全文获取类型
收费全文 | 24443篇 |
免费 | 173篇 |
国内免费 | 916篇 |
专业分类
测绘学 | 1410篇 |
大气科学 | 1987篇 |
地球物理 | 4517篇 |
地质学 | 11594篇 |
海洋学 | 1002篇 |
天文学 | 1641篇 |
综合类 | 2161篇 |
自然地理 | 1220篇 |
出版年
2021年 | 1篇 |
2020年 | 4篇 |
2019年 | 1篇 |
2018年 | 4763篇 |
2017年 | 4039篇 |
2016年 | 2581篇 |
2015年 | 235篇 |
2014年 | 82篇 |
2013年 | 25篇 |
2012年 | 993篇 |
2011年 | 2730篇 |
2010年 | 2015篇 |
2009年 | 2317篇 |
2008年 | 1893篇 |
2007年 | 2360篇 |
2006年 | 53篇 |
2005年 | 194篇 |
2004年 | 403篇 |
2003年 | 409篇 |
2002年 | 249篇 |
2001年 | 47篇 |
2000年 | 52篇 |
1999年 | 14篇 |
1998年 | 22篇 |
1995年 | 1篇 |
1986年 | 1篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1976年 | 6篇 |
1965年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
K. R. Muraleedharan P. K. Dinesh Kumar S. Prasanna Kumar Sebin John B. Srijith K. Anil Kumar K. Naveen Kumar S. Gautham V. Samiksha 《Estuaries and Coasts》2018,41(4):1021-1035
Mud bank formation during the southwest monsoon along the southwest coast of India remains an enigma to the researchers and coastal community in spite of several earlier studies. The present study attempts to unravel the mystery through a high-frequency, season-long time-series observation at Alappuzha, located at the southern part of the west coast of India, a region of frequent occurrence of mud bank. Using 7-month-long weekly time-series observation, we identified strong winds and high waves associated with onset of the southwest monsoon and subsequent three episodic atmospheric low-pressure events (LPEs).With the help of in situ time-series data, we show that the strong winds and high waves associated with southwest monsoon pre-conditions the near shore bottom sediment to bring it into suspension. The high amplitude waves associated with the southwest monsoon, while propagating from the deep water to shallow water region, interact with the bottom initiating bottom-sediment movement and its suspension due to wave refraction and shoaling. The sporadic occurrence of the atmospheric LPEs enhances the process of suspension of bottom sediment in the near shore region leading to the formation of fluid mud. Simulations with a cohesive sediment transport model yielded realistic estimates of sediment transport, in the presence of an onshore current, a pre-requisite for transporting the fluid mud toward the coast. The prevailing onshore upwelling current during the southwest monsoon provides the favorable pre-requisite conditions for transporting the fluid mud through depression channel network towards the coast. Once sufficient quantity and thickness of fluid mud is accumulated in the near shore region, it acts as a wave damper for subsequent high monsoon waves, as indicated by the time-series wave data, leading to the formation of tranquil mud bank region. Depression channel networks extending from the shelf to the coast off Alappuzha, Kochi, Ponnani, Beypore, and Ullal were found in the bathymetric charts, thus explaining why mud banks occur only at few locations in spite of the prevalence of similar monsoon conditions. 相似文献
32.
Efficiency in solving the Saint-Venant equations for watershed rainfall-runoff routing is important in flood hydrology. This paper presents a high-efficiency numerical solution of one-dimensional dynamic wave equations (HEDWE) for watershed rainfall-runoff routing, in which the full momentum equation is written as a quadratic equation with only one unknown variable Q, water depth is derived from the continuity equation using the two-step predictor-corrector method, and the discrete scheme is the explicit upwind scheme. The results of numerical tests showed the HEDWE approach has several major advantages. 1) It is a stable numerical method, even for an initially dry area. 2) Its computational efficiency is higher than 4.76E+05 times/s. 3) It can be used for overland flow, river flow, and combinations thereof. The primary disadvantages of the HEDWE approach are its unsuitability for rapidly varying flow, such as dam-break floods. 相似文献
33.
The Role of History-Dependent Rheology in Plate Boundary Lubrication for Generating One-Sided Subduction 总被引:1,自引:0,他引:1
Michio Tagawa Tomoeki Nakakuki Masanori Kameyama Fumiko Tajima 《Pure and Applied Geophysics》2007,164(5):879-907
We have developed a two-dimensional dynamical model of asymmetric subduction integrated into the mantle convection without
imposed plate velocities. In this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at
the plate boundary. We introduce a rheological layer that depends on the history of the past fracture to simulate the effect
of the oceanic crust. The thickness of this layer is set to be as thin as the Earth's oceanic crust. To treat 1-kilometer
scale structure at the plate boundary in the 1000-kilometer scale mantle convection calculation, we introduce a new numerical
method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes. Using our developed
models, we have systematically investigated effects of basic rheological parameters that determine the deformation strength
of the lithosphere and the oceanic crust on the development of the subducted slab, with a focus on the plate motion controlling
mechanism. In our model the plate subduction is produced when the friction coefficient (0.004–0.008) of the modeled oceanic
crust and the maximum strength (400 MPa) of the lithosphere are in plausible range inferred from the observations on the plate
driving forces and the plate deformation, and the rheology experiments. In this range of the plate strength, yielding induces
the plate bending. In this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle
than by the plate strength. To examine the setting of the overriding plate, we also consider the two end-member cases in which
the overriding plate is fixed or freely-movable. In the case of the freely-movable overriding plate, the trench motion considerably
changes the dip angle of the deep slab. Especially in the case with a shallow-angle plate boundary, retrograde slab motion
occurs to generate a shallow-angle deep slab. 相似文献
34.
The characteristics and forcing mechanisms of high-frequency flow variations (periods of minutes to days) were investigated
near Gladden Spit, a reef promontory off the coast of Belize. Direct field observations and a high-resolution (50-m grid size)
numerical ocean model are used to describe the flow variations that impact the initial dispersion of eggs and larvae from
this site, which serves as a spawning aggregation site for many species of reef fishes. Idealized sensitivity model experiments
isolate the role of various processes, such as internal waves, wind, tides, and large-scale flow variations. The acute horizontal
curvature and steep topography of the reef intensify the flow, create small-scale convergence and divergence zones, and excite
high-frequency oscillations and internal waves. Although the tides in this area are relatively small (∼10-cm amplitude), the
model simulations show that tides can excite significant high-frequency flow variations near the reef, which suggests that
the preference of fish to aggregate and spawn in the days following the time of full moon may not be coincidental. Even small
variations in remote flows (2–5 cm s−1) due to say, meso-scale eddies, are enough to excite near-reef oscillations. Model simulations and the observations further
suggest that the spawning site at the tip of the reef provides initial strong dispersion for eggs, but then the combined influence
of the along-isobath flow and the westward wind will transport the eggs and larvae downstream of Gladden Spit toward less
turbulent region, which may contribute to enhanced larval survival. 相似文献
35.
Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Furthermore, the protective coal seam mining technology featured by economic efficiency has been proven to be the most effective and widely applied method for the prevention of coal and gas outburst disasters. However, the determinations of the protective area coal and gas outburst prevention in a pressure-relief boundary area are fundamental issues that research should be focused on. The technical method for determining stress distribution in pressure-relief boundary area during protective coal seam mining is put forward in this paper. The method is based on a stress-seepage coupled relationship within a gas-containing coal seam. The method includes complex lab experiments and on-site measurements at the Qingdong Coal Mine. The final data illustrate that the permeability and vertical stress in the pressure-relief boundary area of the coal sample form a negative exponential function relationship. Additionally, the permeability of the coal sample within the abovementioned area is significantly different compared with that located at the center of the pressure-relief area. In the pressure-relief boundary area, the gas pressure distribution gradient is 0.0375 MPa/m, while the vertical stress distribution gradient registers 0.56 MPa/m. Under this condition, coal and gas outburst disasters are prone to be triggered. Therefore, effective precautions against coal and gas outburst disasters can be put forward in accordance with stress distribution characteristics within the abovementioned “boundary area.” 相似文献
36.
Pan Wu Changyuan Tang Congqiang Liu Lijun Zhu TingQuan Pei Lijuan Feng 《Environmental Geology》2009,57(7):1457-1467
The chemical characteristics, formation and natural attenuation of pollutants in the coal acid mine drainage (AMD) at Xingren
coalfield, Southwest China, are discussed in this paper based on the results of a geochemical investigation as well as geological
and hydrogeological background information. The chemical composition of the AMD is controlled by the dissolution of sulfide
minerals in the coal seam, the initial composition of the groundwater and the water–rock interaction. The AMD is characterized
by high sulfate concentrations, high levels of dissolved metals (Fe, Al, Mn, etc.) and low pH values. Ca2+ and SO4
2− are the dominant cation and anion in the AMD, respectively, while Ca2+ and HCO3
− are present at significant levels in background water and surface water after the drainage leaves the mine site. The pH and
alkalinity increase asymptotically with the distance along the flow path, while concentrations of sulfate, ferrous iron, aluminum
and manganese are typically controlled by the deposition of secondary minerals. Low concentrations of As and other pollutants
in the surface waters of the Xingren coalfield could be due to relatively low quantities being released from coal seams, to
adsorption and coprecipitation on secondary minerals in stream sediments, and to dilution by unpolluted surface recharge.
Although As is not the most serious water quality problem in the Xingren region at present, it is still a potential environmental
problem.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
37.
Accumulation of heavy metals in wild commercial fish from the Baotou Urban Section of the Yellow River,China 总被引:1,自引:0,他引:1
In this work we studied the accumulation of heavy metals in nine species of fish with different life and feeding habitats
which are native and major commercial fish in the Baotou Urban Section of the Yellow River. The results showed that the concentration
of heavy metals was significantly dependent on fish species; the pollution index of heavy metals in different species were
ranked as Hemiculter leucisclus > Carassius auratus auratus > Hemibarbus maculatus > Megalobrama amblycephala > Abbottina rivularis > Cyprinus carpio > Squaliobarbus curriculus > Perccottus glehni > Saurogobio dabryi. Product–moment correlation coefficients among the metal pairs Pb–Zn, Cu–Cd, Cu–Zn, Cu–Pb, Pb–Cd, and Zn–Cd revealed there
was no competitions between metals in each tissue. Correlations between heavy metal concentrations and fish length or weight
indicated that accumulation of the heavy metals by the different fish species was related to their surrounding environments
and their life and feeding habitats. According to the mean bioconcentration factors (BCFs), the heavy metal concentrations
in these nine species were ranked Zn ≫ Cu > Cd ≈ Pb. In this work, the bioaccumulation factors (BAFs) were developed by using
the sum of exchangeable and bound-to-carbonate heavy metals as Cs values. It was found that BAFs better reveal the accumulation
characteristics of the heavy metals in the fish, which might provide an effective method for assessing bioaccumulation of
heavy metals. 相似文献
38.
Veselá Petra Söllner Frank Finger Friedrich Gerdes Axel 《International Journal of Earth Sciences》2011,100(5):993-1027
New geochronological U-Pb (LA-ICP-MS) zircon data and geochemical analyses from the Variscan orthogneisses and metavolcanic rocks in the western Tauern window are presented and used to reconstruct the pre-Alpine evolution of this area. The late- and post-Variscan stage in the Tauern window was characterised by distinct magmatic pulses accompanied by the formation of volcano-sedimentary basins. The magmatic activity started in the Visean (335.4 ± 1.5 Ma) with the intrusion of a K-rich, durbachitic biotite-granite (protolith of the Ahorn gneiss). Following a period of exhumation and erosion, Westfalian–Stefanian volcanics were deposited (Grierkar meta-rhyodacite: 309.8 ± 1.5 Ma; Venntal meta-rhyolite: 304.0 ± 3.0 Ma). A renewed magmatic pulse occurred in the Early Permian, producing large volumes of tonalites and granodiorites (Tux meta-granodiorite: 292.1 ± 1.9 Ma). The youngest magmatism is characterised by pyroclastic and tuffitic deposits (Pfitsch meta-rhyolite: 280.5 ± 2.6 Ma; Schönach valley meta-andesite: 279.0 ± 4.8 Ma). This volcanism was probably related to crustal extensional faulting within an intra-continental graben and horst setting, asthenospheric upwelling and heat flow increase due to the onset of the Permian rifting. The Permo-Triassic peneplanation and subsidence is documented by shallow marine and evaporitic deposits. Probably in the Middle Jurassic times, the area was flooded and in the Late Jurassic the whole area was covered by limestones, representing post-rift sediments on the southern European continental margin. 相似文献
39.
We show in this short note that the method of singular spectrum analysis (SSA) is able to clearly extract a strong, clean, and clear component from the longest available sunspot (International Sunspot Number, ISN) time series (1700?–?2015) that cannot be an artifact of the method and that can be safely identified as the Gleissberg cycle. This is not a small component, as it accounts for 13% of the total variance of the total original signal. Almost three and a half clear Gleissberg cycles are identified in the sunspot number series. Four extended solar minima (XSM) are determined by SSA, the latest around 2000 (Cycle 23/24 minimum). Several authors have argued in favor of a double-peaked structure for the Gleissberg cycle, with one peak between 55 and 59 years and another between 88 and 97 years. We find no evidence of the former: solar activity contains an important component that has undergone clear oscillations of \(\approx90\) years over the past three centuries, with some small but systematic longer-term evolution of “instantaneous” period and amplitude. Half of the variance of solar activity on these time scales can be satisfactorily reproduced as the sum of a monotonous multi-secular increase, a \(\approx90\)-year Gleissberg cycle, and a double-peaked (\(\approx10.0\) and 11.0 years) Schwabe cycle (the sum amounts to 46% of the total variance of the signal). The Gleissberg-cycle component definitely needs to be addressed when attempting to build dynamo models of solar activity. The first SSA component offers evidence of an increasing long-term trend in sunspot numbers, which is compatible with the existence of the modern grand maximum. 相似文献
40.
Olac Fuentes 《Experimental Astronomy》2001,12(1):21-31
In this article we show how machine learning methods can beeffectively applied to the problem of automatically predictingstellar atmospheric parameters from spectral information, a veryimportant problem in stellar astronomy. We apply feedforwardneural networks, Kohonen's self-organizing maps andlocally-weighted regression to predict the stellar atmosphericparameters effective temperature, surface gravity and metallicityfrom spectral indices. Our experimental results show that thethree methods are capable of predicting the parameters with verygood accuracy. Locally weighted regression gives slightly betterresults than the other methods using the original dataset asinput, while self-organizing maps outperform the other methods when significant amounts of noise are added. We also implemented a heterogeneous ensemble of predictors, combining the results given by the three algorithms. This ensemble yields better results than any of the three algorithms alone, using both the original and the noisy data. 相似文献