全文获取类型
收费全文 | 154篇 |
免费 | 9篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 13篇 |
地球物理 | 60篇 |
地质学 | 55篇 |
海洋学 | 17篇 |
天文学 | 11篇 |
综合类 | 2篇 |
自然地理 | 3篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2012年 | 4篇 |
2011年 | 3篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 9篇 |
2005年 | 1篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 4篇 |
1975年 | 2篇 |
排序方式: 共有177条查询结果,搜索用时 31 毫秒
71.
72.
73.
Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients. 相似文献
74.
This paper describes a series of experiments that tested the ability of the electrical resistivity tomography (ERT) method to locate correctly wet and dry fractures in a meso-scale model. The goal was to develop a method of monitoring the flow of water through a fractured rock matrix. The model was a four by six array of limestone blocks equipped with 28 stainless steel electrodes. Dry fractures were created by placing pieces of vinyl between one or more blocks. Wet fractures were created by injecting tap water into a joint between blocks. In electrical terms, the dry fractures are resistive and the wet fractures are conductive. The quantities measured by the ERT system are current and voltage around the outside edge of the model. The raw ERT data were translated to resistivity values inside the model using a three-dimensional Occam's inversion routine. This routine was one of the key components of ERT being tested. The model presented several challenges. First, the resistivity of both the blocks and the joints was highly variable. Second, the resistive targets introduced extreme changes the software could not precisely quantify. Third, the abrupt changes inherent in a fracture system were contrary to the smoothly varying changes expected by the Occam's inversion routine. Fourth, the response of the conductive fractures was small compared to the background variability. In general, ERT was able to locate correctly resistive fractures. Problems occurred, however, when the resistive fracture was near the edges of the model or when multiple fractures were close together. In particular, ERT tended to position the fracture closer to the model center than its true location. Conductive fractures yielded much smaller responses than the resistive case. A difference-inversion method was able to correctly locate these targets. 相似文献
75.
Cosler DJ 《Ground water》2004,42(2):203-222
Nonequilibrium concentration type curves are numerically developed and sensitivity analyses are performed to examine the relationships between effluent concentrations in partially penetrating monitoring/extraction wells, the vertical plume shape, and the mass transfer characteristics of the aquifer. The governing two-dimensional, axisymmetric nonequilibrium solute transport equation is solved in three stages using an operator-splitting approach. In the first two stages, the advection and dispersion terms are solved with the Eulerian-Lagrangian method, based on the backward method of characteristics for advection and the standard implicit Galerkin finite element method for dispersion. In the third step, the first-order, immobile-mobile domain mass transfer term is computed analytically for both two-site and lognormally distributed, multirate models. Effluent concentration variations with time and contour plots of the pore water concentration distribution in the aquifer are compared for a wide range of field- and laboratory-measured mass transfer rates, various plume shapes, and relevant physical/chemical parameter values, including pumping rate, vertical anisotropy ratio, retardation factor, and porosity. The simulation results show that rate-limited mass transfer can have a significant impact on sample and aquifer pore water concentrations during three-dimensional transport to a partially penetrating well. An alternative dimensionless form of the nonequilibrium solute transport equation is derived to illustrate the key parameter groupings that quantify rate-limited sorption effects and show the relative importance of individual parameters. A hypothetical field application example demonstrates the fitting of dimensional type curves to discrete-interval sampling data in order to evaluate the mass transfer characteristics of an aquifer and shows how type curve superposition can be used to model complex plume shapes. 相似文献
76.
Palaeoecology, reference conditions and classification of ecological status: the EU Water Framework Directive in practice 总被引:2,自引:0,他引:2
The European Union's Water Framework Directive (WFD) requires that all Member States within the European Union determine reference conditions for aquatic ecosystems to provide a baseline against which to measure the effects of past and present activities. Reference conditions are subsequently used to classify the ecological status of European waters. The decisions regarding environmental status will be important future elements in the management of European coastal waters. We have developed a number of classification scenarios for total nitrogen (TN) in the overlying waters of the southern part of Roskilde Fjord, Denmark, taking as our basis a palaeoecological reconstruction of fluctuations in TN between 1850 and 1995. We present a provisional classification scheme for the ecological status of Roskilde Fjord, sensu the WFD. Decision(s) regarding the deviation from reference conditions will give a wide range of apparent ecological status from good, through moderate and poor, to bad depending upon the definition of an acceptable deviation from reference conditions. The determination of an acceptable deviation will ultimately be a political decision, and will result in a wide range in the protection of coastal waters in Europe. There is still, however, an urgent need for a sound scientific documentation of the various scenarios for the implementation of the WFD. 相似文献
77.
Ocean surface water [CO2(aq)] variations based on glacial/interglacial changes in sediment delta 13Corg are shown to compare favorably with reconstructions based on ice core [CO2]. In particular, an approximate 80 microatmospheres increase in atmospheric pCO2 during the last glacial-interglacial transition is calculated to correspond to a 3-4 micromolar increase in ocean surface water [CO2(aq)] at atmospheric equilibrium. A widespread marine delta 13Corg decrease of 1-2% accompanied this event and was not preceded by an equivalent isotopic change in surface water total dissolved inorganic carbon. These observations support the hypothesis that [CO2(aq)] influences photosynthetic isotope fractionation between marine inorganic and organic carbon pools, and therefore that plankton/sediment delta 13Corg may serve as a proxy for surface water [CO2(aq)]. 相似文献
78.
Hepatic concentrations of metallothionein [MT] and three metals (Cu, Zn, Cd) were determined in 242 European flounders (Pleuronectes flesus) collected from power stations at Oldbury-upon-Severn and Hinkley Point, located in Severn Estuary and Bristol Channel, UK, respectively, between March 1996 and February 1998. A model involving three-factor analysis of variance (ANOVA) was used to examine variation in MT and metal concentrations with respect to season, year and site; with age-class included as a covariate in the analysis. Hepatic concentrations of MT and Cd (and to some degree, Cu, but not Zn) increased significantly with age. The model explained 38, 25, 17 and 26% of the variation in MT, Cu, Zn and Cd, respectively, with significant effects due to season, and to a lesser extent, to year. Site was only a significant factor for Cd which was higher in fish from Hinkley. Correlation between the individual concentration of MT and each metal alone, or in combination, was poor, and explained only an additional 3.0% of the residual variation in MT, most of which was attributable to Cu (2.7%). Compared to other industrialised estuaries, Cd concentrations were high (>20 micro g-1 in some individuals). The study emphasises the importance of seasonal variation and other factors in biomonitoring programmes and highlights the limitations of using [MT] as a biomarker for metal contamination in flounders from the Severn Estuary. 相似文献
79.
80.
Whiting events: biogenic origin due to the photosynthetic activity of cyanobacterial picoplankton 总被引:3,自引:0,他引:3
Thompson JB Schultze-Lam S Beveridge TJ Des Marais DJ 《Limnology and oceanography》1997,42(1):133-141
An annual whiting event occurs each year in late May to early June in Fayetteville Green Lake, New York. The initiation of this event correlates with exponential growth of the Synechococcus population within the lake. Synechococcus is the dominant (by approximately 4 orders of magnitude) autotrophic organism owing to the oligotrophic condition of the lake. The delta 13C values of the dissolved inorganic C range seasonally from -9.5% in winter to -6.2% in summer due to photosynthetic activity. Calcite precipitates principally in the microenvironment surrounding Synechococcus because of a photosynthetically driven alkalization process and the availability of the cells as nucleation sites. This calcite has a heavier delta 13C value (>4%) than does the dissolved inorganic C of the lake water owing to the cells' preferential uptake of 12C. A conceptual model suggests that photosynthetic activity and cell surface chemistry, together with the substantial surface area that arises from the great abundance of micron-sized cells, allow Synechococcus to dominate the annual whiting events in Fayetteville Green Lake. 相似文献