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11.
The maximum entropy method (MEM) of image reconstructtion is discussed in the context of incomplete Fourier information (as
in aperture synthesis). Several current viewpoints on the conceptual foundation of the method are analysed and found to be
unsatisfactory. It is concluded that the MEM is a form of model-fitting, the model being a non-linear transform of a band-limited
function. A whole family of ’entropies’ can be constructed to give reconstructions which (a) are individually unique, (b)
have sharpened peaks and (c) have flattened baselines. The widely discussed 1nB and - B1nB forms of the entropy are particular
cases and lead to Lorentzian and Gaussian shaped peaks respectively. However, they hardly exhaust the possibilities-for example,
B1/2 is equally good.
The two essential features of peak sharpening and baseline flattening are shown to depend on a parameter which can be controlled
by adding a suitable constant to the zero spacing correlation ρ00. This process, called FLOATing, effectively tames much of the unphysical behaviour noted in earlier studies of the MEM. A
numerical scheme for obtaining the MEM reconstruction is described. This incorporates the FLOAT feature and uses the fast
Fourier transform (FFT), requiring about a hundred FFTs for convergence. Using a model brightness distribution, the MEM reconstructions
obtained for different entropies and different values of the resolution parameter are compared. The results substantiate the
theoretically deduced properties of the MEM.
To allow for noise in the data, the least-squares approach has been widely used. It is shown that this method is biased since
it leads to deterministic residuals which do not have a Gaussian distribution. It is suggested that fitting the noisy data
exactly has the advantage of being unbiased even though the noise appears in the final map. A comparison of the strengths
and weaknesses of the MEM and CLEAN suggests that the MEM already has a useful role to play in image reconstruction. 相似文献
12.
The coupling of magnetoacoustic waves at a plane interface that separates two semiinfinite collisionless fluids is studied. The fluids are characterized by different temperatures along and transverse to the ambient magnetic field. Continuum equations obtained by Chewet. al. (1956) are used and expressions for the reflection and transmission coefficients are derived. Extensive numerical computations are done to study the variation of the reflection coefficient, with the angle of incidence, for various temperature anisotropies of the media. Relevance of these investigations to the magnetosphere-solar wind boundary is discussed. 相似文献
13.
Heavy metals contamination in road dust in Delhi city,India 总被引：1，自引：1，他引：0
B. S. Rajaram P. V. Suryawanshi A. D. Bhanarkar C. V. C. Rao 《Environmental Earth Sciences》2014,72(10):3929-3938
Road dust samples were collected from four different areas having different landuse patterns: industrial, heavy traffic, residential and mixed use in Delhi city of India. The samples were analyzed for Ba, Co, Cr, Cu Fe, Mn, Ni, Pb and Zn by ICP-AES. Results indicate high levels of Co, Cr, Cu, Mn and Ni in samples collected from industrial area. Ba, Pb and Zn showed higher concentration levels in heavy traffic area while Fe did not show any discernible variation between the localities. The concentrations of Fe, Mn, Ba, Zn, Cr, Cu, Pb, Ni and Co showed a decreasing trend. The content of heavy metals was comparable to those in other cities in the world. A multivariate statistical approach which includes Pearson’s correlations and principal component analysis was used to identify the possible sources of metals in the road dust. Enrichment factors were estimated for further confirming the sources of contamination. Significant positively correlations between road dust metals Cu–Mn–Co–Cr–Ni suggest that major common source of origin is industrial activities. A meaningful correlation between Ba and Zn, and a moderate positive correlation between Pb and Ba indicate the influence of traffic activities. Enrichment factors calculation indicated that Pb, Cu, Cr and Zn are moderately enriched whereas Co, Ni and Mn are less enriched while Ba exhibited very low enrichment in the dust samples. The results indicate that industrial and vehicular traffic are the two major sources. Traffic appears to be responsible for the high levels of Zn, Cu and Ba. High concentration of Co, Cr, Cu and Mn may be due to industrial sources. 相似文献
14.
Jatin Rathod Girija Rajaram Radharani Alyana A. Chandrasekhar Reddy D. S. Misra C. G. Patil M. Y. S. Prasad A. G. Ananth 《Journal of Astrophysics and Astronomy》2008,29(1-2):293-302
In the present study, we investigate the possible relationship of IP parameters of solar wind and interplanetary magnetic field with ground-based geomagnetic indices. To carry out the study, we take all the IP shock events listed by Proton Monitor onboard Solar and Heliospheric Observatory (SOHO) during 2005, and plot the time variations of all the IP parameters and geomagnetic parameters (±5 days), centered at the shock arrival time. Next, we obtain scatter plots of absolute values of solar wind parameters such as Vsw, Nsw and Interplanetary Magnetic Field (IMF) components Bx, By, Bz and total B with the values of geomagnetic parameters such as Dst, Kp indices, dayside Magnetopause (MP) distance and Cosmic-Ray Neutron Monitor count (CRNM). The scatter plots show that before the IP shock, the pattern is random with no clear relationship. Following the shock, a clear pattern emerges with a type of relationship being seen — clear for SHARP shocks and less clear for DIFFUSE shocks. A total of 10 shock events for 2005 have been studied. Typical examples of this behaviour are the shock events of January 21, 2005 and May 15, 2005. Our study suggests a definite correlation between changes in the solar wind and interplanetary magnetic field parameters and ground-based geomagnetic response. We are trying to obtain quantitative relationships between these for shock events of 2005. 相似文献
15.
Based on a simple picture of speckle phenomena in optical interferometry it is shown that the recent signal-to-noise ratio
estimate for the so called bispectrum, due to Wirnitzer (1985), does not possess the right limit when photon statistics is
unimportant. In this wave-limit, which is true for bright sources, his calculations over-estimate the signal-to-noise ratio
for the bispectrum by a factor of the order of the square root of the number of speckles. 相似文献
16.
The theory of gravitational lensing of background quasars by stars in the halo of a galaxy is considered. In the limiting
case of small ‘optical depth’, only one star is close enough to the beam to cause strong scattering, and the effect of all
the other stars is treated as a perturbation with both systematic and random components. The perturbation coming from weak
scattering can increase the number of images and the amplification in those cases where the amplification is already high;
such events are preferentially selected in flux limited observations. The theory is applicable to the apparent association
of background quasars with foreground galaxies. A comparison with earlier work on the same problem is given. The relevance
of these results to gravitational lensing by galaxies as perturbed by random inhomogeneities surrounding the ray path is also
briefly discussed. 相似文献
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18.
Total field magnetic data were collected over the Krishna-Godavari basin covering 20, 000 sq.km with an average spacing of 8.5 km. This was mainly to study the long wavelength features related with the deep structures. Aeromagnetic map of the region compared well with the ground maps. The anomaly maps show a combination of NE-SW, NS/NNE-SSW and NW-SE trends. The anomalies of ground data are transformed to isolate the sources at different depths. The second vertical derivative and downward continuation maps bring out clearly the NE-SW and NS/NNE-SSW trends related to the coastal basin and Eastern Ghats implying that they are shallow. These are probably superposed on much deeper NW-SE trending structural features of Pre-Gondwana breakup as evidenced in the Horizontal Gradient of Pseudogravity and upward continuation maps. From the offshore magnetic data it appears that these trends extend up to the Ocean Continent Boundary. It is inferred that the deeper features are associated with rifting of Dharwar and Bastar cratons within the Indian plate, prior to the rifting of India from Gondwanaland. The superposed horst and graben structures are related to the formation of the pull-apart Krishna-Godavari basin as a result of rifting and drifting of India from Gondwanaland. These two structural features are associated with two different tectonic events. 相似文献
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20.
The present study was carried out to investigate the impact of anthropogenic influences on Cuddalore coast, Southeast coast of India, with regard to physicochemical parameters and heavy metal concentration in the surface water and sediment samples of the study area. The samples were collected in different seasons of the year (January–December 2010) and analysed for physicochemical parameters (Temperature, pH, salinity, nitrate, nitrite, ammonia, phosphate and silicate) and heavy metals (Cd, Cu, Pb and Zn) using standard methods. Results showed that physicochemical characteristics and heavy metals concentration in the samples of the study area were varied seasonally and spatially. The concentrations of heavy metals in water and sediment samples of the study area were higher in the monsoon season compared with those of other seasons. The heavy metal concentration in collected samples was found to be above WHO standards. The order of heavy metals in water and sediment samples was Pb > Cu > Cd > Zn. The heavy metal data were analysed through widely using multivariate statistical methods including principle component analysis (PCA) and cluster analysis (CA). CA classified the sampling sites into three clusters based on contamination sources and season. The PCA revealed that the season has a huge impact on the levels, types and distribution of metals found in water and sediment samples. The study also shows the main basis of heavy metals pollution at Cuddalore coast is land based anthropogenic inputs as a result of discharging of waste from industries, municipal, agricultural activities and sewage into estuarine regions, which carries the wastes into coastal area during tidal action. Statistical analyses and experimental data revealed that the Cuddalore coast may cause health risk to the recreational users and fisher folk, ultimately warrants environmental quality management to control heavy metal contamination. 相似文献