全文获取类型
收费全文 | 1020篇 |
免费 | 29篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 89篇 |
地球物理 | 217篇 |
地质学 | 243篇 |
海洋学 | 202篇 |
天文学 | 202篇 |
综合类 | 10篇 |
自然地理 | 86篇 |
出版年
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 15篇 |
2019年 | 24篇 |
2018年 | 31篇 |
2017年 | 24篇 |
2016年 | 35篇 |
2015年 | 24篇 |
2014年 | 31篇 |
2013年 | 61篇 |
2012年 | 40篇 |
2011年 | 40篇 |
2010年 | 43篇 |
2009年 | 43篇 |
2008年 | 49篇 |
2007年 | 49篇 |
2006年 | 40篇 |
2005年 | 30篇 |
2004年 | 26篇 |
2003年 | 25篇 |
2002年 | 29篇 |
2001年 | 15篇 |
2000年 | 28篇 |
1999年 | 26篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 9篇 |
1995年 | 16篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1985年 | 14篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1915年 | 6篇 |
1907年 | 7篇 |
排序方式: 共有1068条查询结果,搜索用时 31 毫秒
81.
Excavation induced seismic events with moment magnitudesM<0 are examined in an attempt to determine the role geology, excavation geometry, and stress have on scaling relations. Correlations are established based on accurate measurements of excavation geometry and methodology, stress regime, rock mass structure, local tectonics, and seismic locations. Scaling relations incorporated seismic moments and source radii obtained by spectral analysis, accounting for source, propagation, and site effects, and using Madariaga's dynamic circular fault model. Observations suggest that the interaction of stresses with pre-existing fractures, fracture complexity and depth of events are the main factors influencing source characteristics and scaling behaviour. Self-similar relationships were found for events at similar depths or for weakly structured rock masses with reduced clamping stresses, whereas a non-similar behaviour was found for events with increasing depth or for heavily fractured zones under stress confinement. Additionally, the scaling behaviour for combined data sets tended to mask the non-similar trends. Overall, depth and fracture complexity, initially thought as second order effects, appear to significantly influence source characteristics of seismic events withM<0 and consequently favour a non-similar earthquake generation process. 相似文献
82.
83.
Hot water effluent from power stations kills juvenile menhaden migrating through Long Island Sound. Since the fish sink to the bottom, losses are unrecorded or at best under-estimated. Other fish are probably also affected. Menhaden in Long Island Sound contribute to the commercial fishery off the eastern seaboard of the United States. At present the losses have little impact on the fishery, but if the projected increase in hot water discharges into the Sound materializes the consequences may be serious. 相似文献
84.
Other Index
List of Forthcoming Papers 相似文献85.
We have computed line profiles and curves of growth for both reflected and transmitted radiation for typical lines in CO2 bands (in the photographic infrared) which occur in the spectrum of Venus. In our model the pressure variation with altitude was considered and the base of the cloud deck was set at the 2 bar level. The temperature was held constant at 250K and a Voigt profile was used for the lineshape. We also assumed that the scale height of the cloud particles was equal to the scale height of the gas. The calculations were made for four values of the scattering optical thickness (τc = 0.1, 1.0, 10, and 100) using a continuum single scattering albedo (which gives a Bond albedo of 0.896 for τc = 100, the value observed for Venus at these wavelengths). Curves of growth are also presented for reflected radiation which has been averaged over the visible disk for three values of the Venus phase angle (0, 86, and 166°). 相似文献
86.
87.
S.?M.?LeeEmail author K.?D.?Min N.?C.?Woo Y.?J.?Kim C.?H.?Ahn 《Environmental Geology》2003,44(2):210-221
The purpose of this study is to develop statistical models for groundwater quality assessment in urban areas using Geographic Information Systems (GIS). To develop the models, the concentrations of nitrate (expressed as nitrogen, NO3-N), which are different according to the type of land use, well depth and distribution of rainfall, were analyzed in the Seoul (the capital of South Korea) area. Data such as land use, location of wells and groundwater quality data for nitrate contamination were collected and a database constructed within GIS. The distribution of NO3-N concentrations is not normal, and the results of the Mann-Whitney U-test analysis show the difference of NO3-N concentration by well depth and by distribution of rainfall. In both the shallow and deep wells, the radius of influence is 200 m in the dry season and 250 m in the rainy season, showing the tendency to increase in the rainy season. The results of correlation and regression analysis indicate that mixed residential and business areas and cropped field areas are likely to be the major contributor of increasing NO3-N concentration. Land uses are better correlated with NO3-N in deep wells than in shallow wells. 相似文献
88.
89.
Turatto M Suzuki T Mazzali PA Benetti S Cappellaro E Danziger IJ Nomoto K Nakamura T Young TR Patat F 《The Astrophysical journal》2000,534(1):L57-L61
The extraordinary SN 1997cy associated with GRB 970514 has been observed photometrically and spectroscopically for nearly 2 yr. At the time of discovery, SN 1997cy was the brightest supernova (SN) ever observed (MV=-20.1, vhel=19,140 km s-1, H0=65 km s-1 Mpc-1). Up to the last available observations (600 days after the gamma-ray burst), the total time-integrated flux was equal to or larger than that expected from the complete thermalization of the gamma-rays produced by 2.3 M middle dot in circle of 56Co. However, starting already on day 60 the luminosity decline is slower than the 56Co decay rate, indicating that the SN ejecta was interacting with circumstellar material (CSM). The interaction appeared to weaken around day 550. The spectra of SN 1997cy are dominated at all epochs by Halpha emission, which shows at least three components of different widths, as in SN 1988Z. Several other lines with different widths are also visible, especially at early epochs. The entire light curve of SN 1997cy is reproduced by a model of the interaction of the very energetic (E=3x1052 ergs) ejecta of a massive star (25 M middle dot in circle) with the CSM, with some contribution from radioactive decays. The CSM could have been ejected with a mass-loss rate of M&d2; approximately 4x10-4 M middle dot in circle yr-1 as the progenitor star evolved from a blue to a red supergiant about 104 yr before the explosion. The lack of oxygen and magnesium lines in the spectra at nebular phases poses a problem for models requiring high-mass progenitors. The possibility that most of the core material of the progenitor has fallen onto a massive black hole so that the reverse shock dies at the inner edge of the H/He envelope is discussed. 相似文献
90.
New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly
granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture
process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded
at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be
captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial
nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation
speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity
(as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation
velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain
acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate
similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments.
It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding
the controls on fracture nucleation. 相似文献