全文获取类型
收费全文 | 2261篇 |
免费 | 48篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 107篇 |
地球物理 | 577篇 |
地质学 | 770篇 |
海洋学 | 238篇 |
天文学 | 372篇 |
综合类 | 10篇 |
自然地理 | 213篇 |
出版年
2021年 | 21篇 |
2020年 | 31篇 |
2019年 | 28篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 63篇 |
2015年 | 44篇 |
2014年 | 55篇 |
2013年 | 120篇 |
2012年 | 64篇 |
2011年 | 114篇 |
2010年 | 78篇 |
2009年 | 95篇 |
2008年 | 82篇 |
2007年 | 92篇 |
2006年 | 85篇 |
2005年 | 90篇 |
2004年 | 76篇 |
2003年 | 53篇 |
2002年 | 74篇 |
2001年 | 40篇 |
2000年 | 38篇 |
1999年 | 43篇 |
1998年 | 28篇 |
1997年 | 31篇 |
1996年 | 32篇 |
1995年 | 41篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 24篇 |
1988年 | 34篇 |
1987年 | 27篇 |
1986年 | 24篇 |
1985年 | 35篇 |
1984年 | 38篇 |
1983年 | 41篇 |
1982年 | 32篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 32篇 |
1978年 | 24篇 |
1977年 | 33篇 |
1976年 | 31篇 |
1975年 | 25篇 |
1973年 | 19篇 |
1972年 | 22篇 |
1971年 | 18篇 |
排序方式: 共有2335条查询结果,搜索用时 15 毫秒
951.
George Papathanassiou 《Engineering Geology》2008,96(1-2):94-104
Liquefaction Potential Index (LPI) is used as an assessing tool of liquefaction susceptibility. Based on the existing correlation of the LPI values with the liquefaction potential of a site, liquefaction hazard maps have been compiled. The evaluation of liquefaction susceptibility of the soil layers that had been taken into account for the calculation of LPI value, on the existing classifications, was based on the “Chinese criteria”. However, Seed et al. [Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M., Riemer, M.F., Sancio, R.B., Bray, J.D., Kayen, R.E., Faris, A., 2003. Recent advances in soil liquefaction engineering: a unified and consistent framework. 26th annual ASCE L.A. Geotechnical Spring Seminar, Long Beach, California. 71 pp.] concluded that those criteria are conservative and proposed a modification of them.The basic aim of this paper is to examine the impact of these modifications to the existing LPI-based calibrations of severity scale of liquefaction-induced failures and to propose a new one if it is needed, using box–whisker plot method. The data set that was analyzed is compiled by borings with SPT from liquefied and non-liquefied sites in Taiwan, in Turkey and in Greece.Moreover, a LPI-based formula is proposed, as an outcome from the application of logistic regression analysis to the data set, which can be used for predicting the liquefaction surface occurrence. In addition, a discriminant function is suggested for classifying a site as liquefiable or not. In this approach, the LPI and the thickness of the non-liquefiable cap layer are taken into account as the classification parameters. 相似文献
952.
George H. Shaw 《Chemie der Erde / Geochemistry》2008,68(3):235-264
The chemistry of Earth's atmosphere during its first 2–2.5 Ga bears on several branches of geoscience including the origin of prebiotic molecules and life itself, early surface processes, the “faint young sun” problem, carbon isotope systematics, and the transition to an oxidized surface. The geologic record, as sparse as it is for this era, presents several difficulties for attempts to model the atmosphere and its changes through time. The prevailing view for the past 50 years has centered around a moderately oxidized atmosphere of CO2 and N2, and most modeling efforts have been directed at reconciling geologic data, and atmospheric and chemical constraints, with such a composition. Improvements in modeling of early Earth processes and increased knowledge of Archean geology, including new geochemical methods and data, have largely helped support this view of the early atmosphere over the last 25 years, but have also left several nagging questions unanswered. How was a sufficient reservoir (and concentration) of prebiotic molecules produced? What were the major reservoirs for carbon, and how did they develop their isotopic signatures? Is there a solution to the problem of the “faint young sun”? Why was surface oxidation delayed following the advent of oxygenic photosynthesis? Lately, some attempts at answering these questions have suggested the importance of more reducing capacity at the early Earth's surface, but without abandoning the idea of a mainly CO2–N2 atmosphere. It may be that returning to ideas of the early atmosphere current during the 1940s and earliest 1950s could help resolve some of these problems. Such an approach may not only be consistent with the atmospheres of the other terrestrial planets, but may help answer significant questions about the surface history of Mars. 相似文献
953.
Someshwar Das Raghavendra Ashrit Gopal Raman Iyengar Saji Mohandas M. Das Gupta John P. George E. N. Rajagopal Surya Kanti Dutta 《Journal of Earth System Science》2008,117(5):603-620
Performance of four mesoscale models namely, the MM5, ETA, RSM and WRF, run at NCMRWF for short range weather forecasting has been examined during monsoon-2006. Evaluation is carried out based upon comparisons between observations and day-1 and day-3 forecasts of wind, temperature, specific humidity, geopotential height, rainfall, systematic errors, root mean square errors and specific events like the monsoon depressions.It is very difficult to address the question of which model performs best over the Indian region? An honest answer is ‘none’. Perhaps an ensemble approach would be the best. However, if we must make a final verdict, it can be stated that in general, (i) the WRF is able to produce best All India rainfall prediction compared to observations in the day-1 forecast and, the MM5 is able to produce best All India rainfall forecasts in day-3, but ETA and RSM are able to depict the best distribution of rainfall maxima along the west coast of India, (ii) the MM5 is able to produce least RMSE of wind and geopotential fields at most of the time, and (iii) the RSM is able to produce least errors in the day-1 forecasts of the tracks, while the ETA model produces least errors in the day-3 forecasts. 相似文献
954.
D.J. Mossman W.E.L. Minter A. Dutkiewicz D.K. Hallbauer S.C. George Q. Hennigh T.O. Reimer F.D. Horscroft 《Precambrian Research》2008,164(3-4):173-186
In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes. 相似文献
955.
Thomas K. Kelty An Yin Batulzii Dash George E. Gehrels Angela E. Ribeiro 《Tectonophysics》2008,451(1-4):290-311
Understanding the development of the Central Asian Orogenic System (CAOS), which is the largest Phanerozoic accretionary orogen in the world, is critical to the determination of continental growth mechanisms and geological history of central Asia. A key to unraveling its geological history is to ascertain the origin and tectonic setting of the large flysch complexes that dominate the CAOS. These complexes have been variably interpreted as deep-marine deposits that were accreted onto a long-evolving arc against large continents to form a mega-accretionary complex or sediments trapped in back-arc to fore-arc basins within oceanic island-arc systems far from continents. To differentiate the above models we conducted U–Pb geochronological analyses of detrital-zircon grains from turbidites in the composite Hangay–Hentey basin of central Mongolia. This basin was divided by a Cenozoic fault system into the western and eastern sub-basins: the Hangay Basin in the west and Hentey basin in the east. This study focuses on the Hentey basin and indicates two groups of samples within this basin: (1) a southern group that were deposited after the earliest Carboniferous ( 339 Ma to 354 Ma) and a northern group that were deposited after the Cambrian to Neoproterozoic ( 504 Ma to 605 Ma). The samples from the northern part of the basin consistently contain Paleoproterozoic and Archean zircon grains that may have been derived from the Tuva–Mongol massif and/or the Siberian craton. In contrast, samples from the southern part of the basin contain only a minor component of early Paleozoic to Neoproterozoic zircon grains, which were derived from the crystalline basement bounding the Hangay–Hentey basin. Integrating all the age results from this study, we suggest that the Hangay–Hentey basin was developed between an island-arc system with a Neoproterozoic basement in the south and an Andean continental-margin arc in the north. The initiation of the southern arc occurred at or after the early Carboniferous, allowing accumulation of a flysch complex in a long-evolving accretionary complex. 相似文献
956.
957.
Peripheral communities of the Eastern Lau Spreading Center and Valu Fa Ridge: community composition,temporal change and comparison to near‐vent communities 下载免费PDF全文
Arunima Sen Stacy Kim Alex J. Miller Kyle J. Hovey Stéphane Hourdez George W. Luther III Charles R. Fisher 《Marine Ecology》2016,37(3):599-617
Western Pacific hydrothermal vents will soon be subjected to deep‐sea mining and peripheral sites are considered the most practical targets. The limited information on community dynamics and temporal change in these communities makes it difficult to anticipate the impact of mining activities and recovery trajectories. We studied community composition of peripheral communities along a cline in hydrothermal chemistry on the Eastern Lau Spreading Center and Valu Fa Ridge (ELSC‐VFR) and also studied patterns of temporal change. Peripheral communities located in the northern vent fields of the ELSC‐VFR are significantly different from those in the southern vent fields. Higher abundances of zoanthids and anemones were found in northern peripheral sites and the symbiont‐containing mussel Bathymodiolus brevior, brisingid seastars and polynoids were only present in the northern peripheral sites. By contrast, certain faunal groups were seen only in the southern peripheral sites, such as lollipop sponges, pycnogonids and ophiuroids. Taxonomic richness of the peripheral communities was similar to that of active vent communities, due to the presence of non‐vent endemic species that balanced the absence of species found in areas of active venting. The communities present at waning active sites resemble those of peripheral sites, indicating that peripheral species can colonize previously active vent sites in addition to settling in the periphery of areas of venting. Growth and mortality were observed in a number of the normally slow‐growing cladorhizid stick sponges, indicating that these animals may exhibit life history strategies in the vicinity of vents that differ from those previously recorded. A novel facultative association between polynoids and anemones is proposed based on their correlated distributions. 相似文献
958.
Richard?M.?IversonEmail authorView authors OrcID profile David?L.?George 《Acta Geotechnica》2016,11(6):1465-1468
A paper recently published by Bartelt and Buser (hereafter identified as “the authors”) aims to clarify relationships between granular dilatancy and dispersive pressure and to question the effective stress principle and its application to shallow granular avalanches (Bartelt and Buser in Act Geotech 11:549–557, 2). The paper also criticizes our own recent work, which utilizes the concepts of evolving dilatancy and effective stress to model the initiation and dynamics of water-saturated landslides and debris flows. Here we first explain why we largely agree with the authors’ views of dilatancy and dispersive pressure as they apply to depth-integrated granular avalanche models, and why we disagree with their views of effective stress and pore-fluid pressure. We conclude by explaining why the authors’ characterization of our recently developed D-Claw model is inaccurate. 相似文献
959.
960.
The presence of clouds can restrict the potential uses of remote sensing satellite imagery in extracting information and interpretation. Automatic detection and removal of clouds which hide significant information in the image is an important task in remote sensing. Hence, our aim is to detect clouds and restore the missing information in order to make the image ready for further analysis and applications. Due to the difference in nature and appearance, thick and thin clouds are dealt separately. Thick cloud is detected using an efficient Fuzzy C-Means (FCM) clustering algorithm, while thin cloud is detected using a simple region growing technique. In order to reconstruct the missing pixels, we utilize the prior knowledge about the statistics of the specific image class. Kernel principal component analysis (KPCA)-based image model is obtained using a set of training images. Missing area in the image is restored after an iterative projection operation and gradient descent algorithm. In short, an image lying out of the modelled image space is iteratively modified to obtain the restored image and that would be in the image space according to the obtained nonlinear low-dimensional and sparse KPCA image model. To illustrate the performance of the proposed method, a thorough experimental analysis on FORMOSAT multi-spectral images is done using MATLAB platform. When compared to the two recent existing techniques, our proposed method is superior and makes a promising tool for thick and thin cloud removal in multi-spectral satellite images. 相似文献