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621.
El Sayed Ibrahim Selim 《Arabian Journal of Geosciences》2012,5(4):713-722
Abu Madi Messinian Formation occupied a prominent issue as the most exploratory target in the search for natural gas in the Nile Delta. Interpretation of the available onshore seismic reflection data of the northeastern part of the Nile Delta reveals that there is a series of troughs, depressions, and mounded structures within the Abu Madi Formation (Messinian time). Onlap, downlap, and truncation relationship of reflections were observed very close to fault locations in the Miocene time. Many channels have been imaged clearly and described on the seismic reflection data of the onshore Nile Delta. The depths to these channels range between 2.5?s in the southeast to 3.1?s (t.w.t.) while their width ranges from 1?km to more than 5?km. These channels were formed by two factors: (1) submarine erosion and redeposition, during the relative sea level falls and (2) tectonic effect. Many of these channels followed more or less the trends of the historic branches of the ancient Nile Delta. The classification of these channels is based on some factors: (1) channel morphology (depth and width), (2) channel shape, and (3) tectonic effect. 相似文献
622.
Doklady Earth Sciences - 相似文献
623.
Samuel Angiboust Mostafa Fayek Ian M. Power Alfredo Camacho Georges Calas Gordon Southam 《Mineralium Deposita》2012,47(8):859-874
Epithermal uranium deposits of the Sierra Pe?a Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ~20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ34S values (~?24.5‰) in pyrites and the presence of low δ13C (~?24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments. 相似文献
624.
Georg Amthauer Miodrag K. Pavi?evi? Rade Jelenkovi? Ahmed El Goresy Bla?o Boev Predrag Lazi? 《Mineralogy and Petrology》2012,105(3-4):157-169
The LORandite EXperiment (acronym LOREX) is the only geochemical solar neutrino experiment still actively persued. The Tl-mineral lorandite, TlAsS2, occurs in the ore deposit Allchar, Macedonia, close to the border of Greece. The polychronous and polygenetic Sb-As-Tl-Au Allchar deposit was formed by complex physico-chemical processes occurring in a heterogeneous geological environment and by interaction of polyphase hydrothermal fluids with the surrounding magmatic, sedimentary and metamorphic rocks. The genesis of ore mineralization is related to the products of polyphase magmatic activity of Pliocene age (~6.5 to ~1.8?Ma) and its spatial location was controlled by magmatic, structural and lithological factors. The Allchar deposit comprises several orebodies of various shapes, textural-structural characteristics and element associations. Thallium mineralization, which is of significance for the LOREX project, has been proved in two locations: (i) ore body Crven Dol in the northern part and (ii) ore body Centralni Deo in the central part of the Allchar deposit. The age of Tl-mineralization is 4.22?Ma at the Crven Dol locality and 4.31?Ma at Rudina near to the Centralni Deo locality. The present depth of ore mineralization from the present soil surface is about 30?m to 140?m, whereas the paleodepth of its formation is considerably bigger. Using the method of quantitative geomorphological analysis, and AMS- and MS-measurements of cosmogenic radionuclides 26Al, 21Ne, and 3He, the erosion rate has been established to be ~70?m/Ma in the broader area of the Crven Dol locality and ~130?m/Ma in the Centrani Deo of the Allchar deposit. On the basis of these erosion rates and ages of Tl-mineralization, we have calculated the paleo-depth of Lorandite to be ca 180?m?±?35?m to 420?m?±?80?m. Geochemical and mineralogical investigations on lorandite in particular its trace elements (Pb, U, and Th), the quantity of lorandite in the two ore bodies, the geological age and the paleo depth of Tl-mineralization have provided encouraging results and indicate the feasibility of the LOREX project. 相似文献
625.
Cavity expansion theories are employed in a wide range of geotechnical applications including interpretation of pressure meter tests, evaluation of shaft capacity of piles, and pulling forces for horizontal directional drilling. Most of these theories assume infinite medium and isotropic stress field, which may not be justified for many applications. The main objectives of this paper are two folds: to investigate the effects of the free surface, stress gradient, and in situ stress anisotropy on the displacements during the expansion phase of cavities embedded in dilatant sands; and to establish correction factors to account for these effects. The investigation was conducted using two-dimensional finite element analyses. It was found that the cavity expansion theory due to Yu and Houlsby (Geotechnique 41:173–183, 1991) can be used reliably for cases subjected to an initial isotropic stress and embedment depth to diameter ratio of 20 or higher. However, it becomes inaccurate for shallow embedment depth and/or stress anisotropy conditions. An analytical procedure to account for the effects of embedment and/or stress anisotropy was proposed. The applicability of the proposed procedure was demonstrated for a wide range of soil properties and geometrical configurations. The results obtained confirmed its ability to estimate the cavity pressures within 10?% of the values obtained using FEA calculations. 相似文献
626.
Approaches to improve the governance of social–ecological systems are difficult to define in situations where governance is weak, that is, involving limited interactions between the actors and weak management of natural resources. This article analyzes an action research process implemented in the Chaouia coastal region of Morocco, where weak governance of the social–ecological system led to a groundwater and agricultural crisis. A dialogue between local actors was set up with the aim of identifying strategies to address the crisis. First separately and then together, farmers’ groups and staff members of public organizations analyzed the existing situation, scenarios for the future of the area, and strategies to cope with the crisis. Contrary to the expectations of the participants, farmers and staff members of public organizations had productive discussions. This approach clarified how social learning can be achieved and governance improved in this situation of weak governance. 相似文献
627.
Ocean Dynamics - Third-generation models employ a host of parameterization schemes to consider the input wind forcing and the wave energy dissipation under different physical settings and... 相似文献
628.
E. A. Saad S. A. El Wakil H. M. Haggag H. M. Machali 《Astrophysics and Space Science》1986,125(1):1-11
Equations for radiation, scattered anisotropically from dispersive media are obtained for general boundary conditions. The Padé approximant technique is used to calculate the reflection coefficients for these media. Numerical results are given for diffuse reflected boundary condition. 相似文献
629.
Diffuse reflection and transmission coefficients in a plane parallel medium are calculated for a Rayleigh phase-function averaged over polarization and Rayleigh polarized phase-function. This is calculated by imbedding the finite medium into a semi-infinite scattering and absorbing medium. Numerical calculations for semi-infinite albedo are compared with Pomraning results. The albedos for finite medium are calculated via the imbedding equations which converge for large τ to the value of semi-infinite medium. 相似文献
630.
O. M. El Mekki 《Solar physics》1983,85(1):83-91
Hydromagnetic planetary-gravity waves propagating on a β-plane through a zonal flow and transverse magnetic field are examined for instability. Such instabilities may be related to same physical phenomena in the atmospheres of the Sun and planets and in the Earth's core. It is found that the onset of instability depends on the directions of the vertical and transverse wave-numbers and the zonal flow. It is also shown that as the magnetic field intensity is kept uniform instability can onset provided that the zonal flow strength does not exceed a certain factor, which depends on the parameters of the medium, and then the zonal wavenumbers that can become unstable are limited to a given range. If the basic Alfvén wave speed is allowed to vary whereas the zonal flow is kept uniform the zonal wavenumbers that can exhibit instability are again limited but the basic Alfvén wave speed can assume any value. 相似文献