spectral components. The highs and lows observed in those maps were derived in terms of a number of prominent megastructures
e.g., gravity linears, 85°E and 90°E ridges, the Andaman trench complex etc. Satellite-derived gravity profiles along 12°N
latitude match well with the existing structures. 相似文献
69.
70.
Results of numerical simulation studies of processes associated with Stimulated Electromagnetic Emission (SEE) produced during ionospheric heating experiments are presented. A one-dimensional magnetized electrostatic Particle-In-Cell (PIC) simulation model with uniform plasma density is used to investigate electrostatic wave generation in the region where the pump frequency ω0 approximately equals the upper hybrid frequency ωuh. In particular, the simulation plasma is driven with a uniform oscillating electric field to represent the long wavelength pump wave and power spectra of the electrostatic waves produced are taken. The pump wave frequency and amplitude are varied to consider the effects on the simulation power spectrum. The upper hybrid frequency in the model is varied through harmonics of the electron cyclotron frequency Ωce to consider the effects of stepping the pump frequency through cyclotron harmonics. The power spectrum from the simulation plasma is richly structured. The resulting power spectra show sidebands upshifted and downshifted from the pump frequency by multiples of the lower hybrid frequency ωlh. The structure of the spectrum is highly sensitive to the proximity of the upper hybrid frequency to the cyclotron harmonic frequencies. 相似文献