Influence of attenuation and site on microearthquakes’ spectra in Shillong region, of Northeast, India: A case study     

Rajib Biswas  Saurabh Baruah  Dipok K. Bora 《Acta Geophysica》2013,61(4):886-904

We examine the influence of attenuation and site on the spectra of microearthquakes having origin within the Shillong region. The ratios of spectral amplitudes at lower and higher frequencies are measured for three different stations at varying epicentral distances to estimate Q value for both P- and S-wave in near and sub-surface layer. The average estimates of Q _P and Q _S are found to be 178 and 195. The ratio of Q _S to Q _P emerges to be greater than unity in major parts of the Shillong area, suggesting dominance of dry crust prevailing in Shillong region. The variation in corner frequencies for these spectra is inferred to be characteristics of the site. Besides, the disparity in spectral content with reference to hard rock site yields the inference that the incoming seismic signals get amplified considerably while traversing from southern part to northeastern part of Shillong, best outlined at 2 to 5 Hz, which is well corroborated by the existing lithology.  相似文献   

Diversity and Distribution of Microzooplankton Tintinnid (Ciliata: Protozoa) in the Core Region of Indian Sundarban Wetland          下载免费PDF全文

Dibyendu Rakshit  Santosh K. Sarkar  Kamala K. Satpathy  Palani Sankar Ganesh  Nallamuthu Godhantaraman  Jayanta K. Biswas 《洁净——土壤、空气、水》2016,44(10):1278-1286

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Deccan plume,lithosphere rifting,and volcanism in Kutch,India     

Gautam Sen  Michael Bizimis  Reshmi Das  Dalim K. Paul  Arijit Ray  Sanjib Biswas 《Earth and Planetary Science Letters》2009,277(1-2):101-111

Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO₂-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO₂-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.  相似文献   

Comments on “Monitoring of impact of anthropogenic inputs on water quality of mangrove ecosystem of Uran,Navi Mumbai,west coast of India” published in Marine Pollution Bulletin (2013), 75: 291–300 by Prabhakar R. Pawar     

K.K. Satpathy  S. Biswas 《Marine pollution bulletin》2014

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A. K. Biswas 《GeoJournal》1991,25(1):118

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The flattening of the X-ray spectrum of sco X-1 at energies beyond 40 keV     

P. C. Agrawal  S. Biswas  G. S. Gokhale  V. S. Iyengar  P. K. Kunte  R. K. Manchanda  B. V. Sreekantan 《Astrophysics and Space Science》1971,10(3):500-507

The measurements of the hard X-ray spectrum of Sco X-1 in the energy interval 20–150 keV in three balloon flights from Hyderabad, India are reported. These results show conclusively that the spectrum of Sco X-1 is very flat in the energy interval 40–150 keV and the measured fluxes beyond 60 keV are several orders of magnitude higher than those expected on the basis of an extrapolation of bremsstrahlung spectrum from a thin hot plasma at a temperature corresponding tokT=5 keV, which is applicable for Sco X-1 for energies <40 keV. The results are compared with those of other investigators of hard X-rays from Sco X-1, and the implication of the results is briefly discussed.  相似文献   

Time-dependentH-,X- andY-functions for an anisotropically-scattering atmosphere     

G. Biswas  S. Karanjai 《Astrophysics and Space Science》1990,164(1):15-36

We have considered a homogeneous atmosphere scattering anisotropically with Dirac -function type time-dependent incidence. We used the method of integral operator developed by Ambartsumian and the theory ofN-solutions developed by Busbridge to find the correspondingH-function (in semi-infinite atmosphere) andX- andY-functions (in finite atmosphere).  相似文献   

The spectrum of diffuse cosmic X-rays in the 20–125 keV range     

R. K. Manchanda  S. Biswas  P. C. Agrawal  G. S. Gokhale  V. S. Iyengar  P. K. Kunte  B. V. Sreekantan 《Astrophysics and Space Science》1972,15(2):272-283

Diffuse cosmic X-rays in the energy range 20–125 keV were measured in four balloon flights from Hyderabad, India during 1968–70 using almost identical X-ray telescopes mounted on oriented platforms. The results from these flights show that the spectrum of the diffuse cosmic X-rays can be represented by the form dN/dE=29E ^–2.1±0.3 photons/(cm² sr s keV) in 20–125 keV interval after corrections for photoelectric absorption and Compton scattering effects in the atmosphere. The best fit spectrum of all published results in the energy interval 20–200 keV can be represented by the form dN/dE=36E ^–2.1±0.1 photons/(cm² sr s keV) after similar corrections are effected, and there is no need for a change of spectral index in this energy interval. The intensity at 20 keV obtained from the above spectrum agrees well with that given by the spectral form dN/dE=10E ^–1.7±0.1 photons/(cm² sr s keV) in the energy interval 1–20 keV in several rocket experiments. Therefore it is concluded that if there is a break in the spectrum, it occurs between 10 and 20 keV with a change of spectral index by about 0.5, or the index is continuously changing from 1.7±0.1 to 2.1±0.1 in 10–20 keV interval. The implications of the results are briefly discussed.  相似文献   

Effect of PDOP on performance of Kalman Filters for GNSS-based space vehicle position estimation     

Biswas  Sanat K.  Qiao  Li  Dempster  Andrew G. 《GPS Solutions》2017,21(3):1379-1387

GPS Solutions - A theoretical performance analysis of Kalman Filters for Global Navigation Satellite System GNSS-based space vehicle position estimation in varying Position Dilution of Precision...  相似文献   

Variation of γ-Ray and Particle Fluxes At the Sea Level During the Total Solar Eclipse of 24 October, 1995     

Abhijit Bhattacharyya  Sukumar Biswas  Barun K. Chatterjee  Mala Das  Pradipta K. Das  Tapan K. Das  Tarun K. De  M.H. Engineer  Rabi N. Mukherjee  Sibaji Raha  S.C. Roy  Swapan K. Saha  A.K. Sen  Bikash Sinha  Debapriyo Syam 《Astrophysics and Space Science》1997,250(2):313-326

A Total Solar Eclipse (TSE) was observed from Diamond Harbour (lat. 22.2°N, long 88.2°E) on 24 October 1995. The variation of -ray intensity was measured in the energy range of 0.3–3.0 MeV for different time spans throughout the period of the eclipse. A CR-39 detector was used to look at the change in the fluxes of neutral and charged particles. The maximum drop ( 25%) in the intensity of -ray was observed in the range 2.5–3 MeV during TSE. The CR-39 results showed the appearance of a good number of tracks and a small variation of proton and neutron flux of 10% which was not significant statistically. Low energy -ray fluxes at sea level originate from the secondary electron-photon components of cosmic rays in the atmosphere; its modulation by TSE is interpreted as follows. The cooling of the atmosphere in the path of the umbra induces a reduction of the height of the main production layer of the nuclear component, as a result of which, fewer µ^± mesons (from the decay of the^± mesons) decay to e^±. This leads to a small reduction in the flux of electron-photon component at sea level which originates from this branch; the main branch of e - component from ⁰ decay remains nearly unaffected. As the total mass of air remains the same, little or no change in the slow proton or the neutron flux at sea level is expected. These are consistent with the present observations. For a better understanding, further studies of this new phenomenon during future TSE are suggested.  相似文献