A second derivative method of resistivity sounding

The second vertical derivatives of gravity and magnetic potential are widely used in geophysical prospecting because of their better resolution. On the same basis an attempt has been made to obtain the expressions for the second vertical derivative of the electrical potential and to compute its nature for comparison. Derivative responses over a two-layered earth and also over an anticlinal structure have been computed and it is shown that the second vertical derivative sounding could be employed for greater accuracy in finding out the thickness of such beds or the inclination of the sides of the anticline and its depth when compared with normal resistivity sounding     

Coronas in olivine metagabbros from the Proterozoic Chotanagpur terrain at Mathurapur, Bihar, India

The Proterozoic Mathurapur olivine metagabbros possess several types of coronas due to subsolidus reactions between igneous mineral pairs. viz. olivine-plagioclase, ilmenite-plagioclase and pyroxenes-plagioclase. Microprobe analyses of coexisting primary and coronitic minerals from different corona domains, indicate attainment of equilibrium. Mineral chemical data of primary (reactants) and coronitic (products) minerals from the present metagabbros and also from similar studies in the literature are critically examined to evaluate the chemistry of corona reactions by mixing calculations assuming boundary migration. The mass-balanced corona equations are consistent with the allochemical system and do not show volume imbalance. However, SiO₂ and Al₂O₃ in olivine-plagioclase coronas remain immobile, which indicates that the corona reactions therein proceeded with minimum structural rearrangements of (Si.Al)---O bonds.     

Internal wave characteristics in the eastern Arabian Sea during summer monsoon

The time series BT profiles and surface winds and atmospheric pressure, collected in the deep waters off Ratnagiri and Karwar during summer monsoon were utilized to document the characteristics of internal waves (IW). Low-frequency (≤2, cycle per day (cpd)) IW off Ratnagiri are found to propagate at 83 cm/s with wavelengths of 45 km and wave heights upto 40 m. These parameters for high-frequency (>2 cpd) IW off Karwar correspond to 99 cm/s, 3 km and 23 m. The IW off Karwar appear to leave the station at 70° (±10°) (measured from the horizontal). The data sets were further analysed to address the harmonic composition of the IW and identify the possible sources for the observed IW fields. Power spectra of the IW indicated energy peaks at inertial (0·6 cpd) and tidal (1 and 2 cpd) frequencies off Ratnagiri and in the high-frequency band of 0·5–2·0 cycles per hour off Karwar. The coherence between the IW and wind/tide is found to be good at several frequencies within the IW spectrum. This feature probably suggests tides as a source for the IW of tidal frequencies and winds and tides as a joint source for the IW at the remaining frequencies.     

Geophysical anomalies associated with uranium mineralization from Beldih mine, South Purulia Shear Zone, India

Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW-SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ～100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ.     

Role of groundwater protection planning in groundwater governance: A case study from North India

In the present case study, impact of urbanization and industrial development on the shallow groundwater regime of Saharanpur town of Uttar Pradesh in India is examined with the aim of planning groundwater protection for better governance. The hazardous physicochemical and bacteriological parameters and heavy metals detected in the shallow aquifer include harmful pathogens like fecal coliforms, heavy metals like cadmium, chromium, nitrates and sulphates. An assessment of ground water vulnerability using the well known DRASTIC method has confirmed that the shallow groundwater in some central and southern localities of Saharanpur town fall in the medium risk zones. Further, using field data of 32 electrical resistivity soundings, the protective capacity of the unconfined aquifer is assessed in terms of a ‘total longitudinal conductance’ of the semi-pervious to impervious sediments overlying the unconfined aquifer. However, some areas aligned along a northwest-southeast and in the western parts of the town seem to have relatively higher protective capacity against infiltrating waste pollutants. A ground water protection planning map prepared by combining the DRASTIC map and the ‘potentially hazardous pollutants’ map has brought out the need to install eleven new groundwater quality monitoring wells in the town at locations near the line sources and point sources of pollution. This approach can be readily employed by the decision makers in framing sound guidelines for groundwater protection and governance.     

Rapid computation of magnetic anomalies and demagnetization effects caused by bodies of arbitrary shape

Summary With the aid of formulae derived for the field caused by a finite rectangular prism, a machine method is developed for rapid computation of magnetic anomalies due to a body of any shape. Conversely, the method could be utilised for determination of the magnetization vector from the observed anomalies. An example is given to demonstrate the applicability, accuracy and speed of the method. The method is shown to be suitable also for the evaluation of magnetic terrain effects.In addition, a new method is presented which enables high speed calculation of demagnetization effect caused by a body of arbitrary shape where the magnetization in general may be inhomogeneous. Applicability and accuracy of the method are discussed. The method is applied to study the effect of susceptibility on inhomogeneity of the induced magnetization in a cube specimen.For a body of any shape with low susceptibility, a new concept of average demagnetization factor is introduced. Average demagnetization factors so calculated for a cylindrical specimen are tabulated.

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Zusammenfassung Unter Anwendung der Formeln für das Feld eines Quaders wird eine Methode entwickelt um die durch einen Körper beliebiger Form erzeugten magnetischen Anomalien mit Hilfe eines Rechenautomaten zu berechnen. Umgekehrt kann die Methode verwendet werden, um aus den beobachteten Anomalien den Magnetisierungsvektor zu bestimmen. An einem Beispiel wird die Anwendbarkeit, die Genauigkeit und die Schnelligkeit der Methode erläutert. Die Methode kann auch für die Berechnung von magnetischen Terraineffekten verwendet werden.Zudem wird eine neue Methode dargelegt, welche eine rasche Berechnung des Entmagnetisierungseffektes eines Körpers beliebiger Gestalt und inhomogener Magnetisierung ermöglicht. Anwendbarkeit und Genauigkeit der Methode werden besprochen. Die Methode wurde verwendet, um den Einfluss der Suszeptibilität auf die Inhomogenität der induzierten Magnetisierung in einem Würfel zu untersuchen.Für Körper beliebiger Form mit kleiner Suszeptibilität wls neuer Begriff der mittlere Entmagnetisierungsfaktor eingeführt. Mittlere Entmagnetisierungsfaktoren für eine zylindrische Probe werden tabelliert.

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Part I of the dissertation Theretical study of the magnetic attraction due to rock bodies and experimental investigation of the stability of rock magnetism submitted to the Swiss Federal Institute of Technology (ETH), Zurich, for the degree of Doctor of Natural Sciences.     

Primary productivity and phytoplankton size fraction dominance in a temperate North Atlantic estuary

The composition, productivity, and standing crop of net (>20 μm) and nano-(<20 μm) phytoplankton of Peconic Bay, Long Island, New York was examined from June 1978 through May 1979. Nanoplankton, primarily small solitary flagellates, chlorophytes, and diatoms, dominated from May through September accounting for 88.5% of the productivity and 88.1% of the standing crop (measured as chlorophyll a). An apparent net plankton bloom began in December and continued through March. The dominant organism through most of the winter bloom was the chain-forming diatom Skeletonema costatum (Grev.) Cl. Net plankton at this time represented 66.4% of the standing crop. For both size fractions, productivity/chlorophyll a (g C per g chl a per d, integrated through the euphotic zone) was a function of light energy over the year with the exception of a few sampling dates during the post-winter bloom period. Assimilation numbers (g C per g chl a per h at saturating light intensities) were a function of temperature between 0 and 20°C. Nitrogen deficiency did not appear to be a factor in regulating phytoplankton growth rate through the euphotic zone, as ratios of ¹⁴C assimilation for dark bottles enriched with NH₃ and with no enrichment exhibited no relationship to environmental dissolved inorganic nitrogen concentrations. Zooplankton grazing pressure appeared to have been an important factor in regulating the upper limit of phytoplankton biomass and in influencing size fraction dominance. Dominance of one phytoplankton size fraction over the other on any given date was not based on physiological differences between the two groups since both fractions were composed of the same species. Apparent net phytoplankton blooms (in terms of productivity and chlorophyll a) were artifacts of increased chain lengths of nanoplankton diatoms such as Skeletonema costatum, and to a lesser extent, Thalassiosira nordenskioldii Cl. and Detonula confervacea (Cl.) Gran, rather than to the dominance of large, solitary cells.