Volume 90, Issue 3
November 1981, pages 197-460
pp 197-209 November 1981
Characteristic curves for the complete interpretation of a wide range of parameters of a vertical cylinder and a horizontal circular disc are presented. It is established that the distance of the inflexion point on the gravity profile of a vertical cylinder from the point of maximum anomaly [i.e. the origin] is approximately equal to the radius of the vertical cylinder, thus enabling one to demarcate the boundary of the vertical cylinder directly from the contour map based on the cluster of contours. Two gravity profiles, one across an anomalous zone in central Alberta, Canada and the other over Borsad area, India are interpreted by the curves presented and the results are shown.
pp 211-215 November 1981
Using single borehole dilution technique, and tritiated water as tracer, lateral flow rate of water near the phreatic line has been measured at 24 points along the banks of an unlined canal. Permeability values range from 0.5×10−6 m sec−1 to 34×10−6 m sec−1; the median value being 5×10−6m sec−1. The seepage loss is estimated to be 5×10−5 m3 sec−1 m−1. For the entire canal the value is 0.74 m3 sec−1 which is 12% of the total flow. Limitations and merits of the technique are discussed.
pp 217-226 November 1981
Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb−Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.
pp 227-235 November 1981
The surface ice taken from the snout of the Nehnar glacier (Kashmir) in western Himalaya has been dated using radioisotopes32Si and210Pb to be 500 years. Based on the age distribution of ice and the expected activity of32Si in the fallout, the average rate of glacier movement over a period of the last few centuries is estimated to be about 6 m/yr.
The data obtained on32Si and210Pb activities in the surface ice samples in the ablation zone support our previous observation about the existence of five zones of alternately high and low activity of210Pb, which probably is a consequence of complex dynamics of Nehnar glacier.
The vertical profile of210Pb activity in an ice core correlates directly with the total beta activity. This radioactive horizon at an altitude of 4140 m appears to be located at a depth of 10–12m, which is lower compared to the 2–3 m observed earlier at an altitude of 4150 m.
pp 237-244 November 1981
Short-period events such as bays and SSCs have been analysed to investigate the nature of conductivity anomalies at two Indian magnetic observatories: Ujjain (UJJ) and Jaipur (JAI). The induction vectors calculated for these events indicate a high electrical conductivity contrast to the north-west of these stations. To determine an exact cause of anomaly, the direction of induced electric currents is estimated from horizontal disturbance vector$$(\vartriangle \vec B_A )$$ taking Alibag (ABG) as a normal station. These induced currents are found to flow south-west near JAI and westward near UJJ. It is suggested that possible cause of anomaly at JAI is a sub-surface conductivity contrast (possibly asthenospheric upwelling) along the Aravalli belt where high heat flow measurements have been reported. At UJJ, an approximately east-west conductor north of it seems to be responsible for conductivity anomalies.
pp 245-262 November 1981
The literature on influences of solar activity on the Indian weather and climate is reviewed since the discovery of sunspot cycle. Fluctuations in solar activity are undoubtedly a factor affecting weather and climate. Although the results of some of the studies are conflicting, Indian weather and climate is, in general, inversely related to sunspots. However, the areal extent of floods in India seems to expand and contract in phase with the Hale double sunspot cycle, suggesting that the flood rhythm is in some manner controlled by long-term solar activity related to solar magnetic effects. All the evidences of solar influences on weather and climate may have practical implications in improving long-range forecasting of weather and climate, once the physical coupling mechanisms and their modification by other factors are clearly understood. Some of the promising plausible physical mechanisms for explaining solar effects on weather and climate are also discussed.
pp 263-271 November 1981
Atmospheric temperature profiles have been derived using 53·74 GHz, 54·96 GHz and 57·95 GHz channels data from the Microwave Sounding Unit onboard TIROS-N. For this purpose regression coefficients have been derived using MONEX-79 radiosonde data during the period May to July 1979. The temperature profiles derived at a few selected places were closer to radiosonde profiles than the profiles derived using standard regression coefficients for the tropical region supplied by NOAA.
pp 273-280 November 1981
Magnetic properties, namely intensity of natural remanent magnetisation (NRM) and magnetic susceptibility, of mafic and ultramafic formations of Eastern Ghats belt in the Godavari Districts, Andhra Pradesh are studied. The formations sampled for the study are, iron-ore, chromite-ore, amphibolite, gabbro, charnockite and khondalite gneiss. Mineralogical differences in iron-ore and the degree of martitisation are reflected in magnetic susceptibility. NRM and susceptibility of chromite-ore are feeble. The gabbro exhibits strong magnetic properties because of its contiguity with iron formation. The Koenigsberger ratio is found to indicate relative palaeomagnetic stability.
pp 281-290 November 1981
Measurements of the water vapour content over two possible sites for a millimeter-wave radio-observatory in South India are described. The results of the survey, made with an infrared spectral hygrometer built at the Meudon Observatory, France are compared with those of similar surveys made at some other sites in Europe and America.
pp 291-304 November 1981
A direct linear relationship between water vapour content of lower, middle and upper-middle troposphere respectively with the radiances for 8.3, 7.3 and 6.7 μm spectral observations has been attempted. From the atmospheric simulations such a relationship is found to exist with a good degree of correlation and is seen to be quite insensitive to temperature changes. Such an approach is used to interpret the water vapour imageries obtained from TIROS-N sounder through construction of detailed water vapour distribution maps.
pp 305-326 November 1981
Using shallow water equations on an equatorial beta plane, the nonlinear dynamics of the equatorial waves is investigated. A general mathematical procedure to study the nonlinear dynamics of these waves is developed using the asymptotic method of multiple scales. On faster temporal and spatial scales the equations describe the equatorial wavesviz, the Rossby waves, Rossby gravity waves, the inertia gravity waves and the Kelvin waves. Assuming that the amplitude of these waves are functions of slower time and space scales, it is shown that the evolution of the amplitude of these waves is governed by the nonlinear Schrodinger equation. It is then shown that for the dispersive waves like Rossby waves and Rossby-gravity waves, the envelope of the amplitude of the waves has a ‘soliton’ structure.
pp 327-336 November 1981
Starting with an outline of spectral radiance calculation for infrared bands, a simulation study has been presented for 11.1, 8.3, 7.3 and 6.7 μm. Detailed response of the spectral radiance to the meteorological parameters has been studied by generating simulated atmospheric temperature and moisture profiles for Indian atmospheres. It has been found that the responses of the 8.3, 7.3 and 6.7 μm bands are maximum for lower troposphere, middle troposphere and upper troposphere respectively, and these spectral radiances are linearly related to the water vapour content of the relevant layers. The thermal IR window at 11.1 μm is found to be sensitive to not only surface temperature and total water vapour content but also to the boundary layer values of mean temperature and water vapour content.
pp 337-344 November 1981
The model of stellar origin of the anomalous component in the low energy cosmic rays for He to Fe ions observed in space vehicles is studied in the light of recent results. The model of heliospheric origin by Fisket al which has several attractive features cannot explain the long-term variations of intensity observed during 1974 to 1978 as pointed out by Nagashima and Morishita. The stellar origin model of Durgaprasad and Biswas, on the other hand, can easily account for the sudden appearance of the anomalous component in 1972 and its large decrease in intensity in 1978 on the basis of polarity reversal of the solar magnetic field as discussed by Nagashima and Morishita (1980). In this work, we show that in the stellar model energetic ions of He, C, N, O, etc. could originate in O-type stars which manifest very strong stellar wind with mass loss rate of 3·10−6 M⊙ per year. These have terminal velocities of about 1200 to 4000 km/sec and are typically a few times their escape velocity. These velocities correspond to ion energies of 10 to 100 keV/amu. These ions are in partly ionised state and are accelerated in the interstellar shock fronts to about 1 to 50 MeV/amu and thus account for the observed anomalous component of low energy cosmic rays.
pp 345-358 November 1981
The dominant component of nuclear tracks observed in meteoritic minerals poor in uranium is produced by cosmic ray very heavy (vh:Z>20) nuclei. Studies of cosmic ray tracks and other cosmogenic effects in meteorites give us information on the irradiation history of these meteorites and enable us to estimate the extent of ablation during their atmospheric transit, and hence their pre-atmospheric masses. In a specific type of meteorite, known asgas-rich meteorite, one finds individual grains and xenoliths that have received solar flare and galactic cosmic ray irradiation prior to the formation of these meteorites. Detailed studies of these exotic components give insight into the accretionary processes occurring in the early history of the solar system. Some of the important results obtained from such studies and their implications to meteoritics are summarized.
pp 359-382 November 1981
Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (Js,Ro)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years.
Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,28Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid.
Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid.
Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.
pp 383-388 November 1981
Recent work on fission track studies of meteorite samples to obtain cooling rates of metetorite parent bodies is reviewed. The cooling rates of chondrites are in excess of 1oK/106 yr. Fission track studies of phosphate grains in mesosiderites do not support the extremely slow cooling rates of 0°1oK/108 yr for these meteorites, inferred from metallographic studies. The accumulating evidence from fission track studies indicates a gross underestimation of the cooling rates of meteorites as determined by the metallographic techniques.
pp 389-401 November 1981
The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica beltsi.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India.
pp 403-436 November 1981
The precision, meaning, and accuracy of the fission track (ft) dating method are reviewed from an examination of the recent literature as well as previously unpublished data from the author's laboratory.
It is concluded that forapparentft ages (i.e. ages derived from the canonical age equation) a precision (2σ level) of the order of ±4% to ±5% can be reached provided that (i) uranium is sufficiently homogeneously distributed in the dated samples, at least locally; and (ii) a large enough number of tracks can be counted.
Modelft ages,i.e. ages for which partial geological track annealing is taken into account, have variable degrees of precision. While model ages obtained with the track-size method seem, as evaluated from the literature, to have usually a limited precision of the order of ±30% (2σ), plateau ages usually have a precision better than ±5% at a 2σ confidence level. Because it provides an objective test on the accuracy of track identification, as well as some insight of the variability of closing temperatures between various samples of a given mineral phase, the Isochronal Plateau (icp) method, when applicable, will be preferred (Poupeauet al 1980a). However, for phases which could be damaged by heating at relatively high temperatures, as for example hydrated glass shards from tephra, an Isothermal Plateau (itp) approach is to be preferred.
Due to uncertainties about the value of the238U spontaneous fission decay constant λf as well as difficulties inherent in the dosimetry of thermal neutrons in nuclear reactors, theft method of dating is not an independent one. Presently, it relies on the existence of geological standards (volcanic rocks) of known age, allowing to determine anoperational ‘λf’ value (Naeseret al 1980). Accordingly, the accuracy of anft age is limited by the accuracy on the age of the standard. It should be better than ∼5%.
For volcanic, hypovolcanic rocks, and shallow intrusives, theft method dates the time of formation, provided they were not further reheated. More generally, the track method providescooling ages. Closing temperatures calculated from laboratory experiments vary from ≲300°C to 100°C, according to minerals, for slow cooling rates (∼1°C/m.y.). For apatites, recent geological calibrations (Naeseret al 1980; Gleadow and Duddy 1980) confirmed laboratory extrapolations. The association of theft method with other geochronometers is therefore critical to the study of the cooling history of old cratons as well as to the evaluation of uplift/erosion rates in recent belts.
pp 437-460 November 1981
Solid state nuclear track detectors (ssntd) were introduced as an important research tool in nuclear science and technology in the early 1960s. In this paper an attempt is made to give an overview of some of the important applications ofssntd in the study of fission-related phenomena. The areas covered are: (a) spontaneous fission half-lives, (b) compound nuclear life-time measurements, (c) fission cross-section, excitation functions and fission fragment angular distributions, (d) fission isomers, (e) search for superheavy elements and (f) absolute fission yield measurements. In each case a few examples of experimental work carried out in various laboratories including the Bhabha. Atomic Research Centre (barc), Bombay are discussed to highlight the significant contributions these studies have made to our understanding of nuclei and nuclear fission. The important role played byssntd in each of the above areas of fission studies is illustrated. Some specific cases are cited where the innovative use ofssntd has lead to results of profound significance in fission physics. A general review of the impact of these studies on our present understanding of nuclei and nuclear fission as well as a brief outline of the problems and future prospects are also given in the paper.
Volume 128 | Issue 8
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