• Volume 93, Issue 2

      July 1984,   pages  83-176

    • Scintillations ofvhf radio beacons from two satellites

      R G Rastogi

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      Analysis of scintillations ofvhf beacons from two very closely spaced geostationary satellites shows that the drift of irregularities is generally westward at the initial stages and changes to eastward during the later part of the night when the irregularities are fully developed or decaying.

    • Ozone in the mesosphere and lower thermosphere

      L S Hingane

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      One-dimensional photochemical diffusive model has been used to obtain the altitude and temporal variation of O3 in the range of 50 to 150km height. More than 60 chemical reactions and photo-dissociatons of oxygen-hydrogen atmosphere are incorporated. Their contribution in the formation or distribution directly or indirectly, is critically examined.

      In the altitude profile of O3 concentration, it is observed that there is slight increase in concentration around 85km which is due to the increase in production rate at that height through the following reaction as: O+O2+M→O3+M (M=total atmospheric concentration) and the sharp decrease above 90km is also through the same reaction. Two altitude profiles of ozone concentration, one from photochemical equilibrium and other from non-equilibrium model are obtained. It is seen that in the mesosphere these two profiles are comparable. Further it is also seen that the average night time concentration is more than that in the day. Results obtained are compared with experimental values.

    • Zonal winds and temperature structure at the upper levels during poor and good monsoon

      L S Hingane

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      Global analyses of zonal wind field and thermal field structure at standard pressure levels of 200,150 and 100 mb have been carried out in India during July 1979—a poor monsoon year and July 1975—a good monsoon year. More than 250 stations in the belt 60°N and 60°S were selected. Contrasting features of the zonal wind field structure and thermal field are brought out, and it is shown that monsoon activity is reflected in the upper level and is controlled by planetary scale.

    • Wind dependence of quasi-specular sea scatter

      Abhijit Sarkar Lakshmi Bhaduri

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      Earlier investigations have shown that a power law relationship of the typeσ=aWb exists between the scattering coefficientσ and sea surface wind speedW at microwave frequencies. The coefficientsa andb are usually quoted for different frequencies, look angles, polarizations etc. This paper attempts to define the angular dependence using such a power law relationship in the quasi-specular range.

    • Measurements of reflection coefficients of stratified layers using X-band bistatic scatterometers

      K K Jha K P Singh R N Singh

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      Laboratory modelling of earth’s subsurface stratification has been carried out using X-band microwave bistatic scatterometer system. Look angle variation of reflectivity for various subsurface layers under dry and wet conditions have been measured. From measured reflectivity data and the reciprocity theorem the emissivity and the brightness temperature variations have been computed, and the data are in good agreement with reported results. The importance of laboratory and field measurements and its remote sensing application has been discussed.

    • Migration of the dip equator in the Indian region

      AVS Murty K Ahmed D R K Rao

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      The position of the dip equator (de) is worked out in the Indian region based on fairly close geomagnetic measurements made during a survey in the year 1981. Thede is located about 24 km south of its 1971 position along the 77·5°E meridian. Also, thede positions for the epochs 1971 and 1981 show closeness on the east coast of India when compared to its west coast positions. Thede positions inferred from the repeat observations since 1909 are used for estimating the pattern of migration and the results are discussed.

    • Isotopic and related studies of Antarctic ice samples

      N Bhandari S K Bhattacharya V N Nijampurkar D Sengupta V G Shah A K Singhvi P N Shukla K M Suthar C P Vohra

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      The ice samples obtained from Dakshin Gangotri, Antarctica show the presence of nuclear debris, attributed mainly to French nuclear explosions. Cosmogenic7Be occurs at levels of 30 dpm/L. The vertical profile ofδD in 6 m long drill core ranges between −130 and −180‰ compared to Standard Mean Ocean Water (SMOW). No systematic change with depth is seen. Small amounts of dust obtained by filtering melt water show presence of metallic spherules. Absence of elements characteristic of meteoritic or cometary debris suggests that most of them are of volcanic or industrial origin.

    • Geochemical analyses of termite mounds as a prospecting tool for tin deposits in Bastar, MP—a preliminary study

      K Surya Prakash Rao S V Raju

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      Tin deposits in the form of cassiterite occur in precambrian zoned pegmatite bodies intrusive into Archean metabasic and metasedimentary rocks in Bastar, MP. The associated economic minerals include columbite-tantalite, lepidolite, beryl etc. As a major part of the area is covered by a thick overburden, termite mounds have been used as geochemical sampling medium in view of their significant potentiality in tropical areas with overburden, and also because of the low cost and ease of sampling. Though many elements have been used as pathfinders for tin, tin itself acts as the best indicator of its deposits.

      The present work has given encouraging results, and therefore can be applied for tin prospecting and possibly for heavy metals like Nb, Ta, W, etc., in this and other areas where termite mounds have a good spatial distribution.

    • Alkali granite-syenite-carbonatite association in Munnar Kerala, India; implications for rifting, alkaline magmatism and liquid immiscibility

      N G K Nair M Santosh P K Thampi

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      Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO2, Al2O3 and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K2O, K2O/Na2O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.

    • Magnetometric surveys over some iron-ore occurrences in the Eastern Ghats belt of Godavari Districts, Andhra Pradesh

      V Bhaskara Rao A Lakshmipathi Raju

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      Iron-ore with magnetite as the chief ore mineral occurs in the Precambrians of Eastern Ghats. Vertical magnetometric surveys were carried out to delineate some of the ore bands, in the Ghats belt of Godavari Districts, Andhra Pradesh, around Addatigala, Devipatnam and Tekuru. Interpretation of the magnetic anomalies was based on the tabular models. Ore bands are at shallow depths of around 5m. The deduced inclinations of magnetisation suggest that the magnetisation is largely remanent. The intensities of magnetisation are in the range of 5 to 10·5×10−3 emu and agree well with the laboratory measurements on the ore samples. At Devipatnam and Tekuru the magnetic background seems to be high. At Tekuru the ore band appears to be very limited in depth extent. The limited depth extent coupled with high magnetic background explains the anomaly which is not so prominent. It is concluded that in such areas, it is only the high grade magnetite ore bands of considerable depth extent that can be successfully delineated by the magnetic method.

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