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351.
The temperature gradient in the lower mantle is fundamental in prescribing many transport properties, such as the viscosity, thermal conductivity and electrical conductivity. The adiabatic temperature gradient is commonly employed for estimating these transport properties in the lower mantle. We have carried out a series of high-resolution 3-D anelastic compressible convections in a spherical shell with the PREM seismic model as the background density and bulk modulus and the thermal expansivity decreasing with depth. Our purpose was to assess how close under realistic conditions the horizontally averaged thermal gradient would lie to the adiabatic gradient derived from the convection model. These models all have an endothermic phase change at 660 km depth with a Clapeyron slope of around −3 MPa K−1, uniform internal heating and a viscosity increase of 30 across the phase transition. The global Rayleigh number for basal heating is around 2×106, while an internal heating Rayleigh number as high as 108 has been employed. The pattern of convection is generally partially layered with a jump of the geotherm across the phase change of at most 300 K. In all thermally equilibrated situations the geothermal gradients in the lower mantle are small, around 0.1 K km−1, and are subadiabatic. Such a low gradient would produce a high peak in the lower-mantle viscosity, if the temperature is substituted into a recently proposed rheological law in the lower mantle. Although the endothermic phase transition may only cause partial layering in the present-day mantle, its presence can exert a profound influence on the state of adiabaticity over the entire mantle. 相似文献
352.
In this paper, the results of a series of extensive measurements of multifrequency radio wave absorption in theD-region of the ionosphere during the epoch of the minium period of the solar cycle are presented. Experimental results for Udaipur, a low latitude station (24°35′N) are compared with the theoretically expected results. It is found that the experimental values of frequency indexm and cos χ indexn agree well with those obtained theoretically. The mean values ofm andn for the whole period of observation are found to be 1·76 and 1·4 respectively. The seasonal variation of the values ofm andn is also discussed. 相似文献
353.
Malkiat Singh H S Gurm M R Deshpande R G Rastogi G Sethia A R Jain A V Janve R K Rai V M Patwari B S Subbarao 《Journal of Earth System Science》1978,87(3):47-55
Radio beacon from ATS-6 at 140 MHz was used to measure the changes in the polarization angle (Faraday rotation) at Bombay,
Rajkot, Ahmedabad, Udaipur and Patiala during October 1975 to July 1976. In this paper, results of diurnal, seasonal and latitudinal
variations in total electron content (TEC) derived from these measurements are reported. The amplitude of diurnal peak is
found to be higher at Rajkot, Ahmedabad and Udaipur as compared to that at Patiala or Bombay, indicating that the peak of
Appleton anomaly in the latitudinal variation of TEC was close to the latitude of Ahmedabad. The diurnal maximum of TEC occurs
around the same time during summer and winter months. The peak electron content shows a semiannual variation at all the stations
with large values in equinoxes as compared to winter and summer. The TEC at Bombay shows a seasonal anamoly with high values
in winter as compared to summer. The paper describes the development of latitudinal anomaly with the time of the day for different
seasons. This anomaly is maximum during 1000 to 1800 LT and is located between 12° and 14° N (dip latitude) in summer and
equinoxes and at about 10°N in winter. 相似文献
354.
355.
An attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the
northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among
the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence.
The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene
bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears
to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water
(AABW).
Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in
the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic
circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment
accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and
cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by
low abundance ofU. proboscidea and more diverse and equitable benthic fauna.
The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative
abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary. 相似文献
356.
357.
Natural Hazards - The study dealt with seawater intrusion process in a coastal aquifer system in the Mahanadi river delta region in the east coast of India along the Bay of Bengal. The aquifers of... 相似文献
358.
Huai Zhang Yaolin Shi David A. Yuen Zhenzhen Yan Xiaoru Yuan Chaofan Zhang 《Pure and Applied Geophysics》2008,165(3-4):475-496
Modeling tsunami wave propagation is a very challenging numerical task, because it involves many facets: Such as the formation of various types of waves and the impingement of these waves on the coast. We will discuss the different levels of approximations made in numerical modeling of 2-D and 3-D tsunami waves and their relative difficulties. In this paper new attempts are proposed to evaluate the hazards of tsunami’s and visualization of large-scale numerical results generated from tsunami simulations. Specialized low-level computer language, based on a parallel computing environment, is also employed here for generating FORTRAN source code for finite elements. This code can then be run very efficiently in parallel on distributed computing systems. We will also discuss the need to study tsunami waves with modern software and visualization hardware. 相似文献
359.
Klaus Regenauer-Lieb David A. Yuen Florian Fusseis 《Pure and Applied Geophysics》2009,166(10-11):1885-1908
The total rate of rock deformation results from competing deformation processes, including ductile and brittle mechanisms. Particular deformation styles arise from the dominance of certain mechanisms over others at different ambient conditions. Surprisingly, rates of deformation in naturally deformed rocks are found to cluster around two extremes, representing coseismic slip rates or viscous creep rates. Classical rock mechanics is traditionally used to interpret these instabilities. These approaches consider the principle of conservation of energy. We propose to go one step further and introduce a nonlinear far-from-equilibrium thermodynamic approach in which the central and explicit role of entropy controls instabilities. We also show how this quantity might be calculated for complex crustal systems. This approach provides strain-rate partitioning for natural deformation processes occurring at rates in the order of 10-3 to 10-9 s-1. We discuss these processes using examples of landslides and ice quakes or glacial surges. We will then illustrate how the mechanical mechanisms derived from these near-surface processes can be applied to deformation near the base of the seismogenic crust, especially to the phenomenon of slow earthquakes. 相似文献
360.
Soil characterization using electrical resistivity tomography and geotechnical investigations 总被引:1,自引:0,他引:1
Electrical Resistivity Tomography (ERT) has been used in association with Standard Penetration Test (SPT) and Dynamic Cone Penetration Test (DCPT) for Geotechnical investigations at two sites, proposed for thermal power plants, in Uttar Pradesh (UP), India. SPT and DCPT tests were conducted at 28 points and two ERT profiles, each measuring 355 m long, were recorded using 72 electrodes deployed at 5 m spacing. Electrical characterization of subsurface soil was done using borehole data and grain size analysis of the soil samples collected from boreholes. The concept of electrical resistivity variation with soil strength related to the grain size distribution, cementation, porosity and saturation has been used to correlate the transverse resistance of soil with the number of blow counts (N-values) obtained from SPT and DCPT data. It was thus observed that the transverse resistance of soil column is linearly related with the number of blow counts (N-values) at these sites. The linear relationships are site-specific and the coefficients of linear relation are sensitive to the lithology of subsurface formation, which was verified by borehole data. The study demonstrates the usefulness of the ERT method in geotechnical investigations, which is economic, efficient and less time consuming in comparison to the other geotechnical methods, such as SPT and DCPT, used for the purpose. 相似文献