全文获取类型
收费全文 | 71433篇 |
免费 | 1419篇 |
国内免费 | 658篇 |
专业分类
测绘学 | 1731篇 |
大气科学 | 5055篇 |
地球物理 | 14309篇 |
地质学 | 25377篇 |
海洋学 | 6208篇 |
天文学 | 16418篇 |
综合类 | 214篇 |
自然地理 | 4198篇 |
出版年
2022年 | 396篇 |
2021年 | 705篇 |
2020年 | 786篇 |
2019年 | 833篇 |
2018年 | 1898篇 |
2017年 | 1773篇 |
2016年 | 2245篇 |
2015年 | 1319篇 |
2014年 | 2193篇 |
2013年 | 3777篇 |
2012年 | 2284篇 |
2011年 | 3129篇 |
2010年 | 2622篇 |
2009年 | 3523篇 |
2008年 | 3281篇 |
2007年 | 3068篇 |
2006年 | 2894篇 |
2005年 | 2367篇 |
2004年 | 2273篇 |
2003年 | 2132篇 |
2002年 | 1945篇 |
2001年 | 1811篇 |
2000年 | 1730篇 |
1999年 | 1393篇 |
1998年 | 1486篇 |
1997年 | 1392篇 |
1996年 | 1113篇 |
1995年 | 1148篇 |
1994年 | 966篇 |
1993年 | 876篇 |
1992年 | 861篇 |
1991年 | 761篇 |
1990年 | 861篇 |
1989年 | 724篇 |
1988年 | 656篇 |
1987年 | 816篇 |
1986年 | 669篇 |
1985年 | 860篇 |
1984年 | 933篇 |
1983年 | 868篇 |
1982年 | 839篇 |
1981年 | 711篇 |
1980年 | 664篇 |
1979年 | 616篇 |
1978年 | 607篇 |
1977年 | 562篇 |
1976年 | 543篇 |
1975年 | 508篇 |
1974年 | 512篇 |
1973年 | 472篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
X-band sea-clutter nonstationarity: influence of long waves 总被引:1,自引:0,他引:1
In this paper, we deal with the problem of modeling the backscattering from sea surface for low-grazing-angle and high-resolution radar systems. Based on the electromagnetic two-scale model, we analyzed both the amplitude and frequency modulations induced on the small-scale Bragg resonant waves by the large-scale surface tilt and advection due to the swell presence. Evidence of sea-clutter nonstationarity has been verified and the relationship between the variations of clutter spectral features, such as texture, Doppler centroid, and bandwidth, have been studied by processing real sea-clutter data recorded by the IPIX radar of McMaster University, Hamilton, ON, Canada. An autoregressive nonstationary process is proposed and validated to model the physical phenomenon. 相似文献
872.
Ecotone or Ecocline: Ecological Boundaries in Estuaries 总被引:1,自引:0,他引:1
Two main ecological boundaries, ecotone and ecocline, have been defined in landscape ecology. At this scale, the estuary represents a boundary between rivers and the sea, but there has been no attempt to fit empirical data for estuaries to these boundary models. An extensive data set from the Thames estuary was analysed using multivariate techniques and species-range analysis, in order to investigate whether the ecocline or the ecotone model was most relevant to this estuary. Data for periods of high and low freshwater flow allowed the impact of large-scale fluctuations implicit in both models to be determined.A continuum of assemblages existed along the salinity gradient from freshwater river to the North Sea, with shifts in the ranges of organisms apparent in response to changes in freshwater flow. This pattern closely fits an ecocline model. However, the estuary differs from previously defined ecoclines in having two overlapping gradients in the major stressor: from river to mid-estuary for freshwater species and from sea to mid-estuary for marine species. We propose, therefore, that the estuary represents a two-ecocline model, with fauna inhabiting the mid-estuary being either freshwater or marine species at the edge of their range, rather than ‘ true estuarine organisms ’. This allows a redefinition of the Remane diagram, with estuarine species removed, and supports previous arguments that brackish-water species do not exist. Such two-ecocline models may also exist in other marine systems, such as rocky shores. 相似文献
873.
J. M. Lort W. Q. Limond J. Segoufin Ph. Patriat J. R. Delteil B. Damotte 《Marine Geophysical Researches》1979,4(1):71-89
This paper presents seismic reflection and refraction data from the Mozambique Channel, collected between 1971 and 1973. A deep sedimentary basin (up to 5 km of sediments) opens southwards to the Mozambique Basin, and is bounded to the east by the Davie Ridge and beyond by the marginal plateau of Malagasy. A continuous reflector (C), possibly of Cretaceous age, is identified between layers having seismic interval velocities of 2.4–2.8 km/s and 3.1–3.4 km/s. The deepest sediments have velocities of 4.5–4.9 km/s and overlie a layer with velocity 5.5 km/s, which may be volcanic in the north-east of the Channel.The crust occupying most of the Channel is probably pre-Cretaceous in age, and may be largely continental in nature. This is supported by subdued magnetic anomalies and the possibility of a continuous Karroo sedimentary section across the northern Channel. The oceanic crust of the Mozambique Basin may extend as far north as 24°S, into the western Channel only. The problem of the origin of the Mozambique Channel remains unresolved, although a long sedimentary history indicates that Malagasy may have separated from Mainland Africa prior to Karroo times. The Davie Ridge may possibly represent a relict strike-slip fault, which permitted movement along a north-south line. 相似文献
874.
Seismic stratigraphic and main lithological features of the sedimentary cover overlying the basement of the Alboran Sea were established via the analysis of commercial multichannel seismic surveys, geophysical well logs, and well data. Six seismic stratigraphics units (VI to I), bounded by unconformities, form the marine sediments that range in age from early Miocene to Quaternary. They are dated by extrapolation of commercial drilling results from the northern Alboran Sea. Volcanic activity is recorded within sedimentary sequences of units V to II. Undercompaction features are detected in the two basal units. 相似文献
875.
Mud and fluid migration in active mud volcanoes in Azerbaijan 总被引:7,自引:2,他引:7
Mud volcanic eruptions in Azerbaijan normally last for less than a few hours, and are characterized by vigorous extrusion of mud breccias, hydrocarbon gases, and waters. Recent fieldwork and mapping on four active mud volcanoes show that dormant period activity ranges from quiet to vigorous flow of mud and fluids. Geochemical analyses of expelled waters show a wide range in solute concentrations, suggesting the existence of a complex plumbing system. The mud and fluids have a deep origin, but are sometimes stored in intermediate-depth mud chambers. A mixing model between deep-seated saline waters and shallow meteoric water is proposed. 相似文献
876.
V. M. Kushnir 《Physical Oceanography》1997,8(1):29-38
Experimentally-derived data are used to scrutinize structural peculiarities of the upper boundary of deep near-bottom layer
in the Black Sea. Thermal fluxes through the layer’s upper boundary have been evaluated and compared with the geothermal fluxes.
Although there is significant divergence between the individual evaluations, the quantities at issue generally agree with
one another. It is shown that in evaluating thermal fluxes, it is necessary to consider fine structure of the differential-diffusive
type. Stability of the upper boundary of the deep near-bottom layer has been examined, and diagrams of stability/instability
resulting from the development of convection have been plotted.
Translated by Vladimir A. Puchkin. 相似文献
877.
M.E. vinogradov E.A. Shushkina V.I. Vedernikov N.P. Nezlin V.I. Gagarin 《Deep Sea Research Part II: Topical Studies in Oceanography》1997,44(9-10)
Data collected from 20 years of observations by the P. P. Shirshov Institute of Oceanology (about 250 comprehensive ecosystem stations) are summarized to evaluate the primary production and biomass of the principal groups of plankton in the Pacific Ocean. The stations were classified into geographical regions according to satellite determined criteria. The areas of these regions were evaluated according to CZCS data from 1978 to 1986. The total value of primary production was evaluated as 26·9 Gt C year−1. Taking the “bottle effect” correction into account, this value may be as high as 45.6 Gt C year−1. Total biomass values of phytoplankton (134 Mt C), bacteria (86 Mt C), protozoa (26 Mt C) and mesoplankton (184 Mt C) were also calculated. Seasonal variabilities of all these values were remarkably insignificant. 相似文献
878.
The problem of simulation of possible scenarios of the development of processes running in ecological economic systems is studied as a part of the general problem of control over natural economic complexes. The consumption of mineral, food, and ecological resources is regarded as credits given by the society to the producer and then refunded as a part of his gains. If this nature-protective tax does not compensate the damage to the environment caused by production, then the debt of the producer (called accumulated credit) appears and, finally, the process of consumption of the resources is terminated. A universal simulation model is constructed by using a new method of adaptive balance of causes (ABC method). We present basic concepts of this method and some examples of application of the simulation model to the analysis of various scenarios of consumption of marine bioresources under variable economic conditions for a marine environment with variable assimilation capacity. 相似文献
879.
The flux and compositions of solvent-extractable lipid fractions were measured in particulate matter collected periodically by moored sediment traps in the Santa Monica Basin from 1985–1988. The purpose was to assess the compositional changes during settling, the carbon dynamics in the basin and to estimate the impacts of energy-related by-products on the surface sediments. Sediment traps recorded consistently high lipid fluxes in the eastern slope relative to the central basin, reflecting elevated terrigenous carbon inputs possibly from land-based human activities. Generally, lipid fluxes decrease offshore but increase vertically with water depth below 500m, implying lateral transport of particles. The steep decline of flux in the top 500m of the water is related to the rapid decomposition and mineralization of the marine-derived cellular carbon compounds. Less than 5% of the marine lipid components reach the seabed. In contrast, preferential preservation of terrigenous lipid is clearly evident from the chemistry of deeper traps and surface sediments.The lateral transport of particles is reflected in the presence of higher plant-derived lignin phenols and sewage-derived coprostanol and epicoprostanol in the deep trap material as well as in surface sediment throughout the basin. Petroleum triterpanes characteristic of natural seepage also permeates through the entire basin. Based on the data collected from both the trap particulte matter and surface sediments, a carbon budget for the Santa Monica Basin has been constructed. 相似文献
880.