全文获取类型
收费全文 | 1450篇 |
免费 | 81篇 |
国内免费 | 17篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 109篇 |
地球物理 | 372篇 |
地质学 | 529篇 |
海洋学 | 128篇 |
天文学 | 233篇 |
综合类 | 12篇 |
自然地理 | 138篇 |
出版年
2023年 | 10篇 |
2022年 | 5篇 |
2021年 | 30篇 |
2020年 | 24篇 |
2019年 | 23篇 |
2018年 | 47篇 |
2017年 | 44篇 |
2016年 | 50篇 |
2015年 | 47篇 |
2014年 | 61篇 |
2013年 | 102篇 |
2012年 | 59篇 |
2011年 | 88篇 |
2010年 | 72篇 |
2009年 | 103篇 |
2008年 | 80篇 |
2007年 | 63篇 |
2006年 | 63篇 |
2005年 | 59篇 |
2004年 | 54篇 |
2003年 | 46篇 |
2002年 | 58篇 |
2001年 | 21篇 |
2000年 | 19篇 |
1999年 | 22篇 |
1998年 | 22篇 |
1997年 | 18篇 |
1996年 | 28篇 |
1995年 | 18篇 |
1994年 | 14篇 |
1993年 | 4篇 |
1992年 | 20篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1972年 | 3篇 |
排序方式: 共有1548条查询结果,搜索用时 170 毫秒
161.
162.
Making use of the homogenization method for periodic media, developed in the context of elastoplasticity, a simplified constitutive law is proposed for a stone column-reinforced soil, regarded as a homogeneous but anisotropic medium. The closed-form expressions derived for such a constitutive law allow for its implementation into a f.e.m-based numerical procedure. The computational code so obtained is then applied to simulating the response of a foundation soil reinforced by a group of floating columns, expressed in terms of load–settlement curves drawn up to the ultimate bearing capacity. 相似文献
163.
John J. Matese Patrick G. Whitman Daniel P. Whitmire 《Celestial Mechanics and Dynamical Astronomy》1997,69(1-2):77-87
We investigate the distribution of Oort cloud comet perihelia. The data considered includes comets having orbital elements
of the two highest quality classes with original energies designated as new or young. Perihelion directions are determined
in galactic, ecliptic and geocentric equatorial coordinates. Asymmetries are detected in the scatter and are studied statistically
for evidence of adiabatic galactic tidal dynamics, an impulse-induced shower and observational bias. The only bias detected
is the well-known deficiency of observations with perihelion distances q > 2.5 AU. There is no significant evidence of a seasonal
dependence. Nor is there a substantive hemispherical bias in either ecliptic or equatorial coordinates. There is evidence
for a weak stellar shower previously detected by Biermann which accounts for ≈ 10% of the total observations. Both the q bias
and the Biermann star track serve to weaken the evidence for a galactic tidal imprint. Nevertheless, statistically significant
asymmetries in galactic latitude and longitude of perihelia remain. A latitude asymmetry is produced by a dominant tidal component
perpendicular to the galactic disk. The longitude signal implies that ≈ 20% of new comets need an additional dynamical mechanism.
Known disk non-uniformities and an hypothetical bound perturber are discussed as potential explanations. We conclude that
the detected dynamical signature of the galactic tide is real and is not an artifact of observational bias, impulsive showers
or poor data.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
164.
J. Pritchard J. B. Marquette Patrick Tisserand J. P. Beaulieu E. Lesquoy A. Milsztajn 《Astrophysics and Space Science》2006,304(1-4):227-229
More than 10 years of microlensing survey observations by the EROS Collaboration have monitored several million stars, amongst them several thousand eclipsing binary stars. In this poster we present some of the difficulties and rewards of the study of this immense database.Based on observations made at ESO by the EROS collaboration 相似文献
165.
Patrick F. Roche Christopher Packham Charles M. Telesco James T. Radomski Almudena Alonso-Herrero David K. Aitken Luis Colina Eric Perlman 《Monthly notices of the Royal Astronomical Society》2006,367(4):1689-1698
High spatial resolution spectroscopy at 8–13 μm with T-ReCS on Gemini-S has revealed striking variations in the mid-infrared emission and absorption in the nucleus of the Circinus galaxy (hereafter Circinus) on subarcsecond scales. The core of Circinus is compact and obscured by a substantial column of cool silicate dust. Weak extended emission to the east and west coincides with the coronal line region and arises from featureless dust grains which are probably heated by line emission in the coronal emission zone. The extended emission on the east side of the nucleus displays a much deeper silicate absorption than that on the west, indicating significant columns of cool material along the line of sight and corresponding to an additional extinction of AV ∼ 25 mag . Emission bands from aromatic hydrocarbons are not subject to this additional extinction, are relatively weak in the core and in the coronal line region, and are much more spatially extended than the continuum dust emission; they presumably arise in the circumnuclear star-forming regions. These data are interpreted in terms of an inclined disc-like structure around the nucleus extending over tens of parsecs and possibly related to the inner disc found from observations of water masers by Greenhill et al.. 相似文献
166.
167.
168.
We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the 15N/14N ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that 15NH3 is not fractionated in Jupiter's atmosphere, and that the measured 15N/14N represents Jupiter's global value. Our mean value for the global jovian 15N/14N ratio of (2.22±0.52)×10−3 agrees with previous measurements made by Fouchet et al. (2000, Icarus 143, 223-243) and Owen et al. (2001, Astrophys. J. 553, L77-L79). We argue that the jovian isotopic 15N/14N ratio must represent the solar nitrogen isotopic composition. The solar 15N/14N ratio hence significantly differs from the terrestrial value: (15N/14N)⊕=3.68×10−3. This supports the proposition that terrestrial nitrogen originates from a nitrogen reservoir isolated from the main nitrogen reservoir in the proto-solar nebula. The origin and carrier of this isolated reservoir are still unknown. 相似文献
169.
The dwarf nova oscillations observed in cataclysmic variable (CV) stars are interpreted in the context of a low-inertia accretor model, in which accretion on to an equatorial belt of the white dwarf primary causes the belt to vary its angular velocity. The rapid deceleration phase is attributed to propellering. Evidence that temporary expulsion rather than accretion of gas occurs during this phase is obtained from the large drop in extreme ultraviolet flux.
We show that the quasi-periodic oscillations are most probably caused by a vertical thickening of the disc, moving as a travelling wave near the inner edge of the disc. This alternately obscures and 'reflects' radiation from the central source, and is visible even in quite low inclination systems. A possible excitation mechanism, caused by winding up and reconnection of magnetic field lines, is proposed.
We apply the model, deduced largely from VW Hyi observations, to re-interpret observations of SS Cyg, OY Car, UX UMa, V2051 Oph, V436 Cen and WZ Sge. In the last of these we demonstrate the existence of a 742-s period in the light curve, arising from obscuration by the travelling wave, and hence show that the two principal oscillations are a dwarf nova oscillation and its reprocessed companion. 相似文献
We show that the quasi-periodic oscillations are most probably caused by a vertical thickening of the disc, moving as a travelling wave near the inner edge of the disc. This alternately obscures and 'reflects' radiation from the central source, and is visible even in quite low inclination systems. A possible excitation mechanism, caused by winding up and reconnection of magnetic field lines, is proposed.
We apply the model, deduced largely from VW Hyi observations, to re-interpret observations of SS Cyg, OY Car, UX UMa, V2051 Oph, V436 Cen and WZ Sge. In the last of these we demonstrate the existence of a 742-s period in the light curve, arising from obscuration by the travelling wave, and hence show that the two principal oscillations are a dwarf nova oscillation and its reprocessed companion. 相似文献
170.
Tsunami risk mitigation programs often include iconic evacuation signage to direct locals and visitors to safety during a tsunami event. This paper examines sign placement in Seaside, Oregon, from a visibility perspective. It leverages existing visibility analysis methodologies characterize the visibility of the community’s evacuation signage and reveals patterns in the viewable landscape. Additionally, we develop a topologically 3D approach to visibility analysis using raw LiDAR datasets. This applied work situates a discussion on existing patterns of visibility, how to improve existing signage placement, 2D and 3D representation of landscape, and the importance of visibility analysis. This work aims to stimulate discussion and development of hazard research that incorporates a visibility perspective. 相似文献