A panoptic view of western margin of Sundaland: Causes of seismic vulnerability of Sumatra     

Abhijit Deonath  Basab Mukhopadhyay 《Journal of the Geological Society of India》2013,81(5):637-646

The western margin of Sundaland is affected by two major tectonic events, the India-Eurasia collision and subduction along the Burmese-Andaman-Sunda Arc. Geotectonic analysis of the Burmese-Andaman-Sunda Arc areas reveals that it represents an arc setting exposed both in land and sea environment. Within this broad arc setting, there are individual segments that illustrate discernible trends in terms of structural, seismic and geological characteristics. The Equatorial Region around Sumatra has some peculiar geological/geophysical characteristics that make it a tectonic hotspot. Here a relationship between the active volcanism and the development of forearc basins is established. It is also proposed that the downgoing oceanic plate is more strongly coupled to the overlying plate where it is youngest (～ 40 Ma), has the highest temperature and is topographically most elevated with highest seismic activity. In particular, an increase in convergence rate and presence of youngest oceanic crust appear to be the main controlling factor underpinning the tectonics and surge of recent seismic activity in Sumatra.  相似文献   

Variation in groundwater levels and water quality in Chhatna Block, Bankura district, West Bengal — a GIS approach     

S. K. Nag  P. Ghosh 《Journal of the Geological Society of India》2013,81(2):261-280

Ground water levels and quality in Chhatna Block of West Bengal were studied based on different indices for irrigation and drinking purposes. A detailed hydrogeological investigation was carried out to have an overall idea of the aquifer system of the area. The ground water occurs under shallow to moderately deep water table condition. The groundwater is stored mostly in the weathered residuum and fractured — hard rock. Sodium absorption ratio (SAR), soluble sodium percentage (SSP), residual sodium bi-carbonate (RSBC), electrical conductance (EC), magnesium adsorption ratio (MAR) Kelly’s ratio (KR), total hardness (TH), permeability index (PI) were calculated as derived parameters, to investigate the ionic toxicity. From the results of chemical analysis, it was revealed that the values of Sodium Adsorption Ratio indicate that, ground water of the area falls under the category of low sodium hazard. So, there was neither salinity nor toxicity problem of irrigation water, and hence the ground water can safely be used for long-term irrigation. Plotting of analytical results of the groundwater collected from different areas in Piper’s trilinear diagram (1944) indicate that, the waters of the study area fall under fresh and sulphate rich region of the rhombus.  相似文献   

Petromineralogic and rock magnetic aspects of clastic sedimentation in the Surma basin, Mizoram     

A. G. Badekar  S. J. Sangode  Sumit K. Ghosh  R. P. Tiwari  D. C. Meshram  J. Malsawma  P. Lalnuntluanga 《Journal of the Geological Society of India》2013,82(1):23-37

We present here comprehensive petromineralogic, mineral magnetic and lithologic observations from five stratigraphic sections representing the Barail Group, Middle and Upper units of Bhuban Formation, Bokabil Formation and Tipam Group in the Mizoram area. These stratigraphic units mainly display interplay of the grey and buff colored sandstones of the clast compositions varying from sub-lithic to lithic arenites (Q₇₉F₄L₁₇ to Q₅₅F₃L₄₂) including sedimentary, meta-sedimentary and subordinate igneous rock fragments. The buff sandstones with higher lithic fragments [Q₆₉F₄L₂₇ (Ls₆₁Lm₃₈Lv₁)] are dominated by recycled components with higher clast angularity (VA₁A₁₅SA₅₁SR₂₄R₈WR_0.4) relative to the grey sandstones. Mineral magnetic studies decipher bimodal (ferri- and antiferromagnetic) mineralogy with higher concentration in buff sandstones relative to the unimodal ferrimagnetic nature of the grey sandstones. The study infers that the buff sandstones mark the regressive phases driven by hinterland uplifts; whereas the growth of the grey sandstone facies is marked by transgressive basinal processes. Gradual increase in the frequency and appearance of the buff sandstones in the Surma stratigraphy, therefore, can be related to the evolution of the Indo-Burmese ranges.  相似文献   

Analysis of long-term ozone trend over Delhi and its meteorological adjustment   总被引：1，自引：0，他引：1  

S. Kumari  G. Jayaraman  C. Ghosh 《International Journal of Environmental Science and Technology》2013,10(6):1325-1336

Present study deals with the statistical analysis of long-term ground level ozone (O₃) trend and the influence of meteorological variables on its variation over Delhi, India. Daily mean and maximum of O₃ and meteorological data, obtained from India Meteorological Department, were arranged for the period of 9 years (1998–2006). Based on the preliminary correlation study of all the data with O₃, six variables viz. daily maximum temperature, daily average relative humidity, dew point, wind speed, visibility, and total sunshine were selected. Classical additive time series decomposition technique was used to obtain seasonally adjusted long-term trend. To analyze the masking effect of meteorology, adjustment was made using Kolmogorov–Zurbenko filters followed by stepwise regression analysis to the smoothed series of O₃ maximum and meteorological variables, which showed that long-term trend was independent of sunshine duration. Results indicate a significant increasing trend with annual increase of 1.13 % for O₃ mean and 3 % for O₃ maximum. Annual deseasonalized trend for seasonal cycle shows bimodal oscillations. About 43 % of O₃ variation was explained by the selected meteorological factors and rest of variation attributed to factors like emission of precursor gases, pollutant transport, policy changes, etc. Among the three tested regression models, performance of Model 2 with variable temperature, wind speed, and visibility was found to be best that resulted in lowering of O₃ trend. Large variability (23 %) was explained by the variable visibility depicted that the emission of primary pollutants not only provides the precursor gases but also control the local photochemical reactions.  相似文献   

Groundwater discharge along a channelized Coastal Plain stream   总被引：1，自引：1，他引：0  

Danita M. LaSage  Joshua L. Sexton  Abhijit Mukherjee  Alan E. Fryar  Stephen F. Greb 《Journal of Hydrology》2008,360(1-4):252-264

In the Coastal Plain of the southeastern USA, streams have commonly been artificially channelized for flood control and agricultural drainage. However, groundwater discharge along such streams has received relatively little attention. Using a combination of stream- and spring-flow measurements, spring temperature measurements, temperature profiling along the stream-bed, and geologic mapping, we delineated zones of diffuse and focused discharge along Little Bayou Creek, a channelized, first-order perennial stream in western Kentucky. Seasonal variability in groundwater discharge mimics hydraulic-head fluctuations in a nearby monitoring well and spring-discharge fluctuations elsewhere in the region, and is likely to reflect seasonal variability in recharge. Diffuse discharge occurs where the stream is incised into the semi-confined regional gravel aquifer, which is comprised of the Mounds Gravel. Focused discharge occurs upstream where the channel appears to have intersected preferential pathways within the confining unit. Seasonal fluctuations in discharge from individual springs are repressed where piping results in bank collapse. Thereby, focused discharge can contribute to the morphological evolution of the stream channel.  相似文献   

Estimation of hysteretic energy demand using concepts of modal pushover analysis   总被引：1，自引：0，他引：1  

Tholen Prasanth  Siddhartha Ghosh  Kevin R. Collins 《地震工程与结构动力学》2008,37(6):975-990

Hysteretic energy dissipation in a structure during an earthquake is the key factor, besides maximum displacement, related to the amount of damage in it. This energy demand can be accurately computed only through a nonlinear time‐history analysis of the structure subjected to a specific earthquake ground acceleration. However, for multi‐story structures, which are usually modeled as multi‐degree of freedom (MDOF) systems, this analysis becomes computation intensive and time consuming and is not suitable for adopting in seismic design guidelines. An alternative method of estimating hysteretic energy demand on MDOF systems is presented here. The proposed method uses multiple ‘generalized’ or ‘equivalent’ single degree of freedom (ESDOF) systems to estimate hysteretic energy demand on an MDOF system within the context of a ‘modal pushover analysis’. This is a modified version of a previous procedure using a single ESDOF system. Efficiency of the proposed procedure is tested by comparing energy demands based on this method with results from nonlinear dynamic analyses of MDOF systems, as well as estimates based on the previous method, for several ground motion scenarios. Three steel moment frame structures, of 3‐, 9‐, and 20‐story configurations, are selected for this comparison. Bias statistics that show the effectiveness of the proposed method are presented. In addition to being less demanding on the computation time and complexity, the proposed method is also suitable for adopting in design guidelines, as it can use response spectra for hysteretic energy demand estimation. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

The structure of black hole magnetospheres I. Schwarzschild black holes     

Pranab Ghosh 《Monthly notices of the Royal Astronomical Society》2000,315(1):89-97

We introduce a multipolar scheme for describing the structure of stationary, axisymmetric, force-free black hole magnetospheres in the '3+1' formalism. We focus here on Schwarzschild spacetime, giving a complete classification of the separable solutions of the stream equation. We show a transparent term-by-term analogy of our solutions with the familiar multipoles of flat-space electrodynamics. We discuss electrodynamic processes around disc-fed black holes in which our solutions find natural applications: (i) 'interior' solutions in studies of the BlandfordZnajek process of extracting the rotational energy of holes, and of the formation of relativistic jets in active galactic nuclei and 'microquasars'; (ii) 'exterior' solutions in studies of accretion disc dynamos, disc-driven winds and jets. On the strength of existing numerical studies, we argue that the poloidal field structures found here are also expected to hold with good accuracy for rotating black holes, except for the cases of the maximum possible rotation rates. We show that the closed-loop exterior solutions found here are not in contradiction with the MacdonaldThorne theorem, as these solutions, which diverge logarithmically on the horizon of the hole , only apply to those regions that exclude .  相似文献   

Shear waves in elastic medium with void pores welded between vertically inhomogeneous and anisotropic magnetoelastic semi-infinite media     

Shishir Gupta  Mostaid Ahmed  Abhijit Pramanik 《Acta Geophysica》2017,65(1):139-149

The paper intends to study the propagation of horizontally polarized shear waves in an elastic medium with void pores constrained between a vertically inhomogeneous and an anisotropic magnetoelastic semi-infinite media. Elasto-dynamical equations of elastic medium with void pores and magnetoelastic solid have been employed to investigate the shear wave propagation in the proposed three-layered earth model. Method of separation of variables has been incorporated to deduce the dispersion relation. All possible special cases have been envisaged and they fairly comply with the corresponding results for classical cases. The role of inhomogeneity parameter, thickness of layer, angle with which the wave crosses the magnetic field and anisotropic magnetoelastic coupling parameter for three different materials has been elucidated and represented by graphs using MATHEMATICA.  相似文献   

Lithology-dependent seismic anisotropic amplitude variation with offset correction in transversely isotropic media     

Amir Abbas Babasafari  Deva Prasad Ghosh  Ahmed Mohammed Ahmed Salim  Masoumeh Kordi 《Geophysical Prospecting》2020,68(8):2471-2493

Analysis of amplitude variation with offset is an essential step for reservoir characterization. For an accurate reservoir characterization, the amplitude obtained with an isotropic assumption of the reservoir must be corrected for the anisotropic effects. The objective is seismic anisotropic amplitude correction in an effective medium, and, to this end, values and signs of anisotropic parameter differences (Δδ and Δε) across the reflection interfaces are needed. These parameters can be identified by seismic and well log data. A new technique for anisotropic amplitude correction was developed to modify amplitude changes in seismic data in transversely isotropic media with a vertical axis of symmetry. The results show that characteristics of pre-stack seismic data, that is, amplitude variation with offset gradient, can be potentially related to the sign of anisotropic parameter differences (Δδ and Δε) between two layers of the reflection boundary. The proposed methodology is designed to attain a proper fit between modelled and observed amplitude variation with offset responses, after anisotropic correction, for all possible lithofacies at the reservoir boundary. We first estimate anisotropic parameters, that is, δ and ε, away from the wells through Backus averaging of elastic properties resulted from the first pass of isotropic pre-stack seismic inversion, on input data with no amplitude correction. Next, we estimate the anisotropic parameter differences at reflection interfaces (values and signs of Δδ and Δε). We then generate seismic angle gather data after anisotropic amplitude correction using Rüger's equation for the P-P reflection coefficient. The second pass of isotropic pre-stack seismic inversion is then performed on the amplitude-corrected data, and elastic properties are estimated. Final outcome demonstrates how introduced methodology helps to reduce the uncertainty of elastic property prediction. Pre-stack seismic inversion on amplitude-corrected seismic data results in more accurate elastic property prediction than what can be obtained from non-corrected data. Moreover, a new anisotropy attribute (ν) is presented for improvement of lithology identification.  相似文献   

Studies on Schumann Resonance Phenomena and Some Recent Advancements     

Abhijit Ghosh  Biswas  Debasish  Hazra  Pranab  Guha  Gautam  De  S. S. 《Geomagnetism and Aeronomy》2019,59(8):980-994

Geomagnetism and Aeronomy - Discrete spectra of frequencies at 8, 14, 20, 26, … Hz are generated by electromagnetic emission from lightning sources and can be regarded as excitation of AC...  相似文献