首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   83篇
  免费   3篇
大气科学   2篇
地球物理   34篇
地质学   24篇
海洋学   2篇
天文学   21篇
自然地理   3篇
  2022年   3篇
  2020年   1篇
  2017年   4篇
  2016年   5篇
  2015年   1篇
  2014年   4篇
  2013年   3篇
  2012年   5篇
  2011年   3篇
  2010年   2篇
  2009年   1篇
  2008年   8篇
  2007年   4篇
  2006年   10篇
  2005年   2篇
  2004年   1篇
  2003年   1篇
  2002年   2篇
  2000年   3篇
  1998年   2篇
  1997年   1篇
  1996年   1篇
  1995年   3篇
  1994年   1篇
  1993年   2篇
  1992年   1篇
  1987年   1篇
  1986年   3篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1979年   2篇
  1977年   1篇
排序方式: 共有86条查询结果,搜索用时 31 毫秒
21.
A new method of tide gauge data analysis is described for detecting ground deformation in active volcanic areas. This method was tested in the Phlegraean Fields, an active volcanic area near Naples where, in 1970 after a large ground uplift, a dense tide-gauge network was installed. The results of the tide-gauge analysis were checked with levelling data, and a good agreement between the data sets has been found. The observational continuity of the tide-gauge network gave the time evolution of the ground deformation and showed short time oscillations. The good results obtained at the Phlegraean Fields using the tide-gauge network and the data analysis procedure are encouraging.  相似文献   
22.
The essential features of the ongoing potential pre-eruptive crisis at the Phlegraean Fields begun in August 1982 are summarized and the main problems faced by scientists responsible of volcanic hazards evaluation in such a densely populated area are discussed.  相似文献   
23.
During the summer of 1982 a continuous uplift began at Phlegraean Fields, an active volcanic area in southern Italy (Fig. 1), that persisted up to September 1984. The uplift, which reached a maximum value of about 160 cm in the central part of the Phlegraean caldera, was characterized by variable velocity and occurred within an area that extended about 7 km outward from the town of Pozzuoli (Fig. 1); the surface deformation performs a bell-shape pattern.The uplift was accompanied by horizontal displacements and gravity changes that closely correlate with the described elevation changes.The horizontal displacements displayed an anomalous pattern within a narrow belt about 1 km from the center of the uplift and approximately coincident with the area of maximum seismic activity. The change in gravity is attributed primarily to a free-air effect to which a small Bouguer effect must be added.Several models have been invoked in order to explain the observed phenomenon. The one which gives the best fit to the observed data is an increasing pressure source of radial simmetry, at a constant depth of about 3 km beneath the town of Pozzuoli, and having a diameter of several hundred meters. Migration of magma at depth is believed to be responsible for the observed activity in the Phlegraean Fields caldera.  相似文献   
24.
We examine the sensitivity of 10Be concentrations (and derived denudation rates), to debris-flow and anthropogenic perturbations in steep settings of the Eastern Alps, and explore possible relations with structural geomorphic connectivity. Using cosmogenic 10Be as a tracer for functional geomorphic connectivity, we conduct sampling replications across four seasons in Gadria, Strimm and Allitz Creek. Sampling sites encompass a range of structural connectivity configurations, including the conditioning of a sackung, all assessed through a geomorphometric index (IC). By combining information on contemporary depth of erosion and sediment yield, disturbance history and post-LGM (Last Glacial Maximum) sedimentation rates, we constrain the effects of debris-flow disturbance on 10Be concentrations at the Gadria sites. Here, we argue that bedrock weakening imparted by the sackung promotes high depth of erosion. Consequently, debris flows recruit sediment beyond the critical depth of spallogenic production (e.g., >3 m), which in turn, episodically, due to predominantly muogenic production pathways, lowers 10Be concentration by a factor of 4, for at least 2 years. In contrast, steady erosion in Strimm Creek yields very stable 10Be concentrations through time. In Allitz Creek, we observe two- to fourfold seasonal fluctuations in 10Be concentrations, which we explain as the combined effects of water diversion and hydraulic structures on sediment mixing. We further show that 10Be concentration correlates inversely with the IC index, where sub-basins characterized by high concentrations (long residence times) exhibit low IC values (structurally disconnected) and vice versa, implying that, over millennial time scales a direct relation exists between functional and structural connectivity, and that the IC index performed as a suitable metric for structural connectivity. The index performs comparably better than other metrics (i.e., mean slope and mean normalized channel steepness index) previously used to assess topographic controls on denudation rates in active unglaciated ranges. In terms of landscape evolution, we argue that the sackung, by favouring intense debris-flow activity across the Holocene, has aided rapid postglacial reshaping of the Gadria basin, which currently exhibits a topographic signature characteristic of unglaciated debris-flow systems. © 2020 John Wiley & Sons, Ltd.  相似文献   
25.
The stability of spin-orbit resonances, namely commensurabilities between the periods of rotation and revolution of an oblate satellite orbiting around a primary body, is investigated using perturbation theory. We reduce the system to a model described by a one-dimensional, time-dependent Hamiltonian function. By means of KAM theory we rigorously construct bidimensional invariant surfaces, which separate the three dimensional phase space. In particular with a suitable choice of the rotation numbers of the invariant tori we are able to trap the periodic orbit associated with a given resonance in a finite region of the phase space. This technique is applied to the Mercury-Sun system. A connection with the probability of capture in a resonance is also provided.  相似文献   
26.
We consider Einstein-like gravitational equations with Λ term proportional to the trace of the energy tensor. The possibility of Lagrangian formulation is shortly discussed. The theory is consistent with the present-day observations, both at the local and cosmological level. Horizon-free cosmological solutions are found and discussed. The initial condition S(0) = 0 (where S(t) is the expansion factor and t the cosmic time) requires hyperbolic space. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
27.
The saturated hydraulic conductivity, Ks, is a soil property that has a key role in the partitioning of rainfall into surface runoff and infiltration. The commonly used instruments and methods for in situ measurements of Ks have frequently provided conflicting results. Comparison of Ks estimates obtained by three classical devices—namely, the double ring infiltrometer (DRI), the Guelph version of the constant‐head well permeameter (GUELPH‐CHP) and the CSIRO version of the tension permeameter (CSIRO‐TP) is presented. A distinguishing feature in this study is the use of steady deep flow rates, obtained from controlled rainfall–runoff experiments, as benchmark values of Ks at local and field‐plot scales, thereby enabling an assessment of these methods in reliably reproducing repeatable values and in their capability of determining plot‐scale variation of Ks. We find that the DRI grossly overestimates Ks, the GUELPH‐CHP gives conflicting estimates of Ks with substantial overestimation in laboratory experiments and underestimation at the plot scale, whereas the CSIRO‐TP yields average Ks values with significant errors of 24% in the plot scale experiment and 66% in laboratory experiments. Although the DRI would likely yield a better estimate of the nature of variability than the GUELPH‐CHP and CSIRO‐TP, a separate calibration may be warranted to correct for the overestimation of Ks values. The reasons for such discrepancies within and between the measurement methods are not yet fully understood and serve as motivation for future work to better characterize the uncertainty associated with individual measurements of Ks using these methods and the characterization of field scale variability from multiple local measurements.  相似文献   
28.
The photon-to-baryon ratio in the universe is much greater than unity (about109); this fact is unexplained in the standard model of the universe; it can be explained by a photon creation process related to a time-dependent cosmological term. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
29.
Levels of organic maturity of Mesozoic and Tertiary sequences outcropping in the Central Apennines have been established, using vitrinite reflectance techniques, the Thermal Alteration Index and fluorescence colours of organic matter dispersed in sediments. These results provide new constraints throughout the Meso-Cenozoic evolution of this crustal sector. In exploration geology, vitrinite reflectance provides data on hydrocarbon maturation by constraining organic matter maturity. In sedimentary basin modelling, it is adopted to define the palaeothermal regime. Vitrinite reflectance (Ro) also provides information on the burial history of sedimentary basins and may be employed to estimate tectonic uplift and erosion rates. Thermal Alteration Index (TAI) and fluorescence colour values can be correlated with Ro and may be used to estimate the degree of maturation when vitrinite is absent. Samples derived from the Sabini and Tiburtini Mts, in slope facies between the Latium–Abruzzi carbonate Platform and the Umbria–Marche pelagic Basin; from the Simbruini and Ernici Mts, in carbonate Platform facies, and from upper Miocene turbiditic deposits outcropping between the Olevano–Antrodoco Une, towards the West, and the Marsica slope facies, towards the East. Both the pre-terrigenous Meso-Cenozoic sequences show a low grade of organic maturity: the Sabini and Tiburtini Mts show Ro values that are less than 0.4%, and the Simbruini–Ernici Range show Ro values that range between 0.5% and 0.65%. Field analysis indicates that the cause of these low maturity levels is that thick sequences of turbidites were never deposited during the Neogene evolution of the Apennine thrust belt. Moreover, Upper Miocene turbiditic deposits also show low maturity levels, with Ro values that are less than 0.5%, indicating that these deposits were never overthrusted by huge volumes of rocks, during the chain building. The slight increase in the maturity level recorded in the Marsica area may be related to local heating along shear zones in areas of strike-slip tectonics.  相似文献   
30.
Two models for estimating expected areal‐average infiltration rate, ī, at the hillslope scale are presented. The first relies upon the condition of a negligible infiltration of surface water running downslope (run‐on process) into a previous heterogeneous soil. It is an adapted version of an earlier semi‐analytical model. The second incorporates the run‐on process and is based on a lumped approach that uses an effective saturated hydraulic conductivity. This latter was parameterized in terms of the main characteristics of rainfall and soil. Both the models were tested by comparison with the results carried out by Monte‐Carlo simulations over different soil types. It was found that the first model simulated ī with maximum errors in magnitude typically less than 10%. The second model provided similar errors in the total volume of overland flow, and the rising limb of the hydrograph experienced a distortion. Lastly, satisfactory results were obtained by comparing the model without run‐on with an empirical approach particularly accurate for fine‐textured soils. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号