全文获取类型
收费全文 | 1315篇 |
免费 | 113篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 34篇 |
大气科学 | 67篇 |
地球物理 | 471篇 |
地质学 | 428篇 |
海洋学 | 113篇 |
天文学 | 251篇 |
综合类 | 8篇 |
自然地理 | 59篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 21篇 |
2020年 | 30篇 |
2019年 | 30篇 |
2018年 | 55篇 |
2017年 | 77篇 |
2016年 | 76篇 |
2015年 | 76篇 |
2014年 | 74篇 |
2013年 | 60篇 |
2012年 | 72篇 |
2011年 | 82篇 |
2010年 | 74篇 |
2009年 | 88篇 |
2008年 | 71篇 |
2007年 | 57篇 |
2006年 | 38篇 |
2005年 | 29篇 |
2004年 | 54篇 |
2003年 | 35篇 |
2002年 | 41篇 |
2001年 | 30篇 |
2000年 | 18篇 |
1999年 | 17篇 |
1998年 | 20篇 |
1997年 | 12篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 6篇 |
1993年 | 17篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1982年 | 4篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1969年 | 3篇 |
1955年 | 3篇 |
1952年 | 3篇 |
1948年 | 4篇 |
1940年 | 4篇 |
排序方式: 共有1431条查询结果,搜索用时 0 毫秒
991.
AbstractWidespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded. The floor thrust of the Sarrabus unit, the Villasalto thrust, is a west- directed thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerrei, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units. © Elsevier, Paris 相似文献
992.
The 1963 Vajont Landslide: 50th Anniversary 总被引:3,自引:1,他引:2
993.
The analysis of the transition from the large-scale fluid regime to the short-scale kinetic range of wavelengths in the development of the turbulent cascade of energy is nowadays subject of fervent discussion in the space plasmas scientific community. We make use of Hybrid Vlasov-Maxwell simulations where the full kinetic dynamics of ions is taken into account, while electrons are treated as a fluid. We investigate the development of turbulence in the solar wind, in 1D-3V phase space configuration and in the frequency range across the ion cyclotron frequency. These simulations allow for the analysis of the role of kinetic effects in the short-scale region of the energy spectra in the direction parallel to the background magnetic field. Our numerical results show the presence of a significant electrostatic activity at small wavelengths, triggered by the resonant interaction of ions with longitudinal waves. Our model does not allow to take into account the evolution of the turbulent spectra in the plane perpendicular to the ambient field, due to limited dimensionality in phase space. On the other hand, this model permits to isolate and study the possibility of transferring the electromagnetic large-scale energy on the small-scale kinetic electrostatic component of the spectrum. Peculiar features observed in the spacecraft data in the solar wind are qualitatively reproduced within the hybrid-Vlasov model, such as the generation of perpendicular temperature anisotropy and accelerated longitudinal beams of ions in the distribution of particle velocities as well as the appearance of a marked peak of electrostatic activity in the short-scale termination of the turbulent spectra. 相似文献
994.
Michael Batty Robin Morphet Paolo Masucci Kiril Stanilov 《Journal of Geographical Systems》2014,16(4):363-385
We pose the central problem of defining a measure of complexity, specifically for spatial systems in general, city systems in particular. The measures we adopt are based on Shannon’s (in Bell Syst Tech J 27:379–423, 623–656, 1948) definition of information. We introduce this measure and argue that increasing information is equivalent to increasing complexity, and we show that for spatial distributions, this involves a trade-off between the density of the distribution and the number of events that characterize it; as cities get bigger and are characterized by more events—more places or locations, information increases, all other things being equal. But sometimes the distribution changes at a faster rate than the number of events and thus information can decrease even if a city grows. We develop these ideas using various information measures. We first demonstrate their applicability to various distributions of population in London over the last 100 years, then to a wider region of London which is divided into bands of zones at increasing distances from the core, and finally to the evolution of the street system that characterizes the built-up area of London from 1786 to the present day. We conclude by arguing that we need to relate these measures to other measures of complexity, to choose a wider array of examples, and to extend the analysis to two-dimensional spatial systems. 相似文献
995.
P. Cristofanelli P. di Carlo A. D’. Altorio C. Dari Salisburgo P. Tuccella F. Biancofiore P. Stocchi G. P. Verza T. C. Landi A. Marinoni F. Calzolari R. Duchi Paolo Bonasoni 《Pure and Applied Geophysics》2013,170(11):1985-1999
Tropospheric ozone (O3) is an important atmospheric pollutant and climate forcer. The Mediterranean basin is a hot-spot region in terms of short-term O3 distribution, with frequent episodes of high tropospheric O3, especially during summer. To improve the characterisation of summer O3 variability in the Mediterranean area, during the period 6–27 August 2009 an experimental campaign was conducted at Campo Imperatore, Mt Portella (CMP), a high mountain site (2,388 m a.s.l.) located in the central Italian Apennines. As deduced from analysis of atmospheric circulation, the measurement site was significantly affected by air masses originating over the Mediterranean basin, which affected the measurement site for 32 % of the time. Analysis of average values and diurnal and day-to-day variability revealed that CMP O3 observations (average value 60.0 ± 5.1 ppbv) were comparable with measurements at other European mountain stations, indicating a prevalent effect of meteorological conditions and atmospheric transport on the synoptic scale. In fact, only a small “reverse” diurnal variation typically characterises diurnal O3 variability because of local thermal wind circulation, which sporadically favours transport of air masses rich in O3 from the foothill regions. Statistical analysis of five-day back-trajectory ensembles indicates that synoptic-scale air-mass transport from the Mediterranean Sea usually results in decreasing O3 concentrations at CMP, whereas the highest hourly O3 values are mostly associated with air masses from central continental Europe, eastern Europe, and northern Italy. High O3 concentrations are also related to downward air-mass transport from higher altitudes. Comparison of in-situ O3 variability with tropospheric O3 satellite-based measurements reveals similar features of the two data sets. Together with the results from back-trajectory analysis, this indicates that CMP measurements might usefully improve characterisation of broad-scale O3 variability over the central Mediterranean basin. 相似文献
996.
Luca Carturan Giovanni A. Baldassi Aldino Bondesan Simone Calligaro Alberto Carton Federico Cazorzi Giancarlo Dalla Fontana Roberto Francese Alberto Guarnieri Nicola Milan Daniele Moro Paolo Tarolli 《Geografiska Annaler: Series A, Physical Geography》2013,95(1):79-96
Smaller glaciers (<0.5 km2) react quickly to environmental changes and typically show a large scatter in their individual response. Accounting for these ice bodies is essential for assessing regional glacier change, given their high number and contribution to the total loss of glacier area in mountain regions. However, studying small glaciers using traditional techniques may be difficult or not feasible, and assessing their current activity and dynamics may be problematic. In this paper, we present an integrated approach for characterizing the current behaviour of a small, avalanche‐fed glacier at low altitude in the Italian Alps, combining geomorphological, geophysical and high‐resolution geodetic surveying with a terrestrial laser scanner. The glacier is still active and shows a detectable mass transfer from the accumulation area to the lower ablation area, which is covered by a thick debris mantle. The glacier owes its existence to the local topo‐climatic conditions, ensured by high rock walls which enhance accumulation by delivering avalanche snow and reduce ablation by providing topographic shading and regulating the debris budget of the glacier catchment. In the last several years the glacier has displayed peculiar behaviour compared with most glaciers of the European Alps, being close to equilibrium conditions in spite of warm ablation seasons. Proportionally small relative changes have also occurred since the Little Ice Age maximum. Compared with the majority of other Alpine glaciers, we infer for this glacier a lower sensitivity to air temperature and a higher sensitivity to precipitation, associated with important feedback from increasing debris cover during unfavourable periods. 相似文献
997.
A probabilistic approach for the classification of earthquakes as ��triggered�� or ��not triggered��
Luigi Passarelli Francesco Maccaferri Eleonora Rivalta Torsten Dahm Elias Abebe Boku 《Journal of Seismology》2013,17(1):165-187
The occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ??Anticipated?? earthquakes are generally called ??triggered??; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M?=?6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tj?rnes Fracture Zone, Iceland), which occurred about 3?weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes?? theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90?%. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust. 相似文献
998.
Valentina Ciriello Vittorio Di Federico Monica Riva Francesco Cadini Jacopo De Sanctis Enrico Zio Alberto Guadagnini 《Stochastic Environmental Research and Risk Assessment (SERRA)》2013,27(4):945-954
We perform global sensitivity analysis (GSA) through polynomial chaos expansion (PCE) on a contaminant transport model for the assessment of radionuclide concentration at a given control location in a heterogeneous aquifer, following a release from a near surface repository of radioactive waste. The aquifer hydraulic conductivity is modeled as a stationary stochastic process in space. We examine the uncertainty in the first two (ensemble) moments of the peak concentration, as a consequence of incomplete knowledge of (a) the parameters characterizing the variogram of hydraulic conductivity, (b) the partition coefficient associated with the migrating radionuclide, and (c) dispersivity parameters at the scale of interest. These quantities are treated as random variables and a variance-based GSA is performed in a numerical Monte Carlo framework. This entails solving groundwater flow and transport processes within an ensemble of hydraulic conductivity realizations generated upon sampling the space of the considered random variables. The Sobol indices are adopted as sensitivity measures to provide an estimate of the role of uncertain parameters on the (ensemble) target moments. Calculation of the indices is performed by employing PCE as a surrogate model of the migration process to reduce the computational burden. We show that the proposed methodology (a) allows identifying the influence of uncertain parameters on key statistical moments of the peak concentration (b) enables extending the number of Monte Carlo iterations to attain convergence of the (ensemble) target moments, and (c) leads to considerable saving of computational time while keeping acceptable accuracy. 相似文献
999.
Marco Giancotti Mauro Pontani Paolo Teofilatto 《Celestial Mechanics and Dynamical Astronomy》2012,114(1-2):55-76
Analysis and design of low-energy transfers to the Moon has been a subject of great interest for many decades. This paper is concerned with a topological study of such transfers, with emphasis to trajectories that allow performing lunar capture and those that exhibit homoclinic connections, in the context of the circular restricted three-body problem. A fundamental theorem stated by Conley locates capture trajectories in the phase space and can be condensed in a sentence: “if a crossing asymptotic orbit exists then near any such there is a capture orbit”. In this work this fundamental theoretical assertion is used together with an original cylindrical isomorphic mapping of the phase space associated with the third body dynamics. For a given energy level, the stable and unstable invariant manifolds of the periodic Lyapunov orbit around the collinear interior Lagrange point are computed and represented in cylindrical coordinates as tubes that emanate from the transformed periodic orbit. These tubes exhibit complex geometrical features. Their intersections correspond to homoclinic orbits and determine the topological separation of long-term lunar capture orbits from short-duration capture trajectories. The isomorphic mapping is proven to allow a deep insight on the chaotic motion that characterizes the dynamics of the circular restricted three-body, and suggests an interesting interpretation, and together corroboration, of Conley’s assertion on the topological location of lunar capture orbits. Moreover, an alternative three-dimensional representation of the phase space is profitably employed to identify convenient lunar periodic orbits that can be entered with modest propellant consumption, starting from the Lyapunov orbit. 相似文献
1000.
A series of long karst spring discharges have been analysed to determine trends, fluctuations and relationship to rainfall.
Data come from aquifers in southern Italy, and in some cases cover more than one hundred years of records. Based on yearly
discharge data, hydrological series show the drop of the discharge after 1986, which has been interrupted by the recent wet
years of 2009 and 2010. This drop is connected to the decrease in annual rainfall, but other factors also seem to contribute
to this phenomenon. Based on monthly scale data series, the relationship between meteorological and hydrological droughts
has been analyzed, using the standard precipitation index (SPI). As karst systems are large reservoirs, only longer meteorological
droughts induce groundwater droughts, and the start, duration and time-lag of the hydrological droughts have been evaluated. 相似文献