全文获取类型
收费全文 | 71244篇 |
免费 | 1130篇 |
国内免费 | 659篇 |
专业分类
测绘学 | 1714篇 |
大气科学 | 5030篇 |
地球物理 | 14225篇 |
地质学 | 25230篇 |
海洋学 | 6166篇 |
天文学 | 16332篇 |
综合类 | 208篇 |
自然地理 | 4128篇 |
出版年
2022年 | 396篇 |
2021年 | 704篇 |
2020年 | 778篇 |
2019年 | 825篇 |
2018年 | 1894篇 |
2017年 | 1760篇 |
2016年 | 2237篇 |
2015年 | 1321篇 |
2014年 | 2176篇 |
2013年 | 3770篇 |
2012年 | 2277篇 |
2011年 | 3118篇 |
2010年 | 2605篇 |
2009年 | 3504篇 |
2008年 | 3256篇 |
2007年 | 3046篇 |
2006年 | 2876篇 |
2005年 | 2353篇 |
2004年 | 2253篇 |
2003年 | 2110篇 |
2002年 | 1927篇 |
2001年 | 1800篇 |
2000年 | 1717篇 |
1999年 | 1377篇 |
1998年 | 1470篇 |
1997年 | 1387篇 |
1996年 | 1100篇 |
1995年 | 1142篇 |
1994年 | 962篇 |
1993年 | 872篇 |
1992年 | 855篇 |
1991年 | 759篇 |
1990年 | 856篇 |
1989年 | 717篇 |
1988年 | 650篇 |
1987年 | 814篇 |
1986年 | 663篇 |
1985年 | 855篇 |
1984年 | 922篇 |
1983年 | 862篇 |
1982年 | 831篇 |
1981年 | 707篇 |
1980年 | 663篇 |
1979年 | 606篇 |
1978年 | 603篇 |
1977年 | 552篇 |
1976年 | 539篇 |
1975年 | 499篇 |
1974年 | 506篇 |
1973年 | 468篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
The change in the inertia tensor of the Earth, due to the mass shift following a seismic event, has been computed by several authors for non-rotating earth models. Rotation is taken into account in the present paper, and the additional change in the inertia tensor is computed for an equivalent earth model, in which the axis of geometrical symmetry becomes tilted instead of the axis of greatest inertia. Rotation is thus seen to produce an increase by a factor 1.4 in the amplitude variation of the Chandler wobble, with respect to the non-rotating case, which, when added to the 1.4 amplitude increase due to the precessional re-adjustment of the equatorial bulge, gives a factor of 2 increase of the Chandler wobble amplitude with respect to the case of a rigid earth model. 相似文献
862.
summary . A new catalogue of gravity data from Kenya has been prepared and is briefly described here. New Bouguer anomaly maps have also been compiled and a copy is included. 相似文献
863.
C.M. R. Fowler 《Geophysical Journal International》1978,54(1):167-183
Summary. A structural model of the Mid-Atlantic Ridge at 45° N is proposed on the basis of travel-time data, amplitudes and synthetic seismograms. The crustal structure seems to be similar to that in the FAMOUS area (Fowler). At the ridge axis there is an absorptive zone in the upper mantle, the depth below the seabed to the top of this zone being about 6 km. Away from the ridge axis there is a positive velocity gradient of about 0.04 to 0.05 km/(skm) in the top 5 to 8 km of the upper mantle. Shear waves propagate across the ridge axis, suggesting that there is no sizeable crustal magma chamber. The shear-wave velocity of the uppermost mantle is 4.35 km/s. 相似文献
864.
G.S. Fuis E.L. Ambos W.D. Mooney R.A. Page M.A. Fisher T.M. Brocher J.J. Taber 《Geophysical Journal International》1987,89(1):73-78
Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors. 相似文献
865.
Summary. The space-time elastic wave motion generated by an impulsive line source in a homogeneous anisotropic medium is calculated with the aid of the Cagniard-de Hoop method. Two types of sources are considered in detail, viz. a line source of expansion (model for an explosive source) and a line force (model for a mechanical vibrator). Numerical results are presented for the radiated particle velocity in the medium including those regions of space where shear-wave triplication occurs. There is a marked difference in the time response observed for the two types of sources and for the different positions of the receiver with respect to the source position. These waveform differences are important when the radiated wave is used to determine experimentally the elastic properties of the medium. As compared with the traditional Fourier-integral transform method to handle this problem, the computation time with the present method is considerably less. 相似文献
866.
Lene Møller Madsen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):87-99
The article analyses the results of the scheme for field afforestation (EU-regulation 2080/92) in Denmark. The purpose is twofold, firstly to document the afforestation on arable land taking place both within the scheme and outside the scheme for field afforestation. The former is documented in Ribe and Vejle counties whereas the latter is documented in three smaller areas: Varde, Vorbasse, and Vejle. Secondly, the article argues that in order to evaluate the afforestation not only the location within designated areas hut also the spatial parameters and configuration of new woodlands must be considered. Two methods -'forest-pictures' and ‘gradient-pictures’-are presented and applied in order to quantify the spatial configuration of new woodlands in the landscape. 相似文献
867.
WILLIAM G. CAMPBELL M. ROBBINS CHURCH GARY D. BISHOP DOROTHY C MORTENSON SUZANNE M. PIERSON 《International journal of geographical information science》2013,27(4):349-362
Abstract A geographical information system (GIS) has proved to be a valuable tool in the Direct/Delayed Response Project—a comprehensive examination of the future, long-term chemical response of surface waters to acidic deposition. The role of the GIS within the project includes aggregation, analysis and display of various forms, scales and projections of environmental data related to acidic deposition. The GIS is particularly valuable for effectively communicating key scientific findings and results of regional, national and international importance to a diverse audience. 相似文献
868.
JINFEI WANG PAUL M. TREITZ PHILIP J. HOWARTH 《International journal of geographical information science》2013,27(2):141-157
Abstract Visual interpretation of high-resolution satellite data has been useful for mapping linear features, such as roads and updating land-use changes. However, it would be beneficial to map new road networks digitally from satellite data to update digital databases using semi-automated techniques. In this paper, an algorithm called Gradient Direction Profile Analysis (GDPA) is used to extract road networks digitally from SPOT High Resolution Visible (HRV) panchromatic data. The roads generated are compared with a visual interpretation of the SPOT HRV multispectral and panchromatic data. The technique is most effective in areas where road development is relatively recent. This is due to the spectral consistency of new road networks. As new road networks are those of most interest to the land manager, this is a useful technique for updating digital road network files within a geographical information system of urban areas. 相似文献
869.
Lars Eklundh Ulrik Mårtensson 《International journal of geographical information science》2013,27(3):329-340
Abstract In this article we demonstrate that substantial gains in time can be made when using point sampling rather than contour line digitising for generation of Digital Elevation Models (DEMs). A simple sampling scheme, based on regularly distributed points, was used supplemented with points near break-lines in the terrain. An evaluation of surfaces created with three different interpolation methods at three different resolutions shows that the statistical distribution was better when using points as opposed to contours, and that the accuracy was comparable despite the much smaller amount of input data. 相似文献
870.
U. Morgenstern R. G. Ditchburn E. G. Vologina M. Sturm 《Journal of Paleolimnology》2013,50(3):345-352
We applied the 32Si dating technique to a sediment core from Lake Baikal to obtain the sediment chronology for the last millennium. The core was recovered about 4 km offshore from the north slope of the South Basin in 1,366 m water depth. The sediment material consisted of continuously accumulated diatom-rich geogenic-terrigenous mud, intercalated with a number of dark olive-grey turbidite layers. The sediment layers containing the turbidites were excluded from 32Si sampling to obtain the chronology that is representative of the continuous sedimentation. The initial 32Si activity of 31.3 dpm kg?1 SiO2, measured in sediment trap samples, confirms the trend of 32Si specific activities of biogenic silica found in other Northern Hemisphere lakes. The four sediment core samples from depth 0–48 cm have 32Si specific activities between 23.5 and 0.5 dpm kg?1 SiO2, with corresponding ages between 60 and 860 years and constant sedimentation rate of 0.036 ± 0.004 cm year?1 over the most recent 800 years. 32Si allowed us for the first time to date the uppermost turbidites in the South Basin of Lake Baikal, to 1030, 1310 and 1670 ad. Given these dates, the last long-distance turbidity current triggered by slope instabilities had occurred 330 years before 2000 ad, and the intervals between the 1310 and 1670 ad event and between the 1030 and 1310 ad event were 360 and 280 years, respectively. The 32Si ages allow unprecedented time resolution for reconstruction of the former environmental and climatic conditions during the past millennium. 相似文献