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191.
Sanjit Mitra Anand S. Sengupta Subharthi Ray Rajib Saha Tarun Souradeep 《Monthly notices of the Royal Astronomical Society》2009,394(3):1419-1439
Over the last decade, measurements of the cosmic microwave background (CMB) anisotropy have spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic effects in the increasingly sensitive, high-resolution, 'full' sky measurements from different CMB experiments poses a stiff challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the final precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using pseudo-C l unavoidable for high-resolution data. The debiasing of the pseudo-C l needs account for non-circular beams, together with non-uniform sky coverage. We provide a (semi)analytic framework to estimate bias in the power spectrum due to the effect of beam non-circularity and non-uniform sky coverage, including incomplete/masked sky maps and scan strategy. The approach is perturbative in the distortion of the beam from non-circularity, allowing for rapid computations when the beam is mildly non-circular. We advocate that it is computationally advantageous to employ 'soft' azimuthally apodized masks whose spherical harmonic transform die down fast with m . We numerically implement our method for non-rotating beams . We present preliminary estimates of the computational cost to evaluate the bias for the upcoming CMB anisotropy probes ( l max ∼ 3000) , with angular resolution comparable to the Planck surveyor mission. We further show that this implementation and estimate are applicable for rotating beams on equal declination scans, and can possibly be extended to simple approximations to other scan strategies. 相似文献
192.
Srinanda Chaudhuri Rajarshi Saha Jyotisankar Ray S. K. Bhaduri 《Journal of the Geological Society of India》2009,74(3):385-394
The petrological details of the ultramafic-mafic-alkaline complex related to Sylhet Trap occurring near to Mawpyut (25°25′N:92°10′E)
of Jaintia hills district Meghalaya, northeastern India, are poorly known. Field investigations indicate that the Mawpyut
body occurs as a pluton distinctly intrusive into adjacent low grade metasedimentary Shillong Group of rocks. This body reveals
development of two broad lithotypes namely ultramafic (olivine clinopyroxenite, clinopyroxenite and plagioclase bearing ultramafic)
and mafic (mostly gabbro, orthopyroxene gabbro, olivine gabbronorite, mela gabbro and mela-gabbro-norite) with minor presence
of later syenitic veins. Though, in general, the pluton shows mineralogical variations, the field boundaries among those petrographic
types are not discernible. Careful consideration of major and trace element chemistry of the constituent lithomembers clearly
suggest progressive insitu fractionation of a common parent magma. 相似文献
193.
194.
Subodh K. Saha Subhadeep Halder K. Krishna Kumar B. N. Goswami 《Climate Dynamics》2011,36(11-12):2077-2089
It is proposed that, land?Catmosphere interaction around the time of monsoon onset could modulate the first episode of climatological intraseasonal oscillation (CISO) and may generate significant ??internal?? interannual variation in the Indian summer monsoon rainfall. The regional climate model RegCM3 is used over Indian monsoon domain for 27?years of control simulation. In order to prove the hypothesis, another two sets of experiment are performed using two different boundary conditions (El Ni?o year and non-ENSO year). In each of these experiments, a single year of boundary conditions are used repeatedly year after year to generate ??internal?? interannual monsoon variability. Simulation of monsoon climate in the control model run is found to be in reasonably good agreement with observation. However, large rainfall bias is seen over Arabian Sea and Bay of Bengal. The interannual monsoon rainfall variability are of the same order in two experiments, which suggest that the external influences may not be important on the generation of ??internal?? monsoon rainfall variability. It is shown that, a dry (wet) pre-onset land-surface condition increases (decreases) rainfall in June which in turn leads to an anomalous increase (decrease) in seasonal (JJAS) rainfall. The phase and amplitude of CISO are modulated during May?CJune and beyond that the modulation of CISO is quite negligible. Though the pre-onset rainfall is unpredictable, significant modulation of the post-onset monsoon rainfall by it can be exploited to improve predictive skill within the monsoon season. 相似文献
195.
Harsh Gupta D. Shashidhar K. Mallika N. Purnachandra Rao D. Srinagesh H. V. S. Satyanarayana Satish Saha R. T. B. Naik 《Journal of the Geological Society of India》2011,77(1):5-11
Earthquake activity is monitored in real time at the Koyna reservoir in western India, beginning from August 2005 and successful
short term forecasts have been made of M ∼ 4 earthquakes. The basis of these forecasts is the observation of nucleation that
precedes such earthquakes. Here we report that a total of 29 earthquakes in the magnitude range of 3.5 to 5.1 occurred in
the region during the period of August 2005 through May 2010. These earthquakes could broadly be put in three zones. Zone-A
has been most active accounting for 18 earthquakes, while 5 earthquakes in Zone-B and 6 in Zone-C have occurred. Earthquakes
in Zone-A are preceded by well defined nucleation, while it is not the case with zones B and C. This indicates the complexity
of the earthquakes processes and the fact that even in a small seismically active area of only 20 km × 30 km earthquake forecast
is difficult. 相似文献
196.
Govinda C. Mishra Amitava Saha Mitthan L. Kansal Ravi P. Gupta 《Hydrogeology Journal》2011,19(3):573-589
A semi-analytical solution has been derived for predicting the time of emptying a pond due to seepage. The time for the seeping water to reach the water table since the pond was initially filled has been calculated applying the Green-Ampt infiltration theory. The recharge rate after the wetting front joins the water table has been computed using a non-linear relationship between seepage head and recharge rate proposed by earlier investigators. The maximum rise in the water table beneath the center of the pond consequent to the time-varying recharge is calculated applying kernel coefficients obtained from solution of the linearized Boussinesq equation. It was observed that a pond with 50-m initial diameter at the water surface and 3-m maximum depth of water is dry after 168 days, where the subsoil is sandy clay. If the subsoil happens to be clay, the depth of water in the pond at the end of 9 months, i.e., after completion of the non-monsoon period, is 0.62 m. The maximum mound heights beneath the pond for constant recharge rate and uniform recharging area calculated from the present solution compare well with existing numerical as well as analytical solutions. 相似文献
197.
Heavy metal pollution assessment through comparison of different indices in sewage-fed fishery pond sediments at East Kolkata Wetland,India 总被引:3,自引:0,他引:3
Subhasis Sarkar Phani Bhusan Ghosh Alok Kumar Sil Tapan Saha 《Environmental Earth Sciences》2011,63(5):915-924
The sediments of the raw sewage-fed fishpond system at East Kolkata Wetland (EKW) were analyzed for heavy metal content in
a comprehensive way. Various indices of contamination like enrichment factor (EF), geo-chemical index (I
geo), modified degree of contamination (mDC), and pollution load index (PLI) were assessed. In all cases, instead of literature values, the metal concentrations of less
contaminated sites, separated by the statistical approach of the hierarchical cluster analysis, were used as baseline values.
In the present study, about 70% of the pond sediments are found uncontaminated, 5% display low degree of contamination and
25% are designated as moderate degree of contamination. Both the EF and I
geo indices highlighted that the metals lead (Pb), cadmium (Cd), and chromium (Cr) are responsible for the contamination while
there is little anthropogenic input in cases of Cu, Zn, and Ni. Most of the ponds situated near the main sewage flowing canals
as well as the main traffic highway and close to the solid waste dumping areas recorded higher degree of metal contamination
as evident from spatial variation of mDC and PLI indices in the study area. Indices comparison study clearly indicates that although these are calculated using different
methods, these may or may not produce the same indices values and hence the values should neither be compared nor be averaged.
But all the above indices are directly related to a common term contamination factor (CF). Classification of contamination
levels based on these CF values is found to be similar and this classification is only valid up to the level of high degree
of contamination. Thus, the use of any one of these indices is sufficient to classify the degree of contamination of an area.
However, to evaluate the contamination per metal, both I
geo and EF are effective while, to assess the composite effect of all the metals, PLI is preferable to mDC. 相似文献
198.
Dilip Saha 《Journal of Earth System Science》1995,104(3):419-431
In low temperature deformation of polymineralic rocks the constituent minerals often show contrasting deformation mechanisms.
In naturally deformed arkoses, feldspathic quartzites and grits under greenschist to almandine-amphibolite fades condition,
feldspar deforms by microboudinage (rigid-brittle behaviour), while quartz flows by a combination of dislocation creep, pressure
solution and solution transfer. Boudin segments develop and separate in a phased sequential manner while quartz matrix flows
in a ductile manner, indicating a brittle-ductile toggle during progressive deformation.
Both the pressure solution and dislocation creep flows are volume-conservative. Therefore, a net volume increase during the
above deformations is a necessity, unless compensated by a solution-transfer process. Hydrofracturing probably played a role
in microboudinage formation as the ambient level of differential stress is estimated to be low around 45–75 MPa.
To develop a synthetic flow law for the above type of deformation in arkoses, one needs to consider the significance of different
rate-controlling mechanisms. As feldspar pull-aparts are syntectonically filled with quartz or metamorphic minerals crystallizing
during progressive deformation, successive microboudin segmentation will depend on how fast/slow the matrix quartz moves to
the open crack or the sealing takes place by transfer of appropriate solute components by pressure solution or solution transfer,
the real rate-controlling process. 相似文献
199.
Kshudiram Saha 《Journal of Earth System Science》1993,102(1):241-247
Using the theory of plate tectonics and a concept of climate analogs, the paper speculates that a monsoon type of climate
with warm and wet summer and cold and dry winter might have first appeared over the northern part of India when during its
northward drift across the Tethys Ocean (now the Indian Ocean) it was located over the subtropical belt of the southern hemisphere
some 60 million years before present (BP). The monsoon climate gradually evolved and extended to other parts of India as the
Indian plate after crossing the equator about SO million years BP moved further northward and collided against the north Asian
plate giving rise to the Himalayas along the northern boundary of India some 40 million years BP. Recent studies suggest that
despite short and long period fluctuations, no major secular change or trend has taken place in the monsoon climate of India
since then. 相似文献
200.
Dilip Saha 《Journal of Earth System Science》1990,99(2):339-355
A small thrust sheet, named Pedda Gutta thrust sheet, consisting of calcareous to cherty argillites and cherts, and juxtaposed
against tidal-intertidal cross-bedded quartzites and stromatolitic and sileceous limestone in the eastern Proterozoic belt,
Godavari Valley, exhibits structures comparable in style to those of the external zone of a fold-thrust mountain belt. A wide
spectrum of periodic and aperiodic mesoscopic folds varying from upright ones with rounded hinges and attenuated limbs, through
noncylindrical kinks to whalebacks and sheath-like forms have developed within the small volume of the thrust sheet, the preserved
thickness of which is of the order of 50 metres (comparable in scale to cleavage duplexes). Cleavage development is also heterogeneous
across the width of the sheet. Displacement transfer from faults to folds and vice-versa is a common feature.
On the basis of the distribution of the mesoscopic structures of varying style within the sheet and localization of fault
rocks, three slices (wedges) have been recognized, each bounded on the east by a thrust which is steep at the current erosion
level but interpreted to be of listric form making the thrust network comparable in architecture, though not in scale, to
a hinterland (west) dipping imbricate fan. 相似文献