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21.
Ahmed Salem 《Geophysical Prospecting》2005,53(1):75-82
This paper develops an automatic method for interpretation of magnetic data using derivatives of the analytic signal. A linear equation is derived to provide source location parameters of a 2D magnetic body without a priori information about the nature of the source. Then using the source location parameters, the nature of the source can be ascertained. The method has been tested using theoretical simulations with random noise for two 2D magnetic models placed at different depths with respect to the observation height. In both cases, the method gave a good estimate for the location and shape of the sources. Good results were obtained on two field data sets. 相似文献
22.
A new methodology for deriving the uplift load–displacement response of long driven piles in cohesionless soils is proposed. This method accounts for the effects of the friction fatigue processes during pile driving and the existence of locked-in residual stresses at the end of pile driving before commencing the pile load test. A hyperbolic formulation is utilized to simulate the nonlinear load transfer curves (the so-called t–z curves). The utility of this technique is demonstrated for a field pullout load test on a driven pile in sand. Predicted and measured load–displacement curves showed good agreement, indicating that this approach yields reasonable results as long as representative input parameters are employed. 相似文献
23.
Present environment and historic changes from the record of lake sediments, Dhaka City, Bangladesh 总被引:1,自引:0,他引:1
Changes in the urban environment of Dhaka City have been evaluated from the geochemical compositions of bottom sediments from two lakes, Gulshan-1 and –2. Abundances of Pb, Zn, Fe2O3 and total sulfur in a Gulshan-1 core gradually increase toward the sediment–water interface. Three stages of condition change can be recognized in Gulshan-1, based on trace metal concentration patterns. The basal Stage I corresponds to background Dhaka sedimentation, whereas gradual increase in Stage II represents the beginning of pollution. Marked increases in Pb and Zn in Stage III reflect rapid urban development and increased emissions to the atmosphere. The condition of the Dhaka environment is compared to that of Japan based on Zn–Fe2O3 and Pb–Fe2O3 relations. Zn–Fe2O3 ratios in Stage I Dhaka sediments are similar to normal Japanese sediments, whereas Stage II data lie on the same trend as Japan urban sediments. Dhaka Stage III samples have greater Zn:Fe2O3 ratios than Japanese urban sediments, suggesting Zn pollution from poorly-controlled industrial sources. In contrast, the Dhaka Pb–Fe2O3 trend lies between normal and urban Japanese sediments. Although vehicle emissions in Dhaka are now significant, the data suggest that Pb pollution from this source remains in its early stages. 相似文献
24.
ShahidNaseem ShamimAhmedSheikh ErumBashir KhaulaShirin 《中国地质大学学报(英文版)》2005,16(2):160-169
INTRODUCTION Theinteractionofcomplextectonicphasesinthe IndusbasinduringtheCretaceoushasimpartedascal lopedoutlineinthesedimentarysequences.Thedevel opmentofdiversifiedsedimentaryformationsduringthe Cretaceousensuresexcellentsources(SembarandGoru formations)andreservoirs(MoghalKotandPabfor mations)forhydrocarbons.Numerousplaysandpros pectsofhydrocarbonareassociatedwiththeCretaceous system,consequentlytheIndusbasinisattractiveto petroleumexplorationcompaniesinPakistan(Sheikh andNa… 相似文献
25.
S. Mahmood A. Ahmed A. Hussain M. Athar 《International Journal of Environmental Science and Technology》2005,2(3):193-199
Variability among populations was analyzed in five provenances of Acacia nilotica from spatially variable habitats. Populations of A. nilotica developed in response to their habitat conditions. The level of variability was significantly high among the populations. Phenotypic variability was extremely high for leaf and stipular spine characteristics. The nature of morphological variability for vegetative traits appeared environmentally controlled. The differentiation of leaf and stipular spine expression seems to have an adaptive significance for the species in terms of water economy. Although, seed and pod characteristics are genetically controlled showing a lower proportion of variability but these traits supported r and k-selection that may allow the species to survive under a wide array of contrasting habitats. The study suggested that populations of A. nilotica are differentiated in relation to the heterogeneity of environment. These populations became adapted to their habitat through the variability of morphological expressions. The morphologically differentiated populations of the species had allowed them to maintain themselves in a wide array of environmental situations enabling A. nilotica to occupy ample ecological ranges. 相似文献
26.
Unexpectedly high-PGE chromitite from the deeper mantle section of the northern Oman ophiolite and its tectonic implications 总被引:2,自引:0,他引:2
Unusually high, platinum-group element (PGE) enrichments are reported for the first time in a podiform chromitite of the northern Oman ophiolite. The chromitite contains Б.5 ppm of total PGE, being highly enriched in the IPGE subgroup (Ir, Os and Ru) and strongly depleted in the PPGE subgroup (Rh, Pt and Pd). Its platinum-group minerals (PGMs) are classified into three types arranged in order of abundance: (1) sulphides (Os-rich laurite, laurite-erlishmanite solid solution and an unnamed Ir sulphide), (2) alloys (Os-Ir alloy and Ir-Rh alloy), and (3) sulpharsenides (irarsite and hollingworthite). The high PGE concentrations are observed only in a discordant chromitite deep in the mantle section, which has high-Cr# (>0.7) spinel with an olivine matrix. All the other types of chromitite (in the Moho transition zone (MTZ) and concordant pods in the deeper mantle section) are poor in PGEs and tend to have spinels with lower Cr# (up to 0.6). This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low-Cr#, PGE-poor chromitites at the Moho transition zone and, to a lesser extent, within the mantle, possibly beneath a fast-spreading mid-ocean ridge. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr#, PGE-rich discordant chromitite in the upper mantle, possibly in a supra-subduction zone setting. 相似文献
27.
Geology and Genesis of Barite Deposits of Lasbela and Khuzdar Districts, Balochistan, Pakistan 总被引:1,自引:0,他引:1
Abstract: Geology and genesis of the sediment-hosted barite deposits in the Lasbela and Khuzdar districts of Balochistan, Pakistan have been studied and described. Hot solutions generated in rifting environments during the initial stage of separation of the continental crustal block related to the Gondwanaland fragmentation subsequently reposed in Jurassic rocks, most probably played an important role on barite formation.
Paragenetic relationships indicate that the period of barite mineralization was distinctly later than that of zinc-lead sulphide mineralization in the Lasbela and Khuzdar districts. The barite deposits occur on top and in peripheral to the zinc-lead sulphides. The earliest barite mineralization took place in the clastic rock sequence of the lower Spingwar Formation as cross cutting veins. Younger mineralization as stratabound-replacement type occurred within the platform limestones of Loralai Formation, just above the Spingwar Formation. The youngest mineralization as stratiform is hosted within black shales, mudstones and argillaceous limestones of Anjira Formation at the top of Loralai Formation. In the light of the evidences gathered from field and laboratory investigations a genetic model has been deduced. 相似文献
Paragenetic relationships indicate that the period of barite mineralization was distinctly later than that of zinc-lead sulphide mineralization in the Lasbela and Khuzdar districts. The barite deposits occur on top and in peripheral to the zinc-lead sulphides. The earliest barite mineralization took place in the clastic rock sequence of the lower Spingwar Formation as cross cutting veins. Younger mineralization as stratabound-replacement type occurred within the platform limestones of Loralai Formation, just above the Spingwar Formation. The youngest mineralization as stratiform is hosted within black shales, mudstones and argillaceous limestones of Anjira Formation at the top of Loralai Formation. In the light of the evidences gathered from field and laboratory investigations a genetic model has been deduced. 相似文献
28.
Major, trace, and REE data for three localities of calc?Calkaline older granitoid rocks exposed in the north Eastern Desert of Egypt are presented. These rocks were selected to cover wide compositional spectrum of the Egyptian older granitoid varieties. They are petrographically represented by granodiorite, tonalite, quartz?Cdiorite, and quartz?Cmonzodiorite. The rocks are comparable with the peraluminous, unfractionated calc?Calkaline suites and fall within the volcanic arc and I-type granite fields. So, they can be regarded as belonging to the volcanic arc collision stage (665?C614?Ma). The granitoids are geochemically similar to other rocks recorded from continental margin arc-systems being exhibit light-REE enriched patterns with variable but chiefly positive Eu anomaly. The latter has reverse relationship with the ??REE, which was attributed to the fractionation of hornblende during partial melting. These patterns are comparable with models involving partial melting of amphibolitic source. This source must represent basalts, gabbros, or volcanics of an island arc system that were transformed to the level of the island arc crust during continental growth where the P?CT conditions are suitable for partial melting. Thus, it is plausible that the studied rocks were derived by partial melting of LREE-enriched, garnet-free, and amphibole-bearing (i.e., hydrated) mafic source. Wadi Milaha granitoids are consistent with the derivation by a high degree of partial melting (30?C40%) of amphibolite protolith in the deep crust. However, the two other localities (Wadi Umm Anab and West Gharib) are matching with 20?C30% partial melting. Within each locality, variation in rock types from granodiorite to tonalite is said to be dominated by variable degree of restite separation during magma ascent. The high water and volatile contents in Wadi Milaha granitoids allowed higher degree of partial melting (30?C40%). Moreover, the lower volatile contents in the other two localities (Wadi Umm Anab and West Gharib) gave lower degrees of partial melting (20?C30%). These processes may resemble important geodynamic features of the Arabo-Nubian Shield evolution in the north Eastern Desert of Egypt. 相似文献
29.
Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh 总被引:1,自引:0,他引:1
M. Aziz Hasan Mattias von Br?mssen Prosun Bhattacharya Kazi Matin Ahmed Arif Mohiuddin Sikder Gunnar Jacks Ondra Sracek 《Environmental Geology》2009,57(3):499-511
The shallow alluvial aquifers of the delta plains and flood plains of Bangladesh, comprises about 70% of total land area are
mostly affected by elevated concentrations of arsenic (As) in groundwater exposing a population of more than 35 million to
As toxicity. Geochemical studies of shallow alluvial aquifer in the Meghna flood plain show that the uppermost yellowish grey
sediment is low in As (1.03 mg/kg) compared to the lower dark grey to black sediment (5.24 mg/kg) rich in mica and organic
matter. Sequential extraction data show that solid phase As bound to poorly crystalline and amorphous metal (Fe, Mn, Al)-oxyhydroxides
is dominant in the grey to dark grey sediment and reaches its maximum level (3.05 mg/kg) in the mica rich layers. Amount of
As bound to sulphides and organic matter also peaks in the dark grey to black sediment. Vertical distributions of major elements
determined by X-ray fluorescence (XRF) show that iron (Fe2O3), aluminum (Al2O3) and manganese (MnO) follow the general trend of distribution of As in the sediments. Concentrations of As, Mn, Fe, HCO3
−, SO4
2− and NO3
− in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry. Mineralogical analysis
by X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with energy dispersive X-ray spectrometer (EDS) revealed
dominance of crystalline iron oxides and hydroxides like magnetite, hematite and goethite in the oxidised yellowish grey sediment.
Amorphous Fe-oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of
biotite is playing a critical role as the source of Fe(III)-oxyhydroxides which in turn act as sink for As. Presence of authigenic
pyrite in the dark grey sediment indicates active reduction in the aquifer. 相似文献
30.
N. U. Ain M. Latif K. Ullah S. Adnan R. Ahmed M. Umar M. Azam 《Theoretical and Applied Climatology》2020,140(1):69-89
As a result of climate change and unsustainable land use management in the recent past, droughts have become one of the most devastating climatic hazards whose impacts may prolong from months to years. This study presents analysis of droughts for two major cropping seasons, i.e., Kharif (May–September) and Rabi (October–April), over the Potwar Plateau of Pakistan. The analysis is performed using various datasets viz. observational, reanalysis, and Regional Climate Models (RCMs), for the past (1981–2010) and future (2011–2100) time periods. The following two methods for the identification of dry and wet years, also referred to as drought and wetness, are applied: (1) the percentile rank approach and (2) the drought indices, Standardized Precipitation Index (SPI) and Reconnaissance Drought Index (RDI). Future projections of droughts are investigated using RCM (RegCM4.4 and RCA4) outputs from CORDEX South Asia domain under two Representative Concentration Pathway (RCP) scenarios, RCP4.5 and RCP8.5. Generally, the indices show non-significant decreasing trends of drought severity in the recent past for all cases; however, significant increasing trends are observed for annual (0.006) and Kharif (0.007) cases under RCP4.5 scenario. The analysis of large-scale atmospheric dynamics suggests the significant role of low-level geopotential height anomalies over Tibetan Plateau (northwest of Pakistan) during Kharif (Rabi) season in controlling drought occurrence by transporting moisture from the Bay of Bengal (Arabian Sea). Moreover, composites of vertically integrated moisture transport, moisture flux convergence/divergence, and precipitable water anomalies show their marked contribution in maintaining the drought/wetness conditions over the Potwar region. 相似文献