Volume 95, Issue 2
July 1986, pages 153-309
pp 153-167 July 1986
In the present study, the use of one of the recent dependent domain models of capillary hysteresis in the numerical analysis of intermittent infiltration and redistribution of water in two types of soils (a sand and Rubicon Sandy Loam) has been shown. The numerical results for both the soils have been presented in terms of pressure head depth, moisture content depth and the pressure head-moisture content relationships. The capillary hysteresis model has been found to be very useful for the prediction of both wetting and drying scanning curves of various orders.
pp 169-182 July 1986
Spallation produced radionuclides10Be,22Na and26Al and low energy neutron capture radionuclide60Co have been measured in several fragments of the Dhajala chondrite, their shielding depth in space having been established by measurement of cosmic ray track density. These data enable us to obtain depth profiles of production rates of these nuclides within the meteoroid body. These profiles are discussed in terms of their dependence on size of the meteoroid and variations of galactic cosmic ray fluxes in time and space. The slowing down density of neutrons of 0.1 neutrons/cm3 sec is determined near the centre of the Dhajala meteoroid based on60Co.
pp 183-191 July 1986
Study of several cosmic ray effects, such as VH track density, spallogenic26Al and53Mn activity,21Ne and22Ne/21Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —22Ne/21Ne correlation diagram and cosmic ray flux variations over 106–107 years can be deduced from a three-isotope correlation diagram of26Al,53Mn and21Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 107 years.
pp 193-200 July 1986
In this paper, the results of our observations on Al-method ionospheric absorption of radio waves on 1.8 and 2.2 MHz during the solar eclipse of 16 February 1980 are presented. The absorption decreased by about 41% and 46% of the normal value respectively at the above two frequencies at Ahmedabad following the maximum phase of the eclipse (about 77% of full disc) with a delay of 18 minutes. The quantityAT(f) which is a measure of εN vdh is now examined for better clarity of the influence of the changes in theE-layer. The results are discussed in relation to the observations of the ionizing radiations from the sun, changes in the electron density, recombination rate and absorption in the underlyingD andE regions.
pp 201-208 July 1986
Using artificial samples, it is shown that, even for the method of maximum entropy spectral analysis (mesa) evolved by Burg, a presence of a linear trend gives distortions (frequency shifts) in the low frequency (high periodicity) region and hence, it is advisable to eliminate linear trends before attempting spectral analysis.
The effect of the presence of a non-zero mean is not very definite; but, very large mean values seem to reduce the sharpness of the peaks.
The implications of these findings for the results by Jin and Thomas for some geomagnetic parameters are discussed.
pp 209-221 July 1986
Heating and freezing studies on fluid inclusions in quartz from mineralized quartzfeldspar reef reveal the presence of type A CO2-H2O (H2O>50% by volume), type B CO2-H2O (H2O<50% by volume), type C pure CO2 and type D pure aqueous inclusions. Types A, B and C are primary and/or psuedo-secondary inclusions while type D are secondary. Types A and B homogenize on heating into different phases at similar temperatures ranging between 307 and 476°C, indicating entrapment from boiling hydrothermal solutions. Type D inclusions homogenize into a liquid phase at temperatures between 88 and 196°C. Boiling of hydrothermal solutions led to the formation of a CO2-rich phase of low density and salinity that coexisted with another dense and saline aqueous phase with very little CO2 dissolved in it. Ore and gangue mineral assemblage of primary ores indicate that ore deposition was characterized by logfO2=−34.4 to −30.2 atm, logfS2=−11.6 to −8.8 atm and pH=4.5 to 6.5.
pp 223-236 July 1986
This paper presents a method for determining the nature of the spatial structure of the heat flow anomaly at depth given a few observations of surface heat flow. We use the constraint of the positivity of the heat flow throughout the domain under consideration to arrive at the solution. Both Parker’s ideal body and linear programming approaches have been considered. We find that a few measurements are able to give a reasonable good estimation of the heat flow anomaly at depth. The method has been applied to a real case in the French Massif Central. Results point to the presence of enhanced basal heat flow anomaly.
pp 237-244 July 1986
From the wave refraction diagrams it is delineated that the Jaigad Head and Warori Bluff are the zones of wave energy convergence and the Narvan and Ambwah bays the areas of wave energy divergence. The presence of two distinct mineral phases noticed at the Jaigad, Ambwah and Varvade bays shows that there are two different circulations of sediment movements. The presence of natural barriers restricts the movement of sediment along the coast. The sand bar at the mouth of the Jaigad Bay has different orientations during the monsoon and non-monsoon periods causing obstruction to navigation during the former period.
pp 245-263 July 1986
A mathematical model to calculate the234U/238U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of238U by dissolution and that of234U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of234U/238U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of234U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The238U concentration and234U/238U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.
pp 265-273 July 1986
The effect of whitecaps and foam on wind speed extraction with a pulse limited radar altimeter has been studied using a specular point model. By modelling foam and water as a two-layer media, we have investigated the changes in reflectivity as a function of thickness using electro-magnetic field theory. Our analysis indicated a change of reflectivity from a value of 0.617 at 13.9 GHz normal incidence to a value of 0.10 for a foam thickness of less than 1 cm. The values of reflectivity computed from a two-layer model compared fairly well with these derived using an emissivity model.
The modified specular point model gave an improved relationship between σo and wind speed in comparison to Brown’s model. The Seasat altimeter’s data analysis over the Arabian sea and the Bay of Bengal gave an rms difference of 2.2 m/sec in wind speed retrieval using the present modified model and Brown’s model.
pp 275-284 July 1986
The decrease of density contrast with depth in sedimentary strata is approximated by a quadratic function. The anomaly equation of a trapezoidal model with the quadratic density function is derived. Nonlinear optimization technique using the Marquardt algorithm has been developed and used to interpret a synthetic anomaly profile of the trapezoidal model. The exact values of the coefficients of the quadratic density function are assumed to be known. The convergence of the method is shown by plotting the values of the objective function λ and the various parameters with respect to iteration number. θ and half-width of the trapezoidal model are found to be correlated. The method is also applied to interpret the gravity anomalies over San Jacinto graben, California. Finally, the use of modelling with quadratic density function is discussed.
pp 285-292 July 1986
Results of petrographic, mineralogic and granulometric studies of the sedimentary china clay deposits of Trivandrum district in south Kerala are discussed in terms of their origin and depositional environment. While mineralogic data indicate a khondalitic source for the kaolins as against the hitherto postulated leptynitic source, petrographic evidences suggest that most of the kaolinite were transported from the weathering crust and deposited. Granulometric data and SEM scanning of sand grains from the clays indicate their deposition in a fluvial/low energy littoral environment under conditions of subaqueous agitation.
pp 293-298 July 1986
A time-dependent numerical model allowing a simulation of the electric field and precipitation growth in a thundercloud of finite dimensions is described. It is found that slower growth rate (compared to an infinite thundercloud) of the electric field in a finite thundercloud permits larger size growth and higher terminal velocities of hydrometeors owing to an enhancement in precipitation intensity. Calculations also show that a higher maximum of the electric field is needed to slow down the larger particles produced in a thundercloud of finite dimensions. In particular, these solutions also include contribution of screening charge transport in thundercloud electrification.
pp 299-309 July 1986
Global sea-level pressure distribution has been analysed for the months of April and July for 5 years of contrasting situations of Indian summer monsoon, comprising of two drought years (1972 and 1974), a flood year (1975) and two normal monsoon years (1970 and 1973). Mean monthly sea-level pressure data at about 400 stations have been used in the study. Prominent features of pressure departures from long-term normals have also been noted. It is observed that the month of April shows more prominent contrasting features than July. In April, the high pressure centres over USSR and the North Pacific move considerably eastward during poor monsoon years, while a breakaway cell of Icelandic Low goes deep south. Both the high pressure areas over south Indian Ocean and Australia are stronger in good monsoon years. In July, the subtropical high pressure zone over the southern Indian Ocean is stronger and the Australian high is more eastward, in good monsoon years.
Volume 128 | Issue 8
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