• Volume 100, Issue 1

      March 1991,   pages  1-104

    • Radon modelling and the heat transfer surface area of the British hot dry rock geothermal reservoir

      N Hussain

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      A simple thermal model is developed to evaluate the heat exchange surface area of a hot dry rock geothermal reservoir. This model, in conjunction with the Rn model of Andrews and coworkers, is applied to RH12/RH15 system of the British HDR reservoir. Results suggest that although the estimated Rn transfer surface area represents the actual swept surface area, it is between 25 and 45 times larger than the heat transfer surface area. The difference is explained as due to clustering of several fractures within a range of thermal interaction, over the duration of the circulation. It is also shown that the decline in the measured temperatures of circulation fluids since 1985 to the present is consistent with the heat exchange surface area going through a maximum in 1987.

    • Copper-bearing clay minerals of the oxidized zone of the Rakha-Chapri Block, Singhbhum Copper Belt, India

      S G Tenginkai A G Ugarkar M V Koti A Mookherjee

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      In the oxidized zone of Rakha-Chapri Block of the Singhbhum Copper Belt, alteration of biotite, chlorite and muscovite extends down to ∼ 60 m. Below this level, these minerals are not altered, implying a supergene origin for the clay alteration products. The altered host-rock profile consists of an upper, predominantly kaolinitic zone and a lower illite-chlorite rich zone, with the clay minerals showing an overall tendency to decrease with depth. Kaolinite is the dominant clay mineral, the proportion of which varies considerably with depth, and chlorite, illite and halloysite are the other clay minerals of the oxidized zone. Incipient removal of copper even from the cap rocks, in-situ transformation of sulphides to oxidized compounds, and the unusual mode of occurrence of copper in the oxidized zone are the characteristic features of the Rakha-Chapri Block. Insufficient localized hydrolysis of silicates is considered responsible for relatively low acidity in the oxidized zone as a whole. Copper forms a component of the clay minerals probably as surface adsorbed or/lattice-bound ions.

    • Relationship between shear zones and igneous activity: The Closepet Granite of Southern India

      M Jayananda B Mahabaleswar

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      The late Archaean Closepet granite of southern India is bounded by N-S trending shear zone. At the southern end of the granite both charnockite and granite veins are spatially associated with ductile shears. These shears continue further north and are confined to the contact zones in the central part of the granite outcrop. The main component of the shear zone are highly deformed granite sheets, augen gneisses and mylonites. Field observations and microstructural fabric of mylonites indicate a dextral sense of shear movement. Field evidence suggests that shear deformation was active throughout the evolution of the Closepet granite

    • Standard thermodynamic data for Rhodochrosite from equilibrium decomposition curve

      Ali Reza Fazeli J A K Tareen B Basavalingu G T Bhandage

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      Hydrothermal equilibrium decomposition curve for MnCO3⇌MnO + CO2 in the total CO2 pressure range of 100–1700 bars and temperature range of 500–800°C was studied. The standard thermodynamic data obtained are: ΔH0f= − 894.382 ± 0.74 kj/mol and ΔG0f = − 822.170 ± 0.74 kj/mol. These values are more negative than the reported calorimetric data.

    • Sensitivity of launch systems to gravity field

      S P Satyabala D Guptasarma V K Gaur

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      This paper considers the question of how sensitive inertially guided systems are to variations in the gravity field. There are systems which use other kinds of information for guidance, such as terrain. But, they will not be considered here.

    • Variations in the strength of recurrent geomagnetic activity in solar cycles 11 to 21

      G K Rangarajan

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      From maximum entropy spectral analysis (MESA) of short lengths ofAa indices of geomagnetic activity, the characteristics of the strength of the 27-day and 13.5-day signals for each of the solar cycles 11 to 21 are highlighted. It is shown that the 13.5-day signal is a near permanent feature in geomagnetic activity. The Hale-cycle (22 yr) effect could be seen in the average magnitude of the 27-day signal with greater strengths in the even cycles. No clear annual variation in the strength is noticed, contrary to some earlier known results.

    • Premonsoonal water characteristics and circulation in the east central Arabian Sea

      V Ramesh Babu J S Sastry V V Gopalakrishna D V Rama Raju

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      The hydrographic structure in the east central Arabian Sea during premonsoon period undergoes significant temporal change in the thermal field of upper 100 m, wherein temperature rises by about 0–5°C on an average from May to June. The major contribution in increasing the surface layer temperature comes from surface heat exchange processes, while the horizontal advective process tends to remove the heat from the upper layer. The geostrophic flow patterns are similar from May to June in the major part of the study area while in the coastal areas off Goa a southerly current sets in June in response to coastal upwelling.

    • Evolution of salinity field in the upper layers of the east central Arabian Sea and northern Bay of Bengal during summer monsoon experiments

      R R Rao K V Sanil Kumar

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      The intra-seasonal variability observed in the salinity field of the upper layers at a few locations in the east central Arabian Sea and the northern Bay of Bengal during the summer monsoon seasons of 1977 and 1979 is documented with the aid of short time series (1–2 weeks) of salinity measurements made from USSR and Indian ships deployed during MONSOON-77 (1977) and MONEX-79 (1979) field experiments. In the Arabian Sea a typical subsurface maxima observed beneath the mixed layer base either disappeared or considerably weakened due to strong vertical mixing caused by the monsoonal forcing. In the northern Bay of Bengal the salinity variability in the top 30 m water column was rapid and appeared to be influenced by large amounts of fresh water from rain and probably from the major adjoining rivers. Some simple diagnostic calculations are presented to assess the relative importance of various processes which control the observed salinity variability.

    • Adiabatic nonlinear normal mode initialization for a grid point global model

      H C Upadhyaya A Mokssit

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      The baroclinic primitive equation model used for short and medium range forecasting admits high frequency as well as desirable Rossby modes. These high frequency oscillations are excited by initial imbalances between the observed mass and wind fields. In this paper we determine and describe the normal modes of the linearized version of the general circulation model of Laboratoire de Meteorologie Dynamique, Paris. These normal modes are then used to initialize the model through Machenhauer’s nonlinear correction scheme. The adiabatic nonlinear normal mode initialization technique is shown to be superior to dynamic initialization in terms of elimination of high-frequency oscillations in the forecast. Normal modes of a particular model depend on the finite difference scheme chosen to approximate the governing system of model equations. The results presented correspond to enstrophy-conserving finite difference scheme.

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