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101.
This study uses radiometric analysis (210Pb and 137Cs) of short sediment cores with high-resolution sampling (1-cm interval) to trace sedimentation rates in the Nile Delta lagoons,
particularly since completion of the Aswan High Dam in 1964. A declining trend in 210Pbex as calculated by the CIC model is clearly identified in about 10 cm of the upper-core sediments from the lagoons of Manzala
and Edku, accompanied by two spikes of 137Cs in cores from the lagoons of Burullus and Edku. These findings illustrate average post-dam sedimentation rates ranging
from 0.22 to 0.27 cm a−1 in the lagoons, in contrast with those found previously based on low-resolution sampling. The lower sedimentation rates in
the lagoons are a consequence of a dramatic reduction in riverine sediment load to the coastal area as a result of the damming.
Although widespread erosion occurs along the open estuarine coast, the lagoon setting remains calmer than before due to coastal
diking and freshwater regulation in the delta plain in the past decades. This provides the possibility of continuously preserved
radiometric records in the less-bioturbated lagoon sediments. Dating individual layers using the CRS model has revealed increasing
sedimentation rates in Manzala and Burullus since the 1980s, which can largely be explained as a consequence of the reduction
in lagoon area due to intensifying reclamation. The post-dam sedimentation in the shrinking lagoons may have some adverse
ecological consequences due to finer sediment’s affinity with pollutants. These findings would shed light on the environmental
conservation and socioeconomic development in the Nile Delta region. 相似文献
102.
The tracking performance of a Phase Lock Loop (PLL) is affected by the influence of several error sources. In addition to thermal noise and dynamic stress error, oscillator phase noise can cause significant phase jitter which degrades the tracking performance. Oscillator phase noise is usually caused by two different effects: Allan deviation phase noise is caused by frequency instabilities of the receiver's reference oscillator and the satellite's frequency standard. It can be termed as system-inherent phase noise and is relevant for both static and dynamic applications. “External” phase noise, however, is caused by vibration and is a major problem for dynamic applications. In the context of this paper, both types of phase noise will be modeled and the resulting integrals will be evaluated for PLLs up to the third order. Besides, phase jitter induced by thermal noise and signal dynamics will also be discussed, thus providing all necessary formulas for analyzing the performance of a phase lock loop in case of different forms of stress. Since the main focus is centered on the effects of oscillator phase noise, the overall PLL performance is graphically illustrated with and without consideration of oscillator phase noise. © 2002 Wiley Periodicals, Inc. 相似文献
103.
Climate change caused by carbon emissions continuously threatens sustainable development. Due to China’s immense territory, there are remarkable regional differences in carbon emissions. The construction industry, which has strong internal industrial differences, further leads to carbon emission disparity in China. Policymakers should consider spatial effects and attempt to eliminate carbon emission inequality to promote the sustainable development of the construction industry and realize emission reduction targets. Based on the classic Markov chain and spatial Markov chain, this paper investigates the club convergence and spatial distribution dynamics of China’s carbon intensity in the construction industry from 2005 to 2014. The results show that the provincial carbon intensity in the construction industry is characterized by “convergence clubs” during the research period, and very low-level and very high-level convergence clubs have strong stability. Moreover, the carbon intensity class transitions of provinces tend to be consistent with that of their neighbors. Furthermore, the transition of carbon intensity types is highly influenced by their regional backgrounds. The provinces with high carbon emissions have a negative influence on their neighbors, whereas the provinces with low carbon emissions have a positive influence. These analyses provide a spatial interpretation to the “club convergence” of carbon intensity. 相似文献
104.
By using sonar imaging, this paper presents a new algorithm for the clustering of seabed types based on the self-organizing feature maps (SOFM) neural network. The theory as well as data processing is studied in detail. Some valuable conclusions and suggestions are given 相似文献
105.
106.
Pulsatory characteristics of wind velocity in sand flow over typical underlying surfaces 总被引:1,自引:0,他引:1
ZHANG KeCun QU JianJun ZU RuiPing & FANG HaiYan Key Laboratory of Desert Desertification Cold Arid Regions Environmental Engineering Research Institute Chinese Academy of Sciences Lanzhou China 《中国科学D辑(英文版)》2007,50(2):247-253
Pulsatory characteristics of wind velocity in sand flow over Gobi and mobile sand surface have been investigated experimentally in the wind tunnel. The primary goal of this paper is to reveal the relation- ship between pulsatory characteristics of instantaneous wind speed in sand flow and the motion state of sand grains. For a given underlying surface, pulsation of wind velocities in sand flow on different heights has a good correlation. As the space distance among different heights increases, fluctuation of instantaneous wind speed presents a decreasing trend and its amplitude is closely related to the mo- tion state of sand grains and their transport. Pulsatory intensity increases with the indicated wind speed, but its relative value does not depend on it, only agrees with height. 相似文献
107.
The Tarwal River basin with an area of 6560.20 km2 is located in the eastern part of Iranian Kurdistan Province. This river crosses the Qorveh and Dehgolan plains and joins the Ghezel Ozan River in Zanjan Province. The importance of this river as a source for drinking water and agricultural and industrial uses in the region necessitates the need for research in this field. The main purpose of this study is to identify the natural features of the riverbed from the perspective of river geomorphology and to investigate their impact on water quality and river self-purification capacity. To achieve this, the river style framework was employed. To investigate the effects of each style framework on the river, a total of 20 samples from the entrance and outlet of styles were obtained using Impact Assessment method and sampling standards which were later analyzed for their quality parameters including T, pH, EC, TDS, TSS, Na, Ca, Mg, K, Cl, F, NO2, NO3, SO4, PO4, DO, COD and BOD. The results indicated that the changes in the styles lead to changes in water quality and the impact of each style is greater on the physical parameters than the chemical parameters. The river self-purification capacity varied depending on the style. The maximum and the minimum self-purifications occurred in fine-grained Anabranching and low-sinuosity fine-grained styles, respectively. 相似文献
108.
A geochemical evaluation of Niger Delta organic matter was carried out using supercritical fluid extraction (SFE) sample preparation
procedure. Comparison of geochemical significance of gas chromatographic data of rock extracts of SFE with those of Soxhlet
extraction method from previous studies was made in order to establish the usefulness of SFE in geochemical exploration. The
assessment of geochemical character of the rock samples from the comparison and interpretation of other geochemical parameters
were used to give more insights into understanding the source rocks characteristics of onshore and shelf portions of the Niger
Delta Basin. The results of the gas chromatographic (GC) analysis of the rock extracts across the lithostratigraphic units
show that Pr/Ph, Pr/nC17, Pr/nC18, CPI and odd/even preference ranged from 0.07 to 12.39, 0.04 to 6.66, 0.05 to 13.80, 0.12 to 8.4 and 0.06 to 8.12, respectively.
The Rock-Eval pyrolysis data and geochemical ratios and parameters calculated from the GC data showed that most of the samples
are mature and have strong terrestrial provenance while a few samples have strong marine provenance. The few marine source
rocks are located in the deeper depth horizon. Pr/Ph and standard geochemical plots indicate that most of samples were derived
from organic matter deposited in less reducing conditions, i.e. more of oxidizing conditions while a few samples have predominantly
influence of reducing conditions. The results of trace metal analysis of older samples from Agbada Formation also indicate
marine and mixed organic matter input deposited in less reducing conditions. The results obtained in this study are comparable
with those obtained from previous studies when Soxhlet extraction method was used and also indicated the presence of more
than one petroleum systems in the Niger Delta. 相似文献
109.
Cinematic, photometric observations of the 3B flare of August 7, 1972 are described in detail. The time resolution was 2 s; the spatial resolution was 1–2″. Flare continuum emissivity at 4950 Å and at 5900 Å correlated closely in time with the 60–100 keV non-thermal X-ray burst intensity. The observed peak emissivity was 1.5 × 1010 erg cm?2 s?1 and the total flare energy in the 3900–6900 Å range was ~1030 erg. From the close temporal correspondence and from the small distance (3″) separating the layers where the visible emission and the X-rays arose, it is argued that the hard X-ray source must have had the same silhouette as the white light flare and that the emission patches had cross-sections of 3–5″. There was also a correlation between the location of the most intense visible emissions near sunspots and the intensity and polarization of the 9.4 GHz radio emission. The flare appeared to show at least three distinct particle acceleration phases: one, occurring at a stationary source and associated with proton acceleration gave a very bluish continuum and reached peak intensity at ~ 1522 UT. At 1523 UT, a faint wave spread out at 40 km s?1 from flare center. The spectrum of the wave was nearly flat in the range 4950–5900 Å. Association of the wave with a slow drift of the microwave emission peak to lower frequencies and with a softening of the X-ray spectrum is interpreted to mean that the particle acceleration process weakened while the region of acceleration expanded. The observations are interpreted with the aid of the flare models of Brown to mean that the same beam of non-thermal electrons that was responsible for the hard X-ray bremsstrahlung also caused the heating of the lower chromosphere that produced the white light flare. 相似文献
110.
The viability of a complete structural characterization of civil structures is explored and discussed. In particular, the identification of modal (i.e. natural frequencies, damping ratios and modal shapes) and physical properties (i.e. mass and stiffness) using only the structure’s free decay response is studied. To accomplish this, modal analysis from free vibration response only (MAFVRO) and mass modification (MM) methodologies are engaged along with Wavelet based techniques for optimal signal processing and modal reconstruction. The methodologies are evaluated using simulated and experimental data. The simulated data are extracted from a simple elastic model of a 5 story shear building and from a more realistic nonlinear model of a RC frame structure. The experimental data are gathered from shake table test of a 2-story scaled shear building. Guidelines for the reconstruction procedure from the data are proposed as the quality of the identified properties is shown to be governed by adequate selection of the frequency bands and optimal modal shape reconstruction. Moreover, in cases where the structure has undergone damage, the proposed identification scheme can also be applied for preliminary assessment of structural health. 相似文献