• Volume 95, Issue 3

      November 1986,   pages  311-503

    • In situ stress measurements using hydraulic fracturing method

      T N Gowd S V Srirama Rao K B Chary F Rummel

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      The firstin situ stress measurements in India were carried out in 176 m deep borehole by employing deep-hole wireline hydraulic fracturing equipment. The results reveal that the direction of maximum compression is oriented at N 35° E, and the existence of high-near-surface horizontal stresses compared with the overburden pressure. This can be attributed to the continuing convergence of Indian and Eurasian plates. Earthquakes experienced near Hyderabad could be due to refracturing of some NNE or NE trending mineralized fractures which persistently occur in the area, as these fractures are critically oriented with respect to the present-day stress field.

    • Computation of synthetic seismograms for plane layered earth models including absorption and dispersion phenomena

      K N S Yadav Avadh Ram

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      A computer program package has been written in FORTRAN-IV language and tested successfully on an ICL 1904S computer. This program enables one to compute synthetic seismograms for layered earth models. The provision for studying the effect of absorption and dispersion of seismic waves has been made with Subroutines. The present program utilizes eight Subroutines and requires about 35 K core memory. A set of examples are illustrated for absorption and dispersion models. An exponential decay of amplitude has been used for the absorption model. This method is based on the plane wave propagation in a flat-layered earth system. Normal incident P-waves are used to eliminate the effect of other phases. Change in shape of reflected waves is observed in absorption model due to damping of energy of higher frequencies. Lack of resolution is found between closely spaced reflections at higher frequencies. The effect of dispersion on seismic waves decreases the time of primary reflections as well as amplitudes of the seismic waves.

    • Magnetic anomalies over thin plates and their analysis

      H V Rambabu G D J Sivakumar Sinha

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      The magnetic anomaly over a two-dimensional thin horizontal plate is similar to the first horizontal derivative of the magnetic anomaly over a thick dipping dike of infinite depth extent but with a different direction of magnetization. Hence, the magnetic anomalies of thin plates may be integrated along the profile and the pseudomagnetic potential anomaly thus obtained may be interpreted using any standard method of interpreting dike anomalies. Expressions for the Fourier amplitude and phase spectra of the magnetic anomaly over a thin plate are also derived and procedures to evaluate the parameters of the plate from the spectra are formulated.

    • Analysis of gravimagnetic anomalies by horizontal gradients

      G Ramadass S M Varaprasada Rao A Raja Mohammed

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      A simple technique for the interpretation of gravimagnetic anomalies by a numerical differentiation is presented. The ratio of horizontal gradient to the observed field was used for the location of the origin and depth to the causative body. This technique was tested by using fundamental expressions due to sphere, horizontal cylinder and vertical fault models and field data over a spherical body. It has been found that the depth to the centre of the body can be determined accurately by this technique.

    • Palaeo-tectonic environment of the Precambrian basaltic mafic dykes on the application of chemical discriminants

      T Ramamohana Rao B V Satyanarayana Raju M V Rama Raju

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      Dykes of Chitaldurg and Newer Dolerite I are OFB and represent early attempts to rift and to generate oceanic crust. Shimoga dykes are CAB and belong to orogenic phase. Guntur dykes and Ananthapur-Chittoor dykes are OIB and these are related to ‘hot spot’. Newer Dolerite II is CONB and belongs to healing and filling of fractured crust. The Cuddapah dykes are transitional to all magma types and these represent a compound event of rifting and filling of fractured continental crust.

    • Lineament analysis of Cannanore district and their implication on the tectonic evolution of the area

      Venkatesh Raghavan Narayanaswamy

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      The Cannanore district and the adjoining areas mainly comprise of charnokites, gniesses, high and low-grade schists and various types of igneous intrusives. The lineament fabric of the region indicates that the NNW-SSE, NW-SE, ENE-WSW and NE-SW lineament directions are prominent. It is suggested that the area has undergone at least three distinct phases of tectonic activity. The NW-SE and ENE-WSW lineaments appear to have formed during the phase of NW-SE folding. The NE-SW lineaments may be the result of the cross-folding of the earlier folds. The NNW-SSE lineaments have been related to the Precambrian tectonic activity in South India.

    • An interpretation of induced magnetic variations near Shillong and Gulmarg

      A K Agarwal H B Naik

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      Short-period events such as bays and storm sudden commencements (SSCs) have been analysed to investigate the nature of induced magnetic variations at two Indian magnetic observatories: Shillong and Gulmarg. It seems that near Gulmarg there is obvious connection between the induced magnetic variations and the two large scale features; the main central thrust (MCT) and the main boundary fault (MBF) in the north-west direction. The Dauki fault, an approximately east-west conductor, seems to be responsible for the conductivity anomalies at SHL.

    • On the thermal and redox history of the Yamato diogenite Y-74013

      Amalbikash Mukherjee T A Viswanath

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      The Yamato diogenite, Y-74013, shows a high degree of textural equilibrium with the apparent crystallization sequence: troilite and metal → orthopyroxene → plagioclase. The position of the large chromite crystals in this sequence is unclear. Except chromite, all other minerals have composition similar to common orthopyroxene achondrites. The chromite is more magnesian than in common diogenites, strongly zoned and, on the whole, intermediate in composition between chromites of diogenites and pallasites. Texture, mineral composition data and an equilibrium thermodynamic analysis of the mineral association strongly indicate that the chromite crystallized earlier than the silicates at a much higher temperature (possibly above 1100°C) and rapidly grew in a medium which was progressively enriched in Mg, Al and Ti. But the chromite failed to reach chemical equilibrium, even at its outermost rim, with the orthopyroxene. The calculated equilibrium log fO2 of the Yamato diogenite, −20·21 to −11·08 for temperatures between 880°C and 1500°C is well within the normal oxygen fugacity range of pyroxene achondrites.

    • Natural radioactivity distribution studies in Trivandrum district, Kerala, India

      G K Raju John Mathai G R Ravindra Kumar N G K Nair

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      The distribution pattern of radioactive minerals in Trivandrum district of Kerala has been investigated. The surface radioactivity was measured employing a jeep-mounted four-channel gamma-ray spectrometer coupled to a high volume Na I (T1) crystal detector. The distribution of radioactivity has been correlated with the corresponding litho-units and major structures of the study area. The total countrate from sedimentaries including beach sands and pegmatite rich zones (∼ 3,500 CPS) are significantly higher than that of the laterites and gneisses which are substantiated by laboratory studies.

    • Seasonal and solar cycle variations of night-time anomalous enhancements in total electron content

      N Balan P B Rao K N Iyer

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      Seasonal and solar cycle variations of the various characteristics of night-time anomalous enhancements in total electron content (TEC) of the ionosphere are presented for a low latitude station, Hawaii by considering TEC data for a full solar cycle. All the characteristics of the TEC enhancements have seasonal and solar cycle dependence. TEC enhancement characteristics such as frequency of occurrence, amplitude and duration are positively correlated with solar activity. The possible source mechanisms for the observed enhancements are also discussed.

    • Estimation of evaporation rates over the Arabian Sea from satellite data

      M Venkateswara Rao V Ramesh Babu L V Gangadhara Rao J S Sastry

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      Utilizing both the SAMIR brightness temperatures of Bhaskara II and GOSSTCOMP charts of NOAA satellite series, the evaporation rates over the Arabian Sea for June 1982 are estimated through the bulk aerodynamic method. The spatial distribution of evaporation rates estimated from the satellite data sets coincides well with those obtained from ship observations as well as from climatological data. The accuracy in the estimation of evaporation rates has considerably been improved after the removal of bias in sea surface temperature and is about ±0·8 mm/day.

    • Wind gradients at meteor heights over mid-latitude stations

      P C S Devara G Chandrasekhar M I Ahmed

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      The gradients of amplitude and phase of diurnal and semidiurnal components of zonal and meridional winds in the height intervals of 80–90 km and 90–100 km are studied using the meteor wind radar data collected at Atlanta (34°N, 84°W) during the period August 1974–March 1978. The results are compared with those at Adelaide (35°S, 139°E). It is found that the gradients vary in an opposite manner between the two height intervals.

    • Sea surface temperature variability over North Indian Ocean — A study of two contrasting monsoon seasons

      M R Ramesh Kumar S Sathyendranath N K Viswambharan L V Gangadhara Rao

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      Using the satellite derived sea surface temperature (SST) data for 1979 (bad monsoon) and 1983 (good monsoon), the SST variability for two contrasting monsoon seasons is studied. The study indicates that large negative anomalies off the Somali and Arabian coasts are associated with good monsoon rainfall over India. The strong monsoonal cooling in these regions can be attributed to strong low level winds and intense upwelling. The reappearance of 27°C isotherm off Somali coast in May/June coincides with the onset of southwest monsoon over India. Further, the influence of zonal anomaly of SST off Somalia Coast (SCZASST) and Central Indian Ocean Zonal Anomaly of SST (CIOZASST) with monsoon rainfall over India is brought out. The former is negatively related to the monsoon rainfall over western and central parts of India, whilst CIOZASST is positively related.

    • An overview of climate models

      M Sankar-Rao

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      Climate models and results, especially for paleo-climatic scale are reviewed. It is concluded that the climatic system is more stable than it was thought to be when Budyko-Sellers type models first came into existence. Even a 2% decrease in solar radiation may not result in White Earth due to negative feed-backs. A decrease in CO2 to about 200 ppm can result in White Earth. A doubling of CO2 increases the surface temperature by an order of 1°C. The total climatic system which is nonlinear can exhibit very long period internal oscillations, even of the order of 1,00,000 years though none of the time constants involved are in that range individually.

    • A review of stochastic dynamic method of weather prediction

      P C Joshi D R Sikka

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      The stochastic dynamic method of weather prediction (SDP) has been suggested recently for better understanding of the numerical weather prediction. The SDP is described using a simple one-dimensional advection equation. The salient features of the method, its scope and limitations, are discussed.

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