全文获取类型
收费全文 | 12289篇 |
免费 | 412篇 |
国内免费 | 401篇 |
专业分类
测绘学 | 364篇 |
大气科学 | 1114篇 |
地球物理 | 2894篇 |
地质学 | 4884篇 |
海洋学 | 813篇 |
天文学 | 2382篇 |
综合类 | 111篇 |
自然地理 | 540篇 |
出版年
2022年 | 91篇 |
2021年 | 126篇 |
2020年 | 125篇 |
2019年 | 116篇 |
2018年 | 385篇 |
2017年 | 339篇 |
2016年 | 496篇 |
2015年 | 318篇 |
2014年 | 495篇 |
2013年 | 696篇 |
2012年 | 474篇 |
2011年 | 576篇 |
2010年 | 511篇 |
2009年 | 689篇 |
2008年 | 532篇 |
2007年 | 455篇 |
2006年 | 447篇 |
2005年 | 417篇 |
2004年 | 374篇 |
2003年 | 343篇 |
2002年 | 327篇 |
2001年 | 299篇 |
2000年 | 277篇 |
1999年 | 251篇 |
1998年 | 260篇 |
1997年 | 222篇 |
1996年 | 228篇 |
1995年 | 195篇 |
1994年 | 178篇 |
1993年 | 139篇 |
1992年 | 112篇 |
1991年 | 123篇 |
1990年 | 115篇 |
1989年 | 122篇 |
1988年 | 85篇 |
1987年 | 142篇 |
1986年 | 97篇 |
1985年 | 99篇 |
1984年 | 113篇 |
1983年 | 123篇 |
1982年 | 113篇 |
1981年 | 103篇 |
1980年 | 88篇 |
1979年 | 93篇 |
1978年 | 68篇 |
1977年 | 85篇 |
1976年 | 75篇 |
1975年 | 79篇 |
1974年 | 68篇 |
1973年 | 80篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
Metric and decimetric type III bursts and microwave spike emissions with negative and positive frequency drift rates which were observed with radio spectrometers at Yunnan and Beijing Observatories are presented. The frequencies and heights at which the bidirectional electron beams originated are estimated. Three events reveal a separatrix frequency (at 250, 1300, and 2900 MHz) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both upward and downward directions. These cases may indicate that the changeover frequencies of bidirectional electron beams are within a large band from 250 to 2900 MHz and the frequency bands of separatrices are in very small (4 to 100 MHz) and different bands. These type III bursts appear to be a plasma emission phenomenon from a beam of electrons which seem to have widely separated acceleration regions from the high to the low corona. These cases suggest that current sheets that separate open and closed magnetic fluxes in the low corona, and oppositely directed open field lines in the high corona are possible sites for bidirectional electron acceleration. The regions of magnetic topology from closed to open magnetic field structures should be very large (from about 20000 to 107000 km above the photosphere). 相似文献
992.
The Control Of Coherent Eddies In Vegetation Canopies: Streamwise Structure Spacing, Canopy Shear Scale And Atmospheric Stability 总被引:9,自引:9,他引:0
An analogy has been established between a plane mixing layer and the atmospheric flow near the top of a vegetation canopy. It is based on a common feature, a strong inflection in the mean velocity profile, responsible for hydrodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy was tested using a small data set from thirteen experiments, all in near-neutral conditions. It provided a good prediction of the streamwise spacing w of the dominant canopy eddies (evaluated from time series of vertical velocity) that appears to depend on a shear length scale Ls = U(h)/U'(h), where h is canopy height, U is mean velocity and U' the vertical gradient dU/dz. The present analysis utilizes an extensive data set of approximately 700 thirty-minute runs, from six experiments on two forest sites and a maize crop, with a large range of stability conditions. w was estimated for each run using the wavelet transform as an objective, automated detection method. First, the variations of w and Ls with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatter whereas in stable conditions both variables decrease with increasing stability. It is subsequently found that w is directly related to Ls, in a way close to the neutral prediction w /h = 8.1Ls/h.The Strouhal number Str = Ls /w is then shown to vary with atmospheric stability, weakly in unstable conditions, more significantly in stable conditions. Altogether these results suggest that, to some extent, the plane mixing-layer analogy can be extended to non-neutral conditions. It is argued that the primary effect of atmospheric stability, at least in stable conditions, is to modify the shear length scale Ls through changes in U(h) and U'(h), which in turn determines the streamwise spacing of the active, coherent motions. 相似文献
993.
Hofmann Y. Jahr T. Jentzsch G. Bankwitz P. Bram K. 《Studia Geophysica et Geodaetica》2000,44(4):608-610
The Vogtland and NW Bohemia are characterized geoscientifically by periodically occurrence of swarm earthquakes. The basic geophysical mechanism is not yet sufficiently clarified, just like detail questions to geology in especially the deeper underground. Complex geophysical investigations in the seismoactive region indicate geodynamic phenomena like mass redistribution or stress accumulation and release (Spiák et al., 1998). According to Grünthal (1989) a weakness zone is suggested in the region of the swarm earthquakes. This zone can be caused by fluid-tectonics (Kämpf et al., 1992), a mantle plume (pers. com. J. Svancara, 1999) and/or by the geometry of the geological structures (Neunhöfer & Güth, 1988). A three-dimensional gravimetric model can clear up the underground situation. By means of high-resolution gravimetry a three-dimensional model will be developped for the Vogtland and NW Bohemia region. In the first step a homogeneous Bouguer map of the Vogtland and NW Bohemia was created (fig. 1) containing gravity structures analysed by Ibrmajer & Suk (1989) and Blízkovsky et al. (1985). The used gravimetric data were made available by the Saxonian National Office for Environment and Geology, by the Czech Geological Survey, Prague and by the GGA Hannover.
In the context with the interpretation of the deep-seismic profile MVE 90 a two-dimensional gravimetric modeling was carried out (Behr et al., 1994), too. Anomaly-producing source bodies apparently do not offer themselves in a two-dimensional model, because after Jung (1961) the length of a gravimetric source structure must be about four times larger than it's width. The technique of the three-dimensional gravimetric modeling by means of any polyhedrons was developed by Götze (1976, 1984). Gravimetry is a potential method and supplies an infinite number of solutions, so the model has to be developed close to other geoscientific results. The aim is to construct a high-resolution three-dimensional underground model, which includes the upper earth's crust and the deep-seated structures of the middle and lower crust, too. The determination of the mass distribution in the underground supplies contradicting or supporting facts for geodynamic views in the Vogtland and NW Bohemia for example of Bankwitz et al. (1993). The interpretation of the Bouguer map of the Vogtland and a three-dimensional gravimetric model ought to contribute a substantial, also geodynamic part to understand the origin and the emergence of the swarm earthquakes in this region. 相似文献
994.
Simple straightforward methods are applied to testtheir ability to detect the non-linear response of thesoil. Recordings of the main shock and aftershocks ofthe 1995, Hyogo-ken Nanbu (Kobe) earthquake are used.Non-linear effects are investigated using twodifferent techniques, on a collection of data for 12sites situated on different geological structures inthe Kobe and Osaka areas. The first method used is theso-called receiver functions technique (Langston,1979), which consists of computing the spectral ratiobetween horizontal and vertical components of motion.This ratio has been shown to reveal the fundamentalfrequency of a site (Lachet and Bard, 1994; Lachet etal., 1996; Theodulidis et al., 1995, 1996). For eachsite, recordings of the main shock and a set ofaftershocks are considered. The variation of thisspectral ratio for different values of the maximumacceleration recorded at a site is investigated. Bothvariations of the amplitude of the H/V ratio (due tonon-linear behavior, on the horizontal components inparticular) and of the frequency position of theamplified band-width are observed. The secondtechnique used in this study is related to thevariation of the high frequency content of therecordings during the main-shock and its aftershocks.The high frequency spectral decay of the motion,characterized by parameter, is assumed to berelated mainly to the near-surface attenuation. Itshould then increase with increasing peak velocity, incase of non-linearity. The value of kappa iscalculated for the 12 sites in the Kobe area, fordifferent types of soil conditions, and againdifferent values of peak ground acceleration.Variations of kappa are then related to non-linearbehavior of the soil during the Kobe earthquake. 相似文献
995.
The Kamchatka peninsula, located in the far east of Russia, is a geologically active margin where the Pacific plate subducts beneath the North American and Eurasia plates. This area is characterised by frequent and strong seismic activity (magnitudes up to 8.5) and epicentres are generally distributed offshore along the eastern coast of the peninsula. For many years, hydrogeochemicals have been collected with a mean sampling frequency of three days in the form of the flow rate and the most common ions and gases in the groundwater of three deep wells in the southern area of the Kamchatka peninsula, where the capital city Petropavlovsk is located. Beginning in 1988, five earthquakes with M > 6.5 occurred in this area. These earthquakes were powerful enough to be considered as potential precursor sources in the sense that the stresses and strains building up before them might be expected to cause precursory activity. In order to reveal any possible precursors of these earthquakes, we analysed the hydrogeochemical data collected. We considered any signal having an amplitude three times the standard deviation to be an irregularity and we defined as an anomaly the existence of an irregularity occurring simultaneously in more than one parameter at each well. Then, on the basis of the worldwide past results and the time recurrence of the quoted earthquakes, we chose 158 days as the maximum temporal window between a possible anomaly and the subsequent earthquake. We identified some premonitory anomalies in hydrogeochemical parameters at different wells. On the basis of these results some earthquake forecasting criteria in southern Kamchatka may be tentatively formulated 相似文献
996.
An interval-parameter multi-stage stochastic programming model for water resources management under uncertainty 总被引:1,自引:0,他引:1
In this study, an interval-parameter multi-stage stochastic linear programming (IMSLP) method has been developed for water resources decision making under uncertainty. The IMSLP is a hybrid methodology of inexact optimization and multi-stage stochastic programming. It has three major advantages in comparison to the other optimization techniques. Firstly, it extends upon the existing multi-stage stochastic programming method by allowing uncertainties expressed as probability density functions and discrete intervals to be effectively incorporated within the optimization framework. Secondly, penalties are exercised with recourse against any infeasibility, which permits in-depth analyses of various policy scenarios that are associated with different levels of economic consequences when the promised water-allocation targets are violated. Thirdly, it cannot only handle uncertainties through constructing a set of scenarios that is representative for the universe of possible outcomes, but also reflect dynamic features of the system conditions through transactions at discrete points in time over the planning horizon. The developed IMSLP method is applied to a hypothetical case study of water resources management. The results are helpful for water resources managers in not only making decisions of water allocation but also gaining insight into the tradeoffs between environmental and economic objectives. 相似文献
997.
This paper focuses on the attribute weight issue and advocates use of modi?able attribute weights in terrain‐based environmental analysis and classi?cation. A question was asked: ‘How much will the result of a terrain‐based environmental analysis be affected if the weights of used terrain attributes are changed?’ The literature on landform classi?cation and the fuzzy k‐means method was reviewed in particular to help clarify the background and importance of this weight assignment issue. As an example, the effects of modifying attribute weights were evaluated for fuzzy k‐means landform classi?cation in a case study area. A total of 102 classi?cations were compared with each other and with a soil map, and comparison methods were speci?cally designed to evaluate the differences between these classi?cations. The results show that fuzzy k‐means landform classi?cation is sensitive to weight adjustments of adopted terrain attributes. The sensitivity is particularly high when the attribute weights started to be tuned away from the standard (i.e. uniform) weight of one. Better matching between landform classi?cation and a soil map may be produced when attribute weights are tuned. In all, we advocate the widespread adoption of an exploratory attitude in assigning attribute weights for environmental analysis and classi?cation. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
998.
高原地表的感热和潜热通量在亚洲季风系统中有很重要的作用。由于高原地域辽阔,且自然环境较严酷,不利于建立完善的地面观测系统。因此,卫星遥感观测就成为测算高原整体感热和潜热通量的有效工具。地面场地的观测结果作为地表通量的真实值,对于卫星遥感测算是非常重要的。它也为构建陆面—大气模型提供了科学依据,是卫星资料的资料同化系统中的重要组成部分。
计算场地热量通量有几种不同的处理方法。最简单的方法利用有效的观测和试验的参数,可以给出稳定连续的估计。愈精确的Bowen比或者廓线的观测能给出愈精确的信息。综合了湍流测量及辐射测量、土壤热通量的观测结果的估计对陆面—大气相互作用进行了详细的描述,以适应模式的发展。从1998年开始,这些方法联合应用到青藏高原;场地通量观测方面的成果以及目前对其理解将在本文中做一概述。 相似文献
999.
Oliver Montenbruck Christoph Günther Sebastian Graf Miquel Garcia-Fernandez Johann Furthner Hanspeter Kuhlen 《GPS Solutions》2006,10(2):146-153
In late December 2005 the GIOVE-A test satellite was launched by the European Space Agency (ESA) to secure the frequencies for the Galileo system and to provide a platform for testing the new navigation signals. We performed an initial assessment of these signals using the 30 m deep space antenna of the DLR ground station in Weilheim (Germany). The antenna gain raised the signals above the noise level, thus allowing a detailed analysis even without knowledge of the ranging codes. The present paper covers the analysis of the L1/E1 signals, which includes a discussion of the spectrum, the time domain signal and a decoding of the spreading codes for the Open Service. 相似文献
1000.
A four-component decomposition scheme of the coherency matrix is presented here for the analysis of polarimetric synthetic aperture radar (SAR) images. The coherency matrix is used to deal with nonreflection symmetric scattering case, which is an extension of covariance matrix approach. The same decomposition results have been obtained. The advantage of this approach is explicit expressions of four scattering powers in terms of scattering matrix elements, which serve the interpretation of polarimetric SAR data quantitatively. 相似文献