Mechanistic approach to the shear strength behaviour of allophanic soils

The presence of allophane minerals imparts special engineering features to the volcanic ash soils. This study examines the reasons for the allophanic soils exhibiting unusual shear strength properties in comparison to sedimentary clays. The theories of residual shear strength developed for natural soils and artificial soil mixtures and the unusual surface charge properties of the allophane particle are invoked to explain the high shear strength values of these residual soils. The lack of any reasonable correlation between ′ (effective stress-strength parameter) and plasticity index values for allophanic soils is explained on the basis of the unusual structure of the allophane particle. The reasons as to why natural soil slopes in allophanic soil areas (example, Dominica, West Indies) are stable at much steeper angles than natural slopes in sedimentary clay deposits (London clay areas) are explained in light of the hypothesis developed in this study.     

Diurnal wind variation in the planetary boundary layer (PBL) over Sriharikota,India

Pibal ascents were taken every three hours at a coastal station, Sriharikota (India) on the east coast in four different campaigns each representing a season in India. A diurnal pattern of winds in the PBL winds was found in all seasons but the pattern varies from season to season. The details are described and discussed.     

The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF₁, DhF_1a and DhF₂) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF_*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF₁ structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF₂) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ～2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.     

Middle to late Archaean geology of the eastern Baltic shield,with a note on its similarity and contrast with the Archaean of southern India

The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there.     

Study on penetration depth and its dependence on frequency,soil moisture,texture and temperature in the context of microwave remote sensing

A theoretical study has been made to see the influence of microwave frequency, soil moisture, soil texture and soil temperature on penetration depth in the context of microwave remote sensing. The results are presented in the form of figures and also coefficients of least square fitting. The study reveals that there is a definite dependence of penetration depth on the above parameters. The range of penetration depth has been found to be 0 to 10 cms and varies as a function of several parameters. These results are in agreement with experimental observations.     

On the observed synoptic variability in the thermal structure of the upper northern Bay of Bengal during MONEX-79

The short-term variability observed in the near surface meteorological parameters and in the vertical thermal structure of the upper layers of the northern Bay of Bengal during a weak monsoonal regime is examined with the aid of time series measurements. The variability of the mixed layer depth is interpreted in terms of forced mixing caused by the surface wind stress and free mixing by buoyancy flux, Ekman pumping controlled by the curl of the surface wind stress, convergence associated with a clockwise gyral circulation and stratification caused by freshwater discharges from rivers. The daily-averaged current vectors in the upper layers indicate the presence of clockwise gyral circulation in the polygon area.     

Quantification of crop yields under rainfed conditions using a simple soil water balance model

Summary The concept of water requirement satisfaction index was used to quantify the crop yield under rainfed conditions, choosing pearl millet as an example. The relationship between the water requirement satisfaction index and yield was observed to beY = exp (— 18.023+4.173 lnx), indicating the exponential behaviour of the yield as affected by the water availability to the crop. The correlation between the observed and expected yields was found to be 0.94. Yields were simulated for the years 1901–85 using the mathematical relationship. Simulation results showed that the years with commencement of rainy season from 25th to 28th week are most suited for millet production in the study area. The maximum crop failures were observed with 7 years per decade during the years 1901–40. The results also showed that the concept ofWRSI can be used in assessing the effects of soil moisture stress on crop yields at field level.

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Zusammenfassung Das Konzept des Water Requirement Satisfaction Index (Wasser-Bedarf-Sättigungs-Index,WRSI) wurde verwendet, um Ernteprognosen ohne Bewässerung am Beispiel von Perlenhirse zu erstellen. Das Verhältnis zwischenWRSI und Ernteertrag konnte dabei alsy = exp (— 18.023+4.173 lnx) beschrieben werden, was auf eine Beeinflussung der exponentiellen Entwicklung des Ernteertrags durch die Menge verfügbaren Wassers hinweist. Die Übereinstimmung zwischen erwarteten und beobachteten Erträgen lag bei 0.94. Die Ernteerträge für die Jahre 1901–1985 wurden damit simuliert. Die Ergebnisse dieser Simulation zeigten, daß Jahre, in denen der Beginn der Regenzeit zwischen die 25. und 28. Woche fällt, am besten zur Hirseproduktion in der untersuchten Gegend geeignet scheinen. Maximale Ernteausfälle wurden mit sieben Jahren pro Dekade zwischen 1901 und 1940 registriert. Die Ergebnisse zeigen, daß das Konzept desWRSI auch zur Einschätzung von Auswirkungen der Bodenfeuchtigkeitsbelastung auf Ernteerträge auf dem Feld verwendet werden kann.

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Travel times ofP-waves in North-East India

Summary Based on 90 accurately localized earthquakes in and around North-East India and the local crustal velocity model of Gupta et al. [4], the travel times of P-waves have been determined from the foci of these earthquakes at arbitrarily selected depths of 5, 13, 25, 41 and 50 km to the sites of the seismic stations operated by the Regional Research Laboratory, Jorhat and the National Geophysical Research Institute, Hyderabad, and also to the sites of the Shillong and Tura seismic stations run by the India Meteorological Department, New Delhi. The travel times of P-waves fit a straight line very well with velocities of 5·97 ± 0·31, 6·18 ± 0·01, 6·41 ± 0·03, 7·82 ± 0·07 and 7·95 ± 0·01 km/sec at each of the depths under study. Similar investigations of P* and Pg-waves of 16 earthquakes at a depth of 10 km have revealed velocities of 6·53 ± 0·31 and 5·64 ± 0·34 km/sec, respectively. A simplified two-layered crustal model consisting of an average crustal thickness of 41·5 km with 22·2 and 19·3 km thick layers has been obtained.     

Stratification and convection over Arabian Sea during monsoon 1979 from satellite data

From the temperature and moisture retrievals from satellites, two types of indices were derived: one indicating suppression of convection and the other indicating organized deep convection. Sea surface skin temperature and equivalent potential temperatures up to 500 mbar level of the atmosphere, derived from TIROS-N satellite products, are the basis of the two indices. The maps of these indices for various phases of 1979 monsoon are compared with percentage cloudiness, a product also available from TIROS-N satellite observations. Despite the various limitations of satellite soundings, it is shown that these satellite-derived indices can be used to indicate the strengths of atmospheric convection and inversion over the oceans.