全文获取类型
收费全文 | 4497篇 |
免费 | 775篇 |
国内免费 | 265篇 |
专业分类
测绘学 | 149篇 |
大气科学 | 146篇 |
地球物理 | 789篇 |
地质学 | 1244篇 |
海洋学 | 221篇 |
天文学 | 2301篇 |
综合类 | 106篇 |
自然地理 | 581篇 |
出版年
2025年 | 4篇 |
2024年 | 81篇 |
2023年 | 84篇 |
2022年 | 100篇 |
2021年 | 101篇 |
2020年 | 106篇 |
2019年 | 159篇 |
2018年 | 93篇 |
2017年 | 105篇 |
2016年 | 110篇 |
2015年 | 118篇 |
2014年 | 164篇 |
2013年 | 162篇 |
2012年 | 187篇 |
2011年 | 192篇 |
2010年 | 123篇 |
2009年 | 367篇 |
2008年 | 321篇 |
2007年 | 406篇 |
2006年 | 379篇 |
2005年 | 314篇 |
2004年 | 292篇 |
2003年 | 277篇 |
2002年 | 211篇 |
2001年 | 185篇 |
2000年 | 152篇 |
1999年 | 145篇 |
1998年 | 180篇 |
1997年 | 73篇 |
1996年 | 69篇 |
1995年 | 53篇 |
1994年 | 45篇 |
1993年 | 47篇 |
1992年 | 27篇 |
1991年 | 14篇 |
1990年 | 20篇 |
1989年 | 17篇 |
1988年 | 8篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1971年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有5537条查询结果,搜索用时 0 毫秒
11.
C. Baccigalupi L. Bedini C. Burigana G. De Zotti A. Farusi D. Maino M. Maris F. Perrotta E. Salerno L. Toffolatti A. Tonazzini 《Monthly notices of the Royal Astronomical Society》2000,318(3):769-780
We implement an independent component analysis (ICA) algorithm to separate signals of different origin in sky maps at several frequencies. Owing to its self-organizing capability, it works without prior assumptions on either the frequency dependence or the angular power spectrum of the various signals; rather, it learns directly from the input data how to identify the statistically independent components, on the assumption that all but, at most, one of the components have non-Gaussian distributions.
We have applied the ICA algorithm to simulated patches of the sky at the four frequencies (30, 44, 70 and 100 GHz) used by the Low Frequency Instrument of the European Space Agency's Planck satellite. Simulations include the cosmic microwave background (CMB), the synchrotron and thermal dust emissions, and extragalactic radio sources. The effects of the angular response functions of the detectors and of instrumental noise have been ignored in this first exploratory study. The ICA algorithm reconstructs the spatial distribution of each component with rms errors of about 1 per cent for the CMB, and 10 per cent for the much weaker Galactic components. Radio sources are almost completely recovered down to a flux limit corresponding to ≃0.7 σCMB , where σ CMB is the rms level of the CMB fluctuations. The signal recovered has equal quality on all scales larger than the pixel size. In addition, we show that for the strongest components (CMB and radio sources) the frequency scaling is recovered with per cent precision. Thus, algorithms of the type presented here appear to be very promising tools for component separation. On the other hand, we have been dealing here with a highly idealized situation. Work to include instrumental noise, the effect of different resolving powers at different frequencies and a more complete and realistic characterization of astrophysical foregrounds is in progress. 相似文献
We have applied the ICA algorithm to simulated patches of the sky at the four frequencies (30, 44, 70 and 100 GHz) used by the Low Frequency Instrument of the European Space Agency's Planck satellite. Simulations include the cosmic microwave background (CMB), the synchrotron and thermal dust emissions, and extragalactic radio sources. The effects of the angular response functions of the detectors and of instrumental noise have been ignored in this first exploratory study. The ICA algorithm reconstructs the spatial distribution of each component with rms errors of about 1 per cent for the CMB, and 10 per cent for the much weaker Galactic components. Radio sources are almost completely recovered down to a flux limit corresponding to ≃0.7 σ
12.
Y. Wiaux J. D. McEwen P. Vandergheynst O. Blanc 《Monthly notices of the Royal Astronomical Society》2008,388(2):770-788
A new formalism is derived for the analysis and exact reconstruction of band-limited signals on the sphere with directional wavelets. It represents an evolution of a previously developed wavelet formalism developed by Antoine & Vandergheynst and Wiaux et al. The translations of the wavelets at any point on the sphere and their proper rotations are still defined through the continuous three-dimensional rotations. The dilations of the wavelets are directly defined in harmonic space through a new kernel dilation, which is a modification of an existing harmonic dilation. A family of factorized steerable functions with compact harmonic support which are suitable for this kernel dilation are first identified. A scale-discretized wavelet formalism is then derived, relying on this dilation. The discrete nature of the analysis scales allows the exact reconstruction of band-limited signals. A corresponding exact multi-resolution algorithm is finally described and an implementation is tested. The formalism is of interest notably for the denoising or the deconvolution of signals on the sphere with a sparse expansion in wavelets. In astrophysics, it finds a particular application for the identification of localized directional features in the cosmic microwave background data, such as the imprint of topological defects, in particular, cosmic strings, and for their reconstruction after separation from the other signal components. 相似文献
13.
极不充分采动条件下地表下沉规律及计算方法研究 总被引:8,自引:0,他引:8
随着煤矿开采的不断加深,地表呆动程度(开采宽度D与开采深度H之比DIH)不断降低,并逐渐趋向于极不充分采动,同样长度的工作面对地表的影响程度极不同于浅部开采。因此,需要重新认识深部开采时的地表下沉规律。极不充分采动条件下地表下沉规律具有特殊性。经过对国内外大量观测资料的综合分析,发现当开采宽度小于地表下沉起动距时(极不充分采动),地表的下沉或地表下沉率(q′/q)为零。当开采宽度达到地表移动起动距时,地表下沉缓慢增长;达到临界采动距时,地表下沉急剧增大。地表下沉率(q′/q)与开采宽、深比D/H之间为负指数函数关系。在极不充分采动条件下进行地表下沉预计时必须对概率积分法的下沉系数进行修正。算例证明了本文给出计算方法的可行性。但这种修正方法如何与概率积分法相统一仍需进一步研究。 相似文献
14.
15.
16.
17.
Much recent feminist geographical scholarship emphasizes the utility of qualitative research methods; yet, a significant proportion of feminist research in geography is quantitative. Geographers' engagement with the ideas of feminist theorists has shed new light on the relationship between epistemology, methodology, and objectivity, which in turn has facilitated a reexamination of feminist uses of quantification. In providing a context for the debate over quantitative and qualitative methods, we argue that each has a place in feminist geographical research. 相似文献
18.
二维圆柱坐标下FDTD法对多频电磁波测井的数值模拟 总被引:3,自引:2,他引:3
刘四新 《吉林大学学报(地球科学版)》2004,34(2):283-286
在介质围绕Z轴旋转对称的情况下,三维直角坐标下的麦克斯韦方程可变成两组分别对应TM波和TE波的二维柱坐标下的偏微分方程.采用线圈作为电磁波测井的发射器和接收器的时候,可采用TE波对应的方程,这时相当于磁极子的情况.而采用电偶极子作为发射和接收器的时候,则对应TM波的方程.采用时间和空间均为二阶精确的有限差分方法,将偏微分方程进行差分化.这样,空间的电磁场可由时间域有限差分法(FDTD)来求解. 相似文献
19.
20.
A. Müller 《Geophysical Journal International》2000,142(1):257-269
Error estimates from statistical regression analysis are often obviously too small, leading to doubts about the given equations, the statistical method itself and finally, with resignation, to the conclusion that mathematical equations and reality never agree. However, for magnetotelluric data we have found an almost perfect fit between observed scattering and predicted confidence limits of regression coefficients after accounting for a systematic error—the bias.
Different methods to compensate for bias in magnetotelluric impedance estimation have been described using additional data from a reference station. However, sufficiently accurate reference data are often not available. A new method has been developed that enables bias compensation without additional data. For the new method we derive a linear relationship between the effect of bias and an expression depending on the data fit. From this we extrapolate the solution for the unbiased impedance. The new method assumes a special model of uncorrelated noise as well as an approximation for the structure of the impedance tensor. From each pair of components of the unrotated impedance tensor corresponding to the same output channel, one of the pair can be compensated if its magnitude is large compared to that of the other.
The method has been successfully applied in many cases. We claim that the solution is closer to the true impedance than any solution based on the selection of events. It gives a measure of the partitioning of noise between the electric and magnetic channels.
We applied the method to measurements from the North Anatolian Fault Zone (Turkey) and from the Merapi volcano (Central Java) in the period range 10–2500 s. Different instrumentation was used for the two sets of measurements, but in both cases we used fluxgate magnetometers to measure the magnetic variations. 相似文献
Different methods to compensate for bias in magnetotelluric impedance estimation have been described using additional data from a reference station. However, sufficiently accurate reference data are often not available. A new method has been developed that enables bias compensation without additional data. For the new method we derive a linear relationship between the effect of bias and an expression depending on the data fit. From this we extrapolate the solution for the unbiased impedance. The new method assumes a special model of uncorrelated noise as well as an approximation for the structure of the impedance tensor. From each pair of components of the unrotated impedance tensor corresponding to the same output channel, one of the pair can be compensated if its magnitude is large compared to that of the other.
The method has been successfully applied in many cases. We claim that the solution is closer to the true impedance than any solution based on the selection of events. It gives a measure of the partitioning of noise between the electric and magnetic channels.
We applied the method to measurements from the North Anatolian Fault Zone (Turkey) and from the Merapi volcano (Central Java) in the period range 10–2500 s. Different instrumentation was used for the two sets of measurements, but in both cases we used fluxgate magnetometers to measure the magnetic variations. 相似文献