全文获取类型
收费全文 | 68133篇 |
免费 | 993篇 |
国内免费 | 438篇 |
专业分类
测绘学 | 1432篇 |
大气科学 | 3938篇 |
地球物理 | 13422篇 |
地质学 | 25215篇 |
海洋学 | 6218篇 |
天文学 | 15890篇 |
综合类 | 174篇 |
自然地理 | 3275篇 |
出版年
2022年 | 599篇 |
2021年 | 924篇 |
2020年 | 1037篇 |
2019年 | 1163篇 |
2018年 | 2247篇 |
2017年 | 2122篇 |
2016年 | 2324篇 |
2015年 | 1084篇 |
2014年 | 2163篇 |
2013年 | 3476篇 |
2012年 | 2439篇 |
2011年 | 3098篇 |
2010年 | 2805篇 |
2009年 | 3328篇 |
2008年 | 2993篇 |
2007年 | 3171篇 |
2006年 | 2874篇 |
2005年 | 1801篇 |
2004年 | 1725篇 |
2003年 | 1779篇 |
2002年 | 1603篇 |
2001年 | 1507篇 |
2000年 | 1318篇 |
1999年 | 1060篇 |
1998年 | 1082篇 |
1997年 | 1121篇 |
1996年 | 879篇 |
1995年 | 874篇 |
1994年 | 789篇 |
1993年 | 698篇 |
1992年 | 653篇 |
1991年 | 687篇 |
1990年 | 731篇 |
1989年 | 634篇 |
1988年 | 601篇 |
1987年 | 640篇 |
1986年 | 579篇 |
1985年 | 776篇 |
1984年 | 813篇 |
1983年 | 802篇 |
1982年 | 741篇 |
1981年 | 680篇 |
1980年 | 655篇 |
1979年 | 590篇 |
1978年 | 597篇 |
1977年 | 574篇 |
1976年 | 503篇 |
1975年 | 514篇 |
1974年 | 495篇 |
1973年 | 554篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
131.
132.
Eight-year-old Loblolly pine (Pinus taeda) plots located in South Carolina were evaluated using high resolution aircraft multispectral scanner data to determine if biomass could be accurately measured. Sixteen pine plots located on both sandy and clay soils were treated with 0, 180, or 360 kg. (approximately 0, 400, or 800 lb.) of nitrogen per plot. Indices of biomass from remote sensing data were significantly correlated with in situ biomass measurements made in each plot. A ratio of infrared (.9–1.1 μm) and red (.65–.70 μm) channels yielded the best correlation. The indices were not sensitive to differences in soil type (sandy or clay). 相似文献
133.
The Ivrea-Verbano Zone in northern Italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the Alpine orogeny. Recent and on-going structurally-oriented geological mapping in this region is providing new information about the geometry of the complex. The central part of the zone is dominated by a large basic complex (the 'mafic formation') which is intrusive into the surrounding gneisses. The foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning, but in the region south-west of Monte Capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c. 10 km. This fold locally inverts the stratigraphy of the layered basic group of the complex, and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust.
Several high-temperature shear zones have now been traced within the country rock for distances up to 20 km. The geometry of these, and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body.
Close parallels can be drawn between the observed structure in the Ivrea-Verbano Zone (after removing the effects of late, low-temperature faulting and folding related to emplacement of the rocks into their present position), and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the US Basin and Range province. 相似文献
Several high-temperature shear zones have now been traced within the country rock for distances up to 20 km. The geometry of these, and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body.
Close parallels can be drawn between the observed structure in the Ivrea-Verbano Zone (after removing the effects of late, low-temperature faulting and folding related to emplacement of the rocks into their present position), and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the US Basin and Range province. 相似文献
134.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
135.
The main analytical properties of the generalized Shkarofsky function and a numerical code for its computation are discussed. The results of a numerical analysis are compared with the results of an asymptotic analysis for parameter values relevant to the problem of whistler-mode propagation in the Earth's magnetosphere. This comparison allows us to specify the range of applicability of different approximations to the generalized Shkarofsky function, which have been used for the analysis of relativistic effects on whistler-mode propagation and instability. 相似文献
136.
Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH4). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
137.
138.
Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations. 相似文献
139.
Arnaud M. Aubourg E. Bareyre P. Br';ehin S. Caridroit R. de Kat J. Dispau G. Djidi K. Gros M. Lachièze-Rey M. Laigneau Y. Laurent B. Lesquoy E. Lavocat Ph. Magneville C. Mazeau B. Milsztajn A. Moscoso L. Pasquaud J. Paul B. Perrin P. Petibon J. Piret Y. Queinnec F. Rich J. Spiro M. de Trogoff J. Vigroux L. Zylberajch S. Ansari R. Cavalier F. Moniez M. Beaulieu J. P. Ferlet R. Grison Ph. Vidal-Madjar A. Adrianzyk G. Berger J. P. Burnage R. Delclite J. C. Kohler D. Magnan R. Richaud A. Guibert J. Moreau O. Tajahmady F. Baranne A. Maurice E. Prévôt L. Gry C. 《Experimental Astronomy》1994,4(3-4):265-278
A 20cm
2
CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 23×23 µm
2
pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO). 相似文献
140.
Seven supracratonic, Proterozoic basins, occupying more than a fifth of the Precambrian exposures in the Indian Peninsula, comprise the Purana basins. A comprehensive review of the current status of knowledge of these voluminous orthoquartzite-carbonate-shale suites in the context of their contemporary lithostratigraphy, depositional environments and structural disposition is presented. Stromatolite biostratigraphy and available geochronological data are compared, to discern their age limits.
These basins contain perhaps one of the most elaborate records of Middle to Late Proterozoic (Riphean-Vendian) sedimentation preserved in an unmetamorphosed and only slightly deformed state. Further sedimentological and structural studies could lead to a better understanding of the Proterozoic craton-margin processes. Their close association with the Middle Proterozoic Mobile Belt of peninsular India is that of two contrasting tectonic regimes, contemporaneously adjoining each other. The existing lithostratigraphic classifications of many of these sequences may not stand the test of process - response considerations as demonstrated by the recent revisions in the stratigraphy of the Cuddapah and Bhima basins. The prolific stromatolitic, micro-organic and trace-fossil communities preserved in them require much more detailed, but cautious study, and may yield information on the Riphean-Vendian biota. However, these studies must be undertaken in association with elaborate geochronological determinations which are sparse at present.
Inadequacy of the existing knowledge of these basins is highlighted, with the view of inviting the attention of the geological community to these unique basins from peninsular India. 相似文献
These basins contain perhaps one of the most elaborate records of Middle to Late Proterozoic (Riphean-Vendian) sedimentation preserved in an unmetamorphosed and only slightly deformed state. Further sedimentological and structural studies could lead to a better understanding of the Proterozoic craton-margin processes. Their close association with the Middle Proterozoic Mobile Belt of peninsular India is that of two contrasting tectonic regimes, contemporaneously adjoining each other. The existing lithostratigraphic classifications of many of these sequences may not stand the test of process - response considerations as demonstrated by the recent revisions in the stratigraphy of the Cuddapah and Bhima basins. The prolific stromatolitic, micro-organic and trace-fossil communities preserved in them require much more detailed, but cautious study, and may yield information on the Riphean-Vendian biota. However, these studies must be undertaken in association with elaborate geochronological determinations which are sparse at present.
Inadequacy of the existing knowledge of these basins is highlighted, with the view of inviting the attention of the geological community to these unique basins from peninsular India. 相似文献