全文获取类型
收费全文 | 680篇 |
免费 | 24篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 45篇 |
大气科学 | 88篇 |
地球物理 | 170篇 |
地质学 | 269篇 |
海洋学 | 33篇 |
天文学 | 64篇 |
综合类 | 4篇 |
自然地理 | 35篇 |
出版年
2023年 | 3篇 |
2021年 | 7篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 25篇 |
2017年 | 24篇 |
2016年 | 45篇 |
2015年 | 26篇 |
2014年 | 26篇 |
2013年 | 40篇 |
2012年 | 33篇 |
2011年 | 35篇 |
2010年 | 33篇 |
2009年 | 40篇 |
2008年 | 28篇 |
2007年 | 26篇 |
2006年 | 14篇 |
2005年 | 24篇 |
2004年 | 13篇 |
2003年 | 17篇 |
2002年 | 16篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 13篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1973年 | 6篇 |
1972年 | 3篇 |
1971年 | 5篇 |
1969年 | 3篇 |
1968年 | 3篇 |
1950年 | 2篇 |
排序方式: 共有708条查询结果,搜索用时 171 毫秒
51.
52.
Measurements of the isotopic composition of nitrogen in the solar system are summarized. We show that the 30% change, during the last 3 to 4 billion years, of in solar-wind-bearing lunar soils and breccias probably does not reflect changes in this ratio at the solar surface. Such changes, whether by spallation or thermonuclear reactions are ruled out by comparing the yields of 15N with those of other rare isotopes such as 9Be, 11B, 3He or 13C, even if an arbitrary degree of solar mixing is introduced. Moreover, we calculate that the solar activity required for producing significant amounts of 15N by spallation at the solar surface should have resulted in a particle bombardment of the Moon of an intensity that would have produced amounts of spallation isotopes (e.g.15N, 21Ne, 38Ar, 131Xe) several orders of magnitude in excess of what is actually found in the whole regolith.We argue that accretion of interstellar matter also does not work as a cause for a significant change of the photospheric ratio. Evidence is presented that the mixing depth at the solar surface on a time scale of ?109 years is (10?2 ?10?1) M⊙ Mixing to this depth renders accretion of interstellar matter as a source of compositional changes at the solar surface inefficient, even if allowance is made for the expected large difference in the accretion rates of condensed and gaseous matter. A quantitative treatment of several alternatives of solar accretion leads to serious contradictions (e.g. with the low Ne abundances in planetary atmospheres or with the amounts of nitrogen that should have been directly accreted by the Moon), and we conclude that accretion during the main sequence life of the Sun is an unlikely source of changes in at the solar surface.A ratio of is our best estimate for average solar system material and for the Sun. We propose that a rare, very light nitrogen component (called LPN) is admixed in varying amounts to planetary matter. Undiluted LPN has not been found in meteorites or planetary atmospheres, but we show that the combined effects of LPN admixture and isotope fractionation can in principle account for the variability of observed in the planetary system. Determination of the Jovian ratio with an accuracy of ~10% would crucially test our interpretation of the nitrogen isotope observations. 相似文献
53.
54.
This paper focuses on the influence of the initial void ratio on the evolution of the passive earth pressure and the formation
of shear zones in a dry sand body behind a retaining wall. For the numerical simulation a rigid and very rough retaining wall
undergoing a horizontal translation against the backfill is considered. The essential mechanical properties of cohesionless
granular soil are described with a micro-polar hypoplastic model which takes into account stresses and couple stresses, pressure
dependent limit void ratios and the mean grain size as a characteristic length. Numerical investigations are carried out with
an initially medium dense and initially loose sand using a homogeneous and random distribution of the initial void ratio.
The geometry of calculated shear zones is discussed and compared with a corresponding laboratory model test. 相似文献
55.
Marc Rütschlin Johannes H. Cloete Iain M. Mason Keith D. Palmer 《Journal of Applied Geophysics》2007,62(4):354-360
A new measurement technique enables the complex dielectric properties of the geological strata comprising the UG1–UG2 (Upper Group 1–Upper Group 2) unit of the Bushveld Complex in South Africa to be determined with unprecedented detail at radio frequencies (RF). Results of non-destructive laboratory measurements of representative diamond drill core samples from the UG1–UG2 unit are presented at 25 MHz. These data establish that the UG1 and UG2 chromitite layers are embedded in rock strata (norite, pyroxenite and anorthosite) which are translucent in the HF spectral band, whereas the chromitite layers themselves exhibit significant velocity contrast, making them good radar reflectors. The data presented here is useful for calibration of the radar system, and for predicting the range and resolution performance of borehole radars operating in both the hanging and footwalls of the economically important platiniferous UG2 reef. 相似文献
56.
57.
58.
H.J Bauer 《Geoforum》1971,2(4):31-41
The lignite mining area of the Rhineland provides an excellent example for planned recultivation of large open-works. In order to properly carry out recultivation measures, it is necessary to conduct basic research on the ecological factors which compose a balanced landscape. The problems involved in reconstructing a landscape, which has been pitted by large-scale open-cut mines, are discussed from the aspects of: planning and construction of a new landscape-relief, proposed agricultural and forestry use, nature preserves and resort areas. The aim of recultivation, from the ecological point of view, is to renew the natural biological potential and to reform the landscape, so as to provide the healthiest environment for Man. 相似文献
59.
Summary Two electromagnetic seismographs HSJ-I, which are coupled with short-period galvanometers, record the velocity of the ground motion in the range of periods from 0.3 to 20 s in the seismological station Moxa. In this way the magnitude of an earthquake can be calculated without respect to the period of the ground motion. The results of such a determination of magnitudes are compared with those reached in the usual manner from short and long-period seismographs recording the ground displacement.Publication No. 45 of the Institut für Geodynamik, 69 Jena (DDR), Burgweg 11, der Deutschen Akademie der Wissenschaften zu Berlin, Forschungsgemeinschaft. Presented at the IUGG-Assembly, Zurich 1967. 相似文献
60.