全文获取类型
收费全文 | 1550篇 |
免费 | 64篇 |
国内免费 | 55篇 |
专业分类
测绘学 | 66篇 |
大气科学 | 53篇 |
地球物理 | 335篇 |
地质学 | 998篇 |
海洋学 | 68篇 |
天文学 | 89篇 |
综合类 | 14篇 |
自然地理 | 46篇 |
出版年
2023年 | 6篇 |
2022年 | 28篇 |
2021年 | 55篇 |
2020年 | 65篇 |
2019年 | 46篇 |
2018年 | 152篇 |
2017年 | 128篇 |
2016年 | 158篇 |
2015年 | 75篇 |
2014年 | 129篇 |
2013年 | 196篇 |
2012年 | 85篇 |
2011年 | 111篇 |
2010年 | 72篇 |
2009年 | 68篇 |
2008年 | 38篇 |
2007年 | 21篇 |
2006年 | 32篇 |
2005年 | 20篇 |
2004年 | 17篇 |
2003年 | 15篇 |
2002年 | 28篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 11篇 |
1997年 | 13篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有1669条查询结果,搜索用时 140 毫秒
991.
Ali Mahmoud Mahmoud Mohamed Shendi Biswajeet Pradhan Fatma Attia 《Central European Journal of Geosciences》2009,1(3):347-367
The North-Western Coast of Egypt (NWCE) represents one of the high priority regions for future development in the country. El-Hammam area is located in the NWCE with an area of 94752 acres and is one of the main challenging regions for sustaianble development. In this study, we have used remote sensing and soil data in combination with GIS tools, for land use sustainable analysis (SLU) in El-Hammam area. The SLU was established based on various factors such as: land capability and suitability, water resources availability, economic return from water and financial return from land and water. A physiographic soil map for the study area was prepared using remote sensing and GIS. Multiple field surveys were carried out for collecting information on various soil map units (SMUs) and their profiles. Laboratory analysis for the collected samples was performed, and then the soil properties were stored as attributes in a geographical soil database linked with the SMUs. Furthermore, land capability assessment was done to define the suitable areas for agricultural production using a capability model built in ALES software. Results indicate that the area currently lacks high capability and moderate capability classes. By improving the soil properties, the soil can attain potential capability; and 55630 acres will become marginally capable. The assessment of soil physical suitability for different land use types (LUTs) were analysed in ALES software, in order to generate the most suitable areas. The results from the land suitability analysis indicated that, 17114 acres are moderately suitable for wheat and sorghum; whereas 15823 acres are moderately suitable for barley and 12752 acres are moderately suitable for maize, olive and figs. Finally, the SLU was investigated based on two scenarios; (1) the most SLU under the conditions of shortage of irrigation water: clover, barley and sorghum against figs, as the irrigation requirements for barley and sorghum are low; (2) the most sustainable land use in the conditions of irrigation availability will be wheat and maize against figs and guava. From the results it is quite evident that GIS combined with modeling approaches are powerful tools for decision making in the study area. 相似文献
992.
Mohamed A. Daoud René C. Maury Jean-Alix Barrat Rex N. Taylor Bernard Le Gall Hervé Guillou Joseph Cotten Joël Rolet 《Lithos》2010,114(3-4):327-336
Major, trace element and isotopic (Sr, Nd, Pb) data and unspiked K–Ar ages are presented for Quaternary (0.90–0.95 Ma old) basalts from the Hayyabley volcano, Djibouti. These basalts are LREE-depleted (Lan/Smn = 0.76–0.83), with 87Sr/86Sr ratios ranging from 0.70369 to 0.70376, and rather homogeneous 143Nd/144Nd (εNd = + 5.9–+ 7.3) and Pb isotopic compositions (206Pb/204Pb = 18.47–18.55, 207Pb/204Pb = 15.52–15.57, 208Pb/204Pb = 38.62–38.77). They are very different from the underlying enriched Tadjoura Gulf basalts, and from the N-MORB erupted from the nascent oceanic ridges of the Red Sea and Gulf of Aden. Their compositions closely resemble those of (1) depleted Quaternary Manda Hararo basalts from the Afar depression in Ethiopia and (2) one Oligocene basalt from the Ethiopian Plateau trap series. Their trace element and Sr, Nd, Pb isotope systematics suggest the involvement of a discrete but minor LREE-depleted component, which is probably an intrinsic part of the Afar plume. 相似文献
993.
Kais Jacob Mohamed Daniel Rey Belen Rubio Federico Vilas Thomas Frederichs 《Quaternary Research》2010,73(3):507-520
Integrated analyses of magnetic, geochemical and textural data on six cores from the northwestern Iberian continental shelf allowed the reconstruction of the paleoenvironmental evolution of this area since the last glacial maximum (LGM). Four sedimentary units were identified, representing a succession from fluvial and subaerial settings to high and finally low-energy marine deposits subsequent to the post-LGM sea-level rise. The uppermost unit was deposited during the Holocene and its magnetic properties were controlled by the interplay between detrital input and early diagenetic reductive dissolution of magnetic minerals. Identification of a primary steady-state early diagenetic signal allowed the recognition of periods of increased detrital input, bounded by intervals of lower detrital input and intensified reductive diagenesis related to intensified upwelling in the area. These paleoenvironmental alternations are consistent with the climatic evolution of the late Holocene. During the Roman Warm Period and Medieval Warm Period, the combined effect of greater humidity and intense agricultural and mining activities led to a greater erosion and transport of detrital sediments to the shelf. In contrast, enhanced diagenetic reduction intervals, caused by upwelling intensification, were roughly coincident with the colder Dark Ages and the Little Ice Age. 相似文献
994.
Improving Short- and Long-term Stability of Underground Gypsum Mine Using Partial and Total Backfill 总被引:1,自引:0,他引:1
Mohamed Marwan Al Heib Christophe Didier Farimeh Masrouri 《Rock Mechanics and Rock Engineering》2010,43(4):447-461
The stability of underground mines represents a key issue for active and abandoned mines. Over the last few years, several collapses of underground mines in France have affected existing buildings and infrastructures. Many factors are generally identified as the cause of failures: pillar ageing, fractures, and pillars’ height to width ratio, etc. Among the treatment techniques available to prevent instability and reduce the deterioration of pillars, backfill is the most frequently used. A research programme, supported by the French Ministry of the Environment, was developed to study the operability of partial and total backfill using waste material in the Livry-Gargan gypsum mine (near Paris, France), where pillar height is 17 m. The paper focuses on: (1) the characterisation of the gypsum and fill material (laboratory and in situ tests), (2) the in situ measurements, involving 5 pillars equipped with 19 pressure cells, since 1999, (3) and numerical modelling of fractured pillars performed in order to improve understanding of the effects of backfill on the stability of room-and-pillar mines. The study clearly shows the operability and the advantages of partial and total backfill for short-term pillar stability. The induced horizontal pressure generated by backfill can reach 200 kPa. The use of numerical modelling also shows the effect of backfill on fractures and that backfill reduces indicatively the shear displacement and the opening of fractures. Numerical modelling helps in identifying the mechanisms of backfill and in a better understanding of the behaviour of backfilled mines. 相似文献
995.
Material engineers are continually confronted by depletion of quality construction materials for road and airfield construction.
Even if good quality construction materials for road and airfield are available, the haul costs may preclude their use. Stabilization
of soils in order to improve strength and durability properties often relies on cement, lime, fly ash, and asphalt emulsion.
These materials are inexpensive, relatively easy to apply, and provide benefits to many different soil types. In addition,
there are a variety of nontraditional soil stabilization/modification additives available from the commercial sector such
as polymer emulsions, acids, lignin derivatives, enzymes, tree resin emulsions, and silicates. These additives may be in liquid
or solid state and are often touted to be applicable for most soils. Polymers may be easy to apply in permeable materials
such as sand and may achieve good stabilization in relatively shorter periods of time. These polymer materials can be used
for stabilizing, soil in road shoulders, slopes, and pads of military and emergency airports. In addition, these types of
materials can be used to prevent the movement of the dune sands on the sides of railroads and stabilizing the dust on the
surface of access roads. Within the present research, two different polymers of wide range of dosages have been applied. Following
results have been achieved: (1) These polymers improve the compressive strength from 0.03 N/mm2 for control sample to 5.2 N/mm2
for improved sample. (2) The optimum curing time of dune sands with different polymers is 7 days. (3) The UC strength of stabilized
samples soars with an increase in the temperature, in the first 24 h of the curing process. (4) When the concentration of
salt increased from 1 to 10 percent, UC strength of stabilized samples decreased. 相似文献
996.
997.
998.
999.
Mohamed Sahnouni Jan Van der Made Melanie Everett 《Quaternary Science Reviews》2011,30(11-12):1303-1317
In contrast to well-studied paleoenvironments of Plio-Pleistocene hominin sites in East Africa, little is known about the ecology of the earliest North African human occupation sites. The recent studies at Ain Hanech and El-Kherba in northeastern Algeria have broadened the range of Plio-Pleistocene hominin ecology to include the earliest known archaeological sites documented in North Africa. Ain Hanech and El-Kherba are significant for yielding savanna-like faunas associated with Oldowan stone tools dated back to approximately 1.8 million years ago. This paper focuses on reconstructing the ecology of Ain Hanech and El-Kherba based on excavated faunas and stable carbon isotope of pedogenic carbonates from El-Kherba stratigraphic profile. The results point out to an overall open paleolandscape and a C3 predominantly paleovegetation both woody and grasses. In addition, the stable-carbon-isotopic evidence shows a clear environmental change through time at El-Kherba, which likely impacted hominin foraging activities in level A. 相似文献
1000.