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591.
Suzanne O'Connell Belen Alonso Kim A. Kastens Andrés Maldonado Alberto Malinverno C. Hans Nelson Alberto Palanques William B. F. Ryan 《Geo-Marine Letters》1985,5(3):149-156
The Valencia Valley is a Quaternary, 200 km long deep-sea valley in the Valencia Trough, Western Mediterranean Sea. A swathmapping survey approximately mid-way along the valley length, where the floor has an average gradient of 1:250 (0.2°), shows valley walls that rise 200 to 350 m above the valley floor, with slopes of 2 to 18°. Sediment forming the walls is undergoing retrogressive, upslope-directed slumping with increasing bedding disruption along steeper walls. The valley exhibits a winding course with steep outer and gentler inner walls around bends, and bedforms on the valley floor. Lateral migration around bends is less than 5 km and the valley is deeply entrenched into Quaternary-bedded sediments. 相似文献
592.
A Deep-sea Channel in the Northwestern Mediterranean Sea: Morphology and seismic structure of the Valencia Channel and its surroundings 总被引:2,自引:0,他引:2
Belen Alonso Miquel Canals Alberto Palanques Jean-Pierre Rehault 《Marine Geophysical Researches》1995,17(5):469-484
The 400 km long Valencia Channel occupies the axis of the Valencia Trough in the Northwestern Mediterranean. Four different types of seismic reflection profiles were used to analyze the morphology and structure of the Valencia Channel with regard to the role played by both margins, Balearic and Iberian, of the Valencia Trough. From a detailed morphoseismic analysis of the Valencia Channel, its upper, middle, and lower courses can be characterized as follows: (1) in the upper course, tributaries are short and only slightly incised, with recent mass-transport deposits occurring on the adjacent continental slopes; (2) in the middle course, the channel deepens, and tributary valleys merge into it; and (3) the lower course begins after a sudden change in the direction of the channel, has a meandering path, is flanked by levees, and is fed by some valleys.During the Pliocene and Quaternary, at least four erosional and filling phases are observed in seismic profiles of the lower course of the Valencia Channel. The varying intensity of mass-transport processes and associated retrogressive slumping, which are related with fluctuations in sediment supply and relative sea-level changes, have played a major role in the formation, maintenance and deepening of the Valencia Channel. In addition to these sedimentary processes, a basement tectonic control and some morphostructural features affect the direction of the Valencia Channel locally. 相似文献
593.
Carlos José Freire Machado Maria Marlúcia Freitas Santiago Luiz Alberto Ribeiro Mendonça Horst Frischkorn Josué Mendes Filho 《Aquatic Geochemistry》2007,13(2):187-196
Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben
in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through
an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used
Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical
inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path.
Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana
aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and
microbiological redox reactions taking place in the aquifer systems also indicates hydraulic interconnection of the superior
and middle aquifer systems through the Santana aquitard. 相似文献
594.
The study area is located in the Pampean region of Argentina, in the northwest of Buenos Aires Province, covering about 50,000 km2, within an area of great plain with particular hydrologic characteristics, where vertical water movements predominate over
horizontal. The objective of this work is the analysis of the relations between different hydrologic scenarios (characterized
by the soil water balance and the water table levels) and the yields of soybeans that have been increasing in recent years.
It is concluded that water table fluctuations are strongly related to the elements of the local hydrologic balance. On the
other hand, the geographic distribution of soybean yields corresponds directly with the values of the soil water reserve during
the critical stages of the growth. After dry periods, the lowering of the water table makes access to water from the root
zone difficult, and the crop is susceptible to the effects of drought. However, after wetter periods the extra contribution
from the water table situated near the surface is favorable for growth, often preventing its deterioration in conditions of
drought. Nevertheless, an excessive rise of the water table can produce disadvantages at seedtime and during harvest. The
importance of an evaluation of the different scenarios that may have significant implications for agricultural activity is
discussed. 相似文献
595.
Non-local stochastic moment equations are used successfully to analyze groundwater flow in randomly heterogeneous media. Here we present a moment equations-based approach to quantify the uncertainty associated with the estimation of well catchments. Our approach is based on the development of a complete second order formalism which allows obtaining the first statistical moments of the trajectories of conservative solute particles advected in a generally non-uniform groundwater flow. Approximate equations of moments of particles’ trajectories are then derived on the basis of a second order expansion in terms of the standard deviation of the aquifer log hydraulic conductivity. Analytical expressions are then obtained for the predictors of locations of mean stagnation points, together with their associated uncertainties. We implement our approach on heterogeneous media in bounded two-dimensional domains, with and without including the effect of conditioning on hydraulic conductivity information. The impact of domain size, boundary conditions, heterogeneity and non-stationarity of hydraulic conductivity on the prediction of a well catchment is explored. The results are compared against Monte Carlo simulations and semi-analytical solutions available in the literature. The methodology is applicable to both infinite and bounded domains and is free of distributional assumptions (and so applies to both Gaussian and non-Gaussian log hydraulic conductivity fields) and formally includes the effect of conditioning on available information. 相似文献
596.
Paolo Antonio Pirazzoli Stéphane Costa Uwe Dornbusch Alberto Tomasin 《Ocean Dynamics》2006,56(5-6):498-512
This paper is based on statistical analysis of hourly tide measurements for some 285 equivalent full years from the stations
of Weymouth, Bournemouth, Portsmouth, Newhaven, Dover and Sheerness in the UK, and of Cherbourg, Le Havre, Dieppe, Boulogne,
Calais and Dunkirk in France. For each tidal value, surge heights have been determined and correlated with hourly or three-hourly
wind and air pressure data from nearby meteorological stations. Major surges in the area are generally produced by storms
associated with wind from north-west or south-west that tend to push oceanic water into the Channel. Recent medium-term climate
evolution does not seem to increase the flooding risk at French stations, where surge-related winds tend to decrease in frequency
and speed (Cherbourg, Dieppe and Boulogne) or show little change (Le Havre). However, the long-term risk of flooding will
increase through the loss in land elevation due to a continuation of the local relative sea-level rise, especially if this
effect will be enhanced by an acceleration in the global sea-level rise predicted by climatic models. The northern side of
the Channel (Weymouth, Bournemouth and Portsmouth) is mainly exposed to southerly winds that show variable trends. It is also
apparently affected by strong subsidence trends during the last two decades. If lasting, such trends can only increase long-term
flooding risk. The flooding risk has not increased near the eastern end of the Channel. The duration of significant cyclonic
events tends to decrease near Cherbourg but tends to increase near Weymouth, with no conclusive trends in other stations (Portsmouth,
Calais and Dunkirk), where extreme surges may occur also in relatively high-air-pressure situations. In conclusion, medium-term
coastal flooding risk seems to increase especially at Weymouth, Bournemouth and Portsmouth, and also, but less so, at Le Havre
and Sheerness. In addition, few extreme surges occurred during the last decades at the time of spring high tide, which would
seem to be a fortunate coincidence or, in some cases, an effect of tide–surge interaction. The risk of occurrence of less
favourable random events in the near future is therefore of concern, and flood potential would greatly increase if the global
sea-level rise expected in the near future is also considered. 相似文献
597.
Giacomo D''Amato Avanzi Roberto Giannecchini Alberto Puccinelli 《Engineering Geology》2004,73(3-4):215-228
On June 19, 1996, an extremely heavy rainstorm hit a restricted area in the Apuan Alps (northwestern Tuscany, Italy). Its max intensity concentrated over an area of about 150 km2 astride the Apuan chain, where 474 mm was recorded in about 12 h (21% of the mean annual precipitation, with an intensity up to 158 mm/h). The storm caused floods and hundreds of landslides and debris flows, which produced huge damage (hundreds of millions of Euros), partially destroyed villages and killed 14 people. This paper reports the results obtained from a detailed field survey and aerial view interpretation. In the most severely involved area, 647 main landslides were investigated, mapped and related to the geologic, geomorphic and vegetational factors of the source areas. This was in order to define the influence of these factors and contribute to an evaluation of the landslide hazard in the study area. An assessment was also made of the total area and volume of material mobilised by landsliding. The study area, about 46 km2 wide, includes three typically mountainous basins, characterised by narrow, deep cut valleys and steep slopes, where many rock types outcrop. Most of the landslides were shallow and linear, referable to complex, earth and debris translational slide, which quickly developed into flow (soil slip–debris flow). Usually, they involved colluvium and started in hollows underlain by metamorphic rock (metasandstone and phyllite), often dipping downslope. Therefore, bedrock lithology and impermeability appeared to be important factors in the localisation of the landslide phenomena. The investigation of the geomorphic and land use features in the source areas also frequently highlighted a rectilinear profile of the slope, a high slope gradient (31–45°) and dense chestnut wood cover. In the area, about 985,000 m2 (2.1% of 46 km2) was affected by landsliding and about 700,000 m2 of this area was covered by chestnut forest. The landslides removed about 7000 trees. The volume of mobilised material was about 1,360,000 m3; about 220,000 m3 remained on the slopes, while the rest poured into the streams. In addition, about 945,000 m3 was mobilised by the torrential erosion in the riverbeds. 相似文献
598.
599.
600.
Ernesto Martínez Alfonso Aranda Yolanda Díaz-De-Mera Ana Rodríguez Diana Rodríguez Alberto Notario 《Journal of Atmospheric Chemistry》2004,48(3):283-299
The reaction of Cl with cyclohexanone (1) was investigated, for the first time, as a function of temperature (273–333 K) and at a low total pressure (1 Torr) with helium as a carrier gas using a discharge flow-mass spectrometry technique (DF-MS). The resulting Arrhenius expression is proposed, k
1= (7.7 ± 4.1) × 10–10 exp[–(540 ± 169)/T]. We also report a mechanistic study with the quantitative determination of the products of the reaction of Cl with cyclohexanone. The absolute rate constant derived from this study at 1 Torr of total pressure and room temperature is (1.3 ± 0.2) × 10–10 cm3 molecule–1 s–1. A yield of 0.94 ± 0.10 was found for the H-abstraction channel giving HCl. In relative studies, using a newly constructed relative rate system, the decay of cyclohexanone was followed by gas chromatography coupled with flame-ionisation detection. These relative measurements were performed at atmospheric pressure with synthetic air and room temperature. Rate constant measured using the relative method for reaction (1) is: (1.7 ± 0.3) × 10–10 cm3 molecule–1 s–1. Finally, results and atmospheric implications are discussed and compared with the reactivity with OH radicals. 相似文献