全文获取类型
收费全文 | 130篇 |
免费 | 6篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 5篇 |
地球物理 | 21篇 |
地质学 | 70篇 |
海洋学 | 6篇 |
天文学 | 14篇 |
综合类 | 15篇 |
自然地理 | 10篇 |
出版年
2024年 | 1篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 9篇 |
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 4篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 8篇 |
2011年 | 8篇 |
2010年 | 6篇 |
2009年 | 5篇 |
2008年 | 6篇 |
2007年 | 2篇 |
2006年 | 3篇 |
2005年 | 7篇 |
2004年 | 5篇 |
2003年 | 9篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有143条查询结果,搜索用时 0 毫秒
81.
This paper presents 2D and 3D upper bound solutions for the problem of tunnel excavation in soft ground. The solution invokes the use of incompressible flow fields derived from the theory of elasticity and the concept of sinks and sources. Comparison is made with previously published results. For some geometries the current calculation results in lower (better) upper bound values; however, the results were generally close to previously published values. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
82.
Ashraf Owis Francesco Topputo Franco Bernelli-Zazzera 《Celestial Mechanics and Dynamical Astronomy》2009,103(1):1-16
The solution of a feedback optimal control problem arising in orbital mechanics is addressed in this paper. The dynamics is
that of a massless body moving in a central gravitational force field subject also to a drag and a radial modulated force.
The drag is linearly proportional to the velocity and inversely proportional to the square of the distance from the center
of attraction. The problem is tackled by exploiting the properties of a suitably devised linearizing map that transforms the
nonlinear dynamics into an inhomogeneous linear system of differential equations supplemented by a quadratic objective function.
The generating function method is then applied to this new system, and the solution is back transformed in the old variables.
The proposed technique, in contrast to the classical optimal control problem, allows us to derive analytic closed-loop solutions
without solving any two-point boundary value problem. Applications are discussed. 相似文献
83.
1IntroductionThe ecosystem of Araucaria-Forest is one of the most important plant-associations in South Brazil. Today,however,the spreading area is a fraction of its natural size,originally spread over 200 km. It spanned from 18°to 30°of southern lati-t… 相似文献
84.
Ashraf Farahat 《Journal of Astrophysics and Astronomy》2010,31(2):81-88
A new numerical model of particle propagation in the Galaxy has been developed, which allows the study of cosmic-ray production
and propagation in 2D. The model has been used to solve cosmic ray diffusive transport equation with a complete network of
nuclear interactions using the time backward Markov stochastic process by tracing the particles’ trajectories starting from
the Solar System back to their sources in the Galaxy. This paper describes a further development of the model to calculate
the contribution of various galactic locations to the production of certain cosmic ray nuclei observed at the Solar System. 相似文献
85.
N. Akhtar M. Tufail M. Ashraf 《International Journal of Environmental Science and Technology》2005,1(4):279-285
The study was conducted for the investigation of amount of radioactivity in the barren and cultivated soil of Bio saline Research Station in Pakka Anna, established by Nuclear Institute for Agriculture and Biology (NIAB) in 1990, 34 km. away from the city of Faisalabad, in the Punjab province of Pakistan. The studies were done on an area of about 100 hectares of two types of virgin and fertilized saline soils. The technique of gamma ray spectrometry was applied using HPGe (high purity germanium) gamma ray detector and a PC based MCA. Activity concentration levels due to 40K, 137Cs, 226Ra and 232Th were measured in 250 saline soil samples collected at a spacing of about 4 hectares at the depth level of 0–25 cm. with a step of 5 cm. depth. Activity concentrations ranges of the concerned radionuclides for both of the soils were as follows: 40K, for virgin and cultivated saline soil was 500–610.2 and Bq/kg 560.2–635.6 respectively; 137Cs, 3.57–3.63 and 1.98–5.15 Bq/kg 238U, 26.3–31.6 and 30.3–38.7 Bq/kg, and 232Th, 50.6–55.3 and 50.6–64.0 Bq/kg respectively. The absorbed dose rate in air lies in the region 63–73 nGyh-1 and 68–83 nGyh-1 for virgin and fertilized soils respectively. This indicates that this region lies in the area of higher radiation background, while comparing with the worlds’ average. The slightly higher value of dose in the fertilized farm may be due to the use of fertilizers for cultivation. Before the radiometric measurements, chemical analysis for concentration of Na, Ca and Mg was also carried out along with the measurement of electrical conductivity and pH of the soil samples. 相似文献
86.
87.
Salman Ashraf Lars BrabynBrendan J. Hicks 《Applied geography (Sevenoaks, England)》2012,32(2):619-628
Remote sensing based mapping of diverse and heterogeneous freshwater environments requires high-resolution images. Data fusion is a useful technique for producing a high-resolution multispectral image from the merging of a high-resolution panchromatic image with a low-resolution multispectral image. Given the increasing availability of images from different satellite sensors that have different spectral and spatial resolutions, data fusion techniques that combine the strengths of different images will be increasingly important to Geography for land-cover mapping. Different data fusion methods however, add spectral and spatial distortions to the resultant data depending on the geographical context; therefore a careful selection of the fusion method is required. This paper compares a technique called subtractive resolution merge, which has not previously been formally tested, with conventional techniques such as Brovey transformation, principal component substitution, local mean and variance matching, and optimised high pass filter addition. Data fusion techniques are grouped into spectral and spatial centric methods. Subtractive resolution merge belongs to a new class of data fusion techniques that uses a mix of both spatial and spectral centric approaches. The different data fusion techniques were applied to a QuickBird image of a semi-aquatic freshwater environment in New Zealand. The results were compared both qualitatively and quantitatively using spectral and spatial error metrics. This research concludes that subtractive resolution merge performed better than all the other techniques and will be a valuable technique for enhancing images for freshwater land-cover mapping. 相似文献
88.
Land use and land cover change in Greater Dhaka, Bangladesh: Using remote sensing to promote sustainable urbanization 总被引:12,自引:0,他引:12
This study evaluates land use/cover changes and urban expansion in Greater Dhaka, Bangladesh, between 1975 and 2003 using satellite images and socio-economic data. Spatial and temporal dynamics of land use/cover changes were quantified using three Landsat images, a supervised classification algorithm and the post-classification change detection technique in GIS. Accuracy of the Landsat-derived land use/cover maps ranged from 85 to 90%. The analysis revealed that substantial growth of built-up areas in Greater Dhaka over the study period resulted significant decrease in the area of water bodies, cultivated land, vegetation and wetlands. Urban land expansion has been largely driven by elevation, population growth and economic development. Rapid urban expansion through infilling of low-lying areas and clearing of vegetation resulted in a wide range of environmental impacts, including habitat quality. As reliable and current data are lacking for Bangladesh, the land use maps produced in this study will contribute to both the development of sustainable urban land use planning decisions and also for forecasting possible future changes in growth patterns. 相似文献
89.
U. von Rad U. Berner G. Delisle H. Doose-Rolinski N. Fechner P. Linke A. Lückge H. A. Roeser R. Schmaljohann M. Wiedicke SONNE / Scientific Parties M. Block V. Damm J. Erbacher J. Fritsch B. Harazim J. Poggenburg G. Scheeder B. Schreckenberger N. von Mirbach M. Drews S. Walter A. Ali Khan A. Inam M. Tahir A. R. Tabrez A. H. Cheema M. Pervaz M. Ashraf 《Geo-Marine Letters》2000,20(1):10-19
The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon
between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field
of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation
front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization
of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate
layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane
and H2S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being
progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively
destabilized and dissociated into hydrate water, methane and H2S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth,
upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed
at water depths of 350 to 800 m.
Received: 14 June 1999 / Revision accepted: 6 February 2000 相似文献
90.
Rania S. MABROKE Osama M. EL-HUSSEINY Abd El-Naem F. A. ZIDAN Al-Azab TAHOUN Ashraf SULOMA 《海洋湖沼学报(英文)》2019,(1):313-320
This study was conducted to evaluate the ef fect of replacing soybean meal(SBM) by low protein ?oc meal(LPFM; 24% CP) in tilapia diets on growth performance, feed utilization and ?sh chemical composition. Three isonitrogenous and isocaloric diets were formulated; control diet(C; without LPFM),FM 25(25% of SBM protein was substituted by LPFM) and FM 50(50% of SBM protein was substituted by LPFM). Nine 55 l circular plastic tanks were stocked by 12 ?sh to form three experimental groups.No dif ferences in tilapia performance were observed between the control and the FM 25 diet. Chemical composition of ?sh did not dif fer signi?cantly among treatments except for protein and selenium contents where the highest values were recorded in the control treatment. The highest mineral content was recorded in ?oc collected from the control tanks, while calcium content showed its highest value in ?oc collected from FM 50 e ? uent. These data indicate that replacing soybean meal with LPFM up to 25%, had no negative ef fect on growth performance and potentially may improve the system sustainability. Meanwhile, the adverse ef fect of more inclusion of LPFM in tilapia diet may be attributed to the higher content of ash. In other word,minerals seem to cause more energy utilization in ?sh fed ?oc meal since it is needed to maintain osmotic homeostasis. 相似文献