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排序方式: 共有119条查询结果,搜索用时 15 毫秒
31.
Airborne Geophysical Exploration for Ground Water 总被引:1,自引:0,他引:1
32.
33.
The localization of plastic deformation in rock is of particular interest in geology in connection with the formation of “ductile” shear zones. It is commonly conjectured that strain softening, as evidenced by a falling stress–strain curve at constant strain rate, is likely to lead to strain localization. Yet observations in torsion tests at constant twist rate fail to show such an effect. However, a more sophisticated analysis using the theory of Fressengeas and Molinari (J. Mech. Phys. Solids 1987, 35, 185–211) for material showing strain-rate dependence of the flow stress does show that, when the boundary conditions are specified in terms of displacements, no localization is predicted in case of strain softening. In contrast, if the boundary conditions are set in terms of forces, localization can be expected for a strain softening material. This prediction needs experimental testing. 相似文献
34.
Summary. A non-steady-state theoretical model is used to predict the present variation of temperature with depth in two boreholes in the Devon Island ice cap, Arctic Canada. The boreholes are 300 m apart and one of them reaches bedrock. The heat transfer equation is solved numerically with the record of past temperatures obtained from measurements of the variations of oxygen—isotope ratio with depth in the cores as surface boundary condition. The effects of ice advection, refreezing of meltwater percolating from the surface (the amount of which is recorded in the cores), heating due to firn compaction and ice deformation, and heat flow in the bedrock below the ice sheet are all included in the model. The free parameters are geothermal heat flux, present surface temperature and heat loss at the surface which depends on the depth of meltwater penetration and other factors. Agreement between observed and predicted temperature—depth profiles is very close. Latent heat released by percolating meltwater is the predominating factor in determining the temperature distribution in the upper half of each borehole. The temperature distribution is insensitive to the value of the factor used to convert oxygen—isotope ratio to temperature. 相似文献
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36.
Trends in climate time series are often nonlinear and temporally-asymmetric, i.e. the trend is different for different seasons and/or hours of the day. Here a method is developed that allows the nonlinearity and temporal asymmetry of a trend to be investigated simultaneously. First, nonlinear trend components are extracted from a univariate time series, by adapting a nonparametric dimension-reduction method. Then, the nonlinear trend components are substituted into a regression model in which the periodic mean component and the periodic variation in the amplitude of the nonlinear trend are modeled using harmonic functions of the seasonal and diurnal periods. Third, trend patterns in the positive and negative anomalies are investigated, by extending the nonlinear trend model using indicator variables. Fourth, a non-local inferential test is developed to test the statistical significance of the trend patterns. The nonlinear trend model is applied to a simulated time series, as well as to long-term high-resolution temperature records from five Southern Hemisphere sites: Lucas Heights, Sydney Airport, Cape Grim, Macquarie Island and Law Dome. Our method should be generally useful for identifying the effect of both climate-related factors and observation/site-related factors on seasonal and diurnal trends in meteorological data series. 相似文献
37.
Diatom assemblages in recent versus pre-industrial sediments were examined in 40 relatively undisturbed lakes from the Experimental
Lakes Area (ELA). The ELA region of northwestern Ontario receives low amounts of acidic deposition and the lakes have been
minimally disturbed by watershed development or other human activities. Consequently, this region represents an important
location to detect possible changes in lakes due to climate change. In over half of the lakes, planktonic taxa (especially
Discostella stelligera) increased between 10 and 40% since pre-industrial times. Changes in diatom assemblages are consistent with taxa that would
benefit from enhanced stratification and a longer ice-free season. We hypothesized that there should be a relationship between
stratification and measured chemical and physical characteristics of the study lakes. Multiple correlation analysis was undertaken
to see the relationship between planktonic taxa and D. stelligera since pre-industrial times and the physical and chemical characteristics of the study lakes. Lake depth was consistently
identified as an important variable. The timing of the increase in planktonic taxa within cores from these lakes will be needed
to rule out other possible regional changes that may also be occurring in the ELA region. 相似文献
38.
A new radiospectrograph at Culgoora 总被引:1,自引:0,他引:1
N. P. Prestage R. G. Luckhurst B. R. Paterson C. S. Bevins C. G. Yuile 《Solar physics》1994,150(1-2):393-396
IPS Radio and Space Services (IPS) commissioned a new radiospectrograph for solar observations in May 1992. The instrument
operates over a frequency range from 18 MHz to 1.8 GHz and samples every three seconds. Digital data acquisition permits a
variety of analysis procedures that were not previously possible. The operation of the new instrument is briefly described
and some example radiospectrograms are presented. 相似文献
39.
Wiltse Brendan Mushet Graham R. Paterson Andrew M. Cumming Brian F. 《Journal of Paleolimnology》2022,67(3):273-287
Journal of Paleolimnology - Increased relative abundances and accumulation rates of the small, centric diatom Discostella have been recorded in numerous paleolimnological investigations of north... 相似文献
40.
Adam Jeziorski Andrew M. Paterson Norman D. Yan John P. Smol 《Journal of Paleolimnology》2008,39(3):421-425
Despite reductions in atmospheric sulphur emissions and the resulting decline in acidic deposition, many lakes on the Canadian
Shield that have experienced acidification are either recovering at a rate slower than expected or not recovering at all.
This lack of recovery is believed to be partly the consequence of the depletion of exchangeable base cations (principally
calcium, Ca) from watershed soils. Although the implications of reduced Ca availability for biological communities remain
poorly understood, the effects on crustacean zooplankton populations may be severe, as Ca is the primary structural component
of the crustacean zooplankton exoskeleton. Because the daphniid resting egg is protected by a modified portion of the exoskeleton
(ephippia) and because these ephippia are well preserved in lake sediments, we investigated whether inter-specific differences
in Ca content are recoverable from Daphnia ephippia. However, using two methods, we did not find a recoverable Ca signal in the ephippia, indicating that future efforts
should be focused upon sedimentary taxonomic assemblage differences, not differences in ephippial Ca levels. 相似文献