全文获取类型
收费全文 | 756篇 |
免费 | 32篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 41篇 |
地球物理 | 171篇 |
地质学 | 451篇 |
海洋学 | 32篇 |
天文学 | 64篇 |
综合类 | 7篇 |
自然地理 | 27篇 |
出版年
2022年 | 7篇 |
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 12篇 |
2018年 | 51篇 |
2017年 | 53篇 |
2016年 | 46篇 |
2015年 | 31篇 |
2014年 | 42篇 |
2013年 | 59篇 |
2012年 | 43篇 |
2011年 | 43篇 |
2010年 | 26篇 |
2009年 | 45篇 |
2008年 | 46篇 |
2007年 | 23篇 |
2006年 | 26篇 |
2005年 | 15篇 |
2004年 | 25篇 |
2003年 | 22篇 |
2002年 | 13篇 |
2001年 | 6篇 |
2000年 | 11篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1968年 | 1篇 |
1966年 | 3篇 |
1961年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有800条查询结果,搜索用时 31 毫秒
671.
Magdy M. S. El Maghraby Ahmad Kh. O. Abu El Nasr Mahmoud S. A. Hamouda 《Environmental Earth Sciences》2013,70(4):1525-1538
Groundwater is the main source of irrigation within south Al Madinah Al Munawarah region. It is also an important source of drinking water in many areas including Madinah city. The wells installed in the aquifer of the study area (south Madinah city) are not currently regulated by the local authorities although they are a key component of water supply. The aquifers in the study area range from unconfined to semi-confined and confined. The main aim of this study is to assess the groundwater in the region for drinking and agricultural uses. For this purpose, hydrochemical analyses of major, minor and trace constituents and nutrients were performed on 29 groundwater samples from the aquifer located about 20 km south of Madinah. The recharge rate of the aquifer of the study area was estimated to be 6.58 % of the annual precipitation using the chloride mass-balance method. Chloride was positively correlated with major ions, which suggests that agricultural activities have some effect on groundwater chemistry through leaching of readily soluble salts from the soil zone. Groundwater of the study area is characterized by dominance of Na over Ca. Chloride was found to be the most dominant anion and replaced by HCO3, thus reflecting geochemical evolution in the study area. The groundwater of the study area is not safe for drinking but can be safely used for salt-tolerant crops. 相似文献
672.
Large igneous provinces (LIPs) are considered a relevant cause for mass extinctions of marine life throughout Earth’s history. Their flood basalts and associated intrusions can cause significant release of SO4 and CO2 and consequently, cause major environmental disruptions. Here, we reconstruct the long-term periodic pattern of LIP emplacement and its impact on ocean chemistry and biodiversity from δ34Ssulfate of the last 520 Ma under particular consideration of the preservation limits of LIP records. A combination of cross-wavelet and other time-series analysis methods has been applied to quantify a potential chain of linkage between LIP emplacement periodicity, geochemical changes and the Phanerozoic marine genera record. We suggest a mantle plume cyclicity represented by LIP volumes (V) of V = ?(350–770) × 103 km3 sin(2πt/170 Ma) + (300–650) × 103 km3 sin(2πt/64.5 Ma + 2.3) for t = time in Ma. A shift from the 64.5 Ma to a weaker ~28–35 Ma LIP cyclicity during the Jurassic contributes together with probably independent changes in the marine sulfur cycle to less ocean anoxia, and a general stabilization of ocean chemistry and increasing marine biodiversity throughout the last ~135 Ma. The LIP cycle pattern is coherent with marine biodiversity fluctuations corresponding to a reduction of marine biodiversity of ~120 genera/Ma at ~600 × 103 km3 LIP eruption volume. The 62–65 Ma LIP cycle pattern as well as excursion in δ34Ssulfate and marine genera reduction suggest a not-yet identified found LIP event at ~440–450 Ma. 相似文献
673.
Ramadan H. Abu-Zied Ali S. Basaham Mohamed A. El Sayed 《Environmental Earth Sciences》2013,68(2):451-469
Al-Arbaeen and Al-Shabab inlets are two Red Sea coastal inlets lying on the mid-coast of Jeddah City, Saudi Arabia. Forty-four surface sediment samples were collected from these inlets and surrounding areas during June 2010. Water depths and the overlying environmental parameters (temperature, salinity, pH and dissolved oxygen) of these samples were measured. Sediment samples were analyzed for variables, such as loss on ignition (LOI, organic matter), CaCO3, heavy metals (Fe, Mn, Cu, Zn, Cd, Ni, Cr and Pb) and benthic foraminifera to assess any changes in the environment of the inlets and surrounding areas. Variables distribution patterns indicated that mud sediments dominated the inlets and enriched by LOI, heavy metals and Ammonia tepida–Quinqueloculina seminula assemblage, whereas coarse (sand–gravel) sediments dominated the substrates of surrounding areas and enriched by CaCO3 and Coscinospira hemprichii–Peneroplis planatus–Varidentella neostriatula assemblage with low values of LOI and heavy metals. Highest concentrations of Cu, Zn, Cd, Ni, Cr and Pb were recorded inside the inlets, especially near the discharge points, and they were positively correlated with the LOI and mud fraction indicating their affinity to anthropogenic materials. However, highest concentrations of Fe and Mn were typically recorded in the whole study area. These metals were positively correlated with the LOI values of the surrounding area, but in the inlets they were negatively correlated with the LOI, indicating an existence of reducing conditions caused by limited dissolved oxygen conditions at bottom waters of the inlets. Changing the environment within the inlets, according to higher concentrations of heavy metals and LOI, is probably responsible for the existence of the low density and diversity of benthic foraminifera and the absence of (reefal) symbiont-bearing species. 相似文献
674.
There are two main granitic rocks cropping out in the study area:1) the syn-orogenic granites are moderately weathered,jointed,exfoliated and characterized by low relief.These rocks are subdivided into tonalite and granodiorite.They are essentially composed of plagioclase,quartz,biotite,hornblende and potash feldspar;and 2) the post-orogenic granites,characterized by high relief terrain and represented by monzogranite,syenogranite and alkali granite.The monzogranites suffered hydrothermal alteration in particular along joints,faults,shear zones and fractures,which recorded the highest values of radioactivity,reflecting the role of post-magmatic alteration processes in the enhancement of radioactivity.The hydrothermal alteration(desilicification and hematitization) resulted in the formation of mineralized(altered) granites.The altered granites are enriched in TiO 2,Al 2 O 3,FeO T,MnO,MgO,Na 2 O,Rb,Sr,Y,Zr,Zn,Ga and Co and depleted in SiO 2,CaO,P 2 O 5,Nb,Pb,Cu,Ni and Cr relative to the fresh monzogranite.The investigated granites contain basic xenoliths as well as pockets of pegmatites.Perthites,quartz,plagioclase and sometimes biotite,represent the essential constituents.Some accessory minerals like zircon are metamicted reflecting their radiogenic nature.The alkali granites are characterized by the presence of aegirine,rebeckite and arfvedsonite.Both syn-and post-orogenic granites show some variations in their bulk chemical compositions.The older granitoids are metaluminous and exhibit characteristics of I-type granites and possess an arc tectonic environment.On the other hand,the younger granites are peraluminous and exhibit the characteristics of post-collisional granites.It is interpreted that radioactivity of the studied rocks is mainly controlled by both magmatic and post-magmatic activities.Frequently,the post-orogenic granites host zoned and unzoned pegmatite pockets.Some of these pockets anomalously attain high radioactivity.The syenogranites and the pegmatites are characterized by high contents of SiO 2 and K 2 O and low CaO and MgO.They have transitional characters from highly fractionated calc-alkaline to alkaline.The alkali granites related to A2-subtype of A-type granites.The post-orogenic granites were originated from magma of dominant crustal source materials and related to post-collisional setting under extensional environment. 相似文献
675.
Fadi Chaaban Hanan Darwishe Barbara Louche Yvonne Battiau-Queney Eric Masson Jamal El Khattabi Erick Carlier 《Environmental Earth Sciences》2012,65(1):183-193
The use of geographic information system (GIS) minimizes the effort and improves the efficiency of numerical models. The GIS
provides a platform for high capacity collection, management, manipulation, analysis, modeling and display of spatial data.
The conceptual model is created using GIS objects including points, arcs and polygons so that it can accurately represent
real world condition. According to the research problem, the geographical model is based on Hypergraph Based Data Structure
method, and a conceptual data model has been created from which a physical data model was elaborated in ArcGIS9.3 platform.
The groundwater modeling system (GMS) provides a powerful tool for hydrodynamics modeling and it is able to solve complex
problems such as the groundwater flow and seawater intrusion. The sand-dune system of Hardelot-Plage (North of France) suffers
from a lack of well-developed foredune. This problem is linked to the almost constant saturation of beach sand which is the
potential source of dune nourishment. In the south of Hardelot, the coastline is slowly, but constantly retreating. To remedy
this situation, a coupling between a GIS and GMS was adopted, in order to find the possible scenarios which could lower the
piezometric surface in the concerned area and allow dune nourishment again. The GMS used supports the Modflow-2000 code. A
direct approach to designing Modflow finite difference model is tedious and less intuitive, specifically for complex boundary
and initial conditions. Therefore, a Modflow model can be developed either using a grid or conceptual model approach. The
preparation of input data modeling is tedious and takes a long time. The model created in GMS was calibrated against the historical
and observed water level data for 1995–2006. Then a hydrodispersive model (MT3d code in GMS) was launched for evaluating sea-water
intrusion. The model was run to generate groundwater and salt concentration scenario during pumping tests. 相似文献
676.
Ibrahim A. Salem Mohamed E. Ibrahim Mohamed Abd El Monsef 《Arabian Journal of Geosciences》2012,5(3):385-406
The present work deals with the geology, mineralogy, geochemistry, and origin of the metagabbroic-hosted manganese deposits at Wadi Maliek in the southern Eastern Desert of Egypt. The manganese veins are found in the shear zones and channel ways of the fault planes within the metagabbroic rocks pointing to those hydrothermal solutions carrying manganese and iron load penetrating along these fractures. These faults are striking N 80° E?CS 80° W with dipping 65°. These veins vary in thickness from 15?cm up to 125?cm wide; each vein may show difference in thickness from bottom to top. Microscopic examinations, X-ray diffraction, infrared spectral, differential thermal (DTA), thermogravimetric (TGA), and ESEM-EDAX analyses revealed that the manganese minerals consist mainly of pyrolusite, psilomelane, and ramsdellite. Goethite and hematite are the common iron minerals. Petrographically, the manganese deposits can be classified into three ore types based on the predominance of manganese and iron minerals: manganese, manganese?Ciron, and iron ore types. The geochemistry of Maliek deposits indicated that the total averages of some major oxides in manganese, manganese?Ciron, and iron ore types are respectively as follows: SiO2 (15.64%, 11.52%, and 20.58%), MnO (39.9%, 17.81%, and 0.77%), FeO* (7.13%, 33.31%, and 37.08%), CaO (5.89%, 5.82%, and 5.32%), and Na2O (1.04%, 1.61%, and 1.53%). With regard to trace elements, the Maliek manganese deposits are rich in Zn, Ba, Pb, Sr, and V. Based on the geological, mineralogical, and geochemical results, the studied manganese deposits are considered to be precipitated from hydrothermal solution. 相似文献
677.
Numerical groundwater modelling as an effective tool for management of water resources in arid areas
Mohamed El Alfy 《水文科学杂志》2013,58(6):1259-1274
AbstractGroundwater, possibly of fossil origin, is used for water supply in some arid regions where the replenishment of groundwater by precipitation is low. Numerical modelling is a helpful tool in the assessment of groundwater resources and analysis of future exploitation scenarios. To quantify the groundwater resources of the East Owienat area in the southwest of the Western Desert, Egypt, the present study assesses the groundwater resources management of the Nubian aquifer. Groundwater withdrawals have increased in this area, resulting in a disturbance of the aquifer’s natural equilibrium, and the large-scale and ongoing depletion of this critical water reserve. Negative impacts, such as a decline in water levels and increase in salinity, have been experienced. The methodology includes application of numerical groundwater modelling in steady and transient states under different measured and abstraction scenarios. The numerical simulation model developed was applied to assess the responses of the Nubian aquifer water level under different pumping scenarios during the next 30 years. Groundwater management scenarios are evaluated to find an optimal management solution to satisfy future needs. Based on analysis of three different development schemes that were formulated to predict the future response of the aquifer under long-term water stress, a gradual increase in groundwater pumping to 150% of present levels should be adopted for protection and better management of the aquifer. Similar techniques could be used to improve groundwater management in other parts of the country, as well as other similar arid regions.
Editor D. Koutsoyiannis; Associate editor X. Chen 相似文献
678.
ABSTRACTA database of 88 superficial sediment samples, distributed in space and time, was collected from the Sidi Chahed Dam (northeast of Meknes city, Morocco) and from four other reference stations (in the same region) located in supposedly uncontaminated environments. Analyses were focused on the physico-chemical characteristics and concentrations of heavy metals (Fe, Mn, As, Cu, Zn, Pb, Cr, Cd). The database was processed by advanced statistical analysis techniques. The method of classification by self-organizing maps (SOM) was used, permitting understanding and visualization of the spatial and temporal distribution of samples. Principal component analysis (PCA) and SOM hierarchical classification (SOM-HC) were used to validate the classification and detect seasonal variations in heavy metal concentrations. Dependencies between different metal tracers were considered and their spatio-temporal distribution is shown, together with the ranking of clusters, according to their pollution levels. Thus, autumn samples were the only ones with high concentrations of As, compared to the four reference stations. This is due to leaching of bare soil by the first stormy rains of autumn. In winter and spring samples, the concentrations of tracers Mn, Zn, Cu, Pb, Cd and Cr were relatively high compared to those of the reference stations. Summer sample concentrations were most comparable with the reference stations, probably due to the scarcity of rainfall and runoff in this season.
Editor Z.W. Kundzewicz Associate editor E. Gargouri 相似文献
679.
Clotaire Michel Philippe Guguen Saber El Arem Jacky Mazars Panagiotis Kotronis 《地震工程与结构动力学》2010,39(4):419-441
In countries with a moderate seismic hazard, the classical methods developed for strong motion prone countries to estimate the seismic behaviour and subsequent vulnerability of existing buildings are often inadequate and not financially realistic. The main goals of this paper are to show how the modal analysis can contribute to the understanding of the seismic building response and the good relevancy of a modal model based on ambient vibrations for estimating the structural deformation under weak earthquakes. We describe the application of an enhanced modal analysis technique (frequency domain decomposition) to process ambient vibration recordings taken at the Grenoble City Hall building (France). The frequencies of ambient vibrations are compared with those of weak earthquakes recorded by the French permanent accelerometric network (RAP) that was installed to monitor the building. The frequency variations of the building under weak earthquakes are shown to be less (∼2%) and therefore ambient vibration frequencies are relevant over the elastic domain of the building. The modal parameters extracted from ambient vibrations are then used to determine the 1D lumped‐mass model in order to reproduce the inter‐storey drift under weak earthquakes and to fix a 3D numerical model that could be used for strong earthquakes. The correlation coefficients between data and synthetic motion are close to 80 and 90% in horizontal directions, for the 1D and 3D modelling, respectively. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
680.
Jean‐Yves Delenne Moulay Saïd El Youssoufi Fabien Cherblanc Jean‐Claude Bnet 《国际地质力学数值与分析法杂志》2004,28(15):1577-1594
We investigate the stress–strain behaviour and failure of a cohesive granular material both by experiments and numerical simulations. The material is an assembly of aluminium rods glued together by means of an epoxy resin. The behaviour of cohesive bonds (force–displacement relationship, failure conditions) is characterized by performing simple loading tests (tension/compression, shear…) on a couple of rods. Then, this local behaviour is introduced in a numerical code based on a discrete element method in order to perform numerical compression tests on large samples. The validation of this approach was the main goal of the present investigation that is essentially achieved by a direct comparison between the numerical results and similar experimental tests. As a basic application, we derive the macroscopic cohesion and friction characteristics of random cohesive materials by systematic numerical simulations in a biaxial geometry. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献