Horizontal geomagnetic field variations at low latitude in South Atlantic Magnetic Anomaly     

N. D. Kaushika  N. B. Trivedi 《Pure and Applied Geophysics》1972,99(1):99-107

Summary Horizontal geomagnetic field variations on international quiet days at a low latitude station situated in the South Atlantic Magnetic Anomaly are studied with reference to the problem whether these variations are caused by the daytime increases or by the night time depressions of the field. It is found that both the mechanisms are operative significantly. The correction for noncyclic change decreases the magnitude of the night time depression. However, it does not account fully for the same. Possible causes for the night depressions are also discussed.  相似文献   

Mechanisms of bottom boundary fluxes in a numerical model of the Shetland shelf     

Arjav Trivedi  Ralf Toumi 《Ocean Dynamics》2017,67(1):1-21

Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect this region to deeper waters. Two proposed ways in which across-slope BBL fluxes can occur, in regions that have a slope current aligned to the bathymetry, are the frictional veering of bottom currents termed the ‘Ekman drain’ and through local wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom flow). We investigate the variability, magnitude and spatial scale of BBL fluxes on the Shetland shelf, which has a prominent slope current, using a high-resolution (～2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at the shelf break near the 200 m isobath and are found to have a seasonal variability with high/low volume transport in winter/summer respectively. By using a multivariate regression approach, we find that the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus mean-flow flux. We estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow, which has a much shorter decorrelation length compared to the Ekman flux. We conclude that both the speed and direction of the interior current determines the daily BBL flux. The wind does not explicitly contribute through local downwelling, but may influence the interior current and therefore implicitly the BBL fluxes on longer timescales.  相似文献   

Holocene vegetation and climate fluctuations in northwest Himalaya,based on pollen evidence from Surinsar Lake,Jammu region,India     

Anjali Trivedi  M. S. Chauhan 《Journal of the Geological Society of India》2009,74(3):402-412

Pollen analysis of a 33.21 m deep sediment core from Surinsar lake in Jammu region has revealed that between 9,500 and 7,700 yr BP (Pollen zone SL-I), the mixed oak-broad-leaved/chirpine forest occurred in the region under a warm and humid climate. The record of aquatic plants viz. Potamogeton, Typha and freshwater alga Botryococcus in appreciable numbers denotes the existence of the lake since the Early Holocene. Subsequently, mixed chirpine/oak-broad-leaved forests appeared in the area around 7,700 to 6,125 yr BP (Pollen zone SL-II) with the dominance of chirpine (Pinus cf. roxburghii) by the onset of cool and dry climate, attributed to decrease in monsoon rainfall. The expansion of oak and its broad-leaved associates between 6,125 and 4,330 yr BP (Pollen zone SL-III) suggests that the climate turned moderately humid and warm, presumably due to enhanced monsoonal effect. The region has witnessed a brief spell of pluvial environment between 4,330 and 4,000 yr BP (Pollen zone SL-IV) as evidenced by the presence of sandy layer at 15.4–14 m depths. The period of 4,000 to 2,100 yr BP (Pollen zone SL-V) is marked by the prevalence of cool and dry climate as depicted by a sharp decline in oak (Quercus cf. incana) and other broad-leaved taxa and a concurrent increase in chirpine. From 2,100 to 800 yr BP (Pollen zone SL-VI) no palaeofloristic inferences could be drawn due to paucity of pollen, however, the presence of sandy deposit at the corresponding level in lithocolumn implies a pluvial episode by this time. Since 800 yr BP to Present (Pollen zone SL-VII) the slight advance in the oak reflects the ameliorating trend of climate, despite the existing cool and dry climate.  相似文献   

Reservoir Geological Uncertainty Reduction: an Optimization-Based Method Using Multiple Static Measures     

Shahed Rahim  Zukui Li  Japan Trivedi 《Mathematical Geosciences》2015,47(4):373-396

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Major ion chemistry of the Ganga source waters: Weathering in the high altitude Himalaya   总被引：2，自引：0，他引：2  

M. M. Sarin  S. Krishnaswami  J. R. Trivedi  K. K. Sharma 《Journal of Earth System Science》1992,101(1):89-98

A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO₃ and SO₄ are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations. The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km²/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.  相似文献   

Characterization of Non-Gaussian Geologic Facies Distribution Using Ensemble Kalman Filter with Probability Weighted Re-Sampling     

Siavash Nejadi  Juliana Leung  Japan Trivedi 《Mathematical Geosciences》2015,47(2):193-225

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Particle precipitation in Brazilian geomagnetic anomaly during magnetic storms     

N.B. Trivedi  D.B. Rai  I.M. Martin  J.M. Da Costa 《Planetary and Space Science》1973,21(10):1699-1704

Particle precipitation in Brazilian geomagnetic anomaly during magnetic storms is investigated using riometer and VLF propagation data. It is found that during large storms the changes in the ionosphere caused by particle precipitation are detectable. There is a good correlation between the behavior of the absorption and the variations of the magnetic field intensity during different phases of a storm. In particular, there seems to be a close relationship between the precipitation of high energy particles and short-period fluctuations of the magnetic field intensity of the order of 5–6 min. During the main phase of the storm, when the field intensity reaches its minimum, the flux of soft electrons also plays a significant role in producing absorption. The nature of precipitation associated with a sudden commencement appears to be more complex; the predominance of low or high energy particle flux may depend on the magnitude of the field increase. The amplitude and phase records of VLF signals also show the effect of the disturbance, but it is difficult to correlate the changes in these records with the features observed on the magnetogram, because only a small part of the propagation path lies in the region of the anomaly. A more detailed analysis of riometer data from different stations and VLF phase and amplitude records for different paths will be helpful in understanding the mechanism of particle precipitation associated with magnetic disturbances. In future experiments it may also be fruitful to look for detectable radiation emitted by the precipitating electrons, for example, Cherenkov and synchrotron radiation.  相似文献   

Estimation of drift limits for different seismic damage states of RC frame staging in elevated water tanks using Park and Ang damage index     

Lakhade  Suraj O.  Kumar  Ratnesh  Jaiswal  O. R. 《地震工程与工程振动(英文版)》2020,19(1):161-177

Damage to elevated water tanks in past earthquakes can be attributed to the poor performance of their supporting frame staging. In order to ascertain the performance of these elevated water tanks, it is crucial to categorize the damage in quantifiable damage states. Among various parameters to quantify the damage states, the top drift of frame staging can be conveniently correlated to the different damage levels. In literature, drift limits corresponding to different damage states of the frame staging of the elevated water tank are not available. In the present study, drift limits for RC frame staging in elevated water tanks corresponding to different seismic damage states have been proposed. Various damage states of the elevated water tank have been determined using the Park and Ang damage index. The Park and Ang damage index utilizes results of both pushover analysis and incremental dynamic analysis. Twelve models of elevated water tanks have been developed considering variation in staging height and tank capacity. Incremental dynamic analysis has been performed using the suite of twelve actual earthquake ground motions. Based on the regression analysis between damage indexes and drift, limiting drift values for each damage state are proposed.  相似文献   

Kerr — Newman metric in cosmological background     

L. K. Patel  Hiren B. Trivedi 《Journal of Astrophysics and Astronomy》1982,3(1):63-67

A new solution of Einstein-Maxwell field equations is presented. The material content of the field described by this solution is a perfect fluid plus sourceless electromagnetic fields. The metric of the solution is explicitly written. This metric is examined as a possible representation of Kerr-Newman metric embedded in Einstein static universe. The Kerr-Newman metric in the background of Robertson-Walker universe is also briefly described.  相似文献   

Petrographic criteria for establishing the copper potential in granitoid plutons     

M. S. Naik  R. K. Trivedi 《Journal of Earth System Science》1987,96(3):221-227

A petrographic investigation and its possible relevance in mineral exploration have been studied for a mineralized granitoid pluton in Malanjkhand. Using a matrix of mineralogical, textural and structural features, the types and intensities of mineralogical changes by post-magmatic metasomatic processes can be deduced from thin sections of the rocks. The resulting petrological score provides a measure of establishing the copper potential of a granitoid pluton.  相似文献