• Volume 92, Issue 2

      July 1983,   pages  99-215

    • Temperature variation in the mesosphere during solar eclipse

      P Chakrabarty D K Chakrabarty A K Saha

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      Rocket-borne observations of the extinction of solar hydrogen Lyman-alpha radiation made during the solar eclipses of 20 May 1966 at Karistos for normal and 44% visibility and of 7 March 1970 at East Quoddy for normal, 10% and 0.6% visibility have been used to study the variation of temperature in the mesosphere during these events. It is seen that near the mesopause the decrease of temperature at Karistos is by 20° K for 44% visibility and at East Quoddy by 100° K for 0.6% visibility. Possible causes of these temperature variationsvis-a-vis molecular oxygen and pressure variations have been briefly discussed.

    • Application of semi-implicit scheme of integration to barotropic prediction

      S S Singh A Bandopadhyay

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      Application of a semi-implicit version of primitive equation barotropic model to predict monsoon depressions is proposed. Forecasts upto 48 hr have been quite satisfactory. The results are compared with those obtained from an explicit version of primitive equation barotropic model developed earlier. The flow patterns from these two versions are similar with few exceptions in isolated pockets, however, the forecast movement obtained from the semi-implicit version is comparable or slightly superior to those obtained from the explicit version. The computational time in semi-implicit and explicit schemes is approximately in the ratio of 1∶2.

    • Stability of a stationary Rossby wave embedded in barotropic monsoon zonal flow

      S K Dash R N Keshavamurty

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      Barotropic stability of a stationary Rossby wave of wavelength 30° longitude superposed on the uniform monsoon zonal flow has been examined. The wave is unstable to perturbations and the growth rate depends on the meridional scale. These perturbations grow by drawing on the kinetic energy of the stationary Rossby wave.

    • Recent foraminifers from the inner shelf of the central West Coast, India: A reappraisal using factor analysis

      Rajiv Nigam Jorn Thiede

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      Setty and Nigam (1980) had described 72 species of benthonic foraminifers from 25 inner shelf stations off central West Coast of India and the results showed somewhat patchy and anamolous distributions. This paper presents the results ofQ-mode factor analysis, which was applied to reduce the number of variables into assemblages. The analysis reveals 4 important foraminiferal assemblages.Ammonia beccarii—Ammonia annectens assemblage,Nonion boueanum—Florilus scaphum assemblage.Trochammina inflata assemblage andBulimina exilis assemblage. They can be related to freshwater run-off and organic matter contents of the sediment.

    • Petrochemistry of granite and granophyres of the Ezhimala igneous complex, Kerala, India

      N G K Nair M Santosh

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      Anorthosite-gabbro-granite-granophyre suite of rocks occurring in the Precambrian migmatitic gneiss of Cannanore district, Kerala and spatially related to the Bavali lineament, constitute the Ezhimala igneous complex (EIC). The granophyres exhibit typical cuneiform and radiating fringe-type intergrowths. Geochemistry indicates a tonalitic—trondhjemitic to granitic composition of the residual magma. Petrochemical characters and lithologic association suggest that the EIC represents a complete differentiation sequence derived by strong fractionation and crystal cumulation of a parent basaltic magma which followed a tholeiitic differentiation trend, with no significant crustal participation as implied by the low initial strontium isotope levels. Occurrence of the complex along the Bavali lineament, which is a major rift-zone, emphasises a rift-controlled magmatism in this part of the Indian shield.

    • Lithological subdivision and petrology of the Great Himalayan Vaikrita Group in Kumaun, India

      K S Valdiya O P Goel

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      The Vaikrita Group made up of coarse mica-garnet-kyanite and sillimanite-bearing psammitic metamorphics constituting the bulk of the Great Himalaya in Kumaun is divisible into four formations, namely theJoshimath comprising streaky, banded psammitic gneisses and schists, the Pandukeshwar consisting predominantly of quartzite with intercalations of schists, thePindari made up of gneisses and schists with lenses of calc-silicate rocks and overwhelmingly injected by Tertiary pegmatites and granites (Badrinath Granite) leading to development of migmatites, and theBudhi Schist comprising biotite-rich calc-schists. The Vaikrita has been thrust along the Main Central Thrust over the Lesser Himalayan Munsiari Formation made up of highly mylonitized low-to meso-grade metamorphics, augen gneisses and phyllonites.

      Petrological studies demonstrate contrasting nature of metamorphism experienced by the Vaikrita and the Munsiari rocks. Sillimanite-kyanite-garnet-biotite-muscovite (±K-feldspar and ± plagioclase).—quartz metapelites and interbanded calc-schists and calc-gneisses with mineral assemblages of calcite-hornblende-grossular garnet, labradorite (An50−An65), (± K-feldspar)-quartz (± biotite), and hornblende-diopside ± labradorite ± quartz, suggest medium to high grade of metamorphism or indicate upper amphibolite facies experienced by the rocks of the Vaikrita Group. The associated migmatites, granite-gneisses and granites of the Pindari Formation were formed largely as a result of anatexis of metapelites and metapsammites. While, the sericite-chlorite-quartz and muscovite-chlorite-chloritoid-garnet-quartz, assemblages in metapelites and epidote-actinolite-oligoclase (An20)-quartz and epidote-hornblende-andesine (An29) ± quartz in the metabasites suggest a low-grade metamorphism (greenschist facies) for the Munsiari Formation, locally attaining the lower limit of medium-grade (epidote-amphibolite) facies.

      The inferred P-T conditions obtained from textural relations of various mineral phases and the stability relationship of different coexisting phases in equilibrium, suggest that the temperature ranged between 600° and 650° C and pressure was over 5 kb for the Vaikrita rocks. The mineral assemblages of the Munsiari Formation indicate comparatively lower P-T conditions, where the temperature reached approximately 450° C and pressure was near 4 kb.

      The rocks of the two groups were later subjected to intense shearing, cataclasis and attendant retrograde metamorphism within the zone of the Main Central (=Vaikrita) Thrust.

    • Very low frequency electromagnetic prospecting method—A few field applications in India

      I B Rama Prasada Rao R Nagendra S L Narasimha Rao J Venkatramaiah

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      The very low frequency electromagnetic prospecting equipment measures different space and phase components of the electromagnetic field in the frequency range 15–25 kHz emanating from distant naval radio stations. The field application of the equipment designed and constructed for this purpose has demonstrated the usefulness of the investigations in groundwater and geological mapping. The amplitude measurements necessitate correction of the diurnal variations of the electromagnetic field at the observation point.

    • Interpretation of gravity anomalies due to some two-dimensional structures: A Hilbert transform technique

      N Sundararajan N L Mohan S V Seshagiri Rao

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      An interpretation procedure is formulated using the Hilbert transform for analysing the gravity effect of (a) two-dimensional horizontal circular cylinder, (b) semi-infinite thin fault block, and (c) a geologic contact. In all three cases the abscissa of the point of intersection of the gravity anomaly curve or the horizontal derivative curve and its Hilbert transform yields directly the depth of the body. The proposed method is tested on theoretical models. Also, the method is applied to the field data of a geologic contact taken over the Himalayan foothills across the Brahmaputra valley.

    • SyntheticP wave seismograms from Longshot, Milrow and Cannikin explosions

      A R Bangiar

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      The effect of spalling and other transfer functions is considered on the amplitude spectra and waveforms of telese mis short periodP wave seismograms generated due to Longshot. Milrow and Cannikin underground nuclear explosions. Source models of Haskell and Seggern were used for computingP wave seismograms from these events. Step-by-step variations of the amplitude spectra and waveforms ofP wave signal as is propagates from source to various seismometer array stations show that the source function, the source crust transfer function, the mantle transfer function and the spalling function are the most important factors that influence shaping of spectra and waveforms ofP seismograms. Comparison of computed and observedP wave seismograms at various seismometer array stations for Longshot explosion shows that computed seismograms do reproduce many of the features of observed seismograms. The results also support the conclusion that overburdenP wave velocity inferred from observed seismograms is less as compared to its value obtained from on-site measurements.

    • Source time function and spectra for underground explosions

      A R Banghar

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      Constants occurring in Seggern and Blandford's analytic expression of reduced displacement potential are computed for shot media of alluvium, salt and tuff. The amplitude spectra as well as impulse response of the source function for models of Haskell and Seggern are studied as a function of yield. It is shown that the maximum in amplitude spectrum shifts to lower frequency with increase in yield for a given model. The duration of impulse response is longer for Seggern's model as compared to Haskell's model for a given yield.

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