全文获取类型
收费全文 | 236篇 |
免费 | 5篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 3篇 |
地球物理 | 32篇 |
地质学 | 120篇 |
海洋学 | 8篇 |
天文学 | 77篇 |
自然地理 | 5篇 |
出版年
2022年 | 1篇 |
2021年 | 4篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2018年 | 18篇 |
2017年 | 12篇 |
2016年 | 15篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 16篇 |
2012年 | 8篇 |
2011年 | 11篇 |
2010年 | 4篇 |
2009年 | 10篇 |
2008年 | 10篇 |
2007年 | 8篇 |
2006年 | 4篇 |
2005年 | 8篇 |
2004年 | 1篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1969年 | 1篇 |
1917年 | 1篇 |
1907年 | 1篇 |
排序方式: 共有247条查询结果,搜索用时 15 毫秒
71.
The paper presents a modified genetic algorithm called adapted genetic algorithm with adjusting population size (AGA-POP) for precise determination the orbital elements of binary stars. The proposed approach is a simple, robust way that can be considered to be a new member in the class of self organizing genetic algorithms. The proposed AGA-POP is applied on the star η Bootis of MK type G0 IV to find a set of optimal orbital elements. This leads to obtain the best fitting of Keplerian and phase curves. The modified method is compared with other different methods such as standard genetic algorithm, adapted genetic algorithm (AGA) and least square methods. Simulation results show the effectiveness of using AGA-POP compared with other different classic genetic algorithms in reducing the computation time. Also, better performances have been achieved when using the proposed technique. 相似文献
72.
In this paper we describe two different methods to expand the second term of the planetary Hamiltonian function. The Jacobi
system of coordinates is adopted leading to a unique evaluation of the Hamiltonian. Previous analytical or semi-analytical
planetary theories suffer from the drawback of computing the perturbation function for each planet, which is quite cumbersome.
The inclinations of planets are referred to a common fixed plane and the longitudes to a common origin. This is necessary
when we deal with n > 2 planets. The treatment is straightforward, and no complexities appear throughout the analysis.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
73.
Pyrite, chalcopyrite, and gold occur in quartz veins in granitic rocks and as scattered and disseminated impregnations in shear zones of the highly altered metavolcanics in the Hamash area, Southeastern Desert, Egypt. The minerals are associated in part with pyrrhotite, digenite, tetrahedrite, chalcocite, bornite, and covellite. Pyrite occurs in two forms: (1) idio- to hypidiomorphic coarse crystals with inclusions of preexisting sulfides, and (2) fine-crystalline aggregates. Chalcopyrite occurs in three forms: (1) idiomorphic coarse crystals, (2) fine-crystalline microinclusions, and (3) xenomorphic relicts. Three genetic phases of sulfide mineralization were identified. They are related to the successive cooling of the crystallizing solutions. Gold was hosted in the older sulfide minerals during a high-temperature disorder phase. Native gold was formed during the latest, decreasing-temperature phase through remobilization of auriferous pyrite. Microprobe analysis confirmed that gold and copper are relatively enriched in the late pyrite. Identified surface-alteration products include goethite, limonite, gold, carbonates, and sulfates of iron and copper. 相似文献
74.
Osman M. Kamel 《Earth, Moon, and Planets》1983,28(3):221-245
In this part we determine the value ofS
1,
and
in terms of the canonical variables of H. Poincaré. A complete solution of the auxiliary system of equations generated by the Hamiltonian
is presented. 相似文献
75.
Osman Demircan 《Astrophysics and Space Science》1978,59(2):313-332
The aim of the present paper is to deduce some further properties of the fundamental quantities inherent in the frequency-domain approach-such as the fractional loss of light
l
0
and momentsA
2m
of the light curves of eclipsing variables; and also to develop an iterative method for the solution of two key eclipse parametersa andc
0 in terms of the observed quantities. This should facilitate practical applications of the methods developed in the preceding papers of this series for the frequency-domain light curve analysis of eclipsing variables. 相似文献
76.
Determining the optimum weight of the armor blocks is of vital importance in the design of conventional breakwaters. The widely used formulae in the literature include the transition region from plunging to surging waves. In this paper, it is aimed to investigate a new design formula without any transition region as an alternative to widely used Van der Meer formulae. The dimensionless parameters of Van der Meer formulae as well as newly generated variables are used as inputs. Nonlinear surface fit best subset regression model is used to find the optimum input combination that keeps the nonlinear relationships. All the input parameters, their second powers, and their two-way interactions are included in the regression analyses to obtain a nonlinear surface fit. Various goodness of fit statistics are applied to check the different perspectives of the model accuracy. It is demonstrated that the proposed model gives a realistic prediction of the stability number for critical data range. Especially for high values of stability number the proposed formula outperforms the benchmark formulae of Van der Meer and Etemad-Shahidi and Bonakdar. The other advantage is that it does not contain any transition region that depends on wave conditions. Besides, there is no need to include “number of waves” and “permeability” parameters into the equation. 相似文献
77.
Hajo Idriss Isam Salih Abdulaziz S. Alaamer M. A. AL-Rajhi Alshfia Osman Tahir Elamin Adreani M. Y. Abdelgalil Nagi I. Ali 《Acta Geophysica》2018,66(4):673-681
This study shows the assessment of radiation hazard parameters due to terrestrial radionuclides in the soil around artisanal gold mining for addressing the issue of natural radioactivity in mining areas. Hence, the levels 238U, 232Th, 40K and 226Ra in soil (using gamma spectrometry), 222Rn in soil and 222Rn in air were determined. Radiation hazard parameters were then computed. These include absorbed dose D, annual effective dose E, radium equivalent activity Raeq, external hazard Hex, annual gonadal dose equivalent hazard index AGDE and excess lifetime cancer risk ELCR due to the inhalation of radon (222Rn) and consumption of radium (226Ra) in vegetation. Uranium (238U), thorium (232Th) and potassium (40K) averages were, respectively, 26, 36 and 685 Becquerel per kilogram (Bq kg?1). Soil radon (4671 Bq m?3) and radon in air (14.77 Bq m?3) were found to be less than worldwide data. Nevertheless, the average 40K concentration was 685 Bq kg?1. This is slightly higher than the United Nations Scientific Committee on the Effects of Atomic Radiation average value of 412 Bq kg?1. The obtained result indicates that some of the radiation hazard parameters seem unsavory. The mean value of absorbed dose rate (62.49 nGy h?1) was slightly higher than average value of 57 nGy h?1 (~?45% from 40K), and that of AGDE (444 μSv year?1) was higher than worldwide average reported value (300 μSv year?1). This study highlights the necessity to launch extensive nationwide radiation protection program in the mining areas for regulatory control. 相似文献
78.
79.
In this study, a Markov Random Field (MRF) approach is used to locate source boundary positions which are difficult to identify
from Bouguer gravity and magnetic maps. As a generalized form of Markov Chains, the MRF approach is an unsupervised statistical
model based algorithm and is applied to the analysis of images, particularly in the detection of visual patterns or textures.
Here, we present a dynamic programming based on the MRF approach for boundary detection of noisy and super-positioned potential
anomalies, which are produced by various geological structures. In the MRF method, gravity and magnetic maps are considered
as two-dimensional (2-D) images with a matrix composed of N1 × N2 pixels. Each pixel value of the matrix is optimized in real time with no a priori processing by using two parameter sets; average steering vector (θ) and quantization level (M). They carry information about the correlation of neighboring pixels and the locality of their connections. We have chosen
MRF as a processing approach for geophysical data since it is an unsupervised, efficient model for image enhancement, border
detection and separation of 2-D potential anomalies. The main benefit of MRF is that an average steering vector and a quantization
level are enough in evaluation of the potential anomaly maps. We have compared the MRF method to noise implemented synthetic
potential field anomalies. After satisfactory results were found, the method has been applied to gravity and magnetic anomaly
maps of Gelibolu Peninsula in Western Turkey. Here, we have observed Anafartalar thrust fault and another parallel fault northwest
of Anafartalar thrust fault. We have modeled a geological structure including a lateral fault, which results in a higher susceptibility
and anomaly amplitude increment. We have shown that the MRF method is effective to detect the broad-scale geological structures
in the Gelibolu Peninsula, and thus to delineate the complex tectonic structure of Gelibolu Peninsula. 相似文献
80.
Osman Demircan 《Astrophysics and Space Science》1980,72(2):281-286
This is a continuation of a previous paper which appeared in this journal (Demircan, 1980b) and aims at ascertaining some other relations between the integral transforms of the light curves of eclipsing binary systems. The appropriate use of these relations should facilitate the numerical computations for an analysis of eclipsing binary light curves by different Fourier techniques. 相似文献