全文获取类型
收费全文 | 24727篇 |
免费 | 320篇 |
国内免费 | 159篇 |
专业分类
测绘学 | 426篇 |
大气科学 | 1348篇 |
地球物理 | 4733篇 |
地质学 | 9638篇 |
海洋学 | 2399篇 |
天文学 | 5587篇 |
综合类 | 48篇 |
自然地理 | 1027篇 |
出版年
2022年 | 269篇 |
2021年 | 468篇 |
2020年 | 440篇 |
2019年 | 522篇 |
2018年 | 1009篇 |
2017年 | 936篇 |
2016年 | 969篇 |
2015年 | 440篇 |
2014年 | 836篇 |
2013年 | 1429篇 |
2012年 | 977篇 |
2011年 | 1201篇 |
2010年 | 1145篇 |
2009年 | 1329篇 |
2008年 | 1134篇 |
2007年 | 1311篇 |
2006年 | 1143篇 |
2005年 | 625篇 |
2004年 | 597篇 |
2003年 | 592篇 |
2002年 | 598篇 |
2001年 | 539篇 |
2000年 | 445篇 |
1999年 | 359篇 |
1998年 | 349篇 |
1997年 | 343篇 |
1996年 | 275篇 |
1995年 | 275篇 |
1994年 | 248篇 |
1993年 | 190篇 |
1992年 | 220篇 |
1991年 | 194篇 |
1990年 | 208篇 |
1989年 | 200篇 |
1988年 | 165篇 |
1987年 | 190篇 |
1986年 | 174篇 |
1985年 | 218篇 |
1984年 | 213篇 |
1983年 | 210篇 |
1982年 | 201篇 |
1981年 | 181篇 |
1980年 | 166篇 |
1979年 | 189篇 |
1978年 | 169篇 |
1977年 | 148篇 |
1976年 | 133篇 |
1975年 | 137篇 |
1974年 | 129篇 |
1973年 | 168篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Cluster analysis is discussed in the context of its application to map various test areas in the sea. By analysing the correspondence of the identified categories to areas with specific hydrological conditions, an attempt is made to relate the types of fine structure to diverse background conditions. With two test areas mapped, a conclusion is made as to the possible application of cluster analysis to this end.Translated by Vladimir A. Puchkin. 相似文献
142.
I. Rychlik 《Ocean Engineering》1996,23(6):447-454
In this note conservative bounds for significant crest height and amplitude obtained from the crossing intensity of a sea are presented. For Gaussian models of a sea level, the Rayleigh approximation for the distributions of amplitude and crest height is proved to provide conservative values for the expected significant wave characteristics. The results are illustrated by examples in which both Gaussian and non-Gaussian models for a sea are considered. 相似文献
143.
The problem of the seasonal variations of temperature and salinity in the intermediate zone located between the West African and the Gulf of Guinea upwellings is considered. The vertical distribution of the phase-amplitude characteristics of the annual and semi-annual variations is examined in good detail and their parameters are defined from the monthly means using the least-squares fit technique. The contribution of isopycnic advection to the formation of seasonal thermohaline variations is studied.Translated by Vladimir A. Puchkin. 相似文献
144.
Spatio-temporal variability of small copepods (especially Oithona plumifera) in shallow nearshore waters off the south coast of South Africa 总被引:1,自引:0,他引:1
Francesca Porri Christopher D. McQuaid William P. Froneman 《Estuarine, Coastal and Shelf Science》2007,72(4):711-720
Although small copepods are one of the main dietary sources for many commercially important fish, their role in the pelagic trophic dynamics has traditionally been underestimated due to the methodology commonly used in plankton sampling. Temporal variation in abundance of adults and nauplii of small copepods (particularly Oithona plumifera) in nearshore waters on the south coast of South Africa was investigated fortnightly over 14 months at site (km) and location (100 m) scales. Sampling was within <500 m of the shore, where depth was ca. 10 m, using vertical hauls of an 80-μm mesh plankton net from 1 m above the seabed to the surface. Twenty-seven adult copepod taxa were recorded, but Oithona spp. was consistently the most abundant. Taxon richness was 7–19 on each sampling occasion. There was strong temporal variation (Oithona varied between 0 and 2300 m−3), but much of this was short-term variability (e.g. between consecutive sampling sessions), with no seasonality or other long-term discernable patterns. There were periods of consistently low numbers, but very high numbers often followed samples with low abundances. Nor was there spatial structure at the location scale, though numbers differed between sites. Despite considerable variability at the location scale within sites, Kenton consistently showed higher densities than High Rocks. Separate analyses, with Bonferroni adjustment, showed that this difference was significant on eight out of 21 occasions for Oithona, six for other pelagic copepods and three for nauplii. This suggests that hydrodynamics favour aggregation of plankton at Kenton. A high degree of short-term variability, with a tendency for aggregation of small zooplankton at certain sites has implications for both pelagic processes and food-web links between the benthic and pelagic environments. 相似文献
145.
In a modern statement, the concept of a parametric integral model consists of integration of the energy balance equation (or wind forcing) over the frequency. The angular distribution of the energy and the frequency spectrum are prescribed. The equation is solved by the numerical method taking into consideration the energy advection and its local pumping with the assigned wind field varying in time and space. To calculate the swell waves one can employ the well-known Longuet-Higgins theorem on the invariance of a two-dimensional spectrum. or the ray discrete technique.UDK 551.466.315Translated by Mikhail M. Trufanov. 相似文献
146.
147.
S. K. P. Coles C. I. Wright D. A. Sinclair P. Van den Bossche 《Marine Georesources & Geotechnology》2002,20(2):87-110
Extensive potassium and phosphorous-rich mineralization occurs on the outer continental shelf of the southern and west coasts of South Africa and Nambia. This article reviews the potential of exploiting these deposits in an environmentally sound manner for the manufacture of fertilizer. At present, reserves of potash and phosphate fertilizer are exploited from terresterial deposits, the majority being surface mined. The use of fertilizer in South Africa has shown no growth in the past 13 years, and, in some years, usage has even declined. On average, over the last decade, South Africa has consumed 2 million Mt of fertilizer (including nitrogen) per annum, the vast majority of phosphate fertilizer being produced by FOSKOR from the Phalaborwa Igneous Complex. Potash fertilizer is imported into South Africa. Although fertilizer consumption is expected to decrease in the short-term, there are good future prospects for the domestic and international fertilizer market. Considerable research into both glauconitic (containing K 2 O) and phosphatic deposits along the southern African continental shelf indicate that these sedimentary deposits have a complex genesis and mineralization. Of the total K 2 O reserves of 1300 million Mt on the southern African margin, 1000 million Mt is located off the southern African west coast, and the remainder situated on the Agulhas Bank. The largest glauconite concentration ( ±300 million Mt K 2 O) off southern Africa lies west of Saldanha Bay, South Africa. The distribution of P 2 O 5 off southern Africa is dominated by the vast deposit between Walvis Bay and Luderitz, Nambia. This reserve is estimated to contain 1000 million Mt of greater than 5% P 2 O 5 in a relatively small area of about 10000 km2. The phosphorite deposit south of Saldanha Bay constitutes a reserve of ±3500 million Mt of apatite and the deposit on the Agulhas Bank comprises 5500 million Mt. The phosphate deposit off Saldanha Bay occurs as an extensive, low to medium grade deposit. Although vast resources of potash and phosphatic minerals occur along the southern African outer continental shelf the expensive nature of marine exploitation may render most of these deposits, especially the phosphates, subeconomic. The low price of fertilizer andextensive natureontheonland deposits, although confined to asmallnumber of countries, mean that it will not be financially viable to extract these deposits. Assuming high grade glauconitic sand with the right composition can be located, the high market price indicates good future prospects for these potash deposits. 相似文献
148.
149.
Christopher L. Dupont James. W. Moffett Robert R. Bidigare Beth A. Ahner 《Deep Sea Research Part I: Oceanographic Research Papers》2006,53(12):1961-1974
Dissolved and particulate concentrations of the biogenic thiols cysteine (Cys), arginine–cysteine (Arg–Cys), glutamine–cysteine (Gln–Cys), γ-glutamate–cysteine (γ-Glu–Cys) and glutathione (GSH) were measured in the subartic Pacific Ocean in the summer of 2003 using high performance liquid chromatography (HPLC) with precolumn derivatization as reported in previous work. In this study, a preconcentration protocol for the derivatized thiols was utilized to extend detection limits of dissolved thiols to picomolar levels. The measured concentrations of particulate and dissolved thiols were uncoupled, with distinctive depth profiles and large differences in the particulate to dissolved ratios between individual compounds. Glutathione was the most abundant particulate thiol whereas the most abundant dissolved thiol was γ-Glu–Cys, with concentrations as high as 15 nM. Given the relatively small pool of intracellular γ-Glu–Cys and the very low dissolved concentrations of GSH, we hypothesize that glutathione released from cells is rapidly converted to the potentially degradation resistant γ-Glu–Cys outside the cell. The relatively high concentrations of other dissolved thiols compared to particulate concentrations implies both biological exudation and slow degradation rates. Some thiols appear to vary with changes in nutrient availability but this effect is difficult to decouple from changes in community structure inferred from pigment analyses. Dissolved thiol concentrations also exceed typical metal concentrations in the subartic Pacific, supporting previous arguments that they may be important in metal speciation. 相似文献
150.
We consider steady, slowly varying water waves propagating on a steady current over a gently sloping bed, so-called current depth refraction. All expressions are correct to second order in wave amplitude. Formulating the energy equation for the fluctuating motion in terms of wave action (wave energy divided by intrinsic angular frequency) results in an expression, where the dissipative term is strikingly similar to wave action itself. It is simply the ‘extra’ dissipation (per unit area) caused by the fluctuating motion (i.e. total dissipation minus the effect of current acting on total mean bed shear stress) divided by the intrinsic angular frequency. We call it ‘wave action dissipation’. An inconsistency in Phillips' (1977) book is pointed out. A new formula for the calculation of wave amplitudes along rays is set forth. 相似文献