全文获取类型
收费全文 | 960篇 |
免费 | 184篇 |
国内免费 | 256篇 |
专业分类
测绘学 | 34篇 |
大气科学 | 222篇 |
地球物理 | 237篇 |
地质学 | 462篇 |
海洋学 | 107篇 |
天文学 | 75篇 |
综合类 | 116篇 |
自然地理 | 147篇 |
出版年
2024年 | 4篇 |
2023年 | 11篇 |
2022年 | 39篇 |
2021年 | 46篇 |
2020年 | 34篇 |
2019年 | 48篇 |
2018年 | 52篇 |
2017年 | 46篇 |
2016年 | 49篇 |
2015年 | 65篇 |
2014年 | 69篇 |
2013年 | 71篇 |
2012年 | 76篇 |
2011年 | 65篇 |
2010年 | 61篇 |
2009年 | 54篇 |
2008年 | 62篇 |
2007年 | 51篇 |
2006年 | 37篇 |
2005年 | 26篇 |
2004年 | 23篇 |
2003年 | 19篇 |
2002年 | 25篇 |
2001年 | 28篇 |
2000年 | 25篇 |
1999年 | 32篇 |
1998年 | 32篇 |
1997年 | 40篇 |
1996年 | 24篇 |
1995年 | 29篇 |
1994年 | 30篇 |
1993年 | 27篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 14篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有1400条查询结果,搜索用时 609 毫秒
401.
Case-based reasoning is an AI technique in which the previous solutions are stored for future use. People are used to guiding themselves according to those routes that are stored in their memories and have been used by them before. It is just based on people’s preference to familiar routes, which are gained through the study of the cognitive activities. We propose to apply the intelligent method based on the case reasoning to path planning. It is impossible for a case base to store all the solutions to all the shortest paths; therefore, part of them should be stored. However, which routes should be stored and which should not be? How do we adapt the cases that have already been stored and how do we acquire the shortest route based on them? All these issues need to be explained by integrating knowledge of the network on account of case-based reasoning techniques. This paper suggests the case-based reasoning in another point. This means finding some irreplaceable links on the basis of the complete analysis of the problems space, which are called the must_be_passed link between the source and destination. Merely compute the shortest path case from those best exit/entry nodes of the grids to the irreplaceable links, and then add them into the case base storing for future use. This method is based on case-based reasoning technique and completely considers the properties of the problem space. In addition to the use of knowledge of the natural grid in the route network, this method is more efficient than existing algorithms on computing efficiency. 相似文献
402.
Mature amphioxus (Branchiostoma belcheri tsingtauense) collected by trawl in June, 1994 from Qingdao Shazikou sea area bottom sands, were cultured under controlled indoor conditions, where they spawned and their eggs were fertilized and hatched into larvae which gradually settled at the bottom and dug into the sands after 40-50 days culture in a water trough outdoors. The total survival rate of the larvae(LSR)was 5.5% before they went into the sands, 1.9% in 4 months, 0.7% in 10 months. The survival rate of the young fish which had dug into the sands (YSR) was 35.6% in 4 months, and 12.6% in 10 months. The amphioxus number tended to be constant from the 11th month on. In the first five months after incubation, the amphioxus body length increased by an average of about 1.5 mm a month, and about 0.4 mm a month from Dec. to May of next year. The maximum length after ten months was 24 mm; the average was about 11 mm. After two years culture in the water trough, the maximum length could reach 34-35 mm, when the amphioxus gonads began to develop. 相似文献
403.
INTRODUCTIONTherecentyears’rapiddevelopmentofthemarinefishcultureindustryinChinahasbroughtaboutproliferationofviraldiseasesintheindustryasshownbythenotablyfrequentoccurrenceofdiseasesordeathsofthemariculturedfish .Electronmicroscopeexaminationoftheaff… 相似文献
404.
As part of the 2007 Tri-Center Field Mission to Japan, a reconnaissance team comprised of fourteen graduate students and three faculty members from three U.S. earthquake engineering research centers, namely, Multidisciplinary Center for Earthquake Engineering Research (MCEER), Mid-America Earthquake Center (MAE), and Pacifi c Earthquake Engineering Research Center (PEER), undertook a reconnaissance visit to the affected area shortly after the 2007 Niigata- Chuetsu Oki earthquake. This mission provided an opportunity to review the nature of the earthquake damage that occurred, as well as to assess the signifi cance of the damage from an educational perspective. This paper reports on the seismological characteristics of the earthquake, preliminary fi ndings of geotechnical and structural damage, and the causes of the observed failures or collapses. In addition, economic and socio-economic considerations and experiences to enhance earthquake resilience are presented. 相似文献
405.
南沙海区沉积物中细菌和古细菌16S rDNA多样性的研究 总被引:10,自引:1,他引:10
采用细菌16SrDNA通用引物和PCR扩增等方法,构建了南海南沙海区沉积物16S rDNA文库,并通过RFLP酶切分型对所获得的70个克隆进行测序。从国际分子生物学数据库中调取相关序列,以PAUP4.0分析软件构建序列同源性矩阵和系统发育树图。结果表明,与细菌文库中克隆相似的微生物属于4个细菌类群:变形细菌(Proteobacteria)(60%)、革兰氏阳性细菌(Gram-positivre bacteria)(13%)、浮霉菌(Planetomycetes)(10%)和无硫绿细菌(Green nonsulfur bacteria)(6%),其中变形细菌(包括δ-、γ-和α-变形细菌)是明显的优势类群。采用Blast程序对所有序列基因数据库进行搜索,发现只有一个克隆与已知序列完全相似,说明文库具有极高的多样性。但是对古细菌文库中所获得的克隆子进行二级结构和序列特征分析的结果表明,这些克隆子均为海洋未获培养的细菌,因此在作者的文库中并未有古细菌的发现。 相似文献
406.
407.
The Jiaozhou Bay is a semi-enclosed bay, Qingdao, China. More than 10 rivers enter the bay, of which most take wastes from industrial and household discharges. According to historical seasonal investigations in May, August, November 1979, the content, distribution, and development of heavy metal mercury are analyzed as a historical reference. Water samples were taken from the surface and bottom. The results revealed clear seasonal and regional changes in both horizontal and vertical directions, and close relation with major discharging rivers and plankton production. The seawater was polluted more seriously in spring than in any other seasons. However, it was the cleanest in winter during which least waste was input with low plankton production. According to historical data, the state of mercury pollution in seawater was worsening in the period, and has been improving in recent years. Terrestrial contamination was the main reason for mercury pollution in the bay. 相似文献
408.
M. L. Mays O. C. St. Cyr E. Quémerais S. Ferron J.-L. Bertaux S. Yashiro R. Howard 《Solar physics》2007,241(1):113-125
In this study, the possibility that coronal mass ejections (CMEs) may be observed in neutral Lyman-α emission was investigated.
An observing campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α scanning photometer on board the Solar and
Heliospheric Observatory (SOHO) dedicated to monitoring the latitude distribution of the solar wind from its imprints on the
interstellar sky background. This was part of SOHO Joint Observing Program (JOP) 159 and was an exploratory investigation
as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance
(≈60 and 120 R
S). The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs (30 R
S). In our first method we used LASCO, white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope
also on SOHO, to identify CME candidates which, subject to certain criteria, should have been observable in SWAN. The criteria
included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we
discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing
campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-α
intensity representing ≈10% of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed
by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and
statistics, and the results. 相似文献
409.
Ya-Juan Lei Li-Ming Song Jin-Lu Qu Cheng-Min Zhang 《Astrophysics and Space Science》2007,310(3-4):307-315
The rms (root mean square) variability is the parameter for understanding the emission temporal properties of X-ray binaries
(XRBs) and active galactic nuclei (AGN). The rms–flux relation with Rossi X-ray Timing Explorer (RXTE) data for the dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our results show that there exist
the linear rms–flux relations in the frequency range 0.1–10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of the dipping case in the low energy band. For
the high energy band, the slopes of the dipping and nondipping cases are hardly constant within errors. The explanations of
the results have been made by means of the “Propagating Perturbation” model of Lyubarskii (Lyubarskii, Y.E., Mon. Not. Roy.
Astron. Soc. 292, 679–685 (1997)). 相似文献
410.
The formation of the Paleogene in the Chengbei fault-step zone of the Dagang off-shore area is very complex and the stratigraphic
division and correlation are relatively difficult. A method of micropaleontology-sequence stratigraphy is proposed herein.
The study considers Well Zhangcan 1 as an example to prove that the scheme of stratigraphic division is feasible by conducting
micropaleontology-sequence stratigraphy in the Guanjiapu area of the Chengbei faultstep zone. The application of micropaleontology-sequence
stratigraphy method indicates that Member 1 of the Shahejie Formation in Well Zhuanghai 808×1 and Member 3 of the Dongying
Formation of the Paleogene exist.
Translated from Acta Micropalaeontologica Sinica, 2005, 22(4): 417–424 [译自: 微体古生物学报] 相似文献