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151.
Recent changes in the cultural and forest landscapes of the Meseta Purépecha in Michoacán, Mexico as a result of forest degradation underscore the complexity of forest change processes in the tropical highlands of Latin America. Differences in community perception and forest structure and composition between the furniture-making and lumber-producing towns of Pichátaro and Sevina, Purépechan indigenous communities located amidst pine forests on Michoacán's volcanic plateau illustrate the dynamics of this process. We base our comparisons on interviews and field measures of forest structure. Our results show dramatic changes in the forests and cultural landscapes of both communities during the past decade. Following high regional timber exports during the early 1990s, Sevina shifted from a self-sufficient to a timber importation community. By comparison, communal forests and individual parcels in Pichátaro continue to provide wood for approximately 300 wood shops. Field data and forest stand maps confirm the perception of forest degradation in both communities. While Pichátaro has maintained a larger and more diverse forest base to date, stand structure data indicate selective harvesting has led to a shift in dominance toward the less economically desirable pine species and oak. Deforestation and degradation of Sevina and Pichátaro's community forests are symptomatic of both the Meseta Purépecha and Mexico in general. Current forest conditions in both communities justify local, regional, and national concerns regarding declining biodiversity and sustainable economies. 相似文献
152.
HOU Yijun GUO Peifang SONG Guiting SONG Jinbao YIN Baoshu & ZHAO Xixi . Institute of Oceanology Chinese Academy of Sciences Qingdao China . Graduate University of the Chinese Academy of Sciences Beijing China 《中国科学D辑(英文版)》2006,49(4):443-448
1 Introduction The probability distribution of ocean wave element is one of the results of application of random process theory to the ocean wave study. A great amount of outcome has obtained in this field[1,2]. In fact, a great deal of research on linear… 相似文献
153.
Most theoretical investigations of seismic wave scattering rely on the assumption that the underlying medium is statistically isotropic. However, deep seismic soundings of the crust as well as geological observations often reveal the existence of elongated or preferentially oriented scattering structures. In this paper, we develop mean field and radiative transfer theories to describe the attenuation and multiple scattering of a scalar wavefield in an anisotropic random medium. The scattering mean free path is found to depend strongly on the propagation direction. We derive a radiative transfer equation for statistically anisotropic random media from the Bethe–Salpeter formalism and propose a Monte Carlo method to solve this equation numerically. At longer times, the energy density is shown to obey a tensorial diffusion equation. The components of the diffusion tensor are obtained in closed form and excellent agreement is found between Monte Carlo simulations and analytical solutions of the diffusion equation. The theory has important potential implications for lithospheric models where scatterers are for example flat structures preferentially aligned along the surface. In this simple geometry, analytical expressions of the Coda Q parameter can be given explicitly in the diffusive regime. Our results suggest that pulse broadening and coda decay are controlled by different parameters, related to the eigenvalues of the diffusion tensor. These eigenvalues can differ by more than one order of magnitude. This theory could be applied to probe the anisotropy of length scales in the lithosphere. 相似文献
154.
Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function 总被引:3,自引:0,他引:3
Michel Campillo 《Pure and Applied Geophysics》2006,163(2-3):475-502
We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of
P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are
in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate
the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties
of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering
enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for
multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green
function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded
at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield.
Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental
mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level
of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation
in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed
scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the
Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the
correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement
with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different
crustal structures opens the way for a `passive imaging' of the Earth's structure. 相似文献
155.
地震地面运动模型的研究 总被引:1,自引:0,他引:1
地震地面运动被模拟成均值为零的两次过滤Gauss白噪声随机过程。第一次过滤削减白噪声的高频含量;第二次过滤削减白噪声的低频含量。根据地震记录的功率谱,使用非线性函数的最小二乘法,确定了两次过滤Gauss白噪声随机过程的功率谱密度函数的参数。 相似文献
156.
157.
Alberto Bellin Yoram Rubin 《Stochastic Environmental Research and Risk Assessment (SERRA)》1996,10(4):253-278
This paper describes a new method for generating spatially-correlated random fields. Such fields are often encountered in hydrology and hydrogeology and in the earth sciences. The method is based on two observations: (i) spatially distributed attributes usually display a stationary correlation structure, and (ii) the screening effect of measurements leads to the sufficiency of a small search neighborhood when it comes to projecting measurements and data in space. The algorithm which was developed based on these principles is called HYDRO_GEN, and its features and properties are discussed in depth. HYDRO_GEN is found to be accurate and extremely fast. It is also versatile: it can simulate fields of different nature, starting from weakly stationary fields with a prescribed covariance and ending with fractal fields. The simulated fields can display statistical isotropy or anisotropy. 相似文献
158.
单图像三维立体图简介 总被引:2,自引:0,他引:2
林存山 《物探化探计算技术》1996,18(1):52-59,70
立体图指的是在观察图画中的深度时,通过双眼的配合能够构造出一幅心理上的立体透视图来。人们常规的做法是使用一对像图来获得立体景色,这样的图要经适当的制作并要用双眼分别各看一张。近年来,借助于计算机产生的随机点图可用来勾绘出实物。这种随机点图如用单只眼睛观看时则是毫无意义的,但如用双眼观看时便可产生出立体景色。原先这样的随机点立体图似乎均要求制成两张图,但最近的工作表明将此种图合二而一的方法已获成功。 相似文献
159.
This work presents a stochastic diagrammatic theory for the calculation of the effective hydraulic conductivity of heterogeneous
media. The theory is based on the mean-flux series expansion of a log-normal hydraulic conductivity medium in terms of diagrammatic
representations and leads to certain general results for the effective hydraulic conductivity of three-dimensional media.
A selective summation technique is used to improve low-order perturbation analysis by evaluating an infinite set of diagrammatic
terms with a specific topological structure that dominates the perturbation series. For stochastically isotropic media the
selective summation yeilds the anticipated exponential expression for the effective hydraulic conductivity. This expression
is extended to stochastically anisotropic media. It is also shown that in the case of non homogeneous media the uniform effective
hydraulic conductivity is replaced by a non-local tensor kernel, for which general diagrammatic expressions are obtained.
The non-local kernel leads to the standard exponential behavior for the effective hydraulic conductivity at the homogeneous
limit. 相似文献
160.
Dionissios T. Hristopulos George Christakos 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(5):369-395
This work presents a stochastic diagrammatic theory for the calculation of the effective hydraulic conductivity of heterogeneous
media. The theory is based on the mean-flux series expansion of a log-normal hydraulic conductivity medium in terms of diagrammatic
representations and leads to certain general results for the effective hydraulic conductivity of three-dimensional media.
A selective summation technique is used to improve low-order perturbation analysis by evaluating an infinite set of diagrammatic
terms with a specific topological structure that dominates the perturbation series. For stochastically isotropic media the
selective summation yeilds the anticipated exponential expression for the effective hydraulic conductivity. This expression
is extended to stochastically anisotropic media. It is also shown that in the case of non homogeneous media the uniform effective
hydraulic conductivity is replaced by a non-local tensor kernel, for which general diagrammatic expressions are obtained.
The non-local kernel leads to the standard exponential behavior for the effective hydraulic conductivity at the homogeneous
limit. 相似文献