全文获取类型
收费全文 | 46954篇 |
免费 | 401篇 |
国内免费 | 1041篇 |
专业分类
测绘学 | 1748篇 |
大气科学 | 3159篇 |
地球物理 | 8812篇 |
地质学 | 20384篇 |
海洋学 | 3219篇 |
天文学 | 6845篇 |
综合类 | 2194篇 |
自然地理 | 2035篇 |
出版年
2022年 | 263篇 |
2021年 | 431篇 |
2020年 | 395篇 |
2019年 | 461篇 |
2018年 | 5633篇 |
2017年 | 4853篇 |
2016年 | 3424篇 |
2015年 | 595篇 |
2014年 | 841篇 |
2013年 | 1307篇 |
2012年 | 1875篇 |
2011年 | 3779篇 |
2010年 | 3035篇 |
2009年 | 3492篇 |
2008年 | 2900篇 |
2007年 | 3543篇 |
2006年 | 1112篇 |
2005年 | 756篇 |
2004年 | 930篇 |
2003年 | 940篇 |
2002年 | 805篇 |
2001年 | 555篇 |
2000年 | 460篇 |
1999年 | 346篇 |
1998年 | 343篇 |
1997年 | 328篇 |
1996年 | 254篇 |
1995年 | 261篇 |
1994年 | 236篇 |
1993年 | 182篇 |
1992年 | 208篇 |
1991年 | 181篇 |
1990年 | 195篇 |
1989年 | 187篇 |
1988年 | 156篇 |
1987年 | 182篇 |
1986年 | 170篇 |
1985年 | 205篇 |
1984年 | 198篇 |
1983年 | 197篇 |
1982年 | 188篇 |
1981年 | 192篇 |
1980年 | 180篇 |
1979年 | 182篇 |
1978年 | 158篇 |
1977年 | 142篇 |
1976年 | 139篇 |
1975年 | 136篇 |
1974年 | 125篇 |
1973年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
291.
Imaging of Buried Archaeological Materials: The Reflection Properties of Archaeological Wood 总被引:1,自引:0,他引:1
Stephanie H.L. Arnott Justin K. Dix Angus I. Best David J. Gregory 《Marine Geophysical Researches》2005,26(2-4):135-144
Effective marine archaeological site management demands detailed information on not only the spatial distribution of artefacts
but also the degradation state of the materials present. Although sonar methods have frequently been used in an attempt to
detect buried wooden shipwrecks they are currently unable to indicate their degradation state. To assess the sensitivity of
acoustic measurements to changes in the degradation state of such material, and hence the potential for sonars to quantify
degradation, laboratory measurements of compressional wave velocity, as well as bulk density for oak and pine samples, in
varying states of decay, were undertaken. These data enabled the calculation of theoretical reflection coefficients for such
materials buried in various marine sediments. As wood degrades, the reflection coefficients become more negative, resulting
in the hypothesis that the more degraded wood becomes, the easier it should be to detect. Typical reflection coefficients
of the order of −0.43 and −0.52 for the most degraded oak and pine samples in sand are predicted. Conversely, for wood exposed
to seawater the predicted reflection coefficients are large and positive for undegraded material (0.35 for oak, 0.18 for pine)
and decrease to zero or slightly below for the most degraded samples. This indicates that exposed timbers, when heavily degraded,
can be acoustically transparent and so undetectable by acoustic methods. Corroboration of these experimental results was provided
through comparison with high resolution seismic reflection data that has been acquired over two shipwrecks. 相似文献
292.
Management of a shallow temperate estuary to control eutrophication: The effect of hydrodynamics on the system’s nutrient loading 总被引:1,自引:0,他引:1
A.I. Lilleb J.M. Neto I. Martins T. Verdelhos S. Leston P.G. Cardoso S.M. Ferreira J.C. Marques M.A. Pardal 《Estuarine, Coastal and Shelf Science》2005,65(4):697-707
The Mondego estuary, a shallow warm-temperate intertidal system located on the west coast of Portugal, has for some decades been under severe ecological stress, mainly caused by eutrophication. Water circulation in this system was, until 1998, mainly dependent on tides and on the freshwater input of a small tributary artificially controlled by a sluice. After 1998, the sluice opening was effectively minimised to reduce the nutrient loading, and the system hydrodynamics improved due to engineering work in the upstream areas. The objective of the present study was to evaluate the effect of the mitigation measures implemented in 1998. Changes to the hydrodynamics of the system were assessed using precipitation and salinity data in relation to the concentrations of dissolved inorganic nutrients, as well as the linkage between dissolved N:P ratios and the biological parameters (phytoplankton chlorophyll a concentrations, green macroalgal biomass and seagrass biomass). Two distinctive periods were compared, over a ten year period: from January 1993 to January 1997 and from January 1999 until January 2003. The effective reduction in the dissolved N:P atomic ratio from 37.7 to 13.2 after 1998 is a result of lowered ammonia, but not the oxidised forms of nitrogen (nitrate plus nitrite), or increased concentrations of dissolved inorganic phosphorus. Results suggest that the phytoplankton is not nutrient limited, yet maximum and mean biomass of green macroalgae was reduced by one order of magnitude after the mitigation measures. This suggests that besides lowering the water residence time of the system, macroalgal growth became nitrogen limited. In parallel to these changes the seagrass-covered area and biomass of Zostera noltii showed signs of recovery. 相似文献
293.
A simple, rapid, precise and reproducible procedure is described for sample preparation, extraction and measurement of total mercury in peaty sediments, plants and animal tissues. Various amounts of a heavy metal containing dried sewage sludge were applied to experimental plots in a New England salt marsh. This treatment resulted in elevation of mercury levels in the surface 5 cm of the soil. Discrepancies in the bulk density of marsh sediments corresponded with interparticle spaces and detritus/mineral ratios in the peat matrix. Mass balance calculations showed that wetlands covered with tall Spartina alterniflora lost mercury considerable faster than higher marsh locations which retained virtually all mercury added. The shortest halflife of mercury calculated was four years. Grain size analyses of peat and sewage sludge suggested that mercury in the higher intertidal range remained associated with sludge components. Biochemical and physical processes affecting the dissipation of mercury from creek side sediments are discussed. 相似文献
294.
Tiago M. Alves Vasilios Lykousis Dimitris Sakellariou Stamatina Alexandri Paraskevi Nomikou 《Geo-Marine Letters》2007,27(1):41-61
Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN), seismic reflection and sediment-core data are used in the analysis
of two tectonic troughs south of Crete, Eastern Mediterranean Sea. Here, up to 1.2 s two-way travel time (TWTT) of strata
have accumulated since the Middle Miocene in association with extension in the South Aegean region. The study area comprises
>100-km- long by >25-km-wide basins filled by sediments subdivided into two seismic units: (1) an upper Unit 1 deposited in
sub-basins which follow the present-day configuration of the southern Cretan margin; (2) a basal Unit 2, more than 500 ms
(TWTT) thick, accumulated in deeper half-graben/grabens distinct from the present-day depocentres. Both units overlap a locally
stratified Unit 3 comprising the pre-Neogene core complex of Crete and Gavdos. In this work, the interpreted seismic units
are correlated with the onshore stratigraphy, demonstrating that denudation processes occurring on Crete and Gavdos in response
to major tectonic events have been responsible for high sedimentation rates along the proximal southern Cretan margin. Consequently,
topographically confined sedimentary units have been deposited south of Crete in the last 12 Ma, including turbidites and
other mass-flow deposits fed by evolving transverse and axial channel systems. Surface processes controlling facies distribution
include the direct inflow of sediment from alluvial-fan systems and incising mountain rivers onto the Cretan slope, where
significant sediment instability processes occur at present. In this setting, seismic profiles reveal eight different types
of stratigraphic contacts on basin-margin highs, and basinal areas show evidence of halokinesis and/or fluid escape. The acquired
data also show that significant changes to the margin’s configuration occurred in association with the post-Alpine tectonic
and eustatic episodes affecting the Eastern Mediterranean. 相似文献
295.
Benthic foraminifers from a core of bottom sediments obtained in the Deryugin Basin at a depth of 1750 m were examined. Three distinct benthic foraminiferal assemblages that replaced one another throughout the last 26000 years were recognized. The assemblage dominated by Alabaminella weddellensis, Uvigerina auberiana, and Cassidulina translucens dwelled here from 26 to 24 ky B.P. The sediments of the glacial period between 26 and 12.18 ky B.P. contain no benthic foraminifers. The composition of the sediments suggests the existence of active bottom vents of gas-bearing fluids during that time. The dominance of Brizalina spissa, Uvigerina peregrina, and Cassidulina laevigata is characteristic of the period from 12.18 to 7 ky B.P., when the productivity of the surface waters in the region was high, and warm, poorly oxygenated waters spread over the bottom of the basin. During the past 7 000 years, the bottom conditions in the sea were similar to the present-day conditions and an assemblage with the dominance of Gyroidina orbicularis, Uvigerina auberiana, and Cassidulina teretis developed in the region. 相似文献
296.
Yoichi?ShimadaEmail author Atsushi?Kubokawa Kay?I.?Ohshima 《Journal of Oceanography》2005,61(5):913-920
Recent observations suggest that the annual mean southward transport of the East Sakhalin Current (ESC) is significantly larger
than the annual mean Sverdrup transport. Motivated by this observational result, transport of a western boundary current has
been investigated using a simple numerical model with a western slope. This transport is defined as the instantaneous barotropic
transport integrated from the western boundary to the offshore point where the barotropic velocity vanishes. The model, forced
by seasonally varying wind stress, exhibits an annual mean of the western boundary current transport that is larger than that
of the Sverdrup transport, as observed. The southward transport from October to March in the model nearly equals the instantaneous
Sverdrup transport, while the southward transport from April to September decreases slowly. Although the Sverdrup transport
in July vanishes, the southward transport in summer nearly maintains the annual mean Sverdrup transport, because the barotropic
Rossby wave cannot intrude on the western slope. This summer transport causes the larger annual mean. Although there are some
uncertainties in the estimation of the Sverdrup transport in the Sea of Okhotsk, the seasonal variation of the southward transport
in the model is qualitatively similar to the observations. 相似文献
297.
This paper discusses the results obtained through numerical modelling of the annual course of circulation and thermohaline field evolution in the tropical Atlantic under the impact of real wind forcing (from March 1982 to March 1983) and compares them with the calculation of seasonal variability induced by the climatic wind. Changes in the pattern of the equatorial currents and related temperature fields induced by the specific peculiarities in the wind field are analysed.Translated by Vladimir A. Puchkin. 相似文献
298.
An automatic sample changer based upon a commercially available 20-port rotary valve has been developed for use in automated seawater nutrient analyses. All parts exposed to seawater are of stainless steel, Teflon®, or Kel-F®. The sampler has proven to be rugged and efficient in both shore and shipboard laboratories. 相似文献
299.
Farn Parungo Clarence Nagamoto Cecilia M. I. R. Girz Jeff Torgerson Zhou Mingyu 《海洋学报(英文版)》1995,14(2):201-207
ObservationofcloudsandsolarradiationoverthePacificOceanasrelationtoglobalclimate¥FarnParungo;ClarenceNagamoto;CeciliaM.I.R.Gi... 相似文献
300.
The flow field generated by a Rankine body moving under a free surface in afinite-depth water is calculated by potential theory. Velocity field generated by a source located at the origin is calculated first by using highly efficient and adaptive quadratures of the QUADPACK library. This solution is used for generating the flow around a Rankine body by locating a source and an equal strength sink along the body axis. Results agree well with the existing literature. 相似文献