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排序方式: 共有242条查询结果,搜索用时 31 毫秒
51.
This paper describes the development of a computational model to predict the response of synthetic-fiber ropes under both monotonic and cyclic loads. The model addresses the issue of damage to a rope cross-section and its effect on rope response. Isotropic damage is assumed and is therefore characterized by a scalar quantity known as the damage index. The damage index is used to quantify the amount of deterioration that takes place in a damaged rope throughout its loading history. For the current study, data obtained from static tension tests are used to determine the evolution relationship for the damage index. This relationship is based on the stiffness ratio between experimental data and simulation of rope response without any source of damage and is assumed to be described by an exponential equation. Numerical simulations of damaged rope behavior compare well with experimental results. 相似文献
52.
M. Cristina Fossi Silvia Casini Letizia Marsili Stefania Ancora Gabriele Mori Giovanni Neri Teresa Romeo Antonella Ausili 《Marine environmental research》2004,58(2-5):425
In this project we investigated the ecotoxicological effects of endocrine disrupters in a four-year survey of the Mediterranean population of swordfish (Xiphias gladius). In the Mediterranean environment, top predators, such as swordfish, accumulate high concentrations of polyhalogenated aromatic hydrocarbons (PHAHs) and toxic metals, potentially incurring high toxicological risk. The effects of organochlorines and trace elements (Hg, Cd and Pb) in 192 swordfish specimens, caught in the Strait of Messina, Sicily, Italy, were investigated using vitellogenin (Vtg), zona radiata proteins (Zrp) and CYP1A (BPMO, EROD) activities. Vtg and Zrp were found to be dramatically induced in some adult male specimens, suggesting that this species is highly exposed to estrogens in the Mediterranean Sea. A role of organochlorines in this induction phenomenon is suggested by the statistically significant correlations between Zrp in plasma and PCB concentrations in muscle (p<0.032) and Vtg in plasma and PCB concentrations in liver (p<0.034) of male specimens. Levels of trace elements in liver were in the following ranges: Hg 1–22, Cd 1–28 and Pb 0–1.6 ppm d.w. These data indicate potential reproductive alterations in large pelagic fish and suggest the need for continuous monitoring to avoid reductions in the population of this fish species of high commercial and ecological interest. 相似文献
53.
A new imaging balloon-borne telescope for hard X-rays in the energy range from 30 to 100 keV is described. The imaging capability is provided by the use of an extended URA-based coded-mask. With only one motor and suitable stop pins, we can rotate a carbon-fiber wheel with most of the mask elements attached to it by 180°, and a bar, which is also part of the mask pattern and is allowed to rotate freely over the wheel, by 90°; this combined rotation creates an antimask of the original mask, except for the central element. This is a novel and elegant manner of providing an antimask without additional weight and complex mechanical manipulations. We show that the use of antimasks is a very effective method of eliminating systematic variations in the background map over the position-sensitive detector area. The expected sensitivity of the instrument for the 30–100 keV range is of the order of 7 × 10-5 photons cm-2 s-1 keV-1, for an integration time of 104 seconds at a residual atmosphere of 3.5 g cm-2. This telescope will provide imaging observations of bright galactic hard X-ray sources with an angular resolution of 2° in a 10° by 10° FOV, which is defined by a collimator placed in front of the detector system. We are particularly interested in the galactic center region, where recent imaging results in X-rays have shown the presence of an interesting source field. Results of computer simulations of the imaging system are reported. 相似文献
54.
Factors controlling the vegetation distribution and coal‐forming environments in a strike‐slip basin. The Pennsylvanian Peñarroya‐Belmez‐Espiel Basin,southern Spain
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Coal‐forming environments require humid to perhumid conditions. Tectonics governs the size, location and availability of coal seams developed in such environments. While large Pennsylvanian paralic basins generated thick and continuous coal seams, many other small coeval basins, which were tectonically active, developed a puzzling succession, with carbonaceous deposits that varied in size, thickness and the nature of the coal‐forming flora. This study, conducted in the Peñarroya‐Belmez‐Espiel coalfield, a Variscan strike‐slip basin in the south of Spain, provides insights into this subject. The coal seams analysed, generated in different depositional environments, have quantitatively different palynological assemblages. Lacustrine coals are dominated by lycopsids; distal alluvial plain/marginal lacustrine coals are dominated by sphenophytes and tree ferns, and middle alluvial fan coals are dominated by sphenophytes, tree ferns and lycopsids. This means that when conditions were favourable for peat accumulation, peat accumulated regardless of the nature of the available flora. 相似文献
55.
Ray-traced slant factors for mitigating the tropospheric delay at the observation level 总被引:3,自引:2,他引:1
Landon?UrquhartEmail author Felipe?G.?Nievinski Marcelo?C.?Santos 《Journal of Geodesy》2012,86(2):149-160
Three-dimensional ray tracing through a numerical weather model has been applied to a global precise point positioning (PPP)
campaign for modeling both the elevation angle- and azimuth-dependence of the tropospheric delay. Rather than applying the
ray-traced slant delays directly, the delay has been parameterized in terms of slant factors, which are applied in a similar
manner to traditional mapping functions, but which can account for the azimuthal asymmetry of the delay. Five strategies are
considered: (1) Vienna Mapping Functions 1 (VMF1) and estimation of a residual zenith delay parameter; (2) VMF1, estimation
of a residual zenith delay and estimation of two tropospheric gradient parameters; (3) three-dimensional ray-traced slant
factors and estimation of a residual zenith delay; (4) using only ray-traced slant factors and no estimation of any tropospheric
parameters and; (5) using both ray-traced slant factors and estimating a residual zenith delay and two tropospheric gradient
parameters. The use of the ray-traced slant factors (solution 3) showed a 3.8% improvement in the repeatability of the up
component when compared to the assumption of a symmetric atmosphere (solution 1), while the estimation of two tropospheric
gradient parameters gave the best results showing an 7.6% improvement over solution 1 in the up component. Solution 4 performed
well in the horizontal domain, allowing for sub-centimeter repeatability but the up component was degraded due to deficiencies
in the modeling of the zenith delay, particularly for stations located at equatorial latitudes. The magnitude of the differences
in the mean coordinates between solution 2 and solution 3, and the strong correlation with the differences between the north
component and the ray-traced gradients (coefficient of correlation of 0.83), as well as the impact of observation geometry
on the gradient solution indicate that the use of the ray-traced slant factors could have an implication on the realization
of reference frames. The estimated tropospheric products from the PPP solutions were compared to those derived from ray tracing.
For the zenith delay, a root mean square (RMS) of 5.4 mm was found, while for the gradient terms, a correlation coefficient
of 0.46 for the N–S and 0.42 for the E–W was found for the north–south and east–west components, suggesting that there are
still important differences in the gradient parameters which could be due to either errors in the NWM or to non-tropospheric
error sources leaking into the PPP-estimated gradients. 相似文献
56.
Amelie Bücker Patricio Crespo Hans-Georg Frede Kellie Vaché Felipe Cisneros Lutz Breuer 《Aquatic Geochemistry》2010,16(1):127-149
We investigated controls on the water chemistry of a South Ecuadorian cloud forest catchment which is partly pristine, and
partly converted to extensive pasture. From April 2007 to May 2008 water samples were taken weekly to biweekly at nine different
subcatchments, and were screened for differences in electric conductivity, pH, anion, as well as element composition. A principal
component analysis was conducted to reduce dimensionality of the data set and define major factors explaining variation in
the data. Three main factors were isolated by a subset of 10 elements (Ca2+, Ce, Gd, K+, Mg2+, Na+, Nd, Rb, Sr, Y), explaining around 90% of the data variation. Land-use was the major factor controlling and changing water
chemistry of the subcatchments. A second factor was associated with the concentration of rare earth elements in water, presumably
highlighting other anthropogenic influences such as gravel excavation or road construction. Around 12% of the variation was
explained by the third component, which was defined by the occurrence of Rb and K and represents the influence of vegetation
dynamics on element accumulation and wash-out. Comparison of base- and fast flow concentrations led to the assumption that
a significant portion of soil water from around 30 cm depth contributes to storm flow, as revealed by increased rare earth
element concentrations in fast flow samples. Our findings demonstrate the utility of multi-tracer principal component analysis
to study tropical headwater streams, and emphasize the need for effective land management in cloud forest catchments. 相似文献
57.
We develop a new computational methodology for solving two‐phase flow in highly heterogeneous porous media incorporating geomechanical coupling subject to uncertainty in the poromechanical parameters. Within the framework of a staggered‐in‐time coupling algorithm, the numerical method proposed herein relies on a Petrov–Galerkin postprocessing approach projected on the Raviart–Thomas space to compute the Darcy velocity of the mixture in conjunction with a locally conservative higher order finite volume discretization of the nonlinear transport equation for the saturation and an operator splitting procedure based on the difference in the time‐scales of transport and geomechanics to compute the effects of transient porosity upon saturation. Notable features of the numerical modeling proposed herein are the local conservation properties inherited by the discrete fluxes that are crucial to correctly capture the fingering patterns arising from the interaction between heterogeneity and nonlinear viscous coupling. Water flooding in a poroelastic formation subject to an overburden is simulated with the geology characterized by multiscale self‐similar permeability and Young modulus random fields with power‐law covariance structure. Statistical moments of the poromechanical unknowns are computed within the framework of a high‐resolution Monte Carlo method. Numerical results illustrate the necessity of adopting locally conservative schemes to obtain reliable predictions of secondary recovery and finger growth in strongly heterogeneous deformable reservoirs. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
58.
Felipe García-Rodríguez Laura Del Puerto Carola Castieira Hugo Inda Roberto Bracco Peter Spechmann Burkhard Wilhelm Scharf 《Limnologica》2001,31(3)
Physical, chemical and biological data from a sedimentary profile of Rocha lagoon (Uruguay) are presented. Analyses of silica remains showed that opal phytoliths, diatoms, sponge spicules and chrysophyte cysts were the most important contributors to biogenic silica. Organic matter, total nitrogen and total phosphorus were used to infer past trophic development. Physical analysis of sediments allowed us to identify five lithological units related to sea level variation and climatic events. 相似文献
59.
60.
Ana Carolina Sarmento Buarque Namrata Bhattacharya-Mis Maria Clara Fava Felipe Augusto Arguello de Souza Eduardo Mario Mendiondo 《水文科学杂志》2020,65(7):1075-1083
ABSTRACTThe city of São Carlos, state of São Paulo, Brazil, has a historical coexistence between society and floods. Unplanned urbanization in this area is a representative feature of how Brazilian cities have developed, undermining the impact of natural hazards. The Gregório Creek catchment is an enigma of complex dynamics concerning the relationship between humans and water in Brazilian cities. Our hypothesis is that social memory of floods can improve future resilience. In this paper we analyse flood risk dynamics in a small urban catchment, identify the impacts of social memory on building resilience and propose measures to reduce the risk of floods. We applied a socio-hydrological model using data collected from newspapers from 1940 to 2018. The model was able to elucidate human–water processes in the catchment and the historical source data proved to be a useful tool to fill gaps in the data in small urban basins. 相似文献