全文获取类型
收费全文 | 28014篇 |
免费 | 907篇 |
国内免费 | 1979篇 |
专业分类
测绘学 | 1790篇 |
大气科学 | 2653篇 |
地球物理 | 5350篇 |
地质学 | 13539篇 |
海洋学 | 1643篇 |
天文学 | 1844篇 |
综合类 | 2458篇 |
自然地理 | 1623篇 |
出版年
2024年 | 45篇 |
2023年 | 136篇 |
2022年 | 234篇 |
2021年 | 274篇 |
2020年 | 266篇 |
2019年 | 281篇 |
2018年 | 4970篇 |
2017年 | 4250篇 |
2016年 | 2786篇 |
2015年 | 465篇 |
2014年 | 321篇 |
2013年 | 246篇 |
2012年 | 1253篇 |
2011年 | 2956篇 |
2010年 | 2231篇 |
2009年 | 2489篇 |
2008年 | 2072篇 |
2007年 | 2506篇 |
2006年 | 197篇 |
2005年 | 321篇 |
2004年 | 497篇 |
2003年 | 483篇 |
2002年 | 345篇 |
2001年 | 135篇 |
2000年 | 145篇 |
1999年 | 142篇 |
1998年 | 143篇 |
1997年 | 97篇 |
1996年 | 113篇 |
1995年 | 73篇 |
1994年 | 63篇 |
1993年 | 72篇 |
1992年 | 59篇 |
1991年 | 51篇 |
1990年 | 34篇 |
1989年 | 16篇 |
1988年 | 24篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 27篇 |
1980年 | 20篇 |
1979年 | 3篇 |
1976年 | 7篇 |
1958年 | 3篇 |
1957年 | 2篇 |
1933年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
271.
OLSR (optimal link state routing) is one of the four basic routing protocols used in mobile ad hoc Networks by the MANET working group of IETF (Internet engineering task force). OLSR, a proactive routing protocol, is based on a multipoint relaying flooding technique to reduce the number of topology broadcast. OLSR uses periodic HELLO packets to neighbor detection. As introduced in Reference [1], the wormhole attack can form a serious threat in wireless Networks, especially against many ad hoc Network routing protocols and location-based wireless security systems. Here, a trust model to handle this attack in OLSR is provided and simulated in NS2. 相似文献
272.
Pulsatory characteristics of wind velocity in sand flow over typical underlying surfaces 总被引:1,自引:0,他引:1
ZHANG KeCun QU JianJun ZU RuiPing & FANG HaiYan Key Laboratory of Desert Desertification Cold Arid Regions Environmental Engineering Research Institute Chinese Academy of Sciences Lanzhou China 《中国科学D辑(英文版)》2007,50(2):247-253
Pulsatory characteristics of wind velocity in sand flow over Gobi and mobile sand surface have been investigated experimentally in the wind tunnel. The primary goal of this paper is to reveal the relation- ship between pulsatory characteristics of instantaneous wind speed in sand flow and the motion state of sand grains. For a given underlying surface, pulsation of wind velocities in sand flow on different heights has a good correlation. As the space distance among different heights increases, fluctuation of instantaneous wind speed presents a decreasing trend and its amplitude is closely related to the mo- tion state of sand grains and their transport. Pulsatory intensity increases with the indicated wind speed, but its relative value does not depend on it, only agrees with height. 相似文献
273.
The Tarwal River basin with an area of 6560.20 km2 is located in the eastern part of Iranian Kurdistan Province. This river crosses the Qorveh and Dehgolan plains and joins the Ghezel Ozan River in Zanjan Province. The importance of this river as a source for drinking water and agricultural and industrial uses in the region necessitates the need for research in this field. The main purpose of this study is to identify the natural features of the riverbed from the perspective of river geomorphology and to investigate their impact on water quality and river self-purification capacity. To achieve this, the river style framework was employed. To investigate the effects of each style framework on the river, a total of 20 samples from the entrance and outlet of styles were obtained using Impact Assessment method and sampling standards which were later analyzed for their quality parameters including T, pH, EC, TDS, TSS, Na, Ca, Mg, K, Cl, F, NO2, NO3, SO4, PO4, DO, COD and BOD. The results indicated that the changes in the styles lead to changes in water quality and the impact of each style is greater on the physical parameters than the chemical parameters. The river self-purification capacity varied depending on the style. The maximum and the minimum self-purifications occurred in fine-grained Anabranching and low-sinuosity fine-grained styles, respectively. 相似文献
274.
The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the
recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev
vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide
(VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb
and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb).
A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized
patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values
of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly
shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely
reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks. 相似文献
275.
Combined analyses of pollen, seeds, woods, micro-charcoal and non-pollen palynomorphs from Stagno di Maccarese, an artificially
dried out coastal basin north of the Tiber delta now occupied by the Fiumicino Airport (Rome, Italy), document marked vegetation
and environmental changes during the last 8300 years. Between 8300 and 5400 cal. a BP dense mixed deciduous and evergreen
forests surrounded a eutrophic freshwater basin. An abrupt change around 5400 cal. a BP marks the transition to a marshy environment,
due to a lowering of the water table. An increase of cereals and micro-charcoals matches the presence of a nearby Eneolithic
settlement. Between 5100 and 2900 cal. a BP there is a remarkable expansion of riparian trees, indicating an increase of the
water level. Between 2900 and 2000 cal. a BP, a new development of marshlands points to a progressive lowering of the lake.
After 2000 cal. a BP, during the Roman exploitation of the area, an expansion of arboreal vegetation is recorded, characterized
by evergreen and deciduous oak-dominated forests, while an extensive chenopods marshland matches the presence of saltworks.
On the whole, the Stagno di Maccarese area appears very unstable, due to changes in lake level, introgression of marine water,
eutrophic phases, flood events, desiccations and openings of the forest vegetation. 相似文献
276.
A geochemical evaluation of Niger Delta organic matter was carried out using supercritical fluid extraction (SFE) sample preparation
procedure. Comparison of geochemical significance of gas chromatographic data of rock extracts of SFE with those of Soxhlet
extraction method from previous studies was made in order to establish the usefulness of SFE in geochemical exploration. The
assessment of geochemical character of the rock samples from the comparison and interpretation of other geochemical parameters
were used to give more insights into understanding the source rocks characteristics of onshore and shelf portions of the Niger
Delta Basin. The results of the gas chromatographic (GC) analysis of the rock extracts across the lithostratigraphic units
show that Pr/Ph, Pr/nC17, Pr/nC18, CPI and odd/even preference ranged from 0.07 to 12.39, 0.04 to 6.66, 0.05 to 13.80, 0.12 to 8.4 and 0.06 to 8.12, respectively.
The Rock-Eval pyrolysis data and geochemical ratios and parameters calculated from the GC data showed that most of the samples
are mature and have strong terrestrial provenance while a few samples have strong marine provenance. The few marine source
rocks are located in the deeper depth horizon. Pr/Ph and standard geochemical plots indicate that most of samples were derived
from organic matter deposited in less reducing conditions, i.e. more of oxidizing conditions while a few samples have predominantly
influence of reducing conditions. The results of trace metal analysis of older samples from Agbada Formation also indicate
marine and mixed organic matter input deposited in less reducing conditions. The results obtained in this study are comparable
with those obtained from previous studies when Soxhlet extraction method was used and also indicated the presence of more
than one petroleum systems in the Niger Delta. 相似文献
277.
Ryoko Tokeshi Kaoru Ichikawa Satoshi Fujii Kenji Sato Shoichiro Kojima 《Journal of Oceanography》2007,63(4):711-720
A method to extract geostrophic current in the daily mean HF radar data in the Kuroshio upstream region is established by
comparison with geostrophic velocity determined from the along-track altimetry data. The estimated Ekman current in the HF
velocity is 1.2% (1.5%) and 48° (38°)-clockwise rotated with respect to the daily mean wind in (outside) the Kuroshio. Furthermore,
additional temporal smoothing is found necessary to remove residual ageostrophic currents such as the inertial oscillation.
After removal of the ageostrophic components, the HF geostrophic velocity agrees well with that from the altimetry data with
rms difference 0.14 (0.12) m/s in (outside) the Kuroshio. 相似文献
278.
C. P. Goff L. van Driel-Gesztelyi P. Démoulin J. L. Culhane S. A. Matthews L. K. Harra C. H. Mandrini K. L. Klein H. Kurokawa 《Solar physics》2007,240(2):283-299
A series of flares (GOES class M, M and C) and a CME were observed in close succession on 20 January 2004 in NOAA 10540. Radio
observations, which took the form of types II, III and N bursts, were associated with these events. We use the combined observations
from TRACE, EIT, Hα images from Kwasan, MDI magnetograms and GOES to understand the complex development of this event. Contrary
to a standard interpretation, we conclude that the first two impulsive flares are part of the CME launch process while the
following long-duration event flare represents simply the recovery phase. Observations show that the flare ribbons not only
separate but also shift along the magnetic inversion line so that magnetic reconnection progresses stepwise to neighboring
flux tubes. We conclude that “tether cutting” reconnection in the sheared arcade progressively transforms it to a twisted
flux tube, which becomes unstable, leading to a CME. We interpret the third flare, a long-duration event, as a combination
of the classical two-ribbon flare with the relaxation process following forced reconnection between the expanding CME structure
and neighboring magnetic fields.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
279.
280.
The variational assimilation theory is generally based on unbiased observations. In practice, however, almost all observations
suffer from biases arising from observational instruments, radiative transfer operator, precondition of data, and so on. Therefore,
a bias correction scheme is indispensable. The current scheme for radiance bias correction in the GRAPES 3DVar system is an
offline scheme. It is actually a static correction for the radiance bias before the process of cost function minimization.
In consideration of its effects on forecast results, this kind of scheme has some shortcomings. Thus, this study provides
a variational bias correction (VarBC) scheme for the GRAPES 3DVar system following Dee’s idea. In the VarBC scheme, the observation
operator is modified and a new control variable is defined by taking the predictor coefficients as the control parameters.
According to the feature of the GRAPES-3DVAR, an incremental formulation is applied and the original bias correction scheme
is maintained in the actual process of observations. The VarBC is designed to co-exist with the original scheme, because it
is a dynamic revision to the observational operator on the basis of the old method, i.e., it adjusts the model state vector
along with the control parameters to an unbiased state in the process of minimization and the assimilation system remains
consistent with available information automatically. Preliminary experimental results show that the mean departures of background-minus-observation
and analysis-minus-observation are reduced as expected. In a case study of the heavy rainfall that happened in South China
on 11–13 June 2008, the 500-hPa geopotential height is better simulated using the analyzed field from the VarBC as the initial
condition. 相似文献