全文获取类型
收费全文 | 1588篇 |
免费 | 179篇 |
国内免费 | 204篇 |
专业分类
测绘学 | 57篇 |
大气科学 | 278篇 |
地球物理 | 485篇 |
地质学 | 643篇 |
海洋学 | 166篇 |
天文学 | 148篇 |
综合类 | 49篇 |
自然地理 | 145篇 |
出版年
2024年 | 8篇 |
2023年 | 13篇 |
2022年 | 42篇 |
2021年 | 65篇 |
2020年 | 48篇 |
2019年 | 40篇 |
2018年 | 88篇 |
2017年 | 83篇 |
2016年 | 138篇 |
2015年 | 95篇 |
2014年 | 118篇 |
2013年 | 113篇 |
2012年 | 81篇 |
2011年 | 88篇 |
2010年 | 91篇 |
2009年 | 89篇 |
2008年 | 76篇 |
2007年 | 53篇 |
2006年 | 59篇 |
2005年 | 41篇 |
2004年 | 39篇 |
2003年 | 39篇 |
2002年 | 48篇 |
2001年 | 31篇 |
2000年 | 54篇 |
1999年 | 47篇 |
1998年 | 38篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 10篇 |
1992年 | 11篇 |
1991年 | 19篇 |
1990年 | 25篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1969年 | 2篇 |
1950年 | 1篇 |
排序方式: 共有1971条查询结果,搜索用时 0 毫秒
131.
The southeast area of the Argentine Pampas is characterized by the presence of an unconfined aquifer in a wide plain. A methodology is proposed that deals with the aquifer vulnerability where the homogeneity of the hydrogeological variables used by traditional methods (in this case, DRASTIC-P) causes vulnerability maps to show more than 80% of the territory under the same class. This absence of discrimination renders vulnerability maps of little use to decision-makers. In addition, the proposed methodology avoids the traditional vague classification (high, low, and moderate vulnerability) which is highly dependent on subjectivity in its association of each class with hydrogeological considerations. That traditional vulnerability assessment methodology was adapted using a geographic information system to reclassify classes, based on the Natural Breaks (Jenks) method. The pixel-to-pixel comparison between the result obtained by the DRASTIC-P and the reclassified classes generates the so-called operational vulnerability index (OVI), which shows four classes, associating each with different hydrogeological requirements to make decisions. 相似文献
132.
Gabriel Auvinet-Guichard Juan F. Rodríguez-Rebolledo José L. Rangel-Núñez 《Acta Geotechnica》2010,5(1):63-68
The drainage system of Mexico City, one of the largest metropolitan areas in the world, is formed by a large network of interceptors
and collectors leading to the main outlet: a 49.3-km-long tunnel with a 6.5 m diameter. To build the access shafts to the
tunnels of the system and the deep sumps of the pumping stations in the very soft clays of the lacustrine area of Mexico valley,
a technique known as “flotation method” has been commonly used since 1969. The paper presents the main construction steps
followed and some aspects of the analysis and design of shafts constructed by this technique. The experience gained obtained
after more than 30 shafts have been constructed by this method, and the actual trends in its application are also discussed. 相似文献
133.
Contamination levels assessment of potential toxic metals in road dust deposited in different types of urban environment 总被引:1,自引:1,他引:0
A total of 42 samples of road dust were collected along ring road, city centre, city side, and freeway in Urumqi, China. Total
concentrations of Cd, Cr, Cu, Ni, Pb, Mn, Be, Co, Zn, and U were determined by using the inductively coupled plasma-mass spectrometry
in order to assess and to compare road dust contamination levels of metals among the four roads. The results show that, among
the four categories of roads, mean concentrations of Co and U vary little. City centre locations show strong enrichments of
Cd, Cu, Pb, and Be. Along the ring road, the highest mean concentrations were found for Cr, Ni, Mn, and Co. However, the highest
concentrations of Zn and U were found along the freeway. The cluster analysis shows that three main groups can be distinguished.
Every group may be associated with different main sources and concentrations of the metals. The results of contamination assessment
reveal that, among all of the potential toxic metals, Cd, Cu, and Zn pollution were obviously heavier with moderate or high
contamination indices for most road dust samples, while Cr, Ni, and Pb contamination were lower along the four categories
of roads. Compared with the city side, Cd, Cu, Pb, Ni, and Zn contamination were heavier along the ring road, the city centre,
and the freeway with high traffic density. Low Pb contamination or no contamination in all the road dust samples may be related
to the increasing usage of lead-free petrol. 相似文献
134.
为了了解XX核电厂周边海域的生态环境状况,筛选潜在威胁核电厂冷源安全的致灾生物,分析影响致灾生物时空分布的主要环境因素,本研究于2018年进行了四个季节的生态调查。结果表明,研究海域浮游植物密度秋季最高,春季优势种为东海原甲藻,其他季节优势种为中肋骨条藻等硅藻。浮游动物密度夏季最高,以桡足类为主,春、夏季优势种还包括球型侧腕水母和薮枝螅水母。底栖动物的密度和生物量在秋季最高,优势种主要为豆形短眼蟹、长吻沙蚕和丝异须虫。浮游植物、浮游动物和底栖动物群落均在夏季最为稳定,群落多样性水平和物种丰度较高,且分布较为均匀。浮游植物群落的细胞丰度与悬浮物和磷酸盐的浓度正相关性最高。浮游动物的密度受温度和盐度的影响较大。底栖动物的群落分布主要受悬浮物和无机营养盐的浓度影响。本研究共筛选出17种威胁XX核电厂冷源安全的潜在致灾生物。海地瓜、棘刺锚参、海葵和球型侧腕水母的风险等级较高,其分布主要受温度和盐度的影响。后续工作中,XX核电厂应根据潜在致灾生物与环境因子的关系加强暴发机制研究、提升监测预警能力,并优化驱赶消杀技术和干扰体系,为核电厂冷源安全提供理论和技术保障。 相似文献
135.
Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna(Vicugna vicugna) and alpaca(Lama pacos). The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined: total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands. 相似文献
136.
David L. Naftz Gregory T. Carling Cory Angeroth Michael Freeman Ryan Rowland Eddy Pazmiño 《Aquatic Geochemistry》2014,20(6):547-571
Density stratification in saline and hypersaline water bodies from throughout the world can have large impacts on the internal cycling and loading of salinity, nutrients, and trace elements. High temporal resolution hydroacoustic and physical/chemical data were collected at two sites in Great Salt Lake (GSL), a saline lake in the western USA, to understand how density stratification may influence salinity and mercury (Hg) distributions. The first study site was in a causeway breach where saline water from GSL exchanges with less saline water from a flow restricted bay. Near-surface-specific conductance values measured in water at the breach displayed a good relationship with both flow and wind direction. No diurnal variations in the concentration of dissolved (<0.45 μm) methylmercury (MeHg) were observed during the 24-h sampling period; however, the highest proportion of particulate Hgtotal and MeHg loadings was observed during periods of elevated salinity. The second study site was located on the bottom of GSL where movement of a high-salinity water layer, referred to as the deep brine layer (DBL), is restricted to a naturally occurring 1.5-km-wide “spillway” structure. During selected time periods in April/May, 2012, wind-induced flow reversals in a railroad causeway breach, separating Gunnison and Gilbert Bays, were coupled with high-velocity flow pulses (up to 55 cm/s) in the DBL at the spillway site. These flow pulses were likely driven by a pressure response of highly saline water from Gunnison Bay flowing into the north basin of Gilbert Bay. Short-term flow reversal events measured at the railroad causeway breach have the ability to move measurable amounts of salt and Hg from Gunnison Bay into the DBL. Future disturbance to the steady state conditions currently imposed by the railroad causeway infrastructure could result in changes to the existing chemical balance between Gunnison and Gilbert Bays. Monitoring instruments were installed at six additional sites in the DBL during October 2012 to assess impacts from any future modifications to the railroad causeway. 相似文献
137.
138.
Francisco Carreño Conde Sandra García Martínez Javier Lillo Ramos Raquel Fernández Martínez Ariana Mabeth-Montoya Colonia 《Environmental Earth Sciences》2014,71(1):61-66
Water resources management of protected sites requires a powerful tool to analyze the process and changes that are occurring in the environment. This paper describes a 3D geomodel design of the Jarama River Detrital Aquifer located in Madrid (Spain). That hydrogeological unit is included in the “Parque Regional de los Cursos Bajos de los Ríos Manzanares y Jarama” (Regional Park of the Lower Courses of Manzanares and Jarama Rivers). The goal of this work is to define a method by which a three-dimensional (3D) model can be created with hydrogeologic geometry real of main aquifer, to accomplish an adequate management of the groundwater resources. All data used in this study were integrated in a geographic database: geological and hydrogeological information, geological map (1:25,000), eleven cross-sections, piezometric maps and a digital elevation model. The constructed 3D model of the Jarama Aquifer shows geometric features and spatial distribution and variations of geologic units. Thus, the 3D model allows the assessment of volumes of each unit, the depth and thickness variations of the main aquifer, and the spatial and temporal variations of water tables. From the 3D model, the most suitable areas (in terms of groundwater protection) for managed recharge and mining works have been identified. 相似文献
139.
M. J. Rodríguez-Peces J. García-Mayordomo J. M. Azañón A. Jabaloy 《Environmental Earth Sciences》2014,72(7):2423-2435
Complete rupture of the Padul Fault represents one of the largest plausible earthquakes in the Sierra Nevada Range, one of the most seismically active regions of Spain. We performed a regional assessment of earthquake-triggered slope instabilities in the western part of the range to determine the most likely types of failures from such an earthquake in the region and suggest where such failures have a higher likelihood of occurring. These results are broadly useful for management of regional life-lines and future development. First, a slope-instability inventory of the Sierra Nevada was produced to identify the most common instability types. Subsequently, the Newmark’s sliding rigid-block methodology, implemented in a geographic information system, was used to obtain the distribution of Newmark displacements in the area considering a M w 6.6 earthquake on the Padul Fault. The Newmark displacements were then compared to the distribution of the inventoried slope instabilities to identify the areas where seismicity could reactivate old slope instabilities or generate new ones, and to identify the involved landslide typology. The most likely seismically induced slope instabilities in the Sierra Nevada are rock falls and rock slides. These types of instabilities could be triggered by Newmark displacements of 2 cm or less. 相似文献
140.
Marc?UlrichEmail author Manuel?Mu?oz Stéphane?Guillot Michel?Cathelineau Christian?Picard Benoit?Quesnel Philippe?Boulvais Clément?Couteau 《Contributions to Mineralogy and Petrology》2014,167(1):952
The weathering of mantle peridotite tectonically exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline magnesite veins and stock-work are widespread in the serpentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO2. In this study, we investigate serpentinite carbonation processes, and related silicification, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpentine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dissolution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite. 相似文献