全文获取类型
收费全文 | 25466篇 |
免费 | 225篇 |
国内免费 | 938篇 |
专业分类
测绘学 | 1433篇 |
大气科学 | 2100篇 |
地球物理 | 4777篇 |
地质学 | 11911篇 |
海洋学 | 1108篇 |
天文学 | 1816篇 |
综合类 | 2166篇 |
自然地理 | 1318篇 |
出版年
2024年 | 4篇 |
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 16篇 |
2020年 | 27篇 |
2019年 | 30篇 |
2018年 | 4784篇 |
2017年 | 4072篇 |
2016年 | 2608篇 |
2015年 | 269篇 |
2014年 | 127篇 |
2013年 | 97篇 |
2012年 | 1021篇 |
2011年 | 2785篇 |
2010年 | 2065篇 |
2009年 | 2383篇 |
2008年 | 1953篇 |
2007年 | 2410篇 |
2006年 | 108篇 |
2005年 | 233篇 |
2004年 | 445篇 |
2003年 | 444篇 |
2002年 | 275篇 |
2001年 | 66篇 |
2000年 | 75篇 |
1999年 | 31篇 |
1998年 | 36篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 27篇 |
1980年 | 22篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 13篇 |
1974年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
141.
K. Vinod kumar R. C. Lakhera Tapas R. Martha R. S. Chatterjee A. Bhattacharya 《Environmental Geology》2008,55(4):789-799
Mass movements such as landslides in mountainous terrains are natural degradation processes and one of the most important
landscape-building factors. Varunawat Parbat overlooking Uttarkashi town witnessed a series of landslides on 23 September
2003 and the debris slides and rock falls continued for 2 weeks. This landslide complex was triggered due to the incessant
rainfall prior to the event, and its occurrence led to the blockage of the pilgrim route to Gangotri (source of the Ganges
river) and evacuation of thousands of people to safer places. Though there was no loss of lives due to timely evacuation,
heavy losses to the property were reported. High-resolution stereoscopic earth observation data were acquired after the incidence
to study the landslide in detail with emphasis on the cause of the landslide and mode of failure. Areas along the road and
below the Varunawat foothill region are mapped for landslide risk. It was found that the foothill region of the Varunawat
Parbat was highly disturbed by man-made activities and houses are dangerously located below steep slopes. The potential zones
for landslides along with the existing active and old landslides are mapped. These areas are critical and their treatment
with priority is required in order to minimise further landslide occurrences. 相似文献
142.
There could be some discontinuities in a soil media such as layers, earth structures, cracks, and fissures so that estimation
of stresses and deformations in these types of soil masses are somewhat different from continuous masses. The discontinuities
in a soil mass could be considered as a special link between two blocks. Transmitted swelling pressures affected by the soil
properties of the backfill especially at the interface between the backfill and retaining wall. In order to estimate the transmitted
swelling pressure distribution behind a retaining wall, using with interface element, a new finite element model and a code
(SWELPRES) have been developed. To evaluate the effect of backfill characteristics and interface element from the transmitted
lateral swelling pressures, four cases of thickness of backfills with or without interface elements are investigated. 相似文献
143.
Eduardo E. Alonso Sarah M. Springman Charles W. W. Ng 《Geotechnical and Geological Engineering》2008,26(6):817-826
Two large-scale “in situ” demonstration experiments and their instrumentation are described. The first test (FEBEX Experiment)
involves the hydration of a compacted bentonite barrier under the combined effect of an inner source of heat and an outer
water flow from the confining saturated granite rock. In the second case, the progressive de-saturation of Opalinus clay induced
by maintained ventilation of an unlined tunnel is analyzed. The paper shows the performance of different sensors (capacitive
cells, psychrometers, TDR’s) and a comparison of fill behaviour with modelling results. The long term performance of some
instruments could also be evaluated specially in the case of FEBEX test. Capacitive sensors provide relative humidity data
during long transient periods characterised by very large variations of suction within the bentonite. 相似文献
144.
Experiments were conducted in an oscillatory water tunnel to investigate what effects temporal inertia has on the resistance of a granular medium. The flow law governing stationary porous media flow is reasonably well accepted and understood but the effects of unsteady flow have commonly been neglected. The present research was designed to assess the magnitude of the acceleration effects on media of uniformly packed spheres of equal diameter and on one sample of randomly placed stone.Oscillatory flow tests were made in a large oscillatory flume tunnel with periods varying from 3 to 12 seconds. The influence of properties of the medium (grain size and porosity) were tested by using spheres with two different diameters and packing each size sphere in different geometric arrangements. Tests made on a stone sample provided a qualitative assessment of the effects of more random material properties.For the experiments described in this paper, the Forchheimer unsteady-stationary flow law described the oscillatory measurements well when velocities and energy losses were maximum. Empirical coefficients determined from steady-stationary flow were generally found to apply to the unsteady flow, however some evidence of dependency on the period of oscillation was noted. 相似文献
145.
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
146.
Noriaki Kimura 《Journal of Oceanography》2007,63(4):685-694
This paper examines the mechanism controlling the short time-scale variation of sea ice cover over the Southern Ocean. Sea
ice concentration and ice velocity datasets derived from images of the Defense Meteorological Satellite Program (DMSP) Special
Sensor Microwave Imager (SSM/I) are employed to reveal this mechanism. The contribution of both dynamic and thermodynamic
processes to the change in ice edge location is examined by comparing the meridional velocity of ice edge displacement and
sea ice drift. In the winter expansion phase, the thermodynamic process of new ice production off the ice edge plays an important
role in daily advances of ice cover, whereas daily retreats are mostly due to southward ice drift. On the other hand, both
advance and retreat of ice edges in the spring contraction phase are mostly caused by the dynamic process of the ice drift.
Based on the above mechanism and the linear relation between the degree of ice production at the ice edge and northward wind
speed, the seasonal advance of ice cover can be roughly reproduced using the meridional velocity of ice drift at the ice edge. 相似文献
147.
Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant.
One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian
evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It seemed
that the two series were triggered by two unrelated, though contemporaneous, processes. The shore-parallel slumps were initiated
by basinwards flow of the Messinian salt, that carried along the overlying Plio-Quaternary sediments. Seawater that percolated
along the detachment faults dissolved the underlying salt to form distinctly disrupted structures. The slope-normal slumps
are located on top of large canyons that cut into the pre-Messinian sedimentary rocks. A layer of salt is found in the canyons,
and the Plio-Quaternary sediments were deposited on that layer. The slumps are bounded by large, NW-trending faults where
post-Messinian faulted offset was measured. We presume that the flow of the salt in the canyons also drives the slope-normal
slumps. Thus thin-skinned halokynetic processes generated the composite post-Tortonian structural patterns of the Levant margin.
The Phoenician Structures are a prime example of the collapse of a distal continental margin due to the dissolution of a massive
salt layer. 相似文献
148.
149.
Seasonal evolution of surface mixed layer in the Northern Arabian Sea (NAS) between 17° N–20.5° N and 59° E-69° E was observed
by using Argo float daily data for about 9 months, from April 2002 through December 2002. Results showed that during April
- May mixed layer shoaled due to light winds, clear sky and intense solar insolation. Sea surface temperature (SST) rose by
2.3 °C and ocean gained an average of 99.8 Wm−2. Mixed layer reached maximum depth of about 71 m during June - September owing to strong winds and cloudy skies. Ocean gained
abnormally low ∼18 Wm−2 and SST dropped by 3.4 °C. During the inter monsoon period, October, mixed layer shoaled and maintained a depth of 20 to
30 m. November - December was accompanied by moderate winds, dropping of SST by 1.5 °C and ocean lost an average of 52.5 Wm−2. Mixed layer deepened gradually reaching a maximum of 62 m in December. Analysis of surface fluxes and winds suggested that
winds and fluxes are the dominating factors causing deepening of mixed layer during summer and winter monsoon periods respectively.
Relatively high correlation between MLD, net heat flux and wind speed revealed that short term variability of MLD coincided
well with short term variability of surface forcing. 相似文献
150.
Christopher T. Mills Robert F. Dias Dennis Graham Kevin W. Mandernack 《Marine Chemistry》2006,98(2-4):197-209
Sediment samples ranging from 0.05 to 278 m below sea floor (mbsf) at a Northwest Pacific deep-water (5564 mbsl) site (ODP Leg 191, Site 1179) were analyzed for phospholipid fatty acids (PLFAs). Total PLFA concentrations decreased by a factor of three over the first meter of sediment and then decreased at a slower rate to approximately 30 mbsf. The sharp decrease over the first meter corresponds to the depth of nitrate and Mn(IV) reduction as indicated by pore water chemistry. PLFA-based cell numbers at site 1179 had a similar depth profile as that for Acridine orange direct cell counts previously made on ODP site 1149 sediments which have a similar water depth and lithology. The mole percentage of straight chain saturated PLFAs increases with depth, with a large shift between the 0.95 and 3.95 mbsf samples. PLFA stable carbon isotope ratios were determined for sediments from 0.05 to 4.53 mbsf and showed a general trend toward more depleted δ13C values with depth. Both of these observations may indicate a shift in the bacterial community with depth across the different redox zones inferred from pore water chemistry data. The PLFA 10me16:0, which has been attributed to the bacterial genera Desulfobacter in many marine sediments, showed the greatest isotopic depletion, decreasing from − 20 to − 35‰ over the first meter of sediment. Pore water chemistry suggested that sulfate reduction was absent or minimal over this same sediment interval. However, 10me16:0 has been shown to be produced by recently discovered anaerobic ammonium oxidizing (anammox) bacteria which are known chemoautotrophs. The increasing depletion in δ13C of 10me16:0 with the unusually lower concentration of ammonium and linear decrease of nitrate concentration is consistent with a scenario of anammox bacteria mediating the oxidation of ammonium via nitrite, an intermediate of nitrate reduction. 相似文献