全文获取类型
收费全文 | 1122篇 |
免费 | 284篇 |
国内免费 | 213篇 |
专业分类
测绘学 | 111篇 |
大气科学 | 101篇 |
地球物理 | 401篇 |
地质学 | 579篇 |
海洋学 | 221篇 |
天文学 | 111篇 |
综合类 | 31篇 |
自然地理 | 64篇 |
出版年
2024年 | 5篇 |
2023年 | 11篇 |
2022年 | 35篇 |
2021年 | 28篇 |
2020年 | 29篇 |
2019年 | 52篇 |
2018年 | 50篇 |
2017年 | 42篇 |
2016年 | 44篇 |
2015年 | 41篇 |
2014年 | 52篇 |
2013年 | 63篇 |
2012年 | 41篇 |
2011年 | 47篇 |
2010年 | 64篇 |
2009年 | 76篇 |
2008年 | 72篇 |
2007年 | 75篇 |
2006年 | 69篇 |
2005年 | 80篇 |
2004年 | 65篇 |
2003年 | 68篇 |
2002年 | 47篇 |
2001年 | 41篇 |
2000年 | 55篇 |
1999年 | 41篇 |
1998年 | 36篇 |
1997年 | 31篇 |
1996年 | 34篇 |
1995年 | 35篇 |
1994年 | 33篇 |
1993年 | 23篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 19篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1619条查询结果,搜索用时 15 毫秒
991.
M. D.?AlbaEmail author M. A.?Castro M.?Naranjo A. C.?Perdigón 《Physics and Chemistry of Minerals》2004,31(4):195-202
The main objective of this paper is to apply an NMR-based methodology to clay minerals that provides resolution enhancement in 27Al NMR spectra, permitting the differentiation of aluminum ions with one same coordination number, but with different chemical environments. For this purpose, we have performed two-dimensional 1H27Al cross-polarization experiments, which facilitate the identification of the different aluminum crystallographic sites by means of the nature of the proton polarization source. For the development of this methodology, we have carried out a systematic study of a set of well-characterized 2:1 phyllosilicates with aluminum in different crystallographic sites. The structural locations of aluminum in the selected layered silicates have been established unequivocally. Once the correlation between all different aluminum and proton sites in smectite was well established, an unknown material was examined, the utility of this methodology being illustrated. This methodology can be applied to a great variety of materials, such as zeolites, ALPOs and mesoporous aluminosilicates, in which a precise determination of the aluminum location is demanded. 相似文献
992.
Patrizio Signanini Vittorio D'Intinosante Maurizio Ferrini Mario L. Rainone 《Geotechnical and Geological Engineering》2004,22(2):227-244
In the seismic microzonation the evaluation of the local amplification issue may be solved using different methodological
approaches. Taking the hint from the studies made within VEL project framework, sponsored by Tuscany Region on Fivizzano's
area we are herewith comparing the results of geotechnical and geophysical integrated multidisciplinary studies with the existing
data available from previous studies, mainly using a macroseismic approach, on the same area. This qualitative method does
not seem to be able to make any forecasting on the scale whereas generally microzoning is performed. On the contrary, the
knowledge of the geological-technical model of the investigated area, obtained by punctual multidisciplinary integrated studies,
allows a direct collection of the data to insert in input step of numerical modelling, aimed to local effects evaluation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
993.
Changes expected in Mexico City's seismic response due to land subsidence caused by groundwater extraction are examined. Settlements of the land surface observed in the recent past are compiled and processed to predict regional subsidence in the near future. This information is used in combination with the iso-depth and iso-period maps stipulated by the building code to estimate the compaction of surface sediments and the shortening of soil periods. The time-progressive evolution of the official seismic zoning maps and the associated site-specific response spectra is predicted. The same is done with spectral response contours expressed in terms of the site and structure periods. The subsidence effects are considered through an impact factor, which represents the increase or decrease in the spectral ordinates with respect to the current values specified for design purposes. The detrimental or beneficial effects depend on the relative values of the structure and site periods. 相似文献
994.
Based on the formulation of a multiple non-isotropic scattering process, a characteristic source time is introduced to define the initial impulse width of energy density at the source. An analytical expression of the initial intensity spectral density of a seismic wave is incorporated into the integral equation of seismic wave energy density. And, a recursive formula of Green’s function is derived to obtain the higher order Green’s function, which is included to describe the stronger non-isotropic scattering process. Then, the effect of the scattering pattern on the energy density envelope is investigated by the modified scattering theory. Significant differences are found in the decay of the energy density envelopes with distances using different scattering patterns. The envelope synthesized by the forward dominated scattering pattern is larger than the results obtained by the isotropic and backward dominated scattering pattern. Different scattering patterns are also used to fit the observation data from the aftershocks of the 2008 Wenchuan earthquake. It is concluded that the envelopes synthesized by the forward scattering pattern can match the data better than the isotropic and backward dominated scattering cases, and a new interpretation of the coda wave is given. Finally, using the forward dominated scattering pattern, the envelope broadening of the observed data is reproduced. 相似文献
995.
Response parameters used to estimate nonstructural damage differ depending on whether deformation‐sensitive or acceleration‐sensitive components are considered. In the latter case, seismic demand is usually represented through floor spectra, that is response spectra in terms of pseudo‐acceleration, which are calculated at the floor levels of the structure where the nonstructural components are attached to. Objective of this paper is to present a new spectrum‐to‐spectrum method for calculating floor acceleration spectra, which is able to explicitly account for epistemic uncertainties in the modal properties of the supporting structure. By using this method, effects on the spectra of possible variations from nominal values of the periods of vibration of the structure can be estimated. The method derives from the extension of closed‐form equations recently proposed by the authors to predict uniform hazard floor acceleration spectra. These equations are built to rigorously account for the input ground motion uncertainty, that is the record‐to‐record variability of the nonstructural response. In order to evaluate the proposed method, comparisons with exact spectra obtained from a standard probabilistic seismic demand analysis, as well as spectra calculated using the Eurocode 8 equation, are finally shown. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
996.
土层结构对反应谱平台值的影响 总被引:6,自引:5,他引:6
本文利用目前工程上广泛应用的场地地震反应的一维等效线性化波动方法,计算了不同场地模型在三种不同强度的地震动输入下的地表加速度反应谱。为了研究不同的土层结构对反应谱平台值的影响,本文将计算得到的地表加速度反应谱拟合成建筑抗震设计规范规定的设计反应谱的形式,分析对比了不同的覆盖层厚度、软弱土层对反应谱平台值的影响,得到了一些有价值的研究成果。 相似文献
997.
This paper concerns the design of passive base isolation systems characterized by a bilinear hysteretic behaviour. The study refers to the case where the structure to be isolated (superstructure) vibrates according to the first mode. In this case the whole isolated structure can be modelled by a two‐degree‐of‐freedom system. The base isolation effectiveness has been evaluated for different characteristics of the device, namely mass, strength, elastic and plastic stiffness, by using mainly energetic quantities. The optimum values for the base device have been obtained by minimizing the input energy and the displacement of the superstructure. Conclusions are drawn for superstructures with a fundamental period of 0.5s, a damping ratio of 5% and for three different kinds of earthquake ground motions. The study showed that the seismic input greatly affects the behaviour of the isolated structure, and therefore the design ground motion must be carefully chosen, dependent on the characteristics of the site. A simple procedure that involves mainly linear dynamic analyses is proposed for the design of base devices used in conjunction with superstructures of any fundamental vibration period. The procedure produces good results in spite of its simplicity, and therefore it is suitable for practical use by design engineers. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
998.
CT系统的能谱估计及射束硬化校正算法 总被引:1,自引:0,他引:1
本文给出对CT系统X射线能谱的估计算法和单成分被测物体CT成像的射束硬化校正算法, 并给出和分析了数值模拟结果。 相似文献
999.
1000.
In spite of important differences in structural response to near‐fault and far‐fault ground motions, this paper aims at extending well‐known concepts and results, based on elastic and inelastic response spectra for far‐fault motions, to near‐fault motions. Compared are certain aspects of the response of elastic and inelastic SDF systems to the two types of motions in the context of the acceleration‐, velocity‐, and displacement‐sensitive regions of the response spectrum, leading to the following conclusions. (1) The velocity‐sensitive region for near‐fault motions is much narrower, and the acceleration‐sensitive and displacement‐sensitive regions are much wider, compared to far‐fault motions; the narrower velocity‐sensitive region is shifted to longer periods. (2) Although, for the same ductility factor, near‐fault ground motions impose a larger strength demand than far‐fault motions—both demands expressed as a fraction of their respective elastic demands—the strength reduction factors Ry for the two types of motions are similar over corresponding spectral regions. (3) Similarly, the ratio um/u0 of deformations of inelastic and elastic systems are similar for the two types of motions over corresponding spectral regions. (4) Design equations for Ry (and for um/u0) should explicitly recognize spectral regions so that the same equations apply to various classes of ground motions as long as the appropriate values of Ta, Tb and Tc are used. (5) The Veletsos–Newmark design equations with Ta=0.04 s, Tb=0.35 s, and Tc=0.79 s are equally valid for the fault‐normal component of near‐fault ground motions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献