全文获取类型
收费全文 | 57823篇 |
免费 | 3971篇 |
国内免费 | 2431篇 |
专业分类
测绘学 | 2268篇 |
大气科学 | 5573篇 |
地球物理 | 11768篇 |
地质学 | 21918篇 |
海洋学 | 5969篇 |
天文学 | 11880篇 |
综合类 | 1142篇 |
自然地理 | 3707篇 |
出版年
2024年 | 392篇 |
2023年 | 494篇 |
2022年 | 964篇 |
2021年 | 1309篇 |
2020年 | 1304篇 |
2019年 | 1355篇 |
2018年 | 2008篇 |
2017年 | 1892篇 |
2016年 | 2162篇 |
2015年 | 1421篇 |
2014年 | 2039篇 |
2013年 | 2986篇 |
2012年 | 2161篇 |
2011年 | 2642篇 |
2010年 | 2372篇 |
2009年 | 2888篇 |
2008年 | 2592篇 |
2007年 | 2618篇 |
2006年 | 2428篇 |
2005年 | 1684篇 |
2004年 | 1659篇 |
2003年 | 1598篇 |
2002年 | 1696篇 |
2001年 | 1517篇 |
2000年 | 1338篇 |
1999年 | 1256篇 |
1998年 | 1158篇 |
1997年 | 1187篇 |
1996年 | 964篇 |
1995年 | 919篇 |
1994年 | 849篇 |
1993年 | 716篇 |
1992年 | 631篇 |
1991年 | 591篇 |
1990年 | 553篇 |
1989年 | 594篇 |
1988年 | 512篇 |
1987年 | 563篇 |
1986年 | 510篇 |
1985年 | 578篇 |
1984年 | 693篇 |
1983年 | 605篇 |
1982年 | 592篇 |
1981年 | 525篇 |
1980年 | 460篇 |
1979年 | 455篇 |
1978年 | 459篇 |
1977年 | 394篇 |
1975年 | 367篇 |
1973年 | 376篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
This paper provides a comparative analysis of two separate national surveys of the uptake and use of Geographical Information Systems (GIS) by Health Authorities (HAs) in England and Wales. The surveys were undertaken at two different time points (1991 and 2001), and the intervening period has been characterized by major (and continuing) health sector restructuring. The substantial number of HA mergers between 1991 and 2001 provide a methodological challenge in attempting to make comparisons. A pragmatic approach is adopted and the presentation of results has to be restricted solely to English and Welsh HA data. Our findings show that there has been a considerable increase in the use of GIS since the early 1990s, and there is some evidence of more sophisticated uses for a wider range of policy-related and application tasks. However, we conclude that there is still considerable scope for the wider uptake and use of GIS within the NHS. In addition, our findings provide a baseline with which to monitor the impacts of future organizational restructuring on the uses of GIS within the NHS. We suggest that this is important given the replacement of 95 English HAs with 28 new Strategic HAs (StHAs) from April 2002, and the repercussions of the anticipated shift in GIS responsibility to Primary Care Trusts. 相似文献
124.
In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite
layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates.
P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures
did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite.
Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion
they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical
proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion
they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume
compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures. 相似文献
125.
We studied the surface perturbations of a two-layer ideal fluid induced by a flow past a submerged obstacle (simulated by
a point dipole) in the vicinity of the density-jump layer. It is shown that the formation of two different types of surface
waves is possible behind the flowed past obstacle in the real conditions of the open sea. The comparison of the amplitudes
of the surface perturbations induced by the obstacle located above and below the density jump revealed their significant differences,
which seems to be important for the use of this effect in practical problems. 相似文献
126.
127.
128.
A.N. Antipov Yu.M. Semenov N.K. Elizbarashvili O.Ya. Sayadian R.M. Mamedov 《Geography and Natural Resources》2009,30(3):286-293
We have developed the planning documents for model territories of a different dimension: the landscape program of Adzharia (Georgia), the landscape structure plan for the Lake Sevan watershed basin (Armenia), and the landscape plan for the Shirvan National Park (Azerbaijan). Variants of the solution of regional natural-ecological problems are proposed, and the measures for implementation of the target concepts are substantiated. 相似文献
129.
Apatite fission track analysis was performed on 56 samples from central Spain to unravel the far field effects of the Alpine plate tectonic history of Iberia. The modelled thermal histories reveal complex cooling in the Cenozoic, indicative of intermittent denudation. Accelerated cooling events occurred across the Spanish Central System (SCS) from the Middle Eocene to Recent. These accelerated cooling events resulted in up to 2.8±0.9 km of denudation in the western Sierra de Gredos and 3.6±1.0 km in the central and eastern Gredos (assuming a paleogeothermal gradient of 28±5 °C and a surface temperature of 10 °C). The greatest amount of denudation (5.0±1.6 km) occurred in the Sierra de Guadarrama. Accompanying rock uplift was 4.7±1.0 and 5.9±1.6 km in the eastern Gredos and Guadarrama, respectively. Most denudation in the Gredos occurred from the Middle Eocene to the Early Miocene and can be related to the N–S stress field, induced by the Pyrenean compression. In the Guadarrama, the greatest denudation was Pliocene to Recent of age and seems related to the ongoing NW–SE Betic compression. The fact that the formation of the E–W trending Gredos coincides with the N–S Pyrenean compression and the creation of the present day morphology of the NE–SW trending Guadarrama with the younger NW–SE Betic compression, indicates that they record the far field effects of Alpine plate tectonics on Iberia. The trend of pre-existing lineaments was of major importance in influencing the style and magnitude of these of far field effects. 相似文献
130.
B. D. Johnson A. D. Albani P. C. Rickwood J. W. Tayton 《Australian Journal of Earth Sciences》2013,60(7-8):403-408
The bedrock topography of the Botany Basin has been determined from seismic‐sparker records made in Botany Bay and Bate Bay, and from seismic‐refraction and gravity measurements on the Kurnell Peninsula. Supplementary information has been obtained from boreholes both on land and in the sea. The Cooks and Georges Rivers formerly constituted the main drainage of the Basin and flowed generally southeastwards (beneath the present Kurnell Peninsula) and joined the Port Hacking River east of Cronulla. The depth of the bedrock channel of the former Georges River is 75–80 m b.s.l. at Taren Point, 90–95 m beneath the Kurnell Peninsula and 110–115 m at its junction with the Port Hacking River channel. The bedrock channel of the former Cooks River is about 30 m b.s.l. at Kyeemagh, its present entrance to Botany Bay, and it joined the Georges River at a location now 90 m b.s.l. beneath the Kurnell Peninsula. A second drainage system existed in the north and east of Botany Bay and generated the present mouth beneath which the bedrock is now 110 m b.s.l. This channel followed a southeasterly course parallel to the present northern shore of Botany Bay and was separated from that of the ‘Cooks and Georges Rivers’ by a bedrock ridge which extended from beneath Sydney Airport to the northern extremity of the Kurnell Peninsula. Over much of its length this divide had a depth of about 30 m b.s.l. The formation of the Kurnell Peninsula tombolo led to the diversion of the ‘Cooks/Georges River’ through the mouth of Botany Bay and subsequently led to the development of the bay. This change in the drainage system occurred when the sea was less than 30 m below the present sea level. 相似文献