全文获取类型
收费全文 | 519篇 |
免费 | 28篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 28篇 |
地球物理 | 157篇 |
地质学 | 203篇 |
海洋学 | 53篇 |
天文学 | 54篇 |
综合类 | 3篇 |
自然地理 | 32篇 |
出版年
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 13篇 |
2018年 | 29篇 |
2017年 | 20篇 |
2016年 | 33篇 |
2015年 | 18篇 |
2014年 | 17篇 |
2013年 | 40篇 |
2012年 | 32篇 |
2011年 | 40篇 |
2010年 | 35篇 |
2009年 | 33篇 |
2008年 | 15篇 |
2007年 | 24篇 |
2006年 | 23篇 |
2005年 | 24篇 |
2004年 | 12篇 |
2003年 | 9篇 |
2002年 | 18篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 13篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1969年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有552条查询结果,搜索用时 15 毫秒
541.
Mathias Versichele Tijs Neutens Manuel Claeys Bouuaert Nico Van de Weghe 《Transactions in GIS》2014,18(5):687-703
Certain datasets on moving objects are episodic in nature – that is, the data is characterized by time gaps during which the position of the object is unknown. In this article, a model is developed to study the sparsely sampled network‐constrained movement of several objects by calculating both potential and feasible (i.e. more likely) co‐presence opportunities over time. The approach is applied to the context of a static sensor network, where the location of an object is only registered when passing a sensor location along a road network. Feasibility is incorporated based on the deviation from the shortest path. As an illustration, the model is applied to a large Bluetooth tracking dataset gathered at a mass event. The model output consists of maps showing the temporal evolution of the distribution of feasible co‐presence opportunities of tracked visitors over the network (i.e. the number of visitors that could have been present together). We demonstrate the model's usefulness in studying the movement and distribution of a crowd over a study area with relatively few sampling locations. Finally, we discuss the results with a special emphasis on the distinction between feasible and actual presence, the need for further validation and calibration, and the performance of the implementation. 相似文献
542.
Maya García-Comas Manuel López-Puertas Bianca M. Dinelli Alberto Adriani Angioletta Coradini 《Icarus》2011,214(2):571-583
After molecular nitrogen, methane is the most abundant species in Titan’s atmosphere and plays a major role in its energy budget and its chemistry. Methane has strong bands at 3.3 μm emitting mainly at daytime after absorption of solar radiation. This emission is strongly affected by non-local thermodynamic equilibrium (non-LTE) in Titan’s upper atmosphere and, hence, an accurate modeling of the non-LTE populations of the emitting vibrational levels is necessary for its analysis. We present a sophisticated and extensive non-LTE model which considers 22 CH4 levels and takes into account all known excitation mechanisms in which they take part. Solar absorption is the major excitation process controlling the population of the v3-quanta levels above 1000 km whereas the distribution of the vibrational energy within levels of similar energy through collisions with N2 is also of importance below that altitude. CH4-CH4 vibrational exchange of v4-quanta affects their population below 500 km. We found that the ν3 → ground band dominates Titan’s 3.3 μm daytime limb radiance above 750 km whereas the ν3 + ν4 → ν4 band does below that altitude and down to 300 km. The ν3 + ν2 → ν2, the 2ν3 → ν3, and the 13CH4ν3 → ground bands each contribute from 5% to 8% at regions below 800 km. The ν3 + 2ν4 → 2ν4and ν2 + ν3 + ν4 → ν2 + ν4 bands each contribute from 2% to 5% below 650 km. Contributions from other CH4 bands are negligible. We have used the non-LTE model to retrieve the CH4 abundance from 500 to 1100 km in the southern hemisphere from Cassini-VIMS daytime measurements near 3.3 μm. Our retrievals show good agreement with previous measurements and model results, supporting a weak deviation from well mixed values from the lower atmosphere up to 1000 km. 相似文献
543.
Implementation and integrated numerical modeling of a landslide early warning system: a pilot study in Colombia 总被引:3,自引:2,他引:1
Christian Huggel Nikolay Khabarov Michael Obersteiner Juan Manuel Ramírez 《Natural Hazards》2010,52(2):501-518
Landslide early warning systems (EWS) are an important tool to reduce landslide risks, especially where the potential for
structural protection measures is limited. However, design, implementation, and successful operation of a landslide EWS is
complex and has not been achieved in many cases. Critical problems are uncertainties related to landslide triggering conditions,
successful implementation of emergency protocols, and the response of the local population. We describe here the recent implementation
of a landslide EWS for the Combeima valley in Colombia, a region particularly affected by landslide hazards. As in many other
cases, an insufficient basis of data (rainfall, soil measurements, landslide event record) and related uncertainties represent
a difficult complication. To be able to better assess the influence of the different EWS components, we developed a numerical
model that simulates the EWS in a simplified yet integrated way. The results show that the expected landslide-induced losses
depend nearly exponentially on the errors in precipitation measurements. Stochastic optimization furthermore suggests an increasing
adjustment of the rainfall landslide-triggering threshold for an increasing observation error. These modeling studies are
a first step toward a more generic and integrated approach that bears important potential for substantial improvements in
design and operation of a landslide EWS. 相似文献
544.
Uranium minerals from the San Marcos District, Chihuahua, Mexico 总被引:1,自引:0,他引:1
Manuel Reyes-Cortés Luis Fuentes-Cobas Enrique Torres-Moye Hilda Esparza-Ponce María Elena Montero-Cabrera 《Mineralogy and Petrology》2010,99(1-2):121-132
The mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry. At the San Marcos I deposit, uranophane Ca(UO2)2Si2O7·6(H2O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO2)(V2O8)·3(H2O) were observed. Uranophane, uraninite (UO2+x), masuyite Pb(UO2)3O3(OH)·3(H2O), and becquerelite Ca(UO2)6O4(OH)6 ·(8H2O) are present at the Victorino deposit. Field observations, coupled with analytical data, suggest the following sequence of mineralization: (1) deposition of uraninite, (2) alteration of uraninite to masuyite, (3) deposition of uranophane, (4) micro-fracturing, (5) calcite deposition in the micro-fractures, and (6) formation of becquerelite. The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite. The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State. 相似文献
545.
Effects of length and application rate of rice straw mulch on surface runoff and soil loss under laboratory simulated rainfall 总被引:3,自引:0,他引:3
Misagh Parhizkar Mahmood Shabanpour Manuel Esteban Lucas-Borja Demetrio Antonio Zema Siyue Li Nobuaki Tanaka Artemio Cerdà 《国际泥沙研究》2021,36(4):468-478
Forest land affected by deforestation yields high soil and water losses.Suitable management practices need to be found that can reduce these losses and achieve ecological and hydrological sustainability of the deforested areas.Mulch has been found to be effective in reducing soil losses;straw mulch is easy to apply,contributes soil organic matter,and is efficient since the day of application.However,the complex effects of rice straw mulch with different application rates and lengths on surface runoff and soil loss have not been clarified in depth.The current paper evaluates the efficiency of rice straw mulch in reducing the hydrological response of a silty clay loam soil under high intensity and low frequency rainfall events(tap water with total depth of 49 mm and intensity of 98 mm/h)simulated in the laboratory.Surface runoff and soil loss at three lengths of the straw(10,30,and 200 mm)and three application rates(1,2,and 3 Mg/ha)were measured in 50 cm(width)×100 cm(length)×10 cm(depth)plots with disturbed soil samples(aggregate soil size<4 mm)collected in a deforested area.Bare soil was used as control experiment.Runoff volume and erosion were significantly(at p<0.05)lower in mulched soils compared to control plots.These reductions were ascribed to the water absorption capacity of the rice straw and the protection cover of the mulch layer.The minimum runoff was observed for a mulch layer of3 Mg/ha of straw with a length of 200 mm.The lowest soil losses were found with straw length of10 mm.The models developed predict runoff and erosion based on simple linear functions of mulch application rate and length,and can be used for a suitable hydrological management of soil.It is concluded that,thanks to rice straw mulch used as an organic soil conditioner,soil erosion and surface runoff are significantly(at p<0.05)reduced,and the mulch protection contributes to reduce the risk of soil degradation.Further research is,however,needed to analyze the upscaling of the hydrological effects of mulching from the plot to the hillslope scale. 相似文献
546.
Tine?Derez Tom?Van Der?Donck Oliver?Plümper Philippe?Muchez Gill?Pennock Martyn?R.?Drury Manuel?SintubinEmail author 《Contributions to Mineralogy and Petrology》2017,172(7):56
Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz. 相似文献
547.
548.
Another look at the calculation of fallout tephra volumes 总被引:9,自引:2,他引:9
The calculation of volumes of fallout tephra layers is difficult because of the nonlinear dependence of thickness on area and because of the extrapolations required at the vent and in distal regions. Calculation using the trapezoidal rule, straight lines on log-log plots of area versus thickness, straight lines on plots of log thickness versus area1/2, and the crystal-concentration method are reviewed and the problems with each method discussed. The method using straight lines on plots of log thickness versus area1/2 is the most geologically reasonable because most deposits thin exponentially from source and therefore plot as straight lines using these coordinates. Errors and uncertainties in previous derivations for using this method are discussed and more general formulas presented. The method is also used to gain perspective on the missing distal volumes calculated by the crystal-concentration method compared to those calculated based only on isopach data. 相似文献
549.
Andrey Linhares Bezerra de Oliveira Júlio Carlos Afonso Lilian da Silva Arnaldo Alcover Neto Manuel Castro Carneiro Lílian Irene Dias da Silva Maria Inês Couto Monteiro 《Geostandards and Geoanalytical Research》2019,43(4):689-699
Rare earth elements (REEs) are very important to technological development as well as to geochemical and environmental studies. In this work, hydrofluoric acid (HF) was replaced by condensed phosphoric acid (CPA) in the digestion of geological samples, and the quantification of REEs was performed by inductively coupled plasma‐optical emission spectrometry (ICP‐OES). Six international reference materials (RMs), named DC86318, CGL 111, CGL 124, CGL 126, OKA‐2 and COQ‐1 and three Brazilian ore samples, named Araxá, Catalão and Pitinga were analysed. Only zircon and xenotime, which are potential REE‐bearing minerals, were not completely dissolved. Nevertheless, no REE associated with zircon was detected. The investigated digestion method presented many advantages: It was relatively fast (3 h), avoided fluoride precipitation, it was less hazardous because handling diluted H3PO4 is safer than HF, NH4F or NH4HF2 aqueous solutions, it preserved the quartz fittings of the measurement equipment and the final solution contained lower levels of total dissolved solids than those produced by the fusion method. 相似文献
550.
Manuel A. Caraballo Tobias S. Rötting Francisco Macías José Miguel Nieto Carlos Ayora 《Applied Geochemistry》2009
Passive treatment systems have become one of the most sustainable and feasible ways of remediating acid mine drainage (AMD). However, conventional treatments show early clogging of the porosity or/and coating of the reactive grains when high acidity and metal concentrations are treated. The performance of fine-grained reagents dispersed in a high porosity matrix of wood shavings was tested as an alternative to overcome these durability problems. The system consisted of two tanks of 3 m3 filled with limestone sand and wood shavings, and one tank of 1 m3 with caustic magnesia powder and wood shavings, separated by several oxidation cascades and decantation ponds. The system treated about 1.5 m3/day of AMD containing an average of 360 mg/L Fe, 120 mg/L Al, 390 mg/L Zn, 10 mg/L Cu, 300 μg/L As and 140 μg/L Pb, a mean pH of 3.08 and a net acidity of 2500 mg/L as CaCO3 equivalent. The water reached pH 5 and 6 in the first and second limestone tanks, respectively (suitable to remove trivalent metals); and pH 8–9 in the MgO tank (suitable to remove divalent metals). After 9 months of operation, the system achieved an average removal of 100% Al, Cu, As, Pb, more than 70% Fe, about 25% Zn and 80% acidity. Goethite, schwertmannite, hydrobasaluminite, amorphous Al(OH)3 and gypsum were the main precipitates in the two limestone tanks. Precipitation of divalent metals (Fe (II), Zn, and traces of Cd, Ni and Co) were complete inside the third tank of MgO, but preferential flow along the walls was responsible for its low treatment performance. Goethite, gypsum, Zn-schulenbergite and sauconite are the crystalline solid phases identified in the MgO tank. 相似文献