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排序方式: 共有124条查询结果,搜索用时 328 毫秒
91.
Achim Aretz Kristian Bär Annette E. Götz Ingo Sass 《International Journal of Earth Sciences》2016,105(5):1431-1452
92.
Wind tunnel experiments were performed to examine the behavior of suddenly released volumes of dense gas in a turbulent shear layer. Instantaneous concentrations were measured with hot-wire katherometers. Multiple replications of each cloud volume, density, and velocity combination produced statistics for plume arrival time, arrival of maximum concentration time, plume departure time, and maximum concentrations. Probability distributions and standard deviations of each plume property permit prediction of hazard risks. 相似文献
93.
94.
Abstract The Japan Trench forearc offshore Honshu Island in northeast Japan, where the 130‐m.y.‐old Pacific oceanic plate is presently subducted, was drilled during the Ocean Drilling Program Leg 186. Results from mechanical and sedimentological studies of claystones recovered from Sites 1150 and 1151 in the overlying erosional forearc wedge are reported in the present study. Although many physical properties are similar in the seismic (Site 1150) and aseismic portion (Site 1151) of the shallow forearc, Site 1150 displayed a higher abundance of open fractures, two prominent fault zones and enigmatic pore fluid signatures in the claystones. The abundance of weak mineral phases, together with high smectite contents (from X‐ray diffraction), control the low friction coefficients of 0.33–0.39 of the claystones in ring‐shear experiments. Results from triaxial testing proposed overall low magnitudes of in situ effective vertical stress, with somewhat lower values at Site 1150 than at Site 1151. Similarly, samples from Site 1150 displayed slightly higher pore fluid pressures than those at Site 1151. The high sediment porosities, which are in part also a result of intact diatom tests (from scanning electron microscope), together with the anomalous fluid signatures and elevated pore fluid pressures, could very likely result from upward migration and influx of deep‐seated waters. Dewatering reactions at depth result in enhanced pore fluid pressure transients along out‐of‐sequence thrusts and consequently lower effective stress. At depths greater than that of Leg 186 drilling, elevated pressure–temperature conditions trigger mineral transformation and cementation, which result in increasing friction, unstable sliding and seismic rupture. Such earthquakes could have repeatedly disaggregated the consolidated claystone fabrics at the seismic site, and could be responsible for differences in yield strength and cementation when compared to the aseismic Site 1151. 相似文献
95.
Bao Yang Yafeng Shi Achim Braeuning Jianxun Wang 《Quaternary Science Reviews》2004,23(23-24):2537-2548
Former shorelines and sedimentary records from several lake basins in northwestern China (Xinjiang, Qinghai) give evidence for warm and humid climatic conditions during 40–30 ka BP. Further indications of this favorable climate period are derived from palaeosols from the Ili loess and from cemented calcareous layers on the terraces of the Keriya River at the southern margin of the Tarim Basin and in the Badain Jaran Desert in Inner Mongolia. At that time, annual mean temperature in the Qaidam Basin was 2 °C higher and in the western part of Inner Mongolia even 2–3 °C higher than today. Precipitation in most parts of northwestern China was between 60–300 mm greater than today. These changes were probably a consequence of an increase in ocean surface temperature and evaporation resulting from a higher radiation input at middle and low latitudes caused by changes in the Earth's precessional cycle. As a result of these orbital changes, it is suggested that the intensified westerly circulation was responsible for increased moisture over northwestern China. 相似文献
96.
Patricia Roeser Nadine Dräger Dariusz Brykała Florian Ott Sylvia Pinkerneil Piotr Gierszewski Christin Lindemann Birgit Plessen Brian Brademann Michał Kaszubski Michał Fojutowski Markus J. Schwab Michał Słowiński Mirosław Błaszkiewicz Achim Brauer 《Boreas: An International Journal of Quaternary Research》2021,50(2):419-440
We revise the conceptual model of calcite varves and present, for the first time, a dual lake monitoring study in two alkaline lakes providing new insights into the seasonal sedimentation processes forming these varves. The study lakes, Tiefer See in NE Germany and Czechowskie in N Poland, have distinct morphology and bathymetry, and therefore, they are ideal to decipher local effects on seasonal deposition. The monitoring setup in both lakes is largely identical and includes instrumental observation of (i) meteorological parameters, (ii) chemical profiling of the lake water column including water sampling, and (iii) sediment trapping at both bi-weekly and monthly intervals. We then compare our monitoring data with varve micro-facies in the sediment record. One main finding is that calcite varves form complex laminae triplets rather than simple couplets as commonly thought. Sedimentation of varve sub-layers in both lakes is largely dependent on the lake mixing dynamics and results from the same seasonality, commencing with diatom blooms in spring turning into a pulse of calcite precipitation in summer and terminating with a re-suspension layer in autumn and winter, composed of calcite patches, plant fragments and benthic diatoms. Despite the common seasonal cycle, the share of each of these depositional phases in the total annual sediment yield is different between the lakes. In Lake Tiefer See calcite sedimentation has the highest yields, whereas in Lake Czechowskie, the so far underestimated re-suspension sub-layer dominates the sediment accumulation. Even in undisturbed varved sediments, re-suspended material becomes integrated in the sediment fabric and makes up an important share of calcite varves. Thus, while the biogeochemical lake cycle defines the varves’ autochthonous components and micro-facies, the physical setting plays an important role in determining the varve sub-layers’ proportion. 相似文献
97.
G. Le Petit G. Douysset G. Ducros P. Gross P. Achim M. Monfort P. Raymond Y. Pontillon C. Jutier X. Blanchard T. Taffary C. Moulin 《Pure and Applied Geophysics》2014,171(3-5):629-644
Part I of this publication deals with the analysis of fission product releases consecutive to the Fukushima Dai-ichi accident. Reactor core damages are assessed relying on radionuclide detections performed by the CTBTO radionuclide network, especially at the particulate station located at Takasaki, 210 km away from the nuclear power plant. On the basis of a comparison between the reactor core inventory at the time of reactor shutdowns and the fission product activities measured in air at Takasaki, especially 95Nb and 103Ru, it was possible to show that the reactor cores were exposed to high temperature for a prolonged time. This diagnosis was confirmed by the presence of 113Sn in air at Takasaki. The 133Xe assessed release at the time of reactor shutdown (8 × 1018 Bq) turned out to be in the order of 80 % of the amount deduced from the reactor core inventories. This strongly suggests a broad meltdown of reactor cores. 相似文献
98.
Achim Wirth 《Ocean Dynamics》2009,59(4):551-563
Results from numerical simulations of idealised, 2.5-dimensional Boussinesq, gravity currents on an inclined plane in a rotating
frame are used to determine the qualitative and quantitative characteristics of such currents. The current is initially geostrophically
adjusted. The Richardson number is varied between different experiments. The results demonstrate that the gravity current
has a two-part structure consisting of: (1) the vein, the thick part that is governed by geostrophic dynamics with an Ekman
layer at its bottom, and (2) a thin friction layer at the downslope side of the vein, the thin part of the gravity current.
Water from the vein detrains into the friction layer via the bottom Ekman layer. A self consistent picture of the dynamics
of a gravity current is obtained and some of the large-scale characteristics of a gravity current can be analytically calculated,
for small Reynolds number flow, using linear Ekman layer theory. The evolution of the gravity current is shown to be governed
by bottom friction. A minimal model for the vein dynamics, based on the heat equation, is derived and compares very well to
the solutions of the 2.5-dimensional Boussinesq simulations. The heat equation is linear for a linear (Rayleigh) friction
law and non-linear for a quadratic drag law. I demonstrate that the thickness of a gravity current cannot be modelled by a
local parameterisation when bottom friction is relevant. The difference between the vein and the gravity current is of paramount
importance as simplified (streamtube) models should model the dynamics of the vein rather than the dynamics of the total gravity
current. In basin-wide numerical models of the ocean dynamics the friction layer has to be resolved to correctly represent
gravity currents and, thus, the ocean dynamics. 相似文献
99.
A 1.5-dimensional, 1.5-layer shallow water model and an ensemble Kalman filter are used to evaluate the feasibility of estimating friction parameters and determining friction laws of oceanic gravity currents. The two friction laws implemented are a linear Rayleigh friction and a quadratic drag law. We demonstrate that the assimilation procedure rapidly estimates the total frictional force, whereas the distinction between the two laws is evolving on a slower time scale. We also demonstrate that parameter estimation can, in this way, choose between different parametrisations and help to discriminate between physical laws of nature by estimating the coefficients presented in such parametrisations. 相似文献
100.
Using maximum likelihood to derive various distance-based goodness-of-fit indicators for hydrologic modeling assessment 总被引:1,自引:1,他引:0
Qin-Bo?ChengEmail authorView authors OrcID profile Xi?Chen Chong-Yu?Xu Zhi-Cai?Zhang Christian?Reinhardt-Imjela Achim?Schulte 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(4):949-966
Currently used goodness-of-fit (GOF) indicators (i.e. efficiency criteria) are largely empirical and different GOF indicators emphasize different aspects of model performance; a thorough assessment of model skill may require the use of robust skill matrices. In this study, based on the maximum likelihood method, a statistical measure termed BC-GED error model is proposed, which firstly uses the Box–Cox (BC) transformation method to remove the heteroscedasticity of model residuals, and then employs the generalized error distribution (GED) with zero-mean to fit the distribution of model residuals after BC transformation. Various distance-based GOF indicators can be explicitly expressed by the BC-GED error model for different values of the BC transformation parameter λ and GED kurtosis coefficient β. Our study proves that (1) the shape of error distribution implied in the GOF indicators affects the model performance on high or low flow discharges because large error-power (β) value can cause low probability of large residuals and small β value will lead to high probability of zero value; (2) the mean absolute error could balance consideration of low and high flow value as its assumed error distribution (i.e. Laplace distribution, where β = 1) is the turning point of GED derivative at zero value. The results of a study performed in the Baocun watershed via comparison of the SWAT model-calibration results using six distance-based GOF indicators show that even though the formal BC-GED is theoretically reasonable, the calibrated model parameters do not always correspond to high performance of model-simulation results because of imperfection of the hydrologic model. However, the derived distance-based GOF indicators using the maximum likelihood method offer an easy way of choosing GOF indicators for different study purposes and developing multi-objective calibration strategies. 相似文献