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121.
Summary The analytical expressions used to compute the partial derivatives of phase and group velocity of Rayleigh waves with respect to the P- and S-wave velocity and the density are derived and the related computer code is developed. The results of the analytical computations were satisfactorily tested against numerically determined values. Several examples of partial derivatives for a given structural model are presented. 相似文献
122.
We used an individual-based forest simulator (a gap model) to assess the potential effects of anthropogenic climatic change on conifer forests of the Pacific Northwestern United States. Steady-state simulations suggested that forest zones could be shifted on the order of 500–1000 m in elevation, which could lead to the local extirpation of some high-altitude species. For low-elevation sites, species which currently are more abundant hundreds of kilometers to the south would be favored under greenhouse scenarios. Simulations of transient responses suggested that forest stands could show complex responses depending on initial species composition, stand age and canopy development, and the magnitude and duration of climatic warming. Assumptions about species response to temperature, which are crucial to the model's behaviors, were evaluated using data on species temperature limits inferred from regional distributions. The high level of within-species variability in these data, and other confounding factors influencing species distributions, argue against over-interpreting simulations. We suggest how we might resolve critical uncertainties with further research. 相似文献
123.
124.
Solute transfer across the sediment surface of a eutrophic lake: I. Porewater profiles from dialysis samplers 总被引:6,自引:0,他引:6
Noel R. Urban Christian Dinkel Bernhard Wehrli 《Aquatic Sciences - Research Across Boundaries》1997,59(1):1-25
Porewater profiles often are used to identify and quantify important biogeochemical processes occurring in lake sediments.
In this study, multiple porewater profiles were obtained from two eutrophic Swiss lakes using porewater equilibrators (peepers)
in order to examine spatial and seasonal trends in biogeochemical processes. Variability in profile shapes and concentrations
was small on spatial scales of a few meters, but the uncertainty in calculated diffusive fluxes across the sediment surface
was, on average, 35%. Focusing of Fe and Mn oxides toward the lake center resulted in systematic increases in porewater concentrations
and diffusive fluxes of Fe2+ and Mn2+ with increasing water depth; these fluxes are postulated to be regulated by the pH-dependent dissolution of reduced-metal
phases. Despite higher concentrations of inorganic carbon, NH
4
+
, Si and P in pelagic compared to littoral sites, diffusive fluxes of these substances across the sediment surface increased
only slightly or not at all with increasing water depth. Porewater profiles did reveal temporal changes in Fe2+, Mn2+, Ca2+ and Mg2+ that were an indirect result of the large, seasonal changes in seston deposition, but no clear seasonal variations were found
in diffusive fluxes of nutrients across the sediment surface. The intense mineralization occurring at the sediment surface
was not reflected in the porewater profiles nor in the calculated diffusive fluxes. Calculated diffusive fluxes across the
sediment surface resulted from decomposition occurring primarily in the top 5–7 cm of sediment. Diffusive fluxes from this
subsurface mineralization were equal to the solute release from mineralization occurring at the sediment-water interface.
Buried organic matter acts as a memory of previous lake conditons; it will require at least a decade before reductions in
nutrient inputs to lakes fully reduce the diffusive fluxes into the lake from the buried reservoir of organic matter. 相似文献
125.
Experimental dispersion curves of Rayleigh and Love waves along the Uppsala-Prague profile have been determined using records of several Italian earthquakes. To interpret the dispersion data, results of previous geophysical investigations in this region were first analyzed. Seven blocks of the crust and upper mantle were distinguished along the profile on the basis of deep seismic sounding and other seismic data. Layered models were proposed for these blocks. Computation of Rayleigh and Love waves shows a large differentiation of theoretical dispersion curves for the northern (Precambrian) and southern (Palaeozoic) part of the profile. A laterally inhomogeneous model for theUppsala - Prague profile, composed of the seven blocks, satisfies the surface wave data for the profile. Moreover, a mean layered model for the whole profile has also been proposed. 相似文献
126.
127.
A GIS-based groundwater travel time model to evaluate stream nitrate concentration reductions from land use change 总被引:3,自引:0,他引:3
Excessive nitrate-nitrogen (nitrate) loss from agricultural watersheds is an environmental concern. A common conservation
practice to improve stream water quality is to retire vulnerable row croplands to grass. In this paper, a groundwater travel
time model based on a geographic information system (GIS) analysis of readily available soil and topographic variables was
used to evaluate the time needed to observe stream nitrate concentration reductions from conversion of row crop land to native
prairie in Walnut Creek watershed, Iowa. Average linear groundwater velocity in 5-m cells was estimated by overlaying GIS
layers of soil permeability, land slope (surrogates for hydraulic conductivity and gradient, respectively) and porosity. Cells
were summed backwards from the stream network to watershed divide to develop a travel time distribution map. Results suggested
that groundwater from half of the land planted in prairie has reached the stream network during the 10 years of ongoing water
quality monitoring. The mean travel time for the watershed was estimated to be 10.1 years, consistent with results from a
simple analytical model. The proportion of land in the watershed and subbasins with prairie groundwater reaching the stream
(10–22%) was similar to the measured reduction of stream nitrate (11–36%). Results provide encouragement that additional nitrate
reductions in Walnut Creek are probable in the future as reduced nitrate groundwater from distal locations discharges to the
stream network in the coming years. The high spatial resolution of the model (5-m cells) and its simplicity may make it potentially
applicable for land managers interested in communicating lag time issues to the public, particularly related to nitrate concentration
reductions over time. 相似文献
128.
The influence of foliation on the fragility of granitic rocks, image analysis and quantitative microscopy 总被引:6,自引:0,他引:6
Urban kesson Jimmy Stigh Jan Erik Lindqvist Mattias G ransson 《Engineering Geology》2003,68(3-4):275-288
Fabric-dependent anisotropy is important in understanding the mechanical behaviour of foliated rocks because the foliation creates mechanically weak discontinuities. Using optical microscopy, a foliation index (FIX) was calculated for a group of granitic rocks in southern Sweden that displays a range of textures from foliated to nonfoliated. Image analysis from scanning electron microscope with backscattered detector (SEM/BSE) images was used in order to measure the rock texture of these samples. The analyses were compared with the Los Angeles (LA) test, which measures resistance of rock aggregate to fragmentation. The results demonstrate that the measured grain size, shape and spatial arrangement are important parameters controlling the fragility of the rock. Both the texture and foliation must be taken into account to obtain a significant relationship with the mechanical analyses. We suggest that this alternative method could be a helpful tool for predicting a rock's suitability as an aggregate as it gives a better understanding of the material properties compared to mechanical testing. 相似文献
129.
To assess the importance of heterotrophic microplankton in the Wadden Sea, seasonal distribution and biomass of the main subgroups, that is, heterotrophic dinoflagellates, (separated into thecate and athecate forms), tintinnids, and aloricate ciliates, were studied in 1989 and 1990 in a total of six surveys covering the whole area of the northern German Wadden Sea. Heterotrophic microplankton biomass exhibited high spatial and temporal variation, ranging from 0 μg Cl?1 to 66 μg Cl?1, with maximum concentrations in spring., Mean stocks were lowest in winter (1.6 μg Cl?1) and highest in spring (11.7 μg Cl?1); intermediate concentrations were found in summer (8.5 μg Cl?1). In winter, the heterotrophic microplankton was dominated by tintinnids. In spring and summer, aloricate ciliates and dinoflagellates made up the largest part of the biomass. A pronounced feature was a shift within the dinoprotist group from athecate to thecate forms in summer. In spring, maxima of athecate dinoflagellate carbon were associated with blooms ofPhaeocystis globosa, indicating a close trophic relationship. From rough estimates of the daily grazing potential, based on microheterotrophic biomass and conversion factors from the literature, it may be concluded that heterotrophic microplankton temporarily share a main role in the transfer of food and energy to higher trophic levels within the pelagic system of the Wadden Sea. 相似文献
130.
Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Teplá-Barrandian zone (Czech Republic) record 40Ar/39Ar mineral plateau ages which range between ca. 376 and 362?Ma. Hornblende concentrates from metagabbro (Mariánské Lánzě complex) and fine-grained basement amphibolite display plateaux which define 36Ar/40Ar vs 39Ar/40Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Teplá-Barrandian zone is characterized by an internally discordant 40Ar/39Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdyn? complex records a well-defined 40Ar/39Ar age plateau which corresponds to an isotope-correlation age of ca. 516?Ma. This is interpreted to date post-magmatic cooling following emplacement. 相似文献