• Volume 87, Issue 11

      November 1978,   pages  179-281

    • A numerical technique for simulation of cloud seeding experiments

      A Mary Selvam A S Ramachandra Murty Bh V Ramana Murty

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      Two numerical cloud seeding experiments, using historic rainfall for the Deccan plateau region in Maharashtra state, were performed adopting different simulation techniques. The data used consisted of 1-day total rainfall for the 5-year period 1951–55. A double-area cross-over design with area randomisation was adopted.

      The first experiment, EXP-TR, was based on the simulation technique of Twomey and Robertson which involves about 100 hr of Robotron EC-1040 computer time. The second experiment, EXP-MMM was based on a different simulation technique proposed in the present study.

      The results of EXP-TR and EXP-MMM have shown close agreement. The numerical simulation technique of EXP-MMM is more promising for the following two reasons: (i) the computational time is reduced by about an order of magnitude without compromising the scientific value of the results, and (ii) a direct estimate of the lower limit of the double ratio value which can be detected at 5% level of significance is defined.

      The results of the two numerical experiments suggested that, for the Deccan plateau region, 15 and 20% increases in rainfall due to seeding could be detected with 80% or more probability in 5 years.

    • Periodicity in rainfall at Madras

      G K Rangarajan K N Rao

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      Monthly rainfall data from Madras for the period 1813–1976 are spectrally analysed for detection of significant oscillations using the new data adaptive technique of maximum entropy developed by Burg. Spectra for annual, NE monsoon and SW monsoon rainfall are separately presented and compared with the spectra derived from Fourier transform using FFT. The long series of data are divided into several subsets and analysed to determine the stability of the periodicities. The effect of changing the length of PEF on the resultant spectra is also discussed. The analysis reveals that longer period variations (>20 years) in the rainfall is either absent or non-stationary. Solar-cycle periodicity of 11 years is also missing in the rainfall. A quasibiennial oscillation in the annual and monsoon rainfall is observed in all the subsets and in the full series. Stacked spectrum derived from the spectra of monthly rainfall series for 30-year spans is dominated by the annual line and its harmonics. The biennial pulse is absent in the monthly rainfall series.

    • Susceptibility as a tool for studying magnetic stratigraphy of marine sediments

      B L K Somayajulu C Radhakrishnamurty T J Walsh

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      Eighty six gravity cores collected from the Pacific Ocean by the Scripps Institution of Oceanography have been logged for magnetic susceptibility using a simple and rapid technique. These logs fall into three types: Type 1 showing several highs and lows, Type 2 with a single-broad-hump, and Type 3 showing nearly constant susceptibility with depth.

      Type 1 cores are found to be mainly from sediment-trap (trenches) areas which are close to the active volcanoes and the high peaks probably correspond to a slump or deposition of volcanic material; these events occurred between 0·1 and 2·8 million years ago.

      Type 2 cores are by far the most common, (56 out of 86) and show a maximum deposition of magnetic material (i.e. crest region of the hump) in the range of 0·2 and 1·7 million years. The susceptibility during this period was about a factor of two higher for several cores compared to their respective values during the last 0·1 million years. Oceanwide deposition of volcanic material and/or the atmospherically transported dust rich in magnetic material (cosmic and/or terrestrial) by our planet can account for such an increase. A third possibility may be the change (decrease) in accumulation rates of the sediments during this period.

      In type 3 cores the susceptibility is almost constant with depth and these are randomly distributed (excluding the sediment trap areas) analogous to the case of type 2 cores. A high deposition rate in these areas can alter type 2 into type 3.

      It appears that the maximum of type 2 hump can act as a stratigraphic marker since type 2 cores are the most common ones and are widely distributed over the entire Pacific.

    • Daily variations ofE andF region drifts over Tiruchirapalli

      G D Vyas H Chandra R G Rastogi

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      Daily variations of the E-W and N-S components of ionospheric drifts, measured using closely spaced receiver method over Tiruchirapalli near magnetic equator, are described for the periods 1973–75 forE region and 1974–75 forF region. N-S component is not observed except for a few occasions during summer months for theE region. E-W component is generally eastward during night hours and westward during day hours. There is a large day-to-day variability in the magnitude as well as in the evening reversal time. Daily variations primarily consist of the diurnal component with average amplitude of about 90 m/sec forF region and of about 80 m/sec forE region. The drift speeds are found to decrease with magnetic activity. It is concluded that the abnormal electrojet region extends upto north of Tiruchirapalli in the Indian zone and drifts here can be used as a monitor of electric field.

    • On the origin of ringing irregularities—A meteor hypothesis

      M R Deshpande Hari Om Vats A I Trivedi

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      Isolated ionospheric irregularities produce oscillating (ringing) diffraction patterns on the ground. Typical physical properties such as density, size, etc. of these irregularities have been estimated using the characteristics of these diffraction patterns. These physical properties agree well with those of meteor trail ionisation. In addition it has been found that most of the ringing irregularities occur on meteor shower days. Further, investigations of diurnal and seasonal patterns of ringing irregularities and meteor showers suggest that the meteor showers are the most likely cause of ringing irregularities. Hence it is hypothesised that the ringing irregularities are caused by meteor showers. It is to be noted that the current observations of isolated ionospheric irregularities are near the equator (around 9oN latitude) and they fill an observational gap in meteor showers which existed at equatorial latitudes. It has been observed that there is no marked difference in diurnal variation of meteor showers over the globe whereas there exists a strong latitudinal variation of the meteor showers.

    • Solar x-ray control of the ionospheric absorption measured by the sweep frequency method

      G P Gupta

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      The ionospheric absorption data measured by the sweep frequency method have been compared with the solar x-ray data in the band of 1–8 Å during quiet- and disturbed-sun conditions. The degree of solar x-ray control of the ionospheric absorption and the corresponding limit of solar x-ray flux have been discussed.

    • Nature of magnetic grains in basalts and implications for palaeomagnetism

      C Radhakrishnamurty S D Likhite E R Deutsch G S Murthy

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      Investigations involving temperature dependence of low-field susceptibility and of low- and high-field hysteresis have been carried out on more than one thousand basalt samples of ages ranging from less than 3 million years to more than 1 billion years. Combined application of these measurements makes it possible to distinguish rapidly the effective particle sizes and oxidation states of the magnetic minerals in the rock.

      One interesting finding is that, in basalts of widely different ages, a strongly cation-deficient phase of magnetite having distinct magnetic properties predominates over other types of magnetic grains inferred to be present. Properties attributable to multidomain magnetite were found often in basalts older than Cretaceous, but seem to be present only occasionally among the younger basalts. Despite the fact of a frequent association of magnetite with titanium in basalts, results of our tests carried out on such basalts do not in general show a magnetic behaviour consistent with the magnetic material being a solid-solution titanomagnetite. Some implications of these results for palaeomagnetism are discussed.

    • In situ magnetic measurements on igneous rock bodies

      C Radhakrishnamurty S D Likhite P W Sahasrabudhe

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      A simple method for an in situ measurement of susceptibility and low-field hysteresis on rock bodies like dykes and flows is described. Extensive use of this method indicates that in some dykes and flows, the nature of the magnetic grains varies rapidly across the bodies. Making use of such observations, it seems possible to select stable rock samples for palaeomagnetic work and also understand to some extent the origin of remanent magnetization in different types of rock bodies.

    • Mineralogy of the fuchsites from Gattihosahalli, Chitradurga District

      T C Devaraju K S Anantha Murthy

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      The fuchsites described here occur in the lower portions of a precambrian quartzite bed, which typically encloses small bands and lenses of barytes. Of the two fuchsites studied, one occurs in the main relatively fine grained quartzite and the other forming one of the small schistose segregations in the quartzite. While the commonly measured axial angles average 34o and 32o, small angles of 14o and 19o, which are possibly related to the formation of the mineral under high pressure, have been recorded in both the fuchsites. There is a close similarity in x-ray powder pattern between the two fuchsite samples studied and between these and muscovite. This observation is in accordance with the interpretation that fuchsites exhibit the same two layered mica structure as muscovite 2M1. Chemically the fuchsites of Gattihosahalli are closely similar to the one reported from Manitoba and the cell contents are comparable to di-octahedral micas of muscovite character with Cr substituting for octahedarl Al. It is inferred that the fuchsites of Gattihosahalli are of sedimentary origin and the constituents required for their formation were all present in the shaly component of the sandstone or were partly or completely supplied by volcanic exhalations.

    • Measurement of apparent velocity and azimuth of seismic signals recorded at an array

      A Ram

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      A set of computer programs was written which enables one to determine automatically, the apparent velocity and azimuth of any portion of the seismic wave-train recorded at various seismic arrays. All that is required to use these programs is that the raw data be available on digital tape and that the user have access to any modern computer. The programs are written in Fortran IV and make use of the adaptive processing method whereby the arrival times of the wavefront at each sensor are accurately determined by cross-correlating the observed wavelet of interest with the corresponding wave on the beam trace. The new arrival times are then used to create a new and improved beam and the whole operation is repeated in an iterative manner until convergence takes place.

    • Relative performance of different triangular networks in locating regional seismic sources

      S K Arora T G Varghese T K Basu

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      In regional seismic surveys use of small semipermanent networks of detectors is often made. This study aims at investigating qualitatively the location capability of networks constituted by four (bare minimum) sensors, arranged in different triangular configurations, which may be deployed in a region of known crustal structure. Their relative performances are evaluated on the basis of overall shifts in hypocentre, epicentre and origin time, these being computed by relocating a large number of assumed sources at three representative depths (0, 5 and 10 km) using an iterative method. Eight constructed examples, of which five pertain to sources lying outside a network and three pertain to sources contained within the outer boundaries of a network, provide data for the study. The influence of shape and size of a given network, on hypocentral determination, relative to the overall dimensions of the expected source region has been discussed.

      It is shown that a right-angled triangle (three stations at the vertices and one near the centroid) with its equal orthogonal arms comparable in length with radial distance to epicenters is the best suited configuration for locating sources surrounding the network. Reducing network aperture is found to render the network performance poor. However, if the sources happen to be situated inside the network, the response of one type of network is not appreciably different from that of another but comparatively much better than that of any of the ‘source-outside’ configurations. Nevertheless, a right-angled triangle (three stations at the vertices and one near the centroid) enclosing the sources seems to be a marginally better choice. Notwithstanding network geometry, an increase in the number of detectors brings about a definite improvement in the estimates of source parameters. This is demonstrated by taking six stations instead of four. It is also shown that network performance remains practically invariant with shallow source depth. The location errors are inferred to be mainly due to the effect of crustal layering.

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