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51.
A total of 81 samples (244 specimens) from Upper Cretaceous Indus Molasse and Middle to Upper Cretaceous Dras Flyschoids of the Indus-Tsangpo suture zone in Ladakh (northwest Himalaya) has been studied by thermal demagnetization methods.Both formations showed a characteristic magnetization component indicative for equatorial to low northern palaeolatitudes of acquisition. Similar palaeolatitudes have been obtained before from secondary magnetization components of Early Tertiary age in the Ladakh Intrusives and in the Tibetan Sedimentary Series of central Nepal. The present characteristic components are interpreted likewise as secondary magnetizations which stabilized between 50 and 60 m.y. ago, during Greater India's collision with Asia's southern margin.The Dras Flyschoids show another magnetic component which, in case of primary origin, indicates acquisition at a low southern palaeolatitude. If correct, this interpretation supports recent suggestions for Late Cretaceous obduction of an island arc on Greater India's northern margin. 相似文献
52.
53.
An area of 6000 ha covering four villages was adopted by the Central Soil Salinity Research Institute, Karnal under Operational Research Project to transfer the technology of alkali soil reclamation in the farmer’s fields. This necessitated mapping and characterisation of alkali soils of the area. Use of black and white photographs in 1∶25,000 scale proved effective in delineating alkali soils quickly and inexpensively. The physiography and photo elements like tone, landuse, drainage and parcelling facilitated identification and demarcation of four mapping units viz; P1, P3, P4 and C. The unit P represented nearly flat upland alluvial plain while C represented low lying filled up old channel. The alkali soils confined mainly to the upland plain were recorded in white toned barren stretches and accounted for 36.6 percent of surveyed area. In addition, patchy occurrence of alkali soils admist the cultivated fields giving mottled appearance on the photographs was noticed over 11.3 percent area. In this case the photo scale proved limiting for demarcating alkali and non-alkali soils separately. The prtactical utility of the soil inventory in planning reclamation and management of alkali soil in the area has also been discussed. 相似文献
54.
Natural radionuclides in the uranium and thorium series were measured in solid tidal phases (suspended particles, bottom sediment, surface microlayer colloids) of a salt marsh in lower Delaware. The purpose was to identify potential processes responsible for trace element cycling (sources, redistribution and exchange) in salt water marshes and with their coastal waters. Generally, concentrations of U, Th,210Pb, and210Po on the tidal solid phases suggest a general mechanism by which tidal marshes appear to be trapping the nuclides into their interiors. The processes may include transport of enriched fine particles into the marsh, capture by salt marsh grass and chemical fixation by redox processes at the sediment surface. Specifically, the uranium contents of most of the samples are similar with activity ratios234U238U≧1, indicating a mixture of detrital and nondetrital (authigenic) uranium inputs such as seawater or ground water. Since the230Th daughter is generally deficient by about 50%, the authigenic enrichment process appears to favor uranium and is potentially linked to the extensive diagenetic sulfur redox cycle of salt marsh sediments. The210Po/210Pb activity ratio is less than one on Spartina adsorbed solids, and could suggest a general process in salt marshes which favors210Pb enrichment by atmospheric fallout over enrichment of210Po on time scales of weeks which correspond to complete tide marsh exchange. A228Th/232Th activity ratio of less than unity on the solids adsorbed onto marsh grass suggests a net process whereby diffusive loss of the intermediate daughter228Ra from the adsorbed solids to tidal waters dominates over potential228Th scavenging by suspended sediment. 相似文献
55.
P.V. Sharma 《Ore Geology Reviews》1987,2(4)
Developments which have led to an enormous increase in the use of magnetic surveys for mineral exploration during the past 30 years are reviewed here. Advances in instrumentation and digital compilation of data have come about largely due to the extensive use of the airborne magnetometer as a geologic mapping tool. Currently there is a growing interest in the use of the aeromagnetic gradiometer for exploration surveys in the Precambrian Shield areas. The advantages of gradiometry as a complement to total field surveys is becoming well recognized. Miniaturized electronics has reduced the cost and size of many magnetic survey instruments and ancillary equipments. Advanced compensation techniques have made it possible to make optimum use of the increased sensitivity of magnetometers for various high-resolution applications.Quantitative interpretation of magnetic data in terms of models of causative bodies has advanced largely due to the development of computerized multiparameter inversion methods. Many of these permit interpreter interaction through computer-graphic display system to impose sensible geologic constraints. Several survey contractors have the software and hardware facilities to perform various data enhancement techniques and also interactive modelling. Susceptibility mapping and magnetization mapping techniques are of great potential utility in mineral exploration. There is still a great need for studies into the relationship between rock magnetism and magnetic anomalies. 相似文献
56.
57.
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 相似文献
58.
Primary productivity and phytoplankton size fraction dominance in a temperate North Atlantic estuary
Stephen F. Bruno Robert D. Staker Gurdial M. Sharma Jefferson T. Turner 《Estuaries and Coasts》1983,6(3):200-211
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. 相似文献
59.
T. S. V. Vijaya Kumar J. Sanjay B. K. Basu A. K. Mitra D. V. Bhaskar Rao O. P. Sharma P. K. Pal T. N. Krishnamurti 《Natural Hazards》2007,41(3):471-485
This study entails the implementation of an experimental real time forecast capability for tropical cyclones over the Bay
of Bengal basin of North Indian Ocean. This work is being built on the experience gained from a number of recent studies using
the concept of superensemble developed at the Florida State University (FSU). Real time hurricane forecasts are one of the
major components of superensemble modeling at FSU. The superensemble approach of training followed by real time forecasts
produces the best forecasts for tracks and intensity (up to 5 days) of Atlantic hurricanes and Pacific typhoons. Improvements
in track forecasts of about 25–35% compared to current operational forecast models has been noted over the Atlantic Ocean
basin. The intensity forecasts for hurricanes are only marginally better than the best models. In this paper, we address tropical
cyclone forecasts over the Bay of Bengal for the years 1996–2000. The main result from this study is that the position and
intensity errors for tropical cyclone forecasts over the Bay of Bengal from the multimodel superensemble are generally less
than those of all of the participating models during 1- to 3-day forecasts. Some of the major tropical cyclones, such as the
November 1996 Andhra Pradesh cyclone and October 1999 Orissa super cyclone were well handled by this superensemble approach.
A conclusion from this study is that the proposed approach may be a viable way to construct improved forecasts of Bay of Bengal
tropical cyclone positions and intensity. 相似文献
60.
M. D. Sharma 《Journal of Earth System Science》2007,116(4):357-367
Anisotropic wave propagation is studied in a fluid-saturated porous medium, using two different approaches. One is the dynamic
approach of Biot’s theories. The other approach known as homogenisation theory, is based on the averaging process to derive
macroscopic equations from the microscopic equations of motion. The medium considered is a general anisotropic poroelastic
(APE) solid with a viscous fluid saturating its pores of anisotropic permeability. The wave propagation phenomenon in a saturated
porous medium is explained through two relations. One defines modified Christoffel equations for the propagation of plane
harmonic waves in the medium. The other defines a matrix to relate the relative displacement of fluid particles to the displacement
of solid particles. The modified Christoffel equations are solved further to get a quartic equation whose roots represent
complex velocities of the four attenuating quasi-waves in the medium. These complex velocities define the phase velocities
of propagation and quality factors for attenuation of all the quasi-waves propagating along a given phase direction in three-dimensional
space. The derivations in the mathematical models from different theories are compared in order to work out the equivalence
between them. The variations of phase velocities and attenuation factors with the direction of phase propagation are computed,
for a realistic numerical model. Differences between the velocities and attenuations of quasi-waves from the two approaches
are exhibited numerically. 相似文献