A synthesis map of the sky at 34.5 MHz     

K. S. Dwarakanath  N. Udaya Shankar 《Journal of Astrophysics and Astronomy》1990,11(3):323-410

This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds to all angular scale structures and is suitable for studies of both point sources and extended objects This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Roman Research Institute, Bangalore.  相似文献   

Analysis of submerged platform breakwater by Eigenfunction expansion method   总被引：1，自引：0，他引：1  

Hin-Fatt Cheong  N.Jothi Shankar  S Nallayarasu 《Ocean Engineering》1996,23(8):649-666

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Seismic hazard assessment of Chennai city considering local site effects     

A. Boominathan  G. R. Dodagoudar  A. Suganthi  R. Uma Maheswari 《Journal of Earth System Science》2008,117(2):853-863

Chennai city suffered moderate tremors during the 2001 Bhuj and Pondicherry earthquakes and the 2004 Sumatra earthquake. After the Bhuj earthquake, Indian Standard IS: 1893 was revised and Chennai city was upgraded from zone II to zone III which leads to a substantial increase of the design ground motion parameters. Therefore, a comprehensive study is carried out to assess the seismic hazard of Chennai city based on a deterministic approach. The seismicity and seismotectonic details within a 100 km radius of the study area have been considered. The one-dimensional ground response analysis was carried out for 38 representative sites by the equivalent linear method using the SHAKE91 program to estimate the ground motion parameters considering the local site effects. The shear wave velocity profile was inferred from the corrected blow counts and it was verified with the Multichannel Analysis of Surface Wave (MASW) test performed for a representative site. The seismic hazard is represented in terms of characteristic site period and Spectral Acceleration Ratio (SAR) contours for the entire city. It is found that structures with low natural period undergo significant amplification mostly in the central and southern parts of Chennai city due to the presence of deep soil sites with clayey or sandy deposits and the remaining parts undergo marginal amplification.  相似文献   

Wave run-up and overtopping on smooth and rough slopes of coastal structures     

N. J. Shankar  M. P. R. Jayaratne 《Ocean Engineering》2003,30(2):221-238

A series of hydraulic model tests has been carried out in a glass wave flume to investigate the influences of wave height, wave period, wave steepness, surf similarity parameter, roughness, layer thickness and porosity on wave run-up and overtopping of 1:2 sloped impermeable and permeable breakwaters fronted by a 1:10 gentle, smooth beach slope. The analysis of results involves the correlation between the overtopping energy transfer with the relative wall height and the relationship between wave run-up and overtopping rate. Further, measured wave run-up and overtopping rates are compared with the results given in the Shore Protection Manual (1984), Automated Coastal Engineering System (1992)and results of other investigators.  相似文献   

Wave forces on submarine pipelines near a plane boundary     

Cheong Hin-Fatt  N. Jothi Shankar  K. Subbiah 《Ocean Engineering》1987,14(3)

Forces induced by regular waves on submarine pipelines resting on as well as near a plane boundary and aligned parallel to wave fronts of the oncoming waves are investigated experimentally. The inline hydrodynamic coefficients of drag and inertia are evaluated through the use of Morison equation and the least squares method. The transverse force is analysed in terms of maximum transverse force and transverse root mean square (r.m.s.) coefficients. The resulting inline and transverse hydrodynamic coefficients are correlated with the period parameter or Keulegan-Carpenter number and relative clearance of the pipeline from the plane boundary. The effect of depth parameter on these coefficients and the correlation between maximum transverse force and transverse r.m.s. coefficients are also reported.  相似文献   

Probable gas hydrate/free gas model over western continental margin of India     

Satyavani  N.  Shankar  Uma  Thakur  N.K.  Reddi  S.I. 《Marine Geophysical Researches》2002,23(5-6):423-430

Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.  相似文献   

The GEM project: An international collaboration to survey galactic radiation emission     

S. Torres  V. Cañon  R. Casas  A. Umaña  C. Tello  T. Villela  M. Bersanelli  M. Bensadoun  G. De Amici  M. Limon  G. Smoot  C. Witebsky 《Astrophysics and Space Science》1996,240(2):225-234

The GEM (Galactic Emission Mapping) project is an international collaboration established with the aim of surveying the full sky at long wavelengths with a multi-frequency radio telescope. A total of 745 hours of observation at 408 MHz were completed from an Equatorial site in Colombia. The observations cover the celestial band 0 ^h <<24 ^h , and –24° 22<<+35° 37. Preliminary results of this partial survey will be discussed. A review of the instrumental setup and a 10° resolution sky map at 408 MHz is presented.Presented by S. Torres at the UN/ESA Workshop on Basic Space Sciences: From Small Telescopes to Space Missions, Colombo, Sri Lanka 11–14 January 1996  相似文献   

The Mauritius Radiotelescope     

K. Golap  N. H. Issur  R. Somanah  R. Dodson  M. Modgekar  S. Sachdev  N. Udaya Shankar  Ch. V. Sastry 《Astrophysics and Space Science》1995,228(1-2):373-377

The Mauritius Radiotelescope (MRT) is a T-shaped array of helical antennas with a 2048 m EW arm and a 890 m South arm. The primary objective of the telescope is to produce a sky survey in the declination zone -15° to -65° with a point source sensitivity of 200 mJy and an angular resolution of 4'×4.6'cosec(z) at 151.6 MHz, z being the zenith angle . This paper describes the telescope and the present status  相似文献   

Spatial practices and imaginaries: Experiences of colonial officers and development professionals     

Uma Kothari 《Singapore journal of tropical geography》2006,27(3):235-253

This paper explores the spatiality of colonial and postcolonial power and discourse as produced, performed and imagined by former British colonial service officers and contemporary UK international development professionals. It focuses on two key aspects of spatial practices. The first addresses the spaces inhabited by these colonial officers and development professionals overseas and how their locatedness, embedded or enclavic, shapes relationships to others. The second explores this distinctive social and spatial distancing through their relationship to, and imagined geographies of, home and away and how these are embodied in their institutional and cultural capital. The paper examines the regularities and consistencies that stand out from numerous individual practices through which both former colonial officers and development professionals negotiate the situations in which they live and work. It also specifies how authoritative management, privilege and distance informs their spatial practices despite changing global contexts and a more diverse composition of those who articulate contemporary relationships between 'first' and 'third' worlds. Finally, the paper suggests that the cultures which travelled over colonial space through being performed by colonial officers have been reworked throughout the postcolonial period, belying epochal historical periodizations that conjure up a clear disjuncture between colonial and development eras.  相似文献   

Dissolution channels in quartz and the role of pressure changes in gold and sulfide deposition in the Archean, greenstone-hosted, Hutti gold deposit, Karnataka, India     

Hari Shankar Pandalai  Gajanan Narayanrao Jadhav  Biju Mathew  V. Panchapakesan  K. Krishnam Raju  M. L. Patil 《Mineralium Deposita》2002,38(5):597-624

The pressure, temperature and composition of ore fluids that resulted in gold deposition in the Archean, greenstone-hosted Hutti deposit have been studied using fluid inclusions and the compositions of arsenopyrite and chlorite. Five types of fluids have been identified in fluid inclusions in quartz veins associated with mineralization. They are (1) monophase CO ₂-rich fluid; (2) low-salinity (0 to 14 wt% NaCl equivalent) and high-salinity (16 to 23 wt% NaCl equiv.) aqueous fluids; (3) high-salinity (28 to 40 wt% NaCl equiv.), polyphase aqueous fluids; (4) CO ₂–H ₂O–NaCl fluids of low salinity (0–8 wt% NaCl equiv.); and (5) a few carbonic inclusions with halite±nahcolite. The diversity of entrapped fluid composition is explained in terms of changes in fluid pressure and temperature which affect a more or less uniform supply of primary low-salinity CO ₂–H ₂O–NaCl fluid to the shear zone. Geothermobarometric studies indicate that during mineralization temperature ranged between 360 and 240 °C, and fluid pressure between 3,600 and 1,600 bar. The data are interpreted in terms of the cyclic fault-valve mechanism for active shear zones. Deposition of gold and sulfides has been studied on the basis of constraints from the composition of wall-rock chlorite, ore-mineral assemblages, and textural features. Tubular channels, 20 to 100 µm wide and up to 500 µm long that arise from fractures and C-planes in sheared quartz veins are reported for the first time. The channels have pyrrhotite, arsenopyrite, pyrite and gold at their distal ends, with calcite filling up the remaining part. These channels form in response to increases in T and P, by dissolution of quartz grains, guided by dislocations in them. At the P– T conditions of interest, gold and sulfide deposition takes place in the shears and fractures of quartz veins from CO ₂–H ₂O–NaCl ore fluid of low salinity and pH due to changes in phase compositions that occur during the process of shear failure of the enclosing rocks. In the wall rock where pH is buffered, gold deposition takes place from the predominant Au(HS) ₂ ^- species with progressive sulfide deposition and decrease in SS, from 0.01 to 0.001 mol/kg as T falls from 360 to 240 °C.  相似文献