全文获取类型
收费全文 | 7640篇 |
免费 | 269篇 |
国内免费 | 94篇 |
专业分类
测绘学 | 228篇 |
大气科学 | 647篇 |
地球物理 | 1653篇 |
地质学 | 2890篇 |
海洋学 | 620篇 |
天文学 | 1370篇 |
综合类 | 23篇 |
自然地理 | 572篇 |
出版年
2022年 | 51篇 |
2021年 | 91篇 |
2020年 | 120篇 |
2019年 | 134篇 |
2018年 | 213篇 |
2017年 | 227篇 |
2016年 | 272篇 |
2015年 | 199篇 |
2014年 | 258篇 |
2013年 | 383篇 |
2012年 | 286篇 |
2011年 | 420篇 |
2010年 | 340篇 |
2009年 | 459篇 |
2008年 | 389篇 |
2007年 | 340篇 |
2006年 | 351篇 |
2005年 | 340篇 |
2004年 | 414篇 |
2003年 | 314篇 |
2002年 | 240篇 |
2001年 | 173篇 |
2000年 | 168篇 |
1999年 | 137篇 |
1998年 | 144篇 |
1997年 | 112篇 |
1996年 | 85篇 |
1995年 | 80篇 |
1994年 | 66篇 |
1993年 | 63篇 |
1992年 | 67篇 |
1991年 | 62篇 |
1990年 | 57篇 |
1989年 | 39篇 |
1988年 | 47篇 |
1987年 | 57篇 |
1986年 | 41篇 |
1985年 | 44篇 |
1984年 | 45篇 |
1983年 | 36篇 |
1982年 | 41篇 |
1981年 | 53篇 |
1980年 | 38篇 |
1978年 | 37篇 |
1977年 | 40篇 |
1976年 | 32篇 |
1975年 | 31篇 |
1974年 | 42篇 |
1973年 | 36篇 |
1971年 | 26篇 |
排序方式: 共有8003条查询结果,搜索用时 15 毫秒
111.
Annual Cycle of Zooplankton Biomass, Abundance and Species Composition in the Neritic Area of the Balearic Sea, Western Mediterranean 总被引:7,自引:0,他引:7
Abstract. Seasonal changes in zooplankton biomass, abundance and species composition were studied at a neritic station in the Balearic Sea between April 1993 and May 1994. Sampling was carried out every 10 days in a zone influenced by the main current circulating through the Mallorca channel. Three main peaks of zooplankton biomass and abundance were observed: (1) at the beginning of summer when the thermocline developed, (2) in autumn when the thermocline broke down, and (3) in early spring. The smaller zooplankton fraction (100–250 μm) comprised on average 32 % of the total biomass and 73 % of total abundance. Copepods were the predominant group (64 % of the total abundance) with Clausocalanus, Oithona and Paracalanus being the most abundant genera. Paracalanus parvus, Clausocalanus furcatus, Acartia clausi, Oithona plumifera, Temora stylifera, Centropages typicus and Oncaea mediterranea were found to be the most important species in the area. Other abundant groups were cladocerans (15 %) and meroplankton larvae (12 %), both of which were particularly numerous during the stratified period. The copepod community was characterized by the above‐cited perennial species, which were abundant during the cycle studied. However, the influence of the hydrological conditions of the Balearic Sea, such as the Atlantic water influx and the physical structure of the water column (stratification and mixing), promoted the observed variability in zooplankton as well as the appearance of characteristic species during the annual cycle. 相似文献
112.
113.
Many important areas of Quaternary deltaic sedimentation along the Brazilian coast are practically unknown in the geologic
literature, especially outside Brazil. Our studies show that these areas, previously considered as highly destructive wave-dominated
deltas of Holocene age, were affected by a wave-dominated phase during the Pleistocene, succeeded by highly constructive,
intralagoonal stage in the Holocene, in turn followed by wave-dominated deltaic sedimentation that continues until today.
The geologic evolution of these coastal plains is exemplified here by the Doce River mouth area, State of Espírito Santo,
where relative sea-level fluctuations during the Quaternary played an important role in the construction of the deltaic complex. 相似文献
114.
A two-layered model is considered in which the upper layer is continuously stratified and the lower layer is homogeneous. The system is driven by atmospheric forces. Bottom stress and topography are included in the model. The linear three-dimensional hydrodynamical equations are used to describe the system. Taking the eddy viscosity in the upper layer as inversely proportional to the static stability, the dependent variables are expanded in terms of continuous functions in the vertical (eigenfunctions). Using this method it is possible to compute currents and internal displacements at any depth in the upper layer. The three-dimensional structure of the lower layer is not considered in this model. The equations describing the lower layer are integrated over depth to give depth mean currents. Using a staggered finite-difference grid in the horizontal and a forward time-stepping procedure, numerical test experiments are carried out for a cross section and for a closed rectangular basin. 相似文献
115.
116.
I. Brandariz P. Castro M. Montes F. Penedo M.E. Sastre de Vicente 《Marine Chemistry》2006,102(3-4):291-299
Acid–base equilibrium constants of triethanolamine (TEA) have been determined by potentiometric titrations with a glass electrode, at 25 °C. Ionic strength was kept constant with only one electrolyte (using one of these salts: NaCl, KCl, MgCl2 or CaCl2), with binary mixtures of MgCl2 and CaCl2, and finally, in a solution with a composition approximately similar to that of natural seawater without sulfate. Equilibrium constants have been expressed in function of ionic strength by means of Pitzer equations and interaction parameters proposed in this theory have been obtained. It has been found that acid–base behaviour of TEA depends greatly on the salt used: basicity of TEA is decreased by CaCl2, while it is increased by the other electrolytes used in this work. 相似文献
117.
Jonathan M. Bull Martin Gutowski Justin K. Dix Timothy J. Henstock Peter Hogarth Timothy G. Leighton Paul R. White 《Marine Geophysical Researches》2005,26(2-4):157-169
Chirp sub-bottom profilers are marine acoustic devices that use a known and repeatable source signature (1–24 kHz) to produce
decimetre vertical resolution cross-sections of the sub-seabed. Here the design and development of the first true 3D Chirp
system is described. When developing the design, critical factors that had to be considered included spatial aliasing, and
precise positioning of sources and receivers. Full 3D numerical modelling of the combined source and receiver directivity
was completed to determine optimal source and receiver geometries. The design incorporates four source transducers (1.5–13 kHz)
that can be arranged into different configurations, including Maltese Cross, a square and two separated pairs. The receive
array comprises 240 hydrophones in 60 groups whose group-centres are separated by 25 cm in both horizontal directions, with
each hydrophone group containing four individual elements and a pre-amplifier.
After careful consideration, it was concluded that the only way to determine with sufficient accuracy the source–receiver
geometry, was to fix the sources and receivers within a rigid array. Positional information for the array is given by a Real
Time Kinematic GPS and attitude system incorporating four antennas to give position, heading, pitch and roll. It is shown
that this system offers vertical positioning accuracy with a root-mean-square (rms) error less than 2.6 cm, while the horizontal
positioning rms error was less than 2.0 cm. The system is configured so that the Chirp source signature can be chosen by software
aboard the acquisition vessel.
The complete system is described and initial navigational and seismic data results are presented. These data demonstrate that
the approach of using fixed source-receiver geometry combined with RTK navigation can provide complete 3D imaging of the sub-surface. 相似文献
118.
119.
120.
The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River
mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations
in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from
several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling
velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral
groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River
sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to
be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material
of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon
mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally,
relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine
input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil
Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence,
stronger sedimentation of Amazon sediments would occur only west of the river mouth. 相似文献