全文获取类型
收费全文 | 93748篇 |
免费 | 18610篇 |
国内免费 | 42470篇 |
专业分类
测绘学 | 3869篇 |
大气科学 | 25224篇 |
地球物理 | 24638篇 |
地质学 | 54355篇 |
海洋学 | 21064篇 |
天文学 | 16130篇 |
综合类 | 4085篇 |
自然地理 | 5463篇 |
出版年
2022年 | 588篇 |
2021年 | 1077篇 |
2020年 | 2401篇 |
2019年 | 5851篇 |
2018年 | 6896篇 |
2017年 | 6490篇 |
2016年 | 6831篇 |
2015年 | 5600篇 |
2014年 | 5646篇 |
2013年 | 7796篇 |
2012年 | 6162篇 |
2011年 | 6502篇 |
2010年 | 6259篇 |
2009年 | 6495篇 |
2008年 | 5327篇 |
2007年 | 5253篇 |
2006年 | 4706篇 |
2005年 | 4174篇 |
2004年 | 4572篇 |
2003年 | 4183篇 |
2002年 | 3836篇 |
2001年 | 3406篇 |
2000年 | 2955篇 |
1999年 | 2798篇 |
1998年 | 2921篇 |
1997年 | 2956篇 |
1996年 | 2362篇 |
1995年 | 2283篇 |
1994年 | 2066篇 |
1993年 | 1894篇 |
1992年 | 1694篇 |
1991年 | 1427篇 |
1990年 | 1488篇 |
1989年 | 1274篇 |
1988年 | 1120篇 |
1987年 | 1223篇 |
1986年 | 1001篇 |
1985年 | 1160篇 |
1984年 | 1276篇 |
1983年 | 1126篇 |
1982年 | 1065篇 |
1981年 | 1006篇 |
1980年 | 868篇 |
1979年 | 827篇 |
1978年 | 792篇 |
1977年 | 710篇 |
1976年 | 684篇 |
1975年 | 661篇 |
1974年 | 616篇 |
1973年 | 651篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Based on the shear effect of internal wave currents in stratified fluid, the necessary condition for the possible existence of an internal wave and the sufficient condition under which there will be no internal wave are deduced starting from the continuity equation of incompressible fluid. The above two conditions are verified by the measured results from vertical current meter arraies at some observation stations in the Bohai Sea and the South Huanghai Sea. In the treatment of measured results, first of all, the tidal period components are separated from the measured currents by Fourier-analysis method, and then the internal wave currents of tidal period from the tidal period components in the light of some considerations. The analytical results of observed currents are well consistent with theoretical analysis. Furthermore, one can also deduce some features of internal waves therefrom. Consequently, it is concluded that the observation from a vertical current meter array is an important means to verif 相似文献
992.
AN ECOSYSTEM MODEL ANALYSIS OF THE EFFECT OF MINE TAILINGS ON THE EUPHOTIC ZONE OF A PELAGIC ECOSYSTEM 总被引:1,自引:0,他引:1
A computer analysis was performed on experimental results obtained when mine tailings were added to seawater. The Mixed Upper-Layer Ecotrophie Simulation (MULES) model was tested by changing the extinction coefficient and the abundance of heterotrophic zooflagellates. Increasing the extinction coefficient resulted in a delay of phytoplankton growth, an increase in zooplankton standing stock and better growth of autotrophic flagellates compared with diatoms. Zooflagellates in the ecosystem influence the growth of zooplankton; secondary production by zooplankton was markedly depressed at low levels of zooflagellates. These results are believed to be of general significance for the diagnosis of suspended sediment effects on planktonic ecosystems. 相似文献
993.
The paper describes an analytical method to determine an operability index for a marine vehicle when its geometric characteristics, the geographical area in which the vessel is supposed to operate, and the limiting criteria for operations are known. The computer program based on strip theory provides reliable results not only for conventional hullforms, but also for catamarans, offshore vehicles, etc. The results are presented in the form of figures for different vessels, namely, crane barge, naval vessels and SWATH type catamarans. The authors are of the opinion that the proposed method would provide the designer with a valid tool to improve the seakeeping qualities of a vessel, while taking into account the limiting conditions imposed due to seaway operations. 相似文献
994.
995.
We establish the equations of finite element approximation for three-dimensional ocean current and calculate the Kuroshio current in the eastern sea area off Taiwan Province. The interpolation error of finite element approximation is given as Eq. (3. 14). 相似文献
996.
Seismic reflection profiling demonstrates the importance of mass movement in the north-central California Borderland. Bulk
properties of surficial hemipelagic sediments in the Santa Barbara, Santa Cruz, and San Nicolas basins show that different
types of mass-transported sediments have different properties, although their general sedimentologic characteristics are very
similar. Slump deposits have moderate water contents (48% wet weight basis), high plasticity indices (46%), and moderate activities
(1.6). For debris flows these values are low: 28%, 5%, and 0.3, respectively, and for other types (liquefied flow, mudflows,
and turbidities) the values are high: 54 to 63%, 49 to 58%, and 1.2 to 1.5, respectively. 相似文献
997.
A new approach to high-order Boussinesq-type equations with ambient currents is presented. The current velocity is assumed to be uniform over depth and of the same magnitude as the shallow water wave celerity. The wave velocity field is expressed in terms of the horizontal and vertical wave velocity components at an arbitrary water depth level. Linear operators are introduced to improve the accuracy of the kinematic condition at the sea bottom. The dynamic and kinematic conditions at the free surface are expressed in terms of wave velocity variables defined directly on the free surface. The new equations provide high accuracy of linear properties as well as nonlinear properties from shallow to deep water, and extend the applicable range of relative water depth in the case of opposing currents. 相似文献
998.
Sea ice characteristics between the middle Weddell Sea and the Prydz Bay, Antarctica during the austral summer of 2003 总被引:1,自引:0,他引:1
The antarctic sea ice was investigated upon five occasions between January 4 and February 15, 2003. The investigations included: (1) estimation of sea ice distribution by ship-based observations between the middle Weddell Sea and the Prydz Bay; (2) estimation of sea ice distribution by aerial photography in the Prydz Bay; (3) direct measurements of fast ice thickness and snow cover, as well as ice core sampling in Nella Fjord; (4) estimation of melting sea ice distribution near the Zhongshan Station; and (5) observation of sea ice early freeze near the Zhongshan Station. On average, sea ice covered 14.4% of the study area. The highest sea ice concentration (80%) was observed in the Weddell Sea. First-year ice was dominant (99.7%-99.8%). Sea ice distributions in the Prydz Bay were more variable due to complex inshore topography, proximity of the Larsemann Hills, and/or grounded icebergs. The average thickness of landfast ice in NeUa Fjord was 169.5 cm. Wind-blown snow redistribution plays an important role in affecting the ice thickness in Nella Fjord. Preliminary freezing of sea ice near the Zhongshan Station follows the first two phases of the pancake cycle. 相似文献
999.
In this study, we examined the distribution of polycyclic aromatic hydrocarbons (PAHs) in a contaminated coastal area and the characteristics of the natural organic matter in tandem. We present a detailed study of PAH concentration, distribution, and organic matter characteristics of three core samples from Pensacola Bay, Florida. Solid-state 13C Nuclear Magnetic Resonance (NMR), pyrolysis gas chromatography coupled with mass spectrometry (GC-MS), and tetramethyl ammonium hydroxide (TMAH) thermochemolysis GC-MS were applied to obtain structural details about the sedimentary organic matter. Elemental compositions (carbon and nitrogen) and estimates of black carbon contents are also reported. These coastal sediments were found to contain more PAHs in the upper 15 cm layers than in the bottom 15-25 cm samples. The samples that contained the most PAHs also contained the least amount of aromatic carbon and contained a significant amount of paraffinic carbon. Lignin-derived pyrolysis and TMAH thermochemolysis products were abundant and generally higher in all of the samples in comparison to those reported for modern coastal sediments, indicating a large flux of terrestrial carbon. The black carbon contents were found to range from 4.3% to 6.8%, which are significantly lower than other reports of black carbon in sediments, which represent as much as 65% of the total organic carbon content. The low black carbon content suggests that this type of refractory carbon may not be as responsible for regulating PAH distribution as indicated by other researchers. 相似文献
1000.
Sabra K.G. Roux P. Thode A.M. D'Spain G.L. Hodgkiss W.S. Kuperman W.A. 《Oceanic Engineering, IEEE Journal of》2005,30(2):338-347
Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source. 相似文献